Dr. I.S. DAHIYA
CHIEF SCIENTIST CUM HEAD (RTD.), CCS HARYANA AGRICULTURAL UNIVERSITY, HISAR, INDIA
(MAY, 2009)
This write-up is first part of the series "Learn to live with problematic soils and waters". It is shown that the problem of salt affected soils has become a global issue because of poor land and water management practices as well as insufficient reclamation operations in many parts of the world. Salt affected soils are found mostly in arid and semi arid regions, but in several humid regions as well, in more than 100 countries. There are about 250 million ha as existing and about 750 million ha as potential salt affected lands in the world. The problem is increasing day by day, as the world as a whole is losing at least 3 ha of fertile land every minute due to salinization and sodification. As a reslult, fertile and productive lands are turning into non-productive salilne and sodic soils, which result in less crop production and eventually abandonment of the land. The attention given to salt affected soils by the scientific community; causes of origin of salt affected soils; sources of salts; processes responsible for accumulation of salts in soils; and distribution and characteristics of salt affected soils is discussed in details, so that the information could be useful in the proper identification, reclamation and management of soils with problems caused by the presence of excess salts.
CONTENTS
1. INTRODUCTORY REMARKS
2. ATTENTION PAID TO SALT-AFFECTED SOILS BY THE GLOBAL SCIENTIFIC COMMUNITY
2.1. Historical background
2.2. Scientific activities - international symposia
2.3. Preparing global/regional maps, records and reports
3. CAUSES OF ORIGIN OF PROBLEMATIC SOILS
3.1. Introduction of irrigation: The main cause
3.2. Other causes
4. SALT-AFFECTED SOIL: WHAT IT IS AND HOW IT IS RECOGNIZED (CLASSIFICATION OF SALT-AFFECTED SOILS)
5. WHAT IS THE ORIGIN (SOURCES) OF SALTS AND HOW THEY ARE ACCUMULATED IN SOILS LEADING TO THE FORMATION OF SALINE SOILS
5.1. Origin (sources) of salts
5.1.1. Geologically weathered rocks
5.1.2. Ground water containing salts of geologic or marine origin (fossil salts)
5.1.3. Oceanically derived rainfall and other oceanic sources
5.2. How salts are accumulated in root zone leading to soil salinity
5.2.1. Accumulation from saline ground
5.2.2. Accumulation from salic horizon in soil profile
5.2.3. Accumulation from irrigation and run-off waters due to poor soil physical properties
5.2.4. Accumulation from saline water being used as a source of irrigation
5.2.5. Other ways of salt accumulation
6. HOW EVOLUTION OF SALINE-SODIC AND SODIC SOILS OCCUR
6.1. Evolution through sequential process of salinizaiion, sodification and desalinzation
6.2. Evolution through the direct process of sodification
7. SUMMARY OF THE PROCESSES OF SOIL SALINZATION AND SODIFICATION
8. OCCURRENCE (GEOGRAPHICAL DISTRIBUTION) OF SALT-AFFECTED SOILS
8.1. Area of salt affected soils
8.2. Geographical distribution of salt affected soils
8.2.1. Africa
8.2.2. Asia
8.2.3. Australia
8.2.4. Europe
8.2.5. North America
8.2.6 South America
9. CHARACTERISTICS, LIMITATIONS, LAND USE AND POTENTIALITIES OF SALT-AFFECTED SOILS
9.1. Saline soils
9.2. Sodic soils
10. CONCLUSIONS
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1. INTRODUCTORY REMARKS
Soil and water are the two main resources of the earth and their conservation and management are the only ways to protect these valuable resources. These resources are precious natural resources and are Nature's gift to the mankind. The prosperity of the mankind depends on the richness of these resources. In countries like India, China, Pakistan, Indonesia, Malaysia and many other countries of Asia, Africa and South America, where the population pressure on these natural resources is high, rational utilization of them assumes great importance for the optimum and sustained production with minimum hazards. Essentially, this will mean proper utilization of soil and water resources. These resources, however, have been most recklessly used by humankind in the past to extract more and more from them. Consequently, vast areas in the world have gone waste and have not remained cultivable. This has caused rapid deterioration and degradation of lands.
The present need for more food and fiber entails reclamation and development of new land resources apart from an increase in the agricultural input a necessity for greater production. Irrigated agriculture plays, and will continue to play, a major role in increasing the food supply - especially in arid and semi-arid regions. At present, heavy investments are being made in the development of irrigated farming in countries where the quality of either the soil or the water, or sometimes both, is not good enough to yield an economic return without the addition of reclamation measures or special management practices. Moreover, the deterioration of agricultural production on previously productive lands in the arid and semi-arid zones can be directly attributed to the evolution of salinity and alkalinity (sodicity). The land resource is limited, as the total geographical area is fixed. The amount of land (soil) and land based resources (like water and vegetation) is, thus, finite. Land and its resources are, therefore, scarce in supply. These are irreplaceable and not reproducible. While the land is finite, the population dependent on land and its needs are infinite. The needs have been increasing with time. To make it more clear, an example of Haryana state of India is given here (Fig. 1). The total population of this state in 1901 was 4.62 million. It increased to 5.27, 5.77, 7.59, 10.4, 12.9 and 21.5 million in 1941, 1951, 1961, 1971, 1981 and 2001, respectively. As a result, the per ca pita availability of land declined from 0.96 ha in 1901 to 0.84, 0.78, 0.58, 0.44, 0.34 and 0.21 ha, respectively, in 1941, 1951, 1961, 1971, 1981 and 2001. Similarly, India has a total geographical area of 3.3 million sq km (Fig. 1), which is slightly more than one third of USA and only 2.4% of the world's surface. Although, India occupies
1. INTRODUCTORY REMARKS
Soil and water are the two main resources of the earth and their conservation and management are the only ways to protect these valuable resources. These resources are precious natural resources and are Nature's gift to the mankind. The prosperity of the mankind depends on the richness of these resources. In countries like India, China, Pakistan, Indonesia, Malaysia and many other countries of Asia, Africa and South America, where the population pressure on these natural resources is high, rational utilization of them assumes great importance for the optimum and sustained production with minimum hazards. Essentially, this will mean proper utilization of soil and water resources. These resources, however, have been most recklessly used by humankind in the past to extract more and more from them. Consequently, vast areas in the world have gone waste and have not remained cultivable. This has caused rapid deterioration and degradation of lands.
The present need for more food and fiber entails reclamation and development of new land resources apart from an increase in the agricultural input a necessity for greater production. Irrigated agriculture plays, and will continue to play, a major role in increasing the food supply - especially in arid and semi-arid regions. At present, heavy investments are being made in the development of irrigated farming in countries where the quality of either the soil or the water, or sometimes both, is not good enough to yield an economic return without the addition of reclamation measures or special management practices. Moreover, the deterioration of agricultural production on previously productive lands in the arid and semi-arid zones can be directly attributed to the evolution of salinity and alkalinity (sodicity). The land resource is limited, as the total geographical area is fixed. The amount of land (soil) and land based resources (like water and vegetation) is, thus, finite. Land and its resources are, therefore, scarce in supply. These are irreplaceable and not reproducible. While the land is finite, the population dependent on land and its needs are infinite. The needs have been increasing with time. To make it more clear, an example of Haryana state of India is given here (Fig. 1). The total population of this state in 1901 was 4.62 million. It increased to 5.27, 5.77, 7.59, 10.4, 12.9 and 21.5 million in 1941, 1951, 1961, 1971, 1981 and 2001, respectively. As a result, the per ca pita availability of land declined from 0.96 ha in 1901 to 0.84, 0.78, 0.58, 0.44, 0.34 and 0.21 ha, respectively, in 1941, 1951, 1961, 1971, 1981 and 2001. Similarly, India has a total geographical area of 3.3 million sq km (Fig. 1), which is slightly more than one third of USA and only 2.4% of the world's surface. Although, India occupies
Figure 1: Map of India
(A)
(B)
(C)
(D)
(D) Figure 2: (A) World population, (B) world population growth rate, (C) India population and (D) India population growth rate
Table 1. India population
Year, Population
2000, 1014003817
2001, 1029991145
2002, 1045845226
2003, 1049700118
2004, 1065070607
2004, 1065070607
2005, 1080264388
2006, 1095351995
2007, 1129866154
2008, 1147995898
Table 2: Population growth rate of India
Year, Population growth rate (%)
Year, Population growth rate (%)
2000 , 1.58
2001, 1.55
2002, 1.51
2003, ,1.47
2004, 1.44
2005, 1.40
2006, 1.38
2007, 1.606
2008, 1.578
Table 3: World population
Year, Population
2000, 6080671215
2001, 6157400560
2002, 6233821945
2003, 6302309691
2004, 6379157361
2005, 6446131400
2006, 6525170264
2007, 6602224175
2008, 6677563921
Table 4: World population growth rate
Year, Population growth rate (%)
2000, 1.3
2001, 1.25
2002, 1.23
2003, 1.17
2004, 1.14
2005, 1.14
2006, 1.14
2007, 1.167
2008, 1.159
-------------------------------only 2.4% of the world's land area, it supports over 16% of the world's population. Population and population density of India are shown in Figure 1. Only China has a larger population than India. According to the census of 2001, 1.023 billion people, about 4 times the population of USA, were living on this 2.4% of the world's surface. India's population rose by 21.34% from 1991 to 2001. Fig. 2 and Table 1 show that India's population rose from 1.014 billion in 2000 to 1.148 billion in 2008. Population growth rate of India during this period ranged between 1.38 and 1.62% per annum (Table 2 and Fig. 2), as a result of which the projected population of India would be 1.3 billion by 2020. Thus, per ca pita availability of land in India was just only 0.325 ha in 2000. The figure decreased to 0.288 ha in 2008 and would be 0.254 ha in 2020. According to one estimate, as on January 1, 2008, the population growth rate of more than 100 countries is more than 1% and 86 countries have population growth rate more than that of India's. Similarly, world population increased from 6.08 billion in 2000 to 6.68 billion in 2008 (Table 3, Fig. 2) and world population growth rate ranged between 1.14 and 1.30 %. World land area is 148.94 million sqkm. Thus, per capita land availability in world was 2.45 ha in 2000, which decreased to 2.42, 2.39, 2.36, 2.34, 2.31, 2.28, 2.26 and 2.31 ha in 2001, 2002, 2003, 2004, 2005, 2006, 2007 and 2008, respectively.
On the one hand, the increased population in the world has decreased the per ca pita land holding, on the other hand, it increased the demand for food, sugar, vegetable, oil, cotton and several other land products apart from the increased demand of land for houses and other infrastructures. The increase in population has also resulted in the increase of livestock leading to higher demands for fodder and fuel. Moreover, the pressure on land has also been increasing due to pressure of urbanization. The trend of migration from rural to urban and industrial areas is causing a decrease in the cultivable areas. The land which is used for urbanization and industrialization is highly fertile and productive.
To meet these demands is a challenge to agricultural scientists, field officers, planners and even to the farmers that they will have to produce more each unit area than we are producing today. Global food production will need to increase by 38% by 2025 and by 57% by 2050 if food supply to the growing world population is to be maintained at current levels. Most of the suitable land has been cultivated and expansion into new areas to increase food production is rarely possible or desirable. The aim, therefore, should be an increase in yield per unit of land rather than in the area cultivated. More efforts are needed to improve productivity as more lands are becoming degraded. It is estimated that about 15% of the total land area of the world has been degraded by soil erosion and physical and chemical degradation, including soil salinization. The rise in water table, salinization and erosion with the advent of irrigation and increasing desertification due to deforestation would further degrade our lands, if we do not take proper care. Considerable damage has already been done to our soil and water resources by these problems. In that case, the increasing needs of the ever increasing population would be difficult to meet. Therefore, the need of the hour is not only to improve and reclaim our sick lands, but also to stop our lands from further sickness in future.
On the one hand, the increased population in the world has decreased the per ca pita land holding, on the other hand, it increased the demand for food, sugar, vegetable, oil, cotton and several other land products apart from the increased demand of land for houses and other infrastructures. The increase in population has also resulted in the increase of livestock leading to higher demands for fodder and fuel. Moreover, the pressure on land has also been increasing due to pressure of urbanization. The trend of migration from rural to urban and industrial areas is causing a decrease in the cultivable areas. The land which is used for urbanization and industrialization is highly fertile and productive.
To meet these demands is a challenge to agricultural scientists, field officers, planners and even to the farmers that they will have to produce more each unit area than we are producing today. Global food production will need to increase by 38% by 2025 and by 57% by 2050 if food supply to the growing world population is to be maintained at current levels. Most of the suitable land has been cultivated and expansion into new areas to increase food production is rarely possible or desirable. The aim, therefore, should be an increase in yield per unit of land rather than in the area cultivated. More efforts are needed to improve productivity as more lands are becoming degraded. It is estimated that about 15% of the total land area of the world has been degraded by soil erosion and physical and chemical degradation, including soil salinization. The rise in water table, salinization and erosion with the advent of irrigation and increasing desertification due to deforestation would further degrade our lands, if we do not take proper care. Considerable damage has already been done to our soil and water resources by these problems. In that case, the increasing needs of the ever increasing population would be difficult to meet. Therefore, the need of the hour is not only to improve and reclaim our sick lands, but also to stop our lands from further sickness in future.
2. ATTENTION PAID TO SALT-AFFECTED SOILS BY THE GLOBAL SCIENTIFIC COMMUNITY
2.1. Historical background
The problems of salt affected soils called forth wide international interest among soil scientists even before the foundation of the International Society of Soil Science (ISSS). This interest was identifiable already at the 1st International Conference on Agrogeology (Budapest, Hungary 1909) and became more pronounced at subsequent scientific meetings. After the founding of ISSS (in 1924), the 1st International Congress of Soil Science (Washington, D. C., 1927) adopted a resolution on the organization of the Alkali Sub commission. The scientific activity carried out within the framework of the Alkali Sub commission manifested itself not only on the subsequent International Congresses of Soil Science, but in the organization of special Conferences on soil salinity problems (Groningen 1926, Budapest1929, Copenhagen 1933, Helsinki 1938) as well. In addition to the collection of the existing literature on saline and sodic soils, efforts of the Alkali Sub commission were focused on organizing and coordinating research on international level. The fruitful and successful international cooperation in this field was interrupted by the Second World War. After the War, the agriculture of a number of countries was confronted with the expanding problem of soil salinity and sodicity. The rapid growth of the population required the urgent utilization and amelioration of salt affected soils, especially in the developing countries of the arid and semi arid zones. At the same time, with the increase in irrigated areas the danger of secondary salinization and sodification became more and more aggravating. In many countries agricultural production became seriously hindered or even impossible on vast areas. Scientific activity related to soil salinity and alkalinity problems continued in the postwar period in many countries, even in well-known special scientific centers. Publications on theoretical, methodical and practical questions and of basic importance appeared in the fifties. Widely recognized results of investigation have been presented at the International Congresses of Soil Science since 1950. The absence of sufficient coordination, however, slowed down the exchange of information and the dissemination of scientific knowledge in many respects. In 1964, the Hungarian Academy of Sciences organized - with the cooperation of UNESCO - a Symposium on Sodic Soils in Budapest. During the discussions at the Symposium, the participants repeatedly expressed the necessity of a body to coordinate investigations and ensure the exchange of information, knowledge and experiences related to salt affected soils, and the "Recommendations” also reflected this demand. The 8th International Congress of Soil Science (Bucharest 1964) approved the reactivation of the Alkali Sub commission. The new name: Sub commission on Salt Affected Soils showed not only a terminological difference or a more precise phrasing, but the widening of the sphere of the activities as well.
2.1. Historical background
The problems of salt affected soils called forth wide international interest among soil scientists even before the foundation of the International Society of Soil Science (ISSS). This interest was identifiable already at the 1st International Conference on Agrogeology (Budapest, Hungary 1909) and became more pronounced at subsequent scientific meetings. After the founding of ISSS (in 1924), the 1st International Congress of Soil Science (Washington, D. C., 1927) adopted a resolution on the organization of the Alkali Sub commission. The scientific activity carried out within the framework of the Alkali Sub commission manifested itself not only on the subsequent International Congresses of Soil Science, but in the organization of special Conferences on soil salinity problems (Groningen 1926, Budapest1929, Copenhagen 1933, Helsinki 1938) as well. In addition to the collection of the existing literature on saline and sodic soils, efforts of the Alkali Sub commission were focused on organizing and coordinating research on international level. The fruitful and successful international cooperation in this field was interrupted by the Second World War. After the War, the agriculture of a number of countries was confronted with the expanding problem of soil salinity and sodicity. The rapid growth of the population required the urgent utilization and amelioration of salt affected soils, especially in the developing countries of the arid and semi arid zones. At the same time, with the increase in irrigated areas the danger of secondary salinization and sodification became more and more aggravating. In many countries agricultural production became seriously hindered or even impossible on vast areas. Scientific activity related to soil salinity and alkalinity problems continued in the postwar period in many countries, even in well-known special scientific centers. Publications on theoretical, methodical and practical questions and of basic importance appeared in the fifties. Widely recognized results of investigation have been presented at the International Congresses of Soil Science since 1950. The absence of sufficient coordination, however, slowed down the exchange of information and the dissemination of scientific knowledge in many respects. In 1964, the Hungarian Academy of Sciences organized - with the cooperation of UNESCO - a Symposium on Sodic Soils in Budapest. During the discussions at the Symposium, the participants repeatedly expressed the necessity of a body to coordinate investigations and ensure the exchange of information, knowledge and experiences related to salt affected soils, and the "Recommendations” also reflected this demand. The 8th International Congress of Soil Science (Bucharest 1964) approved the reactivation of the Alkali Sub commission. The new name: Sub commission on Salt Affected Soils showed not only a terminological difference or a more precise phrasing, but the widening of the sphere of the activities as well.
2.2. Scientific activities - international symposia
One of the main activities of the Sub Commission on Salt Affected Soils has been the regular organization of intercongress International Symposia. The aim of these meetings was to enhance the exchange of recent information, knowledge and opinion among scientists on cardinal and current problems related to soil salinity. Further aims were: discussions about the coordinating activities of the Sub commission in relation to global/ regional programmes and to give directions and main tasks of salinity investigations for the forthcoming period. The title and the main topics of each meeting have been closely related to specific problems important for the host country from both agricultural and environmental aspects. Moreover, the Symposia have provided good opportunities for local scientists to present and discuss their scientific results, land use, soil reclamation and management. Altogether 15 intercongress Symposia have been organized in the past 38 years in different countries where salt affected soils occur, with the cooperation and support of the Soil Science Society, the relevant institutions, governmental and other organizations of the host country (Table 5). The success of technical activities and the friendly, collegial atmosphere in case of all of the International Symposia was mainly owing to the local scientists: the professional preparation and organization conducted by the staff of the organizing and collaborating institutions, which did their best to ensure the effective work of the given meeting. During the past four decades, apart from the above mentioned Symposia, the Sub commission on Salt Affected Soils has made considerable efforts to appropriately represent the problem of soil salinity on the subsequent International Congresses of Soil Science. Keynote lectures, plenary papers gave the state-of art and the outlooks of global soil salinity issues. Technical session(s) and specific meetings were devoted to the problem.
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One of the main activities of the Sub Commission on Salt Affected Soils has been the regular organization of intercongress International Symposia. The aim of these meetings was to enhance the exchange of recent information, knowledge and opinion among scientists on cardinal and current problems related to soil salinity. Further aims were: discussions about the coordinating activities of the Sub commission in relation to global/ regional programmes and to give directions and main tasks of salinity investigations for the forthcoming period. The title and the main topics of each meeting have been closely related to specific problems important for the host country from both agricultural and environmental aspects. Moreover, the Symposia have provided good opportunities for local scientists to present and discuss their scientific results, land use, soil reclamation and management. Altogether 15 intercongress Symposia have been organized in the past 38 years in different countries where salt affected soils occur, with the cooperation and support of the Soil Science Society, the relevant institutions, governmental and other organizations of the host country (Table 5). The success of technical activities and the friendly, collegial atmosphere in case of all of the International Symposia was mainly owing to the local scientists: the professional preparation and organization conducted by the staff of the organizing and collaborating institutions, which did their best to ensure the effective work of the given meeting. During the past four decades, apart from the above mentioned Symposia, the Sub commission on Salt Affected Soils has made considerable efforts to appropriately represent the problem of soil salinity on the subsequent International Congresses of Soil Science. Keynote lectures, plenary papers gave the state-of art and the outlooks of global soil salinity issues. Technical session(s) and specific meetings were devoted to the problem.
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Table 5: International symposia organized by the Sub commission A (Salt Affected Soils), ISSS, 1964-2001
Sr. No. Year; Place; Title; Main topics; Co-organizers (0organization directed by....)
1. 1964; Budapest (Hungary); Symposium on Sodic Soils; Genesis, classification, properties, methods of reclamation of salt affected soils; UNESCO, Hungarian Academy of Sciences.
2. 1969; Yerevan (USSR); Symposium on the Reclamation of Sodic and Soda-Saline Soils; General concept of soda salinity, the effect on soil properties, methods of reclamation and utilization, methods of predicting secondary salinization; Ministry of Agriculture, and Research Institute of Soil Science, Acad. Sci. Armenian SSR.
3. 1972; Cairo (Egypt); International Symposium on New Development in the Field of Salt Affected Soils; Salt balance studies, problem of sampling, analysis and data interpretation, soil reclamation, secondary salinization; Ministry of Agriculture and Amelioration , and Soil Science Society of Egypt.
4. 1976; Lubbock (Texas, USA); International Conference on Managing Saline Water for Irrigation. Planning for the Future; Soil properties, diagnostic criteria, irrigation water quality, salt movement in soils and its modeling, water management, salt tolerance of plants; Soil Science Society of America, US Salinity Laboratory, Texas Tech. Univ., US Environmental Protection Agency.
5. 1980; Karnal (Haryana, India); Theoretical and Practical Problems of the Reclamation of Salt Affected Soils; Genesis, classification, diagnostic criteria, reclamation, leaching, chemical amendments, salt movement processes, irrigation water quality, salt tolerance, nutrient conditions in salt affected soils; Indian Council of Agricultural Research (ICAR) and Central Soil Salinity Research Institute (CSSRI).
6. 1984; Bet Dagan (Israel); International Conference on Soil Salinity under Irrigation Processes and Management; Salinity effects on soil properties and crop production, salt transport in irrigated soils, soil salinity under irrigation management, plant nutrition under saline conditions; Israel Society of Soil Science, and the Volcani Center.
7. 1985; Jinan (China); International Conference on the Reclamation of Salt-Affected Soils; Environmental and geochemical aspects of soil salinity, water and salt regime, reclamation and utilization; Agricultural University, Beijing Academy of Agricultural Sciences of P.R. of China.
8. 1987; Karnal (Haryana, India); International Symposium on Saline and Alkali Soils and their Utilization by Afforestation; Genesis, classification, diagnostics, dynamics of salts, methods of amelioration, afforestation, salt tolerance of trees; Indian Council of Agricultural Research, and Central Soil Salinity Research Institute.
9. 1988; Osijek (Yugoslavia); International Symposium on Solonetz soils. Problems, Properties, Utilization; Genesis, classification, mapping, reclamation, utilization, irrigation of solonetz soils; Yugoslav Society of Soil Science, Agric. Faculty University of Osijek.
10. 1989; Nanjing (China); International Symposium of Dynamics of Salt Affected Soils; Water and Salt Movement, prediction of secondary salinization processes; Soil Science Society of P.R. of China, Institute of Soil Science, Acad. Sinica.
11. 1991; Volgograd (USSR); International Symposium on the Genesis and Control of Fertility of Salt Affected Soils; Genesis, diagnostics, reclamation, utilization, control of secondary soil salinization under irrigation; All Union Soil Science Soc. of USSR, and Dokuchaev Soil Institute, Moscow.
12. 1992; Bankok (Thailand); International Symposium on Strategies of Utilizing Salt Affected Soils; Genesis, characteristics, mapping techniques, use of saline water for irrigation, management of saline lands, ecology and environment, salt tolerance of crops, halophytes; Ministry of Agriculture of Thailand, Agricultural Societies of Thailand.
13. 1995; Valencia (Spain); International Symposium on Salt Affected Laggon Ecosystems (ISSALE-95); Different types of salt-affected laggons, their ecology, genesis, soil properties and utilization; Universitat de Valencia, Estudigeneral.
14. 1997; Cairo (Egypt); International Symposium on Salt Affected Soils in the Acids Ecosystem; Land capability classification in view of their reclamation and management, reclamation and management practices in different conditions; University of Ain Shams.
15. 2001; Riverside (USA); International Symposium on Sustained Management of Irrigated Land for Salinity and Toxic Element Control; Mapping and assessment of salinity, management of B, As, Se, modeling of salinity, sodicity, reclamation of saline, sodic or toxic element affected soils, irrigation with low quality water; George E. Brown Jr. Salinity Laboratory.
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2.3. Preparing global/regional maps, records and reports
Following the reactivation of the Sub commission on Salt Affected Soils, on the first Board Meeting (Budapest 1967), the participants agreed that the construction of a map indicating the global distribution of salt affected soils was of pressing importance because it would serve as a basis for international cooperation in the research on the amelioration and utilization of these soils. It was also agreed that this map - prepared on a 1:2,500,000 or 1:5,000,000 scale would be a logical extension of the FAO/UNESCO project for a Soil Map of the World. Wide international cooperation started in the course of the preparation and edition of the World Map of Salt Affected Soils. Working groups were set up for different continents and regions. The basic principles of the grouping and classification of salt affected soils were developed at the Budapest Meeting. Reviews of the given state of the maps of the various continents as well as new suggestions were discussed by the regional coordinators at ISSS Congresses (Adelaide 1968, Moscow 1974), at International Symposia of the Sub commission (Yerevan 1969, Cairo 1972, Lubbock 1976) and on other occasions. Australia was the first continent for which the map, together with an explanatory booklet, was completed and published. The European Working Group took an especially active part in the preparation and edition of the Map of Salt Affected Soils of Europe. The representatives of contributing countries reported on the results, discussed the details of classification, diagnostic criteria, the preparation of maps, and coordinated their activity at regular Working Meetings. During these meetings special attention was paid to the definition of different mapping units, and to the definition and criteria of "potential salt affected soils". In addition to the map indicating the extension of salt affected soils on the continent, in the case of several countries, samples of larger scale maps were also compiled. As the outcome of the successful international cooperation of soil scientists from different European countries, a book was published jointly by Martinus Nijhoff - the Hague and Research Institute for Soil Science and Agricultural Chemistry of the Hungarian Academy of Sciences - Budapest, in 1974 under the title: "Salt Affected Soils in Europe" (Ed.: I. Szabolcs). It included a 1:5,000,000 scale map of salt affected soils in Europe and a 1:500,000 scale map of salt affected soils in Hungary. Although on the small scale map only a very simple schematic classification system could be used, it was applied as a technical basis for regional international cooperation for the amelioration of salt affected soils and on the prevention of soil salinization. At the meeting of the European Working Group in Novi Sad (1968), a proposal was made regarding the organization of a coordinated Solonetz Reclamation Programme for European Countries. Within the framework of the Sub commission on Salt Affected Soils a Working Group was organized to study the possibilities of setting up complex field ISSALE-95 Page 7experiments in countries faced with the responsibility of seeking ways and means for the reclamation of large areas of salt affected soils in order to cope with the steadily growing demand for increased food production. Special attention was paid, as part of the activity of this working group, to drainage and water management problems as well as to the application of different doses of chemical amendments. As a first step, a review was compiled ("European Solonetz Soils and their Reclamation", Ed. I. Szabolcs, Akadémiai Kiadó Budapest, 1971) on the different aspects of salt-affected soils. This information served as a scientific basis for regional field experiments leading to a better understanding of the problems related to the technically and economically most efficient methods of reclaiming salt-affected soils. The former COMECON (CMEA) countries of Eastern Europe (including the European part of USSR) took advantage of the possibility of European international cooperation in the field of soil salinity and alkalinity. They made considerable efforts to realize multilateral international cooperation and to integrate investigations on specific questions and details of soil classification, mapping, characterization of physical, chemical and mineralogical properties, complex amelioration of salt affected soils and related technologies, the prognosis and prevention of secondary salinization and alkalization processes due to irrigation. Scientific-methodical conferences were organized in the participating countries to present and discuss annual results. The success of this cooperation was proved by a joint publication prepared in Russian ("Preparation of large scale soil maps of irrigated territories") OMMI Budapest 1968). In addition to the above-mentioned successful cooperation, the Subcommission initiated and coordinated other international programmes, however, because of different -mainly financial and personal - reasons they have not been completed. In 1992, a proposal was elaborated by the Sub commission for an international programme on the preparation and edition of the Map of Salt Affected Soils of East and South Asia. Although the complete programme has not been realized according to the original proposal, in some countries and sub regions different maps have been prepared and assessments of salt affected soils of the region were attempted, with the support of different projects and headed by Zhao-Qiguo and S. Matsumoto (for East Asia) and S. Arunin (for South Asian countries).
3. CAUSES OF ORIGIN OF PROBLEMATIC SOILS
3.1. Introduction of irrigation: The main cause
Although irrigation is practised on only about one-eighth of the arable land in the word, it supplies perhaps as much as one-fourth of the total output of food and fiber, and is the main stay of agricultural economy of many of arid and semi-arid countries including India and several other Asian, African, Latin-American and other countries. The last century and this first decade of this century have witnessed a remarkable progress in this field all over the world. The irrigated area in the world which was about 92 million ha 1947 increased to 149 million ha in 1959 (62% increase during the period 1947 to 1959), and the figure exceeded 200 million in 1969 (34% increase during the period 1959 to 1969). It increased to 277 million ha in 2003 with an increase of 38.3% since 1969. Similarly, in India, the irrigated area increased from 22.55 million ha in 1947 to 44.21 million ha by 1974 and to 55.8 million ha by 2008.
In spite of spectacular increase in the irrigated areas resulting in higher crop yields, the story of irrigation agriculture is not entirely one of prosperity. The decline of once flourishing civilizations has led some important writers to seriously question the permanence of irrigation agriculture. Salinization of land has threatened civilizations in ancient and modern times. Soil salinization in southern Mesopotamia and in several parts of the Tigris–Euphrates valley destroyed the ancient societies that had successfully thrived for several centuries. In modern times, salt-affected soils are naturally present in more than 100 countries of the world where many regions are also affected by irrigation-induced salinization. Recently, dry-land salinity has become a major issue in natural resource management in several countries like India and Australia and has attracted increasing awareness from both farmers and politicians.
2.3. Preparing global/regional maps, records and reports
Following the reactivation of the Sub commission on Salt Affected Soils, on the first Board Meeting (Budapest 1967), the participants agreed that the construction of a map indicating the global distribution of salt affected soils was of pressing importance because it would serve as a basis for international cooperation in the research on the amelioration and utilization of these soils. It was also agreed that this map - prepared on a 1:2,500,000 or 1:5,000,000 scale would be a logical extension of the FAO/UNESCO project for a Soil Map of the World. Wide international cooperation started in the course of the preparation and edition of the World Map of Salt Affected Soils. Working groups were set up for different continents and regions. The basic principles of the grouping and classification of salt affected soils were developed at the Budapest Meeting. Reviews of the given state of the maps of the various continents as well as new suggestions were discussed by the regional coordinators at ISSS Congresses (Adelaide 1968, Moscow 1974), at International Symposia of the Sub commission (Yerevan 1969, Cairo 1972, Lubbock 1976) and on other occasions. Australia was the first continent for which the map, together with an explanatory booklet, was completed and published. The European Working Group took an especially active part in the preparation and edition of the Map of Salt Affected Soils of Europe. The representatives of contributing countries reported on the results, discussed the details of classification, diagnostic criteria, the preparation of maps, and coordinated their activity at regular Working Meetings. During these meetings special attention was paid to the definition of different mapping units, and to the definition and criteria of "potential salt affected soils". In addition to the map indicating the extension of salt affected soils on the continent, in the case of several countries, samples of larger scale maps were also compiled. As the outcome of the successful international cooperation of soil scientists from different European countries, a book was published jointly by Martinus Nijhoff - the Hague and Research Institute for Soil Science and Agricultural Chemistry of the Hungarian Academy of Sciences - Budapest, in 1974 under the title: "Salt Affected Soils in Europe" (Ed.: I. Szabolcs). It included a 1:5,000,000 scale map of salt affected soils in Europe and a 1:500,000 scale map of salt affected soils in Hungary. Although on the small scale map only a very simple schematic classification system could be used, it was applied as a technical basis for regional international cooperation for the amelioration of salt affected soils and on the prevention of soil salinization. At the meeting of the European Working Group in Novi Sad (1968), a proposal was made regarding the organization of a coordinated Solonetz Reclamation Programme for European Countries. Within the framework of the Sub commission on Salt Affected Soils a Working Group was organized to study the possibilities of setting up complex field ISSALE-95 Page 7experiments in countries faced with the responsibility of seeking ways and means for the reclamation of large areas of salt affected soils in order to cope with the steadily growing demand for increased food production. Special attention was paid, as part of the activity of this working group, to drainage and water management problems as well as to the application of different doses of chemical amendments. As a first step, a review was compiled ("European Solonetz Soils and their Reclamation", Ed. I. Szabolcs, Akadémiai Kiadó Budapest, 1971) on the different aspects of salt-affected soils. This information served as a scientific basis for regional field experiments leading to a better understanding of the problems related to the technically and economically most efficient methods of reclaiming salt-affected soils. The former COMECON (CMEA) countries of Eastern Europe (including the European part of USSR) took advantage of the possibility of European international cooperation in the field of soil salinity and alkalinity. They made considerable efforts to realize multilateral international cooperation and to integrate investigations on specific questions and details of soil classification, mapping, characterization of physical, chemical and mineralogical properties, complex amelioration of salt affected soils and related technologies, the prognosis and prevention of secondary salinization and alkalization processes due to irrigation. Scientific-methodical conferences were organized in the participating countries to present and discuss annual results. The success of this cooperation was proved by a joint publication prepared in Russian ("Preparation of large scale soil maps of irrigated territories") OMMI Budapest 1968). In addition to the above-mentioned successful cooperation, the Subcommission initiated and coordinated other international programmes, however, because of different -mainly financial and personal - reasons they have not been completed. In 1992, a proposal was elaborated by the Sub commission for an international programme on the preparation and edition of the Map of Salt Affected Soils of East and South Asia. Although the complete programme has not been realized according to the original proposal, in some countries and sub regions different maps have been prepared and assessments of salt affected soils of the region were attempted, with the support of different projects and headed by Zhao-Qiguo and S. Matsumoto (for East Asia) and S. Arunin (for South Asian countries).
3. CAUSES OF ORIGIN OF PROBLEMATIC SOILS
3.1. Introduction of irrigation: The main cause
Although irrigation is practised on only about one-eighth of the arable land in the word, it supplies perhaps as much as one-fourth of the total output of food and fiber, and is the main stay of agricultural economy of many of arid and semi-arid countries including India and several other Asian, African, Latin-American and other countries. The last century and this first decade of this century have witnessed a remarkable progress in this field all over the world. The irrigated area in the world which was about 92 million ha 1947 increased to 149 million ha in 1959 (62% increase during the period 1947 to 1959), and the figure exceeded 200 million in 1969 (34% increase during the period 1959 to 1969). It increased to 277 million ha in 2003 with an increase of 38.3% since 1969. Similarly, in India, the irrigated area increased from 22.55 million ha in 1947 to 44.21 million ha by 1974 and to 55.8 million ha by 2008.
In spite of spectacular increase in the irrigated areas resulting in higher crop yields, the story of irrigation agriculture is not entirely one of prosperity. The decline of once flourishing civilizations has led some important writers to seriously question the permanence of irrigation agriculture. Salinization of land has threatened civilizations in ancient and modern times. Soil salinization in southern Mesopotamia and in several parts of the Tigris–Euphrates valley destroyed the ancient societies that had successfully thrived for several centuries. In modern times, salt-affected soils are naturally present in more than 100 countries of the world where many regions are also affected by irrigation-induced salinization. Recently, dry-land salinity has become a major issue in natural resource management in several countries like India and Australia and has attracted increasing awareness from both farmers and politicians.
The failure of these early agricultural civilizations are mainly related to the formation of salt-affected soils, waterlogged soils and other degraded soils caused by water and wind erosion due to improper management. Several million ha of irrigated land, once producing abundantly, has gone out of cultivation in several parts of the world because of these soil problems. Moreover, highly productive soils can be readily transformed into nonproductive soils through a failure to control soil salinity, water logging and soil erosion, thereby further increasing the vast area of land already affected by the problem.
3.2. Other causes
As we have seen from above that the advent of canal and tube well irrigation, several soil problems have developed in irrigated areas of the world. Not simply the advent of irrigation alone, but intensive irrigation facilities and excessive use of canal water have created the problems of water logging, soil salinity and soil erosion. Several other factors such as problems of impeded drainage, undulating topography, salt-laden parent material, improper conveyance of water through unlined canals and irrigation channels, stagnation of water due to field irrigation, poor quality of underground water, blocking of natural drainage through construction of roads, canals, railways, residential and industrial infrastructures, etc., wind erosion and movement of sand dunes, water erosion and flooding, excessive permeability of sandy soils, soil crusting and calcareous soils have also contributed to these problems. . These soil hazards deteriorate the land; disturb the ecological balance and fertility status of soils resulting in low production.
4. SALT-AFFECTED SOIL: WHAT IT IS AND HOW IT IS RECOGNIZED (CLASSIFICATION OF SALT-AFFECTED SOILS)
Salt-affected soils are soils on which most crops cannot make normal growth owing to the presence of excessive soluble salts in the soil solution (saline soils), the presence of exchangeable sodium on surface of the soil particles (sodic soils) or both (saline-sodic soils). In the extreme cases, there may be no plant growth at all.
Saline soils: Soluble salts provide most of the elements essential for plant growth, but as with most essential factors, excessive levels can be injurious. When soluble salts are present in such an excess amount that they injure plants, the soil is said to be saline. Saline soils usually contain a mixture of salt constituents, such as chlorides, sulfates, carbonates and bicarbonates of sodium, calcium and magnesium. The proportions may vary widely from place to place, depending upon the source of salts. Since the various combinations of salts normally found in saline soils affect most field crops about equally, a measure of total salts usually suffices. This is most readily done by measuring the electricity conductivity of saturation extract of the soil (EC) in mmhos/cm, a measure which is closely related to the osmotic pressure of the solution, which determines how readily plan roots can absorb water from the soil. The greater the concentration of ions is a solution, the greater the conductivity of the solution.
Sodic soils: Soils containing excessive are called sodic soils. Sodic soils usually take water slowly, crust when dry, are sticky when wet, have a poor permeability to air and water, and have a black surface. Sodicity of soil is measured as exchangeable sodium percentage (ESP) which is calculated as "{(amount of exchangeable sodium on the soil particles/amount of total exchangeable cations on the soil surface) x100}".
The various terms in common use relating to salt-affected soils has been somewhat variable as to meaning. The following definitions have, however, been widely accepted:
Saline soil: is defined as a soil for which the EC is 4 or more mmhos/cm at 25^o C, the ESP less than 15 and pH is usually below 8.5.
Sodic soil: is defined as a soil for which the EC is less than 4 mmhos/cm, the ESP more than 15 and pH ranges between 8.5 and 10.
Saline-sodic soil: is that which has EC greater than 4 mmhos/cm, the ESP more than 15 and pH above 8.5.
The term salt-affected soil (or salty soils) is, however used as a general term for all the above-mentioned terms. Broadly speaking, based on their amelioration needs, the salt-affected soils have been classified into two main categories:
Saline soils: the soils containing as excess of neutral soluble salts, dominated by chlorides and sulfates of sodium, calcium and magnesium.
Sodic or alkali soils: the soils containing an excess of exchangeable sodium or influenced by sodium salts capable of alkali hydrolysis (carbonates and bicarbonates of sodium) or both.
3.2. Other causes
As we have seen from above that the advent of canal and tube well irrigation, several soil problems have developed in irrigated areas of the world. Not simply the advent of irrigation alone, but intensive irrigation facilities and excessive use of canal water have created the problems of water logging, soil salinity and soil erosion. Several other factors such as problems of impeded drainage, undulating topography, salt-laden parent material, improper conveyance of water through unlined canals and irrigation channels, stagnation of water due to field irrigation, poor quality of underground water, blocking of natural drainage through construction of roads, canals, railways, residential and industrial infrastructures, etc., wind erosion and movement of sand dunes, water erosion and flooding, excessive permeability of sandy soils, soil crusting and calcareous soils have also contributed to these problems. . These soil hazards deteriorate the land; disturb the ecological balance and fertility status of soils resulting in low production.
4. SALT-AFFECTED SOIL: WHAT IT IS AND HOW IT IS RECOGNIZED (CLASSIFICATION OF SALT-AFFECTED SOILS)
Salt-affected soils are soils on which most crops cannot make normal growth owing to the presence of excessive soluble salts in the soil solution (saline soils), the presence of exchangeable sodium on surface of the soil particles (sodic soils) or both (saline-sodic soils). In the extreme cases, there may be no plant growth at all.
Saline soils: Soluble salts provide most of the elements essential for plant growth, but as with most essential factors, excessive levels can be injurious. When soluble salts are present in such an excess amount that they injure plants, the soil is said to be saline. Saline soils usually contain a mixture of salt constituents, such as chlorides, sulfates, carbonates and bicarbonates of sodium, calcium and magnesium. The proportions may vary widely from place to place, depending upon the source of salts. Since the various combinations of salts normally found in saline soils affect most field crops about equally, a measure of total salts usually suffices. This is most readily done by measuring the electricity conductivity of saturation extract of the soil (EC) in mmhos/cm, a measure which is closely related to the osmotic pressure of the solution, which determines how readily plan roots can absorb water from the soil. The greater the concentration of ions is a solution, the greater the conductivity of the solution.
Sodic soils: Soils containing excessive are called sodic soils. Sodic soils usually take water slowly, crust when dry, are sticky when wet, have a poor permeability to air and water, and have a black surface. Sodicity of soil is measured as exchangeable sodium percentage (ESP) which is calculated as "{(amount of exchangeable sodium on the soil particles/amount of total exchangeable cations on the soil surface) x100}".
The various terms in common use relating to salt-affected soils has been somewhat variable as to meaning. The following definitions have, however, been widely accepted:
Saline soil: is defined as a soil for which the EC is 4 or more mmhos/cm at 25^o C, the ESP less than 15 and pH is usually below 8.5.
Sodic soil: is defined as a soil for which the EC is less than 4 mmhos/cm, the ESP more than 15 and pH ranges between 8.5 and 10.
Saline-sodic soil: is that which has EC greater than 4 mmhos/cm, the ESP more than 15 and pH above 8.5.
The term salt-affected soil (or salty soils) is, however used as a general term for all the above-mentioned terms. Broadly speaking, based on their amelioration needs, the salt-affected soils have been classified into two main categories:
Saline soils: the soils containing as excess of neutral soluble salts, dominated by chlorides and sulfates of sodium, calcium and magnesium.
Sodic or alkali soils: the soils containing an excess of exchangeable sodium or influenced by sodium salts capable of alkali hydrolysis (carbonates and bicarbonates of sodium) or both.
The two categories of the salt-affected soils generally occur in the distinct geographic zones, have distinct physio-chemical properties, and require different approaches for their management and reclamation.
Now, the question arises, in which of the two categories, the saline-sodic soils should be included? It has been argued that both the saline-sodic and saline soils should be classified as saline, because in these soils, the plant growth is affected by an excess of salts and not by an excess of exchangeable sodium, or the lack of calcium. Hence, saline sodic soils containing excess of neutral salts (mainly chlorides and sulphates of sodium, calcium and magnesium) and having ESP more than 15 should be included in the category of saline soils. On the other hand, saline-sodic soils containing measurable quantities of sodium carbonates and bicarbonates are essentially sodic inspite of their high total salt concentration.
Although the above two categories account for a very large fraction of salt affected soils the world over, there are transitional or borderline formations which are likely to have properties intermediate between those of the two broad categories. Several local terms in different parts of the world are in vogue to designate such soils. Other categories of salt-affected soils which, though less extensive, are commonly met in different parts of the world are:
Acid-sulfate soils: These are soils that have somewhere within a 50 cm depth a pH below 3.5 to 4.0 that is directly or indirectly caused by sulfuric acid formed by the oxidation of pyrite (FeS2) or, rarely of other reduced sulfur compounds. Potential acid sulfate soils occur in tidal swamps. They have high levels of pyrite, low levels of bases and produce strongly acid sulphate soils when pyrite is oxidized to sulphuric acid after drainage. Pyrite formation is favoured in brackish and saline mangrove swamps dissected by tidal creeks where deposition and build up of coastal sediments is slow. Apart from high salinity, the productivity of acid sulphate soils is restricted due to such soil factors as iron and aluminium toxicities, deficiency of phosphorus, etc.
Degraded sodic soils: Degraded sodic soils are usually considered to be an advanced stage of soil development resulting from the washing out of salts. The details of the type of soil developed as the leaching proceeds depend on local conditions, particularly soil texture and type of clay present. As a result of the leaching processes there is a tendency for the dispersed clay and organic matter to move down the profile resulting in the formation of a dark, extremely compact layer having a sharply defined upper surface and merging gradually into the subsoil with increasing depth. The darker colour of the compact layer compared with the layer above may be due to its higher clay content since it does not always have a higher content of organic matter. The upper soil layers have a loose porous, laminar structure due to loss of clay and the upper surfaces of this layer may be paler than the lower, possibly because of silica being deposited on them. The clay pan cracks on drying into well defined vertical columns having a rounded top and smooth, shiny, well defined sides. These can be broken into units about 10 cm high and 5 cm across with a flat base. Below this the column breaks into rather smaller units with flat tops and bottoms which on light crushing break into angular fragments. As the leaching of these desalinized soils proceeds, the upper horizons deepen and often become slightly acidic in reaction and the amorphous silica content increases. As a further stage of development, it has been suggested that the very characteristic clay pan becomes less pronounced, possibly because of washing down of sandy material from the A horizon in the cracks between the structural units.
There are large areas in western Canada, Australia, USA, India and other countries where soils having profile morphology typical of solonetz/solod soils are found although sodium forms only a minor proportion of the exchangeable ions. It is possible that these soils originally had enough exchangeable sodium for the solonetz-solod morphology to develop in the profile but that most of this sodium has now been lost through leaching.
Other sub-categories: A large number of other sub-categories of salt-affected soils are recognized in different parts of the world depending on the dominance of a particular chemical constituent (e.g. calcium chloride rich soils or soils containing excessive quantities of exchangeable magnesium - magnesium solonetz, gypsiferous soils containing large amount of gypsum i.e. calcium sulphate, etc.) or a particular morphological character of the soil profile, e.g. presence of a structural ‘B’ horizon.
5. WHAT IS THE ORIGIN (SOURCES) OF SALTS AND HOW THEY ARE ACCUMULATED IN SOILS LEADING TO THE FORMATION OF SALINE SOILS
5.1. Origin (sources) of salts
Acid-sulfate soils: These are soils that have somewhere within a 50 cm depth a pH below 3.5 to 4.0 that is directly or indirectly caused by sulfuric acid formed by the oxidation of pyrite (FeS2) or, rarely of other reduced sulfur compounds. Potential acid sulfate soils occur in tidal swamps. They have high levels of pyrite, low levels of bases and produce strongly acid sulphate soils when pyrite is oxidized to sulphuric acid after drainage. Pyrite formation is favoured in brackish and saline mangrove swamps dissected by tidal creeks where deposition and build up of coastal sediments is slow. Apart from high salinity, the productivity of acid sulphate soils is restricted due to such soil factors as iron and aluminium toxicities, deficiency of phosphorus, etc.
Degraded sodic soils: Degraded sodic soils are usually considered to be an advanced stage of soil development resulting from the washing out of salts. The details of the type of soil developed as the leaching proceeds depend on local conditions, particularly soil texture and type of clay present. As a result of the leaching processes there is a tendency for the dispersed clay and organic matter to move down the profile resulting in the formation of a dark, extremely compact layer having a sharply defined upper surface and merging gradually into the subsoil with increasing depth. The darker colour of the compact layer compared with the layer above may be due to its higher clay content since it does not always have a higher content of organic matter. The upper soil layers have a loose porous, laminar structure due to loss of clay and the upper surfaces of this layer may be paler than the lower, possibly because of silica being deposited on them. The clay pan cracks on drying into well defined vertical columns having a rounded top and smooth, shiny, well defined sides. These can be broken into units about 10 cm high and 5 cm across with a flat base. Below this the column breaks into rather smaller units with flat tops and bottoms which on light crushing break into angular fragments. As the leaching of these desalinized soils proceeds, the upper horizons deepen and often become slightly acidic in reaction and the amorphous silica content increases. As a further stage of development, it has been suggested that the very characteristic clay pan becomes less pronounced, possibly because of washing down of sandy material from the A horizon in the cracks between the structural units.
There are large areas in western Canada, Australia, USA, India and other countries where soils having profile morphology typical of solonetz/solod soils are found although sodium forms only a minor proportion of the exchangeable ions. It is possible that these soils originally had enough exchangeable sodium for the solonetz-solod morphology to develop in the profile but that most of this sodium has now been lost through leaching.
Other sub-categories: A large number of other sub-categories of salt-affected soils are recognized in different parts of the world depending on the dominance of a particular chemical constituent (e.g. calcium chloride rich soils or soils containing excessive quantities of exchangeable magnesium - magnesium solonetz, gypsiferous soils containing large amount of gypsum i.e. calcium sulphate, etc.) or a particular morphological character of the soil profile, e.g. presence of a structural ‘B’ horizon.
5. WHAT IS THE ORIGIN (SOURCES) OF SALTS AND HOW THEY ARE ACCUMULATED IN SOILS LEADING TO THE FORMATION OF SALINE SOILS
5.1. Origin (sources) of salts
Soil salinization may originate from a variety (and combination) of frequently interrelated sources. However, weathering of rocks and minerals in the earth's crust is the chief source of all soluble salts present in the soil and sea. Although the salts currently occurring in the ocean arise mainly from the weathering processes of the earth crust, the ocean now functions as an important 'source term' for redistribution of salts. The main origin of salts for a particular area can be any one or combination of the these: (1) The geologically weathered rocks (mineral weathering), (2) oceanically derived rain fall, (3) seawater encroachment, (4) ground water containing salts of geologic or marine origin (fossil salts), and (5) the human activities that add salts to soil including irrigation and saline industrial wastes.
5.1.1. Geologically weathered rocks
As stated above, the main source of all salts in the soil is the primary mineral in the exposed layer of the earth's crust. During the process of chemical weathering which involves hydrolysis, hydration, solution, oxidation, carbonation and other processes, the salt constituents are gradually released and made soluble. The released salts are transported away from their source of origin through surface or ground water streams. Thus, as a result of weathering processes, salts are formed in the soil. But under humid conditions, soluble salts are carried down through the soil profile by percolating rainwater and ultimately are transported to the ocean or to inland seas by streams and rivers. Therefore, inland salt affected soils are rarely formed in humid areas. But under arid and semi arid conditions, these weathering products accumulate in situ and result in the development of salinity or sodicity. In arid regions, leaching is generally more localized. Salts tend to accumulate because of the relative scarcity of rainfall, high evaporation and plant transpiration rates, or land locked topography. This process of formation of salt affected soils as a result of accumulation of salts released during weathering is called primary salinization.
Without leaching, in situ weathering of primary minerals would eventually allow soluble salts to accumulate to hazardous levels, but this degree of accumulation is rare. Salts are leached during weathering. Mafic minerals (dark, Mg- and Fe-rich) minerals, for example, are common in arid-region soils. If present in sufficient quantities, they can increase the salt concentration of slowly percolating waters by as much as 3 to 5 mmol/l. In arid regions, the occasional rains that cause the weathering are usually sufficient to flush out most of the salts.
Weathering minerals are rarely dissolved congruently (in strict proportion to their composition). Instead, they release their most soluble components first. A mineral high in Ca and Mg may, therefore, initially release significant amounts of Na and K to the percolating solution. The water weathering the minerals is usually of sufficient quantity to carry the soluble salts thus created to the sea, to a landlocked lake, to a nearby saline seep, or at least to the average annual depth of wetting of the soil.
5.1.2. Ground water containing salts of geologic or marine origin (fossil salts)
Salt accumulation in arid regions often involves 'fossil salts', derived from either deposits or entrapped solutions in former marine deposits. Salt release may occur naturally or result from human activities. An example of the former is the rise of salt bearing groundwater through an originally impervious cap (which becomes permeable as a result of weathering processes) overlying saline strata. Examples of the latter are building of canals or water works in saline strata and the use of groundwater for irrigation.
As stated above, the main source of all salts in the soil is the primary mineral in the exposed layer of the earth's crust. During the process of chemical weathering which involves hydrolysis, hydration, solution, oxidation, carbonation and other processes, the salt constituents are gradually released and made soluble. The released salts are transported away from their source of origin through surface or ground water streams. Thus, as a result of weathering processes, salts are formed in the soil. But under humid conditions, soluble salts are carried down through the soil profile by percolating rainwater and ultimately are transported to the ocean or to inland seas by streams and rivers. Therefore, inland salt affected soils are rarely formed in humid areas. But under arid and semi arid conditions, these weathering products accumulate in situ and result in the development of salinity or sodicity. In arid regions, leaching is generally more localized. Salts tend to accumulate because of the relative scarcity of rainfall, high evaporation and plant transpiration rates, or land locked topography. This process of formation of salt affected soils as a result of accumulation of salts released during weathering is called primary salinization.
Without leaching, in situ weathering of primary minerals would eventually allow soluble salts to accumulate to hazardous levels, but this degree of accumulation is rare. Salts are leached during weathering. Mafic minerals (dark, Mg- and Fe-rich) minerals, for example, are common in arid-region soils. If present in sufficient quantities, they can increase the salt concentration of slowly percolating waters by as much as 3 to 5 mmol/l. In arid regions, the occasional rains that cause the weathering are usually sufficient to flush out most of the salts.
Weathering minerals are rarely dissolved congruently (in strict proportion to their composition). Instead, they release their most soluble components first. A mineral high in Ca and Mg may, therefore, initially release significant amounts of Na and K to the percolating solution. The water weathering the minerals is usually of sufficient quantity to carry the soluble salts thus created to the sea, to a landlocked lake, to a nearby saline seep, or at least to the average annual depth of wetting of the soil.
5.1.2. Ground water containing salts of geologic or marine origin (fossil salts)
Salt accumulation in arid regions often involves 'fossil salts', derived from either deposits or entrapped solutions in former marine deposits. Salt release may occur naturally or result from human activities. An example of the former is the rise of salt bearing groundwater through an originally impervious cap (which becomes permeable as a result of weathering processes) overlying saline strata. Examples of the latter are building of canals or water works in saline strata and the use of groundwater for irrigation.
In Rajasthan India, a canal built on an underlying gypsum layer has resulted in development of salinity in the area within only a few years of its construction. This has been due to the perched water table and contribution of salts from the underground gypsum water.
So-called fossil salts can introduce large amounts of salinity into soil and ground water. This, for example, was dramatized in 1960s by the Welton-Mohawk irrigation project of Arizona, USA, where saline ground waters were discharged into the Gilla River after irrigation raised the ground water level in a valley underlain by saline deposits. The drainage water mixed with the Colorado River and significantly increased the river's salinity. Downstream farmers in Mexicali, Mexico, were understandably angered when the more saline water damaged their irrigated crops. Fossil salts dissolving in percolating waters contribute materially to the salinity added to the Colorado River from several irrigation projects along its upper reaches.
Fossil salts can also be dissolved when water-storage or water-transmission structures are placed over saline sediments. The Lake reservoir behind Hover Dam in southern Nevada, USA, overlies deposits of gypsiferous sediments. Dissolution of this gypsum substantially increases the salinity of the Colorado River during its seepage through the reservoir.
Under certain situations, seepage resulting from water inflow in upslope areas can cause severe salinity on the down slope areas, especially when the sub-surface water flow takes place through the strata that are rich in salts and/ or marine deposits. Mineral springs owe their salts to such a situation.
Fossil salts can also be dissolved when water-storage or water-transmission structures are placed over saline sediments. The Lake reservoir behind Hover Dam in southern Nevada, USA, overlies deposits of gypsiferous sediments. Dissolution of this gypsum substantially increases the salinity of the Colorado River during its seepage through the reservoir.
Under certain situations, seepage resulting from water inflow in upslope areas can cause severe salinity on the down slope areas, especially when the sub-surface water flow takes place through the strata that are rich in salts and/ or marine deposits. Mineral springs owe their salts to such a situation.
5.1.3. Oceanically derived rainfall and other oceanic sources
Appreciable salt can also be deposited in some areas from the atmosphere. Rain droplets form around tiny condensation nuclei such as salt or dust particles. The total salt concentration of rainfall may be as high as 50 to 200 mg/l near seacoast, but rapidly decreases to only a few mg/l in the continental interior (Table 6). The exact pattern of decrease depends on local topography and weather patterns. Changes in composition of the rainfall also occur. The salts in the rain near the seacoast are high in Na, Cl and Mg. Inland precipitation is dominated by Ca and Mg sulfates and bicarbonates. The quantities of salt added from the atmosphere to arid and semiarid regions may amount to only a few kg per ha per year, but the amounts introduced over periods of tens of thousands of years can be substantial. Inland deposition of NaCl at rate of 20-100 kg/ha/year are quite common and values of 100-200 kg/ha/year for nearby coastal areas have been reported.
Appreciable salt can also be deposited in some areas from the atmosphere. Rain droplets form around tiny condensation nuclei such as salt or dust particles. The total salt concentration of rainfall may be as high as 50 to 200 mg/l near seacoast, but rapidly decreases to only a few mg/l in the continental interior (Table 6). The exact pattern of decrease depends on local topography and weather patterns. Changes in composition of the rainfall also occur. The salts in the rain near the seacoast are high in Na, Cl and Mg. Inland precipitation is dominated by Ca and Mg sulfates and bicarbonates. The quantities of salt added from the atmosphere to arid and semiarid regions may amount to only a few kg per ha per year, but the amounts introduced over periods of tens of thousands of years can be substantial. Inland deposition of NaCl at rate of 20-100 kg/ha/year are quite common and values of 100-200 kg/ha/year for nearby coastal areas have been reported.
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Table 6: Salt content of rain water in GermanyLocation, Distance from coast (km), Cl (mg/l), Na (mg/l)
Westerland, 0.2, 37.6, 18.5
Schleswig, 50, 4.9, 2.4
Braunschweig, 450, 1.9, 0.8
Augustenberg, 800, 0.9, 5
Retz, 50, 3, 1
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In the coastal areas, the soils also get enriched with salts from the sea through inundation of surface soil by sea water during high tide, and ingress of sea water through rivers, estuaries, groun water inflows, etc.
5.2. How salts are accumulated in root zone leading to soil salinity
Salts will accumulate in a soil profile where evaporation exceeds downward flux of water. Continued evaporation from the soil surface, that will cause serious salinization, is possible under the following conditions: (1)Where saline ground water is present at rather shallow depth, (2) where a salic horizon in soil profile comes to upper soil layers due to irrigation, (3) where poor soil physical characteristics prevent sufficient downward water movement, salt contained in irrigation and runoff waters may become concentrated in the root zone, and (4) where amount of saline water applied for irrigation is not sufficient for a fairly large fraction of it to pass beyond the root zone, some of the salts contained in water remains in root zone.
5.2.1. Accumulation from saline ground water present at shallow depths
5.2. How salts are accumulated in root zone leading to soil salinity
Salts will accumulate in a soil profile where evaporation exceeds downward flux of water. Continued evaporation from the soil surface, that will cause serious salinization, is possible under the following conditions: (1)Where saline ground water is present at rather shallow depth, (2) where a salic horizon in soil profile comes to upper soil layers due to irrigation, (3) where poor soil physical characteristics prevent sufficient downward water movement, salt contained in irrigation and runoff waters may become concentrated in the root zone, and (4) where amount of saline water applied for irrigation is not sufficient for a fairly large fraction of it to pass beyond the root zone, some of the salts contained in water remains in root zone.
5.2.1. Accumulation from saline ground water present at shallow depths
Prior to the introduction of irrigation, in any watershed, there exists a balance between rainfall on one hand and stream flow, evaporation and transpiration on the other hand. With the introduction of irrigation, this balance is disturbed because of new contribution to the groundwater in the form of seepage from canals and irrigation channels, mostly earthen and unlined, and percolation of irrigation water from the fields which are often over irrigated at times and not at all irrigated during some stages of crop growth. This coupled with unscientific soil, water crop management practices like bringing proportionately more land under high water requirement crops such as rice; presence of poor quality of ground water which cannot be used for irrigation to check the water table rise; the poor natural sub-surface drainage due to lack of drainage outlets; and other unscientific development activities (construction of roads, railways, irrigation works which nearly completely obstruct natural drainage) results in the gradual rise in groundwater table to the critical level.
According to some estimates, for example, during the period 1974-1998, water table level rising rate was 23 cm/year in one of the rising water table areas of the Indo-Gangetic Plains of northern India (Fig. 1). About 6 million ha of the country (covering about 4% of the total cultivable area) are under water logging due to rise of water table to the critical level of 0-3 m. The underground water in most of these areas is moderately to highly saline. This is happening since long. For example, during 1932 to 1963, there was 16.5 cm average annual rise in water table level in areas commanded by the Western Yamuna Canal in northern India. Similarly, the rate of rise of water table in some areas of Haryana, India irrigated by Bhakra Canal is above 1 m per year. This has been shown by the observations at the Experimental Farm of the Haryana Agricultural University, Hisar, where the water table rose from 16.7 m in 1968 to less than 3 m in 1980. In Haryana state alone, about 21 % of the total cultivated area has a water table level within 3 m of the soil surface.
The critical water table depth varies from less than 80 cm for light textured (sandy), 1-1.5 m for heavy textured (high clay content) and 2-3 m for medium textured soils. Once the water table is within critical level , it acts as a continuous reservoir for evaporation losses which simultaneously results in accumulation of large quantities of salts in the surface soil from saline underground water.
To elaborate the fact that the saline ground water, when present close to the soil surface, contributes to soil salinization, some examples from India are given here. In several areas of the south-western parts of Punjab, Haryna, Uttar Pradesh, Rajasthan and Gujarat, as well as in the coastal areas, the salinity has developed due to this reason. Similar is the case with the Vertisols (black soils) of south India which are being brought under canal irrigation for the first time. It is also true for many areas along the sea coast. In these areas, the quality of ground water is generally poor on account of excess salt content, high SAR and high residual sodium carbonates (RSC). In Haryana state, for example, 63 to 77 % of the area has marginal to saline ground water quality. In general, the quality of ground waters is poor in the arid zones and improves in the regions having comparatively high rain fall. For example, 62, 29 and 10 % of ground waters, respectively, in the arid, semiarid and humid regions of Rajasthan are saline (EC more than 2000 micromhos/cm).
The composition and the salinity of ground waters can change several folds within relatively short distance. Saline soils usually, therefore, occur highly spotted and localized in character. For example, it was observed in a semi-arid, irrigated area of Haryana, India (part of Indo-Gagetic region) that the EC and sodium adsorption ratio -SAR = [Na/{(Ca+Mg)/2}^1/2] - of underground water at salty spot were 30 mmhos/cm and 25, respectively, whereas the corresponding figures for groundwater at a nearby normal spot were 2.3 mmhos/cm and 6, respectively.
According to some estimates, for example, during the period 1974-1998, water table level rising rate was 23 cm/year in one of the rising water table areas of the Indo-Gangetic Plains of northern India (Fig. 1). About 6 million ha of the country (covering about 4% of the total cultivable area) are under water logging due to rise of water table to the critical level of 0-3 m. The underground water in most of these areas is moderately to highly saline. This is happening since long. For example, during 1932 to 1963, there was 16.5 cm average annual rise in water table level in areas commanded by the Western Yamuna Canal in northern India. Similarly, the rate of rise of water table in some areas of Haryana, India irrigated by Bhakra Canal is above 1 m per year. This has been shown by the observations at the Experimental Farm of the Haryana Agricultural University, Hisar, where the water table rose from 16.7 m in 1968 to less than 3 m in 1980. In Haryana state alone, about 21 % of the total cultivated area has a water table level within 3 m of the soil surface.
The critical water table depth varies from less than 80 cm for light textured (sandy), 1-1.5 m for heavy textured (high clay content) and 2-3 m for medium textured soils. Once the water table is within critical level , it acts as a continuous reservoir for evaporation losses which simultaneously results in accumulation of large quantities of salts in the surface soil from saline underground water.
To elaborate the fact that the saline ground water, when present close to the soil surface, contributes to soil salinization, some examples from India are given here. In several areas of the south-western parts of Punjab, Haryna, Uttar Pradesh, Rajasthan and Gujarat, as well as in the coastal areas, the salinity has developed due to this reason. Similar is the case with the Vertisols (black soils) of south India which are being brought under canal irrigation for the first time. It is also true for many areas along the sea coast. In these areas, the quality of ground water is generally poor on account of excess salt content, high SAR and high residual sodium carbonates (RSC). In Haryana state, for example, 63 to 77 % of the area has marginal to saline ground water quality. In general, the quality of ground waters is poor in the arid zones and improves in the regions having comparatively high rain fall. For example, 62, 29 and 10 % of ground waters, respectively, in the arid, semiarid and humid regions of Rajasthan are saline (EC more than 2000 micromhos/cm).
The composition and the salinity of ground waters can change several folds within relatively short distance. Saline soils usually, therefore, occur highly spotted and localized in character. For example, it was observed in a semi-arid, irrigated area of Haryana, India (part of Indo-Gagetic region) that the EC and sodium adsorption ratio -SAR = [Na/{(Ca+Mg)/2}^1/2] - of underground water at salty spot were 30 mmhos/cm and 25, respectively, whereas the corresponding figures for groundwater at a nearby normal spot were 2.3 mmhos/cm and 6, respectively.
5.2.2. Accumulation from salic horizon in soil profile
Irrigation in agriculture also disturbs the distribution of salts in the soil profile. In many arid and semi-arid regions of the world, soil profiles contain the salts of geologic origin at certain depths because of the inadequate natural leaching. The depth at which the zone of salt accumulation occurs depends on soil properties and characteristics of natural rainfall. With the introduction of irrigation and cropping without the provision of sub-surface drainage, the salts from deeper layers are accumulated in the surface layers with the rise in water table. Even a good quality underground water coming from shallow depths will transport the salts from the deep zone of accumulation to soil surface and, thus, causes serious soil salinization. To illustrate this, results of soil analysis of two typical soil profiles from Haryna, India, one each before and after canal irrigation was introduced, are presented in Table 7. The average rain fall of the area is about 350 mm and water table depth before the advent of irrigation was 18 m and at present it varies from 0.5 to 5 m during different months of the year. It is apparent from the data that in these soils there was a zone of salt accumulation at some depths, but with rise in water table level after the introduction of irrigation, the salts have migrated to the upper soil layers and created a serious salinity problem.
The sandy loam and loamy soils developed on the alluvium in a 300-400 mm rainfall zone usually have a shallow salt layer at a depth of 1 to 2 m. Most of the Vertisols in the medium-or deep-black soil areas in south India in a 500-700 mm rainfall zone have an accumulation of salts at a depth of 1 to 1.5 m. When such areas, even without a shallow ground water condition, are irrigated with an inadequate amount of water, the salts are dissolved in water during irrigation move to the soil surface during the subsequent fallow season.
Irrigation in agriculture also disturbs the distribution of salts in the soil profile. In many arid and semi-arid regions of the world, soil profiles contain the salts of geologic origin at certain depths because of the inadequate natural leaching. The depth at which the zone of salt accumulation occurs depends on soil properties and characteristics of natural rainfall. With the introduction of irrigation and cropping without the provision of sub-surface drainage, the salts from deeper layers are accumulated in the surface layers with the rise in water table. Even a good quality underground water coming from shallow depths will transport the salts from the deep zone of accumulation to soil surface and, thus, causes serious soil salinization. To illustrate this, results of soil analysis of two typical soil profiles from Haryna, India, one each before and after canal irrigation was introduced, are presented in Table 7. The average rain fall of the area is about 350 mm and water table depth before the advent of irrigation was 18 m and at present it varies from 0.5 to 5 m during different months of the year. It is apparent from the data that in these soils there was a zone of salt accumulation at some depths, but with rise in water table level after the introduction of irrigation, the salts have migrated to the upper soil layers and created a serious salinity problem.
The sandy loam and loamy soils developed on the alluvium in a 300-400 mm rainfall zone usually have a shallow salt layer at a depth of 1 to 2 m. Most of the Vertisols in the medium-or deep-black soil areas in south India in a 500-700 mm rainfall zone have an accumulation of salts at a depth of 1 to 1.5 m. When such areas, even without a shallow ground water condition, are irrigated with an inadequate amount of water, the salts are dissolved in water during irrigation move to the soil surface during the subsequent fallow season.
The above described two conditions, i.e. transport of salts from deeper layers to upper layers of the soil profile with and without shallow water condition, are considered to be the main causes of soil salinzation in the canal irrigated areas of Punjab, Haryana, Rajasthan and the black-soil regions of south India.
-------------------------------------------Table 7: Soil analysis of two typical profiles before and after introduction of irrigation
Before the introduction of irrigation---After the introduction of irrigation
Depth (cm)--EC (mmhos/cm)--ESP---Depth (cm)-- EC (mmhos/cm)-- ESP
0-10-------- 1.42------------- 10.81---0-20--------- 19.20-------------51.0
10-20-------0.80------------- 5.29----20-40--------14.80------------- 54.7
20-45-------0.84--------------4.29----40-60---------11.60-------------51.7
43-88-------1.50--------------1.72-----60-100--------8.00-------------41.7
88-152------1.72--------------1.82-----100-150-------6.40-------------45.7
152-208----4.84------------- 3.51------150-200-------6.40------------45.5
208-228--- 10.90------------3.65------200-250-------4.00------------33.6
----------------------------------------------
Before the introduction of irrigation---After the introduction of irrigation
Depth (cm)--EC (mmhos/cm)--ESP---Depth (cm)-- EC (mmhos/cm)-- ESP
0-10-------- 1.42------------- 10.81---0-20--------- 19.20-------------51.0
10-20-------0.80------------- 5.29----20-40--------14.80------------- 54.7
20-45-------0.84--------------4.29----40-60---------11.60-------------51.7
43-88-------1.50--------------1.72-----60-100--------8.00-------------41.7
88-152------1.72--------------1.82-----100-150-------6.40-------------45.7
152-208----4.84------------- 3.51------150-200-------6.40------------45.5
208-228--- 10.90------------3.65------200-250-------4.00------------33.6
----------------------------------------------
5.2.3. Accumulation from irrigation and run-off waters due to poor soil physical properties
Salt accumulation in root zone also results from salts contained in the irrigation water, salt-rich flood water from industrial wastes, undulating land facilitating accumulation of water-washed salts in depressions, etc. Since all irrigation and run-off waters contain some salts, salinity may gradually build up even if waters of low salt content are used. This very apt to occur in soils of poor water permeability. The salts added by these waters will not leached (washed down) readily through such soils by an excess of applied water, and consequently will be left in the soil after the water has been removed by evaporation and crop use (i.e. by evapotranspiration). For example, investigations carried out by the workers of Central Soil Salinity Research Institute, Karnal, Haryana, India have revealed that vast areas have been rendered unfit for cultivation in the Indo-Gangetic plains of India as a result of excess salts in soils due to low lying nature of lands coupled with their poor physical characteristics and consequently periodic flooding by water from other areas.
5.2.4. Accumulation from saline water being used as a source of irrigation
The trend in irrigation is in the direction of using all the available water including the underground saline water. The use of irrigation water containing too much salt, or even good irrigation water, can cause soils to become saline if the amount of water applied is not sufficient for a fairly large fraction of it to pass beyond the root zone; some of the salts contained in the water will remain in the root zone. This is particularly true on irrigated areas having very high evaporation rates or very low rainfall. Subsequent irrigations will increase the salt content of the surface soil. See in Table 8, how large amount of salts accumulated in soil profile in only one cropping season under high evaporation rates and low rainfall in a field under wheat irrigated with saline underground water in a desert soil in Jodhpur, Rajasthan, India.
-----------------------------------------
Salt accumulation in root zone also results from salts contained in the irrigation water, salt-rich flood water from industrial wastes, undulating land facilitating accumulation of water-washed salts in depressions, etc. Since all irrigation and run-off waters contain some salts, salinity may gradually build up even if waters of low salt content are used. This very apt to occur in soils of poor water permeability. The salts added by these waters will not leached (washed down) readily through such soils by an excess of applied water, and consequently will be left in the soil after the water has been removed by evaporation and crop use (i.e. by evapotranspiration). For example, investigations carried out by the workers of Central Soil Salinity Research Institute, Karnal, Haryana, India have revealed that vast areas have been rendered unfit for cultivation in the Indo-Gangetic plains of India as a result of excess salts in soils due to low lying nature of lands coupled with their poor physical characteristics and consequently periodic flooding by water from other areas.
5.2.4. Accumulation from saline water being used as a source of irrigation
The trend in irrigation is in the direction of using all the available water including the underground saline water. The use of irrigation water containing too much salt, or even good irrigation water, can cause soils to become saline if the amount of water applied is not sufficient for a fairly large fraction of it to pass beyond the root zone; some of the salts contained in the water will remain in the root zone. This is particularly true on irrigated areas having very high evaporation rates or very low rainfall. Subsequent irrigations will increase the salt content of the surface soil. See in Table 8, how large amount of salts accumulated in soil profile in only one cropping season under high evaporation rates and low rainfall in a field under wheat irrigated with saline underground water in a desert soil in Jodhpur, Rajasthan, India.
-----------------------------------------
Table 8: Soil properties before (October) and after (March) under wheat crop irrigated with saline water
Irrigation water quality = EC - 3 to 10 mmhos/cm, SAR = 9 to 25
Irrigation water quality = EC - 3 to 10 mmhos/cm, SAR = 9 to 25
EC & ESP in root zone before crop sowing = 2 to 3 mmhos/cm, 11 to 25
EC and ESP in root zone after crop harvesting = 4 to 8 mmhos/cm, 11 to 31
------------------------------------Using saline ground water is a compulsion in many areas, because it is an important source of irrigation, only next to the surface water. In the arid and semiarid regions, the saline ground water is the major source of irrigation to supplement the scanty rainfall and poor canal water facilities. The prolonged use of such waters, or even that of good quality waters, for irrigation causes soil salinization in due course, if leaching requirement of the soil is not met with and the drainage is not provided for leaching.
5.2.5. Other ways of salt accumulation
The activities of man extend the range of possible salinity effects to all agricultural enterprises. In areas of intensive agriculture, and quite often in green house operations, high levels of fertilizers may cause an accumulation salts to the point where salinity may seriously affect crop production. The salts accumulated can cause salt injury no less damaging than that resulting from natural salinity.
In some areas, salinity of soil is explained by the fact that the soils were formed from the sediments deposited in the passages under the sea. The salt-affected soils of the Gangetic Delta of West Bengal, India, for example, contain a high proportion of sodium and magnesium, indicating the marine origin of these salts.
Sea water can salinize coastal areas, particularly where violent sweep on shores or even when sea breezes blow salt-spray inland (so called cyclic salts). For instance, on some areas along the sea coasts in Maharashtra, Kerala, Tamil Nadu, Andhra Pradesh and Bengal states of India, soil salinity is usually caused by annual inundation with sea water (Fig. 3).
5.2.5. Other ways of salt accumulation
The activities of man extend the range of possible salinity effects to all agricultural enterprises. In areas of intensive agriculture, and quite often in green house operations, high levels of fertilizers may cause an accumulation salts to the point where salinity may seriously affect crop production. The salts accumulated can cause salt injury no less damaging than that resulting from natural salinity.
In some areas, salinity of soil is explained by the fact that the soils were formed from the sediments deposited in the passages under the sea. The salt-affected soils of the Gangetic Delta of West Bengal, India, for example, contain a high proportion of sodium and magnesium, indicating the marine origin of these salts.
Sea water can salinize coastal areas, particularly where violent sweep on shores or even when sea breezes blow salt-spray inland (so called cyclic salts). For instance, on some areas along the sea coasts in Maharashtra, Kerala, Tamil Nadu, Andhra Pradesh and Bengal states of India, soil salinity is usually caused by annual inundation with sea water (Fig. 3).
Figure 3. Area influenced by sea water intrusions
6. HOW EVOLUTION OF SALINE-SODIC AND SODIC SOILS OCCUR
6.1. Evolution through sequential process of salinizaiion, sodification and desalinzation
6.1. Evolution through sequential process of salinizaiion, sodification and desalinzation
Salt accumulation is the first stage in the sequence of process common to the family of salt-affected soils. The origin and causes of salt accumulation have been presented in Section 5 above. A series of reactions take place concurrently with the salinization of soil profile. Sodium salts often predominate in the early stages of salinization. Calcium and magnesium salts accumulate more slowly. If the soluble cations consist of predominantly sodium salts, a major part of the calcium and magnesium are displaced from the exchange positions of the clay complex and sodium gets adsorbed as a result of cation exchange. This process is known as sodification and soil passes to the saline-sodic stage. As the salt concentration increases, calcium and magnesium get precipitated as carbonates. When waters having appreciable concentration of carbonates and bicarbonates are employed for irrigation in the soil, calcium is precipitated as calcium carbonate. The precipitation of calcium carbonate from irrigation water causes a decrease in soil salinity but an increase in the proportion of sodium in the soil solution and, therefore, increases the adsorption of exchangeable sodium by the soil according to law of mass action.
As exchangeable sodium accumulates on account of above processes, the pH of the soil usually increases and this enhances the precipitation of calcium carbonate. With additional precipitation of calcium carbonate, the proportion of sodium in the soil solution and on the exchange complex increases, further leading to an additional increase in pH and a repetition of the above described processes. This concentration and precipitation process causing accumulation of relatively insoluble calcium carbonate, soluble carbonates and sodium leads the soil to an advanced saline-sodic stage.
Considerable downward movement of salts may further take place by irrigation or rain water. Consequent removal of the excess salts brings about a process of desalinization accompanied by significant changes in the constitution of soil profile. As the excess salts are leached down, the soil colloids get dispersed and move to the subsoil making it dense and compact. This desalinization process leads the soil to sodic stage.
If extensive leaching of a saline-sodic soil occurs in the absence of source of calcium or magnesium, part of the exchangeable sodium is gradually replaced by hydrogen, yet contains enough sodium to give unstable structure. Such a soil is referred to as degraded sodic soil.
6.2. Evolution through the direct process of sodification
Sodic soils, dominated by sodium carbonate-type salts, in the Indo-Gangetic pains of India (Haryana, Punjab, Uttar Pradesh, Bihar and Madhya Pradesh) have been formed through the direct process of sodification. The geographic setting of sodic soils, their historic background, and their physico-chemical, hydrological and hydro-chemical characteristics establish the origin of these soils through the direct process of sodification commencing initially at the surface. These sodic soils usually occupy somewhat lower elevations in the otherwise flat terrain. The weathering of alumino-silicate minerals, in hills as well as in plains, through alkali hydrolysis, provides a steady supply of sodium bicarbonates with runoff water, which accumulate in the undrained low lying basins during the rainy season, July to September. In the post-rainy dry season, the excessive evaporation concentrates the soil solution and causes in increase in its SAR, thereby resulting in an increased adsorption of sodium on the exchange complex by displacing calcium and consequently a rise in soil pH. The displaced calcium precipitates as insoluble carbonates at high pH. This process repeated over long periods has led to the formation of sodic soils in the Indo-Gangetic plains. These soils have, therefore, not been formed through the so-called sequential process of salinization, sodification and desalinization. The wide-spread occurrence of sodic soils even before canal irrigation was introduced indicates that they are not necessarily linked with canal irrigation. Presently, these soils occur both in the canal irrigated and unirrigated regions, thus discarding the belief that the introduction of canal irrigation alone is the sole factor for their formation.
As exchangeable sodium accumulates on account of above processes, the pH of the soil usually increases and this enhances the precipitation of calcium carbonate. With additional precipitation of calcium carbonate, the proportion of sodium in the soil solution and on the exchange complex increases, further leading to an additional increase in pH and a repetition of the above described processes. This concentration and precipitation process causing accumulation of relatively insoluble calcium carbonate, soluble carbonates and sodium leads the soil to an advanced saline-sodic stage.
Considerable downward movement of salts may further take place by irrigation or rain water. Consequent removal of the excess salts brings about a process of desalinization accompanied by significant changes in the constitution of soil profile. As the excess salts are leached down, the soil colloids get dispersed and move to the subsoil making it dense and compact. This desalinization process leads the soil to sodic stage.
If extensive leaching of a saline-sodic soil occurs in the absence of source of calcium or magnesium, part of the exchangeable sodium is gradually replaced by hydrogen, yet contains enough sodium to give unstable structure. Such a soil is referred to as degraded sodic soil.
6.2. Evolution through the direct process of sodification
Sodic soils, dominated by sodium carbonate-type salts, in the Indo-Gangetic pains of India (Haryana, Punjab, Uttar Pradesh, Bihar and Madhya Pradesh) have been formed through the direct process of sodification. The geographic setting of sodic soils, their historic background, and their physico-chemical, hydrological and hydro-chemical characteristics establish the origin of these soils through the direct process of sodification commencing initially at the surface. These sodic soils usually occupy somewhat lower elevations in the otherwise flat terrain. The weathering of alumino-silicate minerals, in hills as well as in plains, through alkali hydrolysis, provides a steady supply of sodium bicarbonates with runoff water, which accumulate in the undrained low lying basins during the rainy season, July to September. In the post-rainy dry season, the excessive evaporation concentrates the soil solution and causes in increase in its SAR, thereby resulting in an increased adsorption of sodium on the exchange complex by displacing calcium and consequently a rise in soil pH. The displaced calcium precipitates as insoluble carbonates at high pH. This process repeated over long periods has led to the formation of sodic soils in the Indo-Gangetic plains. These soils have, therefore, not been formed through the so-called sequential process of salinization, sodification and desalinization. The wide-spread occurrence of sodic soils even before canal irrigation was introduced indicates that they are not necessarily linked with canal irrigation. Presently, these soils occur both in the canal irrigated and unirrigated regions, thus discarding the belief that the introduction of canal irrigation alone is the sole factor for their formation.
The areas dominated by sodic soils are generally associated with ground waters of good quality. But in some areas, sodic and saline sodic shallow ground waters are another source of sodium carbonates and bicarbonates. Rise in groundwater table in such regions where the ground waters have high residual sodium carbonate (RSC - sodium carbonates and bicarbonates minus calcium plus magnesium) further results in sodification. In other cases, use of ground waters with high sodium hazard for irrigation has resulted in the extension of sodic soils.
Thus, the geomorphic setting of the sodic soils in the Indo-Gangetic plains, their historic background, and their hydrological characteristics, clearly establish their origin through the above mentioned direct processes of sodification commencing initially at the soil surface. Hence, they differ from the sodic soils (solonets) occurring in the parts of Europe, former USSR and USA, where soils have evolved through the sequential processes of salinization, alkalization and desalinization. Therefore, much of the talk about the salinization leading to sodification is based on theoretically conjectures rather than on the real field situations as far as the formation of sodic soils in India is concerned.
7. SUMMARY OF THE PROCESSES OF SOIL SALINZATION AND SODIFICATION
The dominant sources of salt are rainfall and rock weathering. Rainwater contains low amounts of salt, but over time, salt deposited by rain can accumulate in the landscape. Wind-transported (aeolian) materials from soil or lake surfaces are another source of salt. Poor quality irrigation water also contributes to salt accumulation in irrigated soils. Seawater intrusion onto land, as occurred in recent tsunami-affected regions, can deposit huge amounts of salts in soils of coastal lands. The particular processes contributing salt, combined with the influence of other climatic and landscape features and the effects of human activities, determine where salt is likely to accumulate in the landscape.
There are three major types of salinity based on soil and groundwater processes found all over the world.
1) Groundwater associated salinity (GAS). In discharge areas of the landscape, water exits from groundwater to the soil surface bringing the salts dissolved in it. The driving force for upward movement of water and salts is evaporation from the soil plus plant transpiration. Generally, the water table in the landscape is at or very close to the soil surface and soil properties at the site allow a maximum rate of water movement through the surface layers. Salt accumulation is high when the water table is less than 1.5 m below the soil surface. However, this threshold depth may vary depending on soil type, soil hydraulic properties and climatic conditions.
2) Non-groundwater-associated salinity (NAS). In landscapes where the water table is deep and drainage is poor, salts, which are introduced by rain, weathering, and aeolian deposits are stored within the soil solum. In drier climatic zones, these salt stores are usually found in the deeper solum layers. However, poor hydraulic properties of shallow solum layers can lead to the accumulation of salts in the topsoil and subsoil layers affecting agricultural productivity. In regions where sodic soils are predominant, this type of salinity is a common feature.
3) Irrigation associated salinity (IAS). Salts introduced by irrigation water are stored within the root zone because of insufficient leaching. Poor quality irrigation water, low hydraulic conductivity of soil layers as found in heavy clay soils and sodic soils, and high evaporative conditions accelerate irrigation-induced salinity. Use of highly saline effluent water and improper drainage and soil management increase the risk of salinity in irrigated soils. In many irrigation regions, rising saline groundwater interacting with the soils in the root zone can compound the problem.
1) Groundwater associated salinity (GAS). In discharge areas of the landscape, water exits from groundwater to the soil surface bringing the salts dissolved in it. The driving force for upward movement of water and salts is evaporation from the soil plus plant transpiration. Generally, the water table in the landscape is at or very close to the soil surface and soil properties at the site allow a maximum rate of water movement through the surface layers. Salt accumulation is high when the water table is less than 1.5 m below the soil surface. However, this threshold depth may vary depending on soil type, soil hydraulic properties and climatic conditions.
2) Non-groundwater-associated salinity (NAS). In landscapes where the water table is deep and drainage is poor, salts, which are introduced by rain, weathering, and aeolian deposits are stored within the soil solum. In drier climatic zones, these salt stores are usually found in the deeper solum layers. However, poor hydraulic properties of shallow solum layers can lead to the accumulation of salts in the topsoil and subsoil layers affecting agricultural productivity. In regions where sodic soils are predominant, this type of salinity is a common feature.
3) Irrigation associated salinity (IAS). Salts introduced by irrigation water are stored within the root zone because of insufficient leaching. Poor quality irrigation water, low hydraulic conductivity of soil layers as found in heavy clay soils and sodic soils, and high evaporative conditions accelerate irrigation-induced salinity. Use of highly saline effluent water and improper drainage and soil management increase the risk of salinity in irrigated soils. In many irrigation regions, rising saline groundwater interacting with the soils in the root zone can compound the problem.
8. OCCURRENCE (GEOGRAPHICAL DISTRIBUTION) OF SALT-AFFECTED SOILS
8.1. Area of salt affected soils
Salt-affected soils are common in arid and semiarid regions, where annual precipitation is insufficient to meet the evapotranspiration needs of plants. As a result, salts are not leached from the soil. Instead, they accumulate in amounts or type detrimental to plant growth. Salt problems are not restricted to arid and semiarid regions, however. They can develop in sub humid and humid regions under appropriate conditions.
The distribution of soils affected by salts at present is closely related to environmental factors such as arid or semi-arid climate, accumulation of products of weathering in groundwater near the surface, etc. On a world-wide scale there is a considerable amount of data, maps and other materials, showing the extent of salt affected soils. Among these, the FAO/Unesco Soil Map of the World (scale 1:5 000 000) should be mentioned particularly, because it is the first to give a world-wide inventory of these soils and their distribution. Salt affected soils cover such an area in many countries in arid and semi-arid regions of Asia, Africa and South America and some countries in humid regions that they cause considerable problems regarding not only the natural environment of these areas, but also the national economy. Thus, the salinization and sodification of soils, which are frequently limiting factors in their exploitation, have been a pressing problem both in regions with humid climates, e.g. Holland, Sweden, Hungary, the former USSR, and in arid and semi-arid regions, e.g. the south western USA, Africa, South America, Australia, India, Pakistan and the Middle East. As was stated earlier, during the recent history of agriculture, the importance of this problem has considerably increased owing to the rapid extension of salinization and sodification of soils, which in many places, has caused a sharp reduction in agricultural production. Many thousands of sqkm of fertile irrigated lands were transformed into saline and alkali deserts during the history of mankind by the influence of improper irrigation. Unfortunately, this has happened not only in the past, but secondary salinization and/or alkalization is showing a disastrous increase, parallel with the construction of new irrigation systems in many countries all over the world, particularly in arid and semi-arid regions, or in regions with mineralized ground-water near the soil surface.
Besides soils affected by salt at present, we have potential salt affected soils. Soils considered potentially salt affected are those which are not, or only to a very low degree, saline and/or sodic at present but human intervention, especially irrigation, could cause their considerable salinization and/or alkalization.
The sodic soils are generally confined to areas with a mean annual rainfall between 550 and 1000 mm. The saline soils are widely distributed in the arid and semi-arid regions where the annual rainfall does not generally exceed 550 mm. The total area covered by salt-affected soils in the world is about 250 million ha. About 0.004 million ha of land annually become unfit for agricultural production because of this problem. It is well known that the majority of irrigated territories in the world are exposed to the hazard of secondary salinization, alkalization and water logging. According to estimates by the UK and affiliated agencies (FAO, Unesco, etc.) more than 50% of all irrigated lands of the world have been damaged by secondary salinization, alkalization and water logging. In the same estimation, many millions of productive hectares in irrigation systems have to be abandoned yearly owing to these causes. In India, for example, 7 million ha of otherwise productive land have been reported unproductive by soil salinity and sodicity in irrigated regions (Figs. 4 and 5) and the area under potential salt-affected soils is about 20 million ha.
The distribution of soils affected by salts at present is closely related to environmental factors such as arid or semi-arid climate, accumulation of products of weathering in groundwater near the surface, etc. On a world-wide scale there is a considerable amount of data, maps and other materials, showing the extent of salt affected soils. Among these, the FAO/Unesco Soil Map of the World (scale 1:5 000 000) should be mentioned particularly, because it is the first to give a world-wide inventory of these soils and their distribution. Salt affected soils cover such an area in many countries in arid and semi-arid regions of Asia, Africa and South America and some countries in humid regions that they cause considerable problems regarding not only the natural environment of these areas, but also the national economy. Thus, the salinization and sodification of soils, which are frequently limiting factors in their exploitation, have been a pressing problem both in regions with humid climates, e.g. Holland, Sweden, Hungary, the former USSR, and in arid and semi-arid regions, e.g. the south western USA, Africa, South America, Australia, India, Pakistan and the Middle East. As was stated earlier, during the recent history of agriculture, the importance of this problem has considerably increased owing to the rapid extension of salinization and sodification of soils, which in many places, has caused a sharp reduction in agricultural production. Many thousands of sqkm of fertile irrigated lands were transformed into saline and alkali deserts during the history of mankind by the influence of improper irrigation. Unfortunately, this has happened not only in the past, but secondary salinization and/or alkalization is showing a disastrous increase, parallel with the construction of new irrigation systems in many countries all over the world, particularly in arid and semi-arid regions, or in regions with mineralized ground-water near the soil surface.
Besides soils affected by salt at present, we have potential salt affected soils. Soils considered potentially salt affected are those which are not, or only to a very low degree, saline and/or sodic at present but human intervention, especially irrigation, could cause their considerable salinization and/or alkalization.
The sodic soils are generally confined to areas with a mean annual rainfall between 550 and 1000 mm. The saline soils are widely distributed in the arid and semi-arid regions where the annual rainfall does not generally exceed 550 mm. The total area covered by salt-affected soils in the world is about 250 million ha. About 0.004 million ha of land annually become unfit for agricultural production because of this problem. It is well known that the majority of irrigated territories in the world are exposed to the hazard of secondary salinization, alkalization and water logging. According to estimates by the UK and affiliated agencies (FAO, Unesco, etc.) more than 50% of all irrigated lands of the world have been damaged by secondary salinization, alkalization and water logging. In the same estimation, many millions of productive hectares in irrigation systems have to be abandoned yearly owing to these causes. In India, for example, 7 million ha of otherwise productive land have been reported unproductive by soil salinity and sodicity in irrigated regions (Figs. 4 and 5) and the area under potential salt-affected soils is about 20 million ha.
Figure 4: Extensive areas of sodic lands lying barren in northern India 
Figure 5: A typical saline soil in northern India
Figure 6: Global distribution of salt-affected soils
According to another estimate, there are over 970 million ha of salt-affected and potentially sat-affected lands in the world (Fig. 6, Table 9). Out of this 250 million ha are saline and sodic soils and approximately 720 million ha are potentially saline and sodic that mark present and potential degradation. In general, about 7% of the total soil surface area of the world is covered by salt-affected lands: Australia 42.3%, Asia 21%, South America 7.6%, Africa 3.5%, North America 0.9%, Central America 0.7% and Europe 4.6%. These figures and the ones given above are just estimates. Exact information on the extent of degraded areas and particularly their degree of deterioration is not available for all of the countries. Another estimate reveals that roughly 20 million ha of European salt-affected soils are sodic and about 7 million ha are saline. Vast amount of these soils are found in Bulgaria, former Czechoslovakia, France, Hungary(Fig. 7), Rumania, former USSR and Yugoslavia, while in Austria(Fig. 8), Greece, Italy and Portugal they are spotted about. The salt-affected and potentially salt-affected areas in the Near-East region are estimated at 31.5 million ha, the bulk of which is distributed as follows: Iran 7.3 (Fig. 9), Pakistan 6.4, Thailand 1.5 (Fig. 10), Somalia 3.0, Egypt 2.0, Iraq 1.8 and Syria 0.2 million ha. In Africa, salt-affected are found in Nigeria, Chad, Mali, Algeria, Morocco and Tanzania. According to a recent survey, in the USA, 5 million ha of irrigated land are estimated to salt-affected, mostly in the 17 western states. According to this survey, as much as one-third of all irrigated lands in the world (or approximately 70 million ) may be plagued by salt problems. When salt problems of nonirrigated semiarid and humid regions, greenhouse crops, mine spoils, and waste disposal areas are added to these figures, the dimensions of the problem are truly impressive.
Figure 7: Salt-affected soils in northeast Hungary
Figure 8: Salt-affected soils in southwest Australia
Figure 9: Severe salinity in Dasht-e-Kavir, Iran (Wasteland)
Figure 10: Salt-affected soils in northeast Thailand
Table 9. Regional distribution of salt-affected and potentially salt-affected (saline and sodic phases) soils in the world (1000 ha)
Regions---------------------Saline---Saline phase--Sodic--Sodic phase--Total
Europe--------------------------------------------------------------------50804
North America------------------------6191---------9564-----------------15755
Mexico & Central America--242------1723--------------------------------1965
South America-------------10461----58949--------14790--44963-------129163
Africa----------------------45579----47705--------1181----5356--------98521
South Asia-----------------47233----36077-----------------1798--------85108
North & Central Asia-------22465----69156--------30062--90003------211686
South East Asia----------------------19983------------------------------19983
Austral Asia----------------16567----792-----------38111--301860-----357330
Total-----------------------140547---240576------94408--443980-----970315
Table 10: World distribution of salt affected areas (1000 ha)
Continent/country, Saline soils, Sodic soils, Total
North America
1. Canada,264, 6974, 72382
2. USA, 5927, 2590, 8517
Mexico and Central America
1. Cuba, 316, 0, 316
2. Mexico, 1649, 0, 1649
South America
1. Argentina, 32473, 53139, 85612
2. Bolivia, 5233, 716, 5949
3. Brazil, 4141, 362, 4503
4. Chile, 5000, 3642, 8642
5. Colombia, 907, 0, 907
6. Ecuador, 387, 0 , 387
7. Paraguay, 20008, 1894, 21902
8. Peru, 21, 0, 21
9. Venezuela, 1240, 0, 1240
Africa
1. Afars and Issas, 1741, 0, 1741
2. Algeria, 3021, 129, 3150
3. Angola, 440, 86, 526
4. Botswana, 5009, 670, 5679
5. Chad, 2417, 5850, 8267
6. Egypt, 7360, 0, 7360
7. Ethiopia, 10608, 425, 11033
8. Gambia, 150, 0, 150
9. Ghana, 200, 118, 318
10. Guinea, 525, 0, 525
11. Guinea-Bissau,194, 0, 194
12. Kenya, 4410, 448, 4858
13. Liberia, 362, 44, 406
14. Libya, 2457, 2457
15. Madagascar, 37, 1287, 1324
16. Mali, 2770, 0, 2770
17. Mauritania, 640, 0, 640
18. Morocco, 1148, 0, 1148
19. Namibia, 562, 1751, 2313
20. Niger, o, 1389, 1389
21. Nigeria, 665, 5837, 6502
22. Rhodesia, 0, 26, 26
23. Senegal, 765, 0, 765
24. Sierra Leone, 307, 0, 307
25. Somalia, 1569, 4033, 5602
26. Sudan, 2138, 2736, 4874
27. Tunisia, 990, 990
28. Cameroon, 0, 671, 671
29. Tanzania, 2954, 583, 3537
30. Zaire, 53, 0, 53
31. Zambia, 0, 863, 863
South Asia
1. Afghanistan, 3103, 0, 3103
2. Bangladesh, 2479, 538, 3017
3. Burma, 634, 0, 634
4. India, 23222, 574, 23796
5. Iran, 26399, 686, 27085
6. Iraq, 6726, 0, 6726
7. Israel, 28, 0, 28
8. Jordan, 180, 0, 180
9. Kuwait, 209, 0, 209
10. Muscat and Oman, 290, 0, 290
11. Pakistan, 10456, 0, 10456
12. Qatar, 225, 0, 225
13. Sarawak, 1538, 0, 1538
14. Saudi Arabia, 6002, 0, 6002
15. Sri Lanka, 200, 0, 200
16. Syria, 532, 0, 532
17. UAE, 1089, 0, 1089
North and Central Asia
1. China, 36221, 437, 36658
2. Mongolia, 4070, 0, 4070
3. USSR, 51092, 119628, 170720
South-East Asia
1. Kampuchea, 1291, 0, 1291
2. Indonesia, 13213, 0, 13213
3. Malaysia, 3040, 0, 3040
4. Vietnam, 938, 0, 938
5. Thailand, 1456, 0, 1456
Australasia
1. Australia, 17269, 339971, 357240
2. Fiji, 90, 90
3. Solomon Islands, 238, 238
Europe
1. Czechoslovakia,--,--,106
2.France,--,--,250
3. Hungary,--,--,1272
4. Italy,--,--, 450
5. Rumania,--,--, 250
6. Spain,--,--, 840
7. Yugoslavia,--,--, 255
8.2. Geographical distribution of salt affected soils
Above given table 10 gives world distribution of salt affected areas according to another estimation. The data include both salt affected and potentially salt affected lands. According to a more recent report published by FAO in 2000, the total global area of salt-affected soils including saline and sodic soils was 831 million ha, extending over all the continents including Africa, Asia, Australasia, and the Americas. Figures 11 to 18 give geographical distribution of salt affected soils in some countries. Their diastribution in different continents is discussed below:
8.2.1. Africa
It has been estimated that about 2% of the land area of Africa is salt affected. In the North Afrcan region, the salt accumulation is the main cause of low agricultural production. Impeded drainage affects very large areas in and adjacent to Nile Valley. Soil salinity and sodicity are major problems in Egyptian agriculture, affecting about 800,000 ha of cultivated soils. In Sudan, soil salinity is a major problem in the Gezira and in Northern Province.
8.2.2. Asia
Among the countries of South and South-East Asia, the problem is greater in India, Pakistan, Indonesia, Malaysia, Bangladesh, Kampuchea, Thailand, Vietnam, Philippines (Figs. 11 -18). In India, estimates of the total area affected are 6.1, 7, 8.5 and 23.8 million ha. Four geographical categories of salt-affected soils are found in India: Sodic soils of the Indo-Gangetic regions (Haryana, Punjab, Uttar Pradesh, Bihar, Rajasthan and Madhya Pradesh) covering 35% (2.5 million ha) of the total 7 million ha area of the salt-affected soils, saline soils of arid and semi-arid regions (Gujarat, Rajasthan, Punjab and Haryana) occupying 14.2% (1.00 million ha) area, saline soils of the medium and deep black soils regions (Karnataka, Madhya Pradesh, Andhra Pradesh, Maharashtra and Tamil Nadu) with 20.4% (1.42 million ha) of the total area, and coastal saline soils of arid and humid regions (Gujarat, West Bengal, Orissa, Andhra Pradesh, Tamil Nadu, Kerala and West Bengal) accounting for 30.4% (2.12 million ha) of the total salt-affected area. In Pakistan, there are about 2 million ha of salt affected soils in the Indus Valley and about 1.5 million ha in Punjab.
In China, there are 20 million ha of salt affected soils. Another estimate puts it at 36.7 million ha. The salinity is widespread in the Sungari Valley in Manchuria and in the Hwang-Ho Delta, particularly near the Yellow Sea. These soils also occur in many arid and semi arid regions of the country. Salinity originating from marine chlorides is near the cost.
Saline and sodic soils occur widely in Asiatic USSR, with large areas in the river valleys of eastern and western Siberia in the Uails region and in the Araxes Valley in Armenia.
The middle Eastern countries are much affected by salinity. In Syria, it is found in the Palmyra region and in the Euphrates, El-khabour and Dan valleys. Salinity has been a problem in Iraq since ancient times, affecting most of the Euphrates and Tigris valleys, whether irrigated or not. The areas of salt affected soils in Afghanistan are saline rather than alkali. The largest areas of salt affected soils in this region , however, are in Iran and Saudi Arabia.
Figure 11: Problem soils in India.
Figure 12: Problem soils in Pakistan.
Figure 13: Problem soils in Indonesia
Figure 14: Problem soils in Bangladesh.
Figure 15: Problem soils in Kampuchea.
Figure 16: Problem soils in Thailand.
Figure 17: Problem soils in Vietnam.
Figure 18: Problem soils in Philippines. 8.2.3. Australia
Salt affected soils are wide-spread in many parts of Australia, both as naturally occurring sodic and saline soils and those formed as a result of man's activities in sensitive dry land situations. These soils are not widely cultivated and left as natural reserves. Inland salinity is an increasing problem in irrigated areas. According to one estimate, 17,269 and 339,971 ha of saline and sodic soils, respectively, occur in Australia. Later, area under saline soils has been put to 167,ooo ha.
2.2.4. Europe
Salt affected soils are found in the alluvial regions of the Danube, Dnieper and Don in the former USSR. They also occur in eastern Europe in Hungary, Czechoslovakia, Rumania and Yugoslavia, where they account for 10 % of land area. In western Europe, Spain has about 600,000 ha in Andalusia, Aragon and Catalonia. The Netherlands are a special case in that though much of land was originally under sea water and, thus, saline, it has subsequently been drained and leached free of salts through a system of polders and intensive drainage.
2.2.5. North America
Salt affected soils are found in 17 western states of USA, notably Arizona, New Mexico, Texas and Utah, and in parts of Canada. There are significant areas of saline soils of varying origin in Cuba and in Mexico.
2.2.6. South America
Salt affected soils are found in most countries of South America, particularly in Argentina, Brazil and Chile. Of some interest, though affecting a relatively small area, is the salinity in the coastal belt of Peru, 2000 km long and 10 to 25 km wide, initially of marine origin but now becoming steadily more extensive because of water logging due to irrigation and poor drainage facilities. It has been estimated that crop production was reduced by salt accumulation in 25 to 30 % of this belt.
9. CHARACTERISTICS, LIMITATIONS, LAND USE AND POTENTIALITIES OF SALT-AFFECTED SOILS
The two main groups of salt-affected soils differ not only in their chemical characteristics but also in their geographical and geochemical distribution, as well as in their physical and biological properties. The two categories also require different approaches for their reclamation and agricultural utilization. The distinguishing features of these two broad groups of salt-affected soils are presented in Table 11.
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Table 11. Distinguishing features of saline and sodic soils
1. Chemical characteristics
1)Saline soils are dominated by neutral soluble salts consisting of chlorides and sulfates of sodium, calcium and magnesium. Appreciable quantities of neutral soluble salts generally absent in sodic soils. Measurable to appreciable quantities of salts capable of alkaline hydrolysis, e.g. Na2CO3, present.
2) pH of saturated soil paste is less than 8.5 in saline soils. pH of the saturated soil paste is more than 8.5 in sodic soils.
3) An electrical conductivity of the saturated soil extract of more than 4 mmhos/cm at 25 °C is the generally accepted limit above which soils are classed as ‘saline’. Their ESP is less than 15.
An ESP of 15 or more is the generally accepted limit above which soils are classed as ‘sodic’. Electrical conductivity of the saturated soil extract is generally less than 4 mmhos/cm at 25 °C but may be more if appreciable quantities of sodium carbonates, etc. are present.
4) There is generally no well-defined relationship in saline soils between pH of the saturated soil paste and exchangeable sodium percentage (ESP) of the soil or the sodium adsorption ratio (SAR) of the saturation extract. In sodic soils, there is a well defined relationship between pH of the saturated soil paste and the exchangeable sodium percentage (ESP) of the soil or the SAR of the saturation extract for an otherwise similar group of soils such that the pH can serve as an approximate index of soil sodicity (alkali) status.
5) Although Na is generally the dominant soluble cation, the soil solution also contains appreciable quantities of divalent cations, e.g. Ca and Mg in saline soils. Sodium is the dominant soluble cation in sodic soils. High pH of the soils results in precipitation of soluble Ca and Mg such that their concentration in the soil solution is very low.
6) Saline soils may contain significant quantities of sparingly soluble calcium compounds, e.g. gypsum. Gypsum is nearly always absent in sodic soils.
2. Physical
1) In saline soils, in the presence of excess neutral soluble salts, the clay fraction is flocculated and the soils have a friable structure. Excess exchangeable sodium and high pH result in the dispersion of clay in sodic soils and the soils have an unstable structure.
2) Permeability of deeply ploughed saline soils to water and air and other physical characteristics are generally comparable to normal soils. Permeability of soils to water and air is restricted. Physical properties of the soils become worse with increasing levels of exchangeable sodium/pH.
3. Effect on plant growth
In saline soils plant growth is adversely affected: (a) chiefly through the effect of excess salts on the osmotic pressure of soil solution resulting in reduced availability of water; and (b) through toxicity of specific ions, e.g. Na, Cl, B, etc. In sodic soils plant growth is adversely affected: (a) chiefly through the dispersive effect of excess exchangeable sodium resulting in poor physical properties; (b) through the effect of high soil pH on nutritional imbalances including a deficiency of calcium; and (c) through toxicity of specific ions, e.g. Na, CO3, Mo, etc.
4. Soil improvement
Improvement of saline soils essentially requires removal of soluble salts in the root zone through leaching and drainage. Application of amendments may generally not be required.
Improvement of sodic soils essentially requires the replacement of sodium in the soil exchange complex by calcium through use of soil amendments and leaching and drainage of salts resulting from reaction of amendments with exchangeable sodium.
5. Geographic distribution
Saline soils tend to dominate in arid and semi-arid regions. Sodic soils tend to dominate in semi-arid and sub-humid regions.
6. Ground-water quality
Groundwater in areas dominated by saline soils has generally high electrolyte concentration and a potential salinity hazard. Groundwater in areas dominated by sodic soils has generally low to medium electrolyte concentration and some of it may have residual sodicity so has a potential sodicity hazard.
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The description given below, includes above mentioned and some additional features of saline and sodic soils.
9.1. Saline soils
In most of the saline soils, the whole profile is salt-laden. In strongly saline soils, a white and 1 to 5 cm thick salt layer occurs on soil surface during the dry period. The salt content in the profile gradually decreases with depth. The excessive salinity in most of these soils is due to the dominance of sodium chloride followed by sodium sulfate and chlorides and sulpates of calcium and magnesium. The pH value is mostly less than 8.5 but not less than 7.0, with the exception of some coastal saline soils like those found in Kerala, India, which are distinctly acidic. In some gypsiferous soils, like those found in Gujarat and Rajasthan states of India, calcium sulphate is the dominant salt.
Due to excessive neutral salts, these soils are usually flocculated and have good physical properties. The structure is, however, not stable due to lack organic matter. As the soils are mostly uncultivated, a hard layer develops below the fluffy salty surface layer (0-10 cm) due to the compaction caused by the hydrostatic pressure of the flood water during rainy season, the prolonged fallowing, and the trembling by farm animals and farm machinery. This layer restricts water movement, if deep ploughing is not practised.
Limitations in saline soils arise from the high amount of soluble salts present in the soil solution. High amount of these salts interferes with crop growth through (1) restricting the availability of water due to high osmotic pressure of the soil solution (physiological effect, (2) creating the imbalance between plant nutrients, and (3) creating toxicity of certain elements like iron, manganese, boron, chloride, sodium and lithium.
The conditions like periodic submergence or water logging and the presence of shallow brackish ground water are the severe limitations, as these cause further salinization. Low permeability due to the presence of a hard dense layer below the fluffy salty layer in some areas and a heavy -texture in others, low fertility, impeded drainage and seepage from farm canals are the other major problems of these soils.
Slightly saline soils are moderately cultivated with some salt resistant crops like rice, barley, wheat, sugarcane, oats, berseem and sesbania. There is stunted patchy and poor plant growth. Plants growing on such soils may appear stunted and have thickened leaves and dark green colour. Strongly saline soils are poorly cultivated in small and scattered patches. On waste lands, salt tolerant grass vegetation and barren spots are common. There is complete failure in severely affected areas.
Saline soils have potential productivity due to abundant of nutrient reserve, loamy soil texture and ideal moisture retention and transmission characteristics after reclamation. Soil that has been saline for several years will usually be fertile, with high nitrogen, phosphorus and potassium. These nutrients build up in salty areas when there is little removal by crops and the area is fertilized each year.
9.2. Sodic soils
Preponderance of carbonates and bicarbonates of sodium, high pH (above 9 or 9.5) and low or high EC are diagnostic characteristic of these soils. There is a white encrustation on the surface during dry months, which often shows dark/black colour on wetting. The clay and organic fractions are dispersed. Dispersed organic matter accumulates at the surface of poorly drained areas as water evaporates and imparts a black colour to the surface on wetting, hence the name “black alkali" soils. When wet, the soil become sticky and on drying it is cloddy, hard, compact and difficult to work with. Rainwater does not move down and remains standing for a long period. There is a barren look on the land.
The excessive sodium carbonate in the soil solution causes the precipitation of calcium of the exchange complex as insoluble calcium carbonate, thus causing a high amount of exchangeable sodium. The ESP values of 80-90 or more are common in the surface layers of these soils. These soils have excessive soluble salts in the upper 0-30 cm soil surface layer. Generally, sodicity and salinity decreases with depth.
The high pH and ESP values induce deficiencies and toxicities of several nutrient and non-nutrient elements (chemical effect).
As stated above, the soils are highly dispersed and consequently have poor physical properties, resulting in the restricted water and air movement. The high dispersibility renders the soils prone to erosion and high runoff during rain, which causes floods and damage to crops in the adjoining fields. It also causes the prolonged water stagnation on soil surface and the surface crusting on drying. The soils are very hard when dry and produce big clods on ploughing. Thus, these soils are a particularly difficult management problem.
The poor soil physical and water transmission properties impede the growth of plants (soil physical effect).
Thus, we have seen that the limitations of these soils stem from the presence of high pH, high ESP, negligible permeability, prolonged water stagnation on soil surface following irrigation and rain, very hard surface after drying and pH induced deficiencies and toxicity of several nutrients.
The development of poor physical conditions coupled with nutritional disorders, therefore, results in stunted or no crop growth on these soils. Only salt tolerant crops like rice, barley, berseem, sesbania and some grasses can grow on them giving extremely poor yields.
Loamy textural family in most of cases, optimum soil depth, abundant mineral nutrient reserve and thick sweet water aquifer in most cases and availability of gypsum for their reclamation are favorable characteristics contributing to a high productivity of these soils.
10. CONCLUSIONS
9.1. Saline soils
In most of the saline soils, the whole profile is salt-laden. In strongly saline soils, a white and 1 to 5 cm thick salt layer occurs on soil surface during the dry period. The salt content in the profile gradually decreases with depth. The excessive salinity in most of these soils is due to the dominance of sodium chloride followed by sodium sulfate and chlorides and sulpates of calcium and magnesium. The pH value is mostly less than 8.5 but not less than 7.0, with the exception of some coastal saline soils like those found in Kerala, India, which are distinctly acidic. In some gypsiferous soils, like those found in Gujarat and Rajasthan states of India, calcium sulphate is the dominant salt.
Due to excessive neutral salts, these soils are usually flocculated and have good physical properties. The structure is, however, not stable due to lack organic matter. As the soils are mostly uncultivated, a hard layer develops below the fluffy salty surface layer (0-10 cm) due to the compaction caused by the hydrostatic pressure of the flood water during rainy season, the prolonged fallowing, and the trembling by farm animals and farm machinery. This layer restricts water movement, if deep ploughing is not practised.
Limitations in saline soils arise from the high amount of soluble salts present in the soil solution. High amount of these salts interferes with crop growth through (1) restricting the availability of water due to high osmotic pressure of the soil solution (physiological effect, (2) creating the imbalance between plant nutrients, and (3) creating toxicity of certain elements like iron, manganese, boron, chloride, sodium and lithium.
The conditions like periodic submergence or water logging and the presence of shallow brackish ground water are the severe limitations, as these cause further salinization. Low permeability due to the presence of a hard dense layer below the fluffy salty layer in some areas and a heavy -texture in others, low fertility, impeded drainage and seepage from farm canals are the other major problems of these soils.
Slightly saline soils are moderately cultivated with some salt resistant crops like rice, barley, wheat, sugarcane, oats, berseem and sesbania. There is stunted patchy and poor plant growth. Plants growing on such soils may appear stunted and have thickened leaves and dark green colour. Strongly saline soils are poorly cultivated in small and scattered patches. On waste lands, salt tolerant grass vegetation and barren spots are common. There is complete failure in severely affected areas.
Saline soils have potential productivity due to abundant of nutrient reserve, loamy soil texture and ideal moisture retention and transmission characteristics after reclamation. Soil that has been saline for several years will usually be fertile, with high nitrogen, phosphorus and potassium. These nutrients build up in salty areas when there is little removal by crops and the area is fertilized each year.
9.2. Sodic soils
Preponderance of carbonates and bicarbonates of sodium, high pH (above 9 or 9.5) and low or high EC are diagnostic characteristic of these soils. There is a white encrustation on the surface during dry months, which often shows dark/black colour on wetting. The clay and organic fractions are dispersed. Dispersed organic matter accumulates at the surface of poorly drained areas as water evaporates and imparts a black colour to the surface on wetting, hence the name “black alkali" soils. When wet, the soil become sticky and on drying it is cloddy, hard, compact and difficult to work with. Rainwater does not move down and remains standing for a long period. There is a barren look on the land.
The excessive sodium carbonate in the soil solution causes the precipitation of calcium of the exchange complex as insoluble calcium carbonate, thus causing a high amount of exchangeable sodium. The ESP values of 80-90 or more are common in the surface layers of these soils. These soils have excessive soluble salts in the upper 0-30 cm soil surface layer. Generally, sodicity and salinity decreases with depth.
The high pH and ESP values induce deficiencies and toxicities of several nutrient and non-nutrient elements (chemical effect).
As stated above, the soils are highly dispersed and consequently have poor physical properties, resulting in the restricted water and air movement. The high dispersibility renders the soils prone to erosion and high runoff during rain, which causes floods and damage to crops in the adjoining fields. It also causes the prolonged water stagnation on soil surface and the surface crusting on drying. The soils are very hard when dry and produce big clods on ploughing. Thus, these soils are a particularly difficult management problem.
The poor soil physical and water transmission properties impede the growth of plants (soil physical effect).
Thus, we have seen that the limitations of these soils stem from the presence of high pH, high ESP, negligible permeability, prolonged water stagnation on soil surface following irrigation and rain, very hard surface after drying and pH induced deficiencies and toxicity of several nutrients.
The development of poor physical conditions coupled with nutritional disorders, therefore, results in stunted or no crop growth on these soils. Only salt tolerant crops like rice, barley, berseem, sesbania and some grasses can grow on them giving extremely poor yields.
Loamy textural family in most of cases, optimum soil depth, abundant mineral nutrient reserve and thick sweet water aquifer in most cases and availability of gypsum for their reclamation are favorable characteristics contributing to a high productivity of these soils.
10. CONCLUSIONS
1. Salinization and sodification is the accumulation of water-soluble salts in the soil solum or regolith to a level that imparts on agricultural production, environmental health, and economic welfare.
2. The above brief write up regarding origin, geological distribution and characteristics of salt-affected soils does convey the extent (250 million ha salt-affected and 720 million ha potentially salt-affected soils) and complexicity of the salt hazards in the world. This problem is of very great magnitude in irrigated, arid and semiarid regions considering the vastness of the salt-affected soils. Salt-affected soils occur in more than 100 countries of the world with a variety of extents, nature, and properties. No climatic zone in the world is free from salinization and sodification, although the general perception is focused on arid and semiarid regions.
3.Thus, salinity and sodicity problems are, however, not only restricted to our arid and semiarid regions, but also met within the inland and coastal humid areas in several countries. The situation is further alarming because large areas are still going out of cultivation very rapidly in most of these regions.
4. Salinization and sodification are complex processes involving the movement of salts and water in soils during seasonal cycles and interactions with ground water. While mineral weathering, rainfall, aeolian deposits, and stored salts are the sources of salts, surface and groundwaters can redistribute the accumulated salts and may also provide additional sources. Thus, three main natural sources of soil salinity are geologically weathered rocks (mineral weathering), oceanically derived rain fall, and ground water containing salts of geologic or marine origin (fossil salts). The human activities that add salts to soil include irrigation and saline industrial wastes. Seawater encroachment can also harm soils. High and fluctuating water tables have been found responsible for the creation of many of our salt problem soils. Since all irrigation waters contain some salts, irrigation water itself has created salt problems in some areas. Also increased use of supplemental irrigation with quite saline water has created problems where none existed before. Special salinity problems along our sea coasts have developed because of inundations by sea water, tidal action, cyclic salt sprays, salty rains or presence of saline underground sea water.
5. In most of the saline soils, the whole profile is salt-laden, a white salt layer occurs on soil surface during the dry period, the salt content in the profile decreases with depth. Salinity in these soils is due to the dominance of sodium chloride followed by sodium sulpfate and chlorides and sulfates of calcium and magnesium with pH value less than 8.5. In some areas, gypsiferous, acid sulphate soils and sodium chloride dominating soils also occur. These soils are usually flocculated and have good physical properties, but the structure is not stable due to lack organic matter. High amount of soluble salts present in the soil solution interferes with crop growth through (1) restricting the availability of water due to high osmotic pressure of the soil solution (physiological effect0, (2) creating the imbalance between plant nutrients, and (3) creating toxicity of certain elements like iron, manganese, boron, chloride, sodium and lithium. The conditions like periodic submergence or water logging and the presence of shallow brackish ground water are the severe limitations, as these cause further salinization. Saline soils are moderately cultivated with some salt resistant crops and grasses with stunted patchy and poor plant growth. There is complete failure in severely affected areas. Saline soils have potential productivity due to abundant of nutrient reserve, loamy soil texture and ideal moisture retention and transmission characteristics after reclamation. Improvement of saline soils essentially requires removal of soluble salts in the root zone through leaching and drainage.
2. The above brief write up regarding origin, geological distribution and characteristics of salt-affected soils does convey the extent (250 million ha salt-affected and 720 million ha potentially salt-affected soils) and complexicity of the salt hazards in the world. This problem is of very great magnitude in irrigated, arid and semiarid regions considering the vastness of the salt-affected soils. Salt-affected soils occur in more than 100 countries of the world with a variety of extents, nature, and properties. No climatic zone in the world is free from salinization and sodification, although the general perception is focused on arid and semiarid regions.
3.Thus, salinity and sodicity problems are, however, not only restricted to our arid and semiarid regions, but also met within the inland and coastal humid areas in several countries. The situation is further alarming because large areas are still going out of cultivation very rapidly in most of these regions.
4. Salinization and sodification are complex processes involving the movement of salts and water in soils during seasonal cycles and interactions with ground water. While mineral weathering, rainfall, aeolian deposits, and stored salts are the sources of salts, surface and groundwaters can redistribute the accumulated salts and may also provide additional sources. Thus, three main natural sources of soil salinity are geologically weathered rocks (mineral weathering), oceanically derived rain fall, and ground water containing salts of geologic or marine origin (fossil salts). The human activities that add salts to soil include irrigation and saline industrial wastes. Seawater encroachment can also harm soils. High and fluctuating water tables have been found responsible for the creation of many of our salt problem soils. Since all irrigation waters contain some salts, irrigation water itself has created salt problems in some areas. Also increased use of supplemental irrigation with quite saline water has created problems where none existed before. Special salinity problems along our sea coasts have developed because of inundations by sea water, tidal action, cyclic salt sprays, salty rains or presence of saline underground sea water.
5. In most of the saline soils, the whole profile is salt-laden, a white salt layer occurs on soil surface during the dry period, the salt content in the profile decreases with depth. Salinity in these soils is due to the dominance of sodium chloride followed by sodium sulpfate and chlorides and sulfates of calcium and magnesium with pH value less than 8.5. In some areas, gypsiferous, acid sulphate soils and sodium chloride dominating soils also occur. These soils are usually flocculated and have good physical properties, but the structure is not stable due to lack organic matter. High amount of soluble salts present in the soil solution interferes with crop growth through (1) restricting the availability of water due to high osmotic pressure of the soil solution (physiological effect0, (2) creating the imbalance between plant nutrients, and (3) creating toxicity of certain elements like iron, manganese, boron, chloride, sodium and lithium. The conditions like periodic submergence or water logging and the presence of shallow brackish ground water are the severe limitations, as these cause further salinization. Saline soils are moderately cultivated with some salt resistant crops and grasses with stunted patchy and poor plant growth. There is complete failure in severely affected areas. Saline soils have potential productivity due to abundant of nutrient reserve, loamy soil texture and ideal moisture retention and transmission characteristics after reclamation. Improvement of saline soils essentially requires removal of soluble salts in the root zone through leaching and drainage.
6. Dominance of carbonates and bicarbonates of sodium, high pH (above 9 or 9.5) and low or high EC are diagnostic criteria of sodic soils. There is a white encrustation on the surface during dry months, which often shows dark/black colour on wetting. When wet, the soil become sticky and on drying it is cloddy, hard, compact and difficult to work with. Rainwater does not move down and remains standing for a long period. The ESP values of 80-90 or more are common in the surface layers of these soils. The high pH and ESP values induce deficiencies and toxicities of several nutrient and non-nutrient elements. Poor physical properties restrict water and air movement, make the soils prone to erosion and high runoff during rain, which causes floods and prolonged water stagnation and damage to crops. Thus, these soils are a particularly difficult management problem. The poor soil physical and water transmission properties impede the growth of plants. Only salt tolerant crops like rice, barley, berseem, sesbania and some grasses can grow on them giving extremely poor yields. Loamy textural family in most of cases, optimum soil depth, abundant mineral nutrient reserve and thick sweet water aquifer in most cases and availability of gypsum for their reclamation are favorable characteristics contributing to a high productivity of these soils.
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