Thus, urgency and seriousness of these problems have been discussed in this Write-up. It has been shown that salt affected soils exert a variety of harmful effects on crop production and soil properties; socio-economic conditions of the affected people; and environment in a number of ways throughout the world.
6.3.6.3. How to confirm?
Both the soil and plant can be tested for B toxicity. The optimal toxicity limits of B in leaves have to be interpreted with caution:
1) There is a steep concentration gradient of B within a leaf blade, from low values at the leaf base to high values at the tip.
2) Critical toxicity levels in field-grown rice are lower than those of plants grown in the greenhouse because of B leaching from leaves due to rain.
3) The effect on yield differs significantly among varieties.
The critical toxicity limits of B in the soil, for example for rice, are as follows:
1) >4 mg B kg-10.05N HCl
2) >5 mg B kg-1hot water- soluble B
3) >2.5 mg B L-1soil solution
6.3.6.4. Why and where it occurs?
B toxicity is relatively rare - especially in irrigated rice systems. It is caused by:
1) Large B concentration in soil solution because of the use of B-rich saline groundwater and high temperature (e.g., in arid regions, very deep tube wells, or wells in areas affected by geothermal activities).
2) Large B concentration in soil solution because of B-rich parent material. B content is high in some marine sediments, plutonic rocks, and other volcanic materials (e.g., tuff), but the content in igneous rocks is low.
3) Excess application of borax or large applications of municipal waste (compost).
B toxicity is most common in arid and semiarid regions, but has also been reported in other areas. Soils prone to B toxicity include the following types:
1) Soils formed on volcanic parent material, usually associated with the use of saline irrigation water pumped from deep wells containing a large B concentration (e.g., IRRI farm, Los Baños, and Albay, Philippines).
2) Some coastal saline soils.
6.3.6.5. When damage is important?
The damage is important throughout the growth cycle of the crop.
6.3.6.6. Economic importance
B toxicity is relatively rare - especially in canal irrigated systems - being more common in arid and semiarid regions.
6.3.6.7. Mechanism of damage
The physiology of B tolerance and B toxicity is not well understood. B uptake is closely related to the B concentration of the soil solution and the rate of water transpiration. When the B concentration in the soil solution is large, B is distributed throughout the plant in the normal transpiration stream, causing the accumulation of B in leaf margins and leaf tips. Excess B appears to inhibit the formation of starch from sugars or results in the formation of B-carbohydrate complexes, resulting in retarded grain filling but normal vegetative growth. Varieties with a large B requirement are less susceptible to B toxicity and vice versa.
6.3.7. Other ions
They are generally toxic when their concentrations exceed relatively low levels including Fluoride, selenate, lithium and others. These tend to be localized in occurrence, and are not usually, or necessarily associated with a general saline condition.
6.3.8. Diagnosing potential for specific ion toxicities by soil testing
The potential for specific ion toxicities can be diagnosed by testing soil using the saturated paste method (Table 9).
6.4.1. Effects on nutrient availability and uptake by plants
6.4.1.1. General description
Apart from the effect on water availability to plants and the possible toxic effect of some constituents, excess neutral soluble salts in soils may also interfere with the normal nutrition of crops in saline soils. At a given level of salinity, growth and yield of crops are likely to be depressed more when nutrition is disturbed than when it is normal. At moderate salt concentrations in the soil solution, plants generally try to exclude unwanted ions, as far as possible, and promote the uptake of nutrients. With increasing salt concentration, the uptake of sodium and chloride ions increases sharply. This luxury consumption of ions is essential for the plants to compensate for the increased outside osmotic pressure but is responsible for growth retardation. Excessive uptake of certain ions, in turn, often results in reduced uptake of some essential plant nutrients causing nutrient imbalances and deficiencies. Thus, although the available status of a nutrient in a soil might not be in a deficient range per se, its application might compensate for the decreased uptake by plants resulting from the antagonistic effect of excess uptake of certain ions. Results of several studies tend to show that deficiencies of the elements K and Ca appear to play an important role in the observed growth depressions in many saline soils. Proper fertilization of soils of low or medium salinity should serve to:
- supplement nutrients that are present in insufficient amounts;
- supplement nutrients that, although present in sufficient amounts, are not taken up in adequate amounts due to antagonistic effects, e.g. K or Ca; and decrease the uptake of harmful ions, e.g. K against Na or phosphate against chloride.
High salinity may interfere with the growth and activity of the soil’s microbial population and thus indirectly affect the transformation of essential plant nutrients and their availability to plants. Reduced symbiotic N fixation due to the toxic effect of salts on rhizobia has been reported. It has been observed that normal rhizobia associated with pea can tolerate a maximum salinity up to 4.5 mmhos/cm. Other factors likely to influence the N-fertilization of crops grown in saline soils include high leaching losses of N as NO3, decreased nitrification rates due to high salinity and the direct toxic effect of ions such as chloride on the bacterial activity. In many saline soils, water tables close to the surface can greatly modify the nutritional needs- of crops. Studies brought out that it was possible to compensate for high water tables by applying N fertilizers to cereals (Figs 9 and 10), sugar beets, potatoes, etc.
There have been only a limited number of studies on the effect of salinity on the nutrition of crops in respect of micronutrients. A disturbed balance in the uptake and composition of major nutrients is bound to influence the plant composition of micronutrients. Besides the generally known toxic effects of boron there is a need to understand better the behaviour of Fe, Mn, Zn, Cu, etc., in relation to soil salinity particularly with a view to establishing limiting values - so far only developed for normal soils. Figures 9 and 10 schematically demonstrate how a well-adjusted fertilization could improve the yields of crops.
Figure 11 shows the inverse relationship observed between available soil phosphorus and the chloride content of wheat straw in pot studies. It has been observed by some workers that the application of P increased yields of crops markedly and alleviated salt injury symptoms in oats and barley. They observed a 400 percent increase in yield with a saline soil in spite of its having high available P. Another report showed that much higher responses of applied phosphorus occurred on a moderately saline than on a non-saline soil of comparable available P status. In another study, the beneficial effect of applied P to wheat and barley crops was limited up to an EC of 9 mmhos/cm. These observations show that higher plant responses to applied P occur on moderately saline than on non-saline soils. Responses to applied P-fertilizers in saline soils cannot be explained on the basis of soil test values alone as the saline soils, even when containing high amounts of extractable P have shown positive responses to applied phosphorus. This is because in saline soils the availability of P is more a function of plant root length and area (which is restricted due to salinity) and the negative effect of excess chlorides on P absorption by roots. Application of judicious quantities of P-fertilizers in saline soils helps to improve crop yields by directly providing phosphorus and by decreasing the absorption of toxic elements like Cl.
On moderately saline soils, the application of potassic fertilizers may increase the crop yields, either by directly supplying K or by improving its balance with respect to Na, Ca and Mg. However under high salinity conditions, it is difficult to exclude Na effectively from the plant by use of K-fertilizers.
6.4.1.2. Specific description
We have, thus, seen that a specific ion present in excess may also inhibit plant growth because of its effect on normal plant nutrition. Because different crops and their varieties vary widely in their nutritional requirements for given nutrients and in their ability to absorb specific nutrients, the nutritional effects differ markedly from species to species and variety to variety. Thus, some species and some varieties of a particular crop may be immune to nutritional disturbances caused by a given type of salinity, whereas others may be severely affected.
6.4.1.2.1. Cations
It is possible that sodium accumulation within plant may be associated with depression in the uptake and utilization of other cations. At present, there exists little clear cut evidence that strictly saline soils may induce nutritional effects through sodium accumulation. However, high Na concentrations have been found to increase K leakage and decrease root elongation. When sodium is present in high concentration in the soil solution, transpiration rate is reduced in proportion to salinity. Sodium also affects the respiratory pathway of roots. High sodium in solution causes calcium deficiency symptoms in several crops like tomato, pepper, and celery plants. This is most likely caused by sodium replacing calcium from membranes of root cells.
High calcium concentrations may produce nutritional imbalance in some species unless compensated by some other ions such as sodium or potassium. Some varieties of beans and carrots are affected in this way by high calcium accumulations which restrict the uptake of essential potassium.
Excessive concentrations of magnesium may be accompanied by decreased absorption of calcium and potassium, such an effect is usually eliminated by the simultaneous presence of moderately high concentrations of calcium along with magnesium. Cases have been reported in which relatively high levels of potassium have induced characteristic symptoms of iron-chlorosis and magnesium deficiency.
6.4.1.2. 2. Anions
The uptake of essential anions like nitrate, phosphate and sulphate is rarely affected to a critical degree by an increase in concentration of chloride. The stability in nutrient anion absorption may be related to the rapid conversion of these anions into organic compounds.
The sulphate ion generally restricts the absorption of calcium promoting the uptake of sodium. Calcium, however, rarely reaches the point of deficiency due to sulphate accumulations except in case of some lettuce varieties. By promoting the uptake of sodium, sulphate may induce sodium toxicity in susceptible species.
Bicarbonate accumulation has, been found to inhibit the intake of calcium by some plants. In beans, for example, it induces a pronounced depression in calcium absorption, and an increase in potassium uptake. In beets, intake of calcium and magnesium is depressed markedly, that of potassium depressed slightly and intake of sodium increases.
6.4.2. Effects on soil fertility
In addition to soil physiological stresses and restriction of nutrient absorption by an excessive accumulation of a specific ion, which decrease crop yields, salt-affected soils may themselves be deficient in nutrients due to their neglected nature and other reasons. Thus, salinity also adversely affects soil fertility. Salt-affected soils generally exist in the hyperthermic temperature regime, conducive for rapid decomposition of organic matter; thus, contain very low organic matter and are poor in fertility Most salt-affected soils are deficient in nitrogen, phosphorus, potassium, calcium, magnesium, manganese, and, at some times, copper, zinc and iron. However, some saline soils are medium to high in potassium. Zinc deficiency is widespread and sulfur and boron deficiency are becoming important in several areas. However, in saline soils, boron, lithium, fluorine, selenium and molybdenum could be toxic. Boron, lithium and fluorine are phyto-toxic, whereas selenium and molybdenum may not adversely affect plant growth but the crops grown on them, particularly forages, may contain excessive concentration of these elements causing health hazard to grazing animals.
As stated earlier, high salinity decreases microbial activity and hence the rate of N mineralization and, thus, necessitate higher rates of N fertilizers. Most Rhizobium species are relatively unaffected at soil salinity levels that are less than the tolerance threshold values reported for most leguminous crops. At soil salinities greater than their threshold, their ability to survive and fix nitrogen may be severely affected. This is particularly important, since legumes that are already weakened by salinity stress will be deprived of essential N fertilization as well. However, there appears to be wide range of tolerance to salinity by the various species of rhizobia. Some strains of R. meliloti can survive soil water salinities greater than that of sea water (about 46 mmhos/cm), but most strains of R. japonicum grow poorly at salinities of 12 mmhos/cm. Studies comparing various species of rhyzobium report the salt tolerance of R. meliloti > R. trifoli > R. leguminosarum > R. japonicum. The salt effect on rhizobia appears to be ion specific, with chloride salts of sodium, potassium, magnesium being more toxic than corresponding sulfate salts. In addition, magnesium ions inhibit growth og rhizobia at a much lower concentration than sodium or potassium.
Also, due to the adverse effects of salinity on transformations of soil and applied fertilizer N, crops respond to much higher levels of N in these soils compared with normal soils (Figs. 9 and 10). Green manuring can improve fertilizer N use efficiency by crops. As described earlier, in saline soils, phosphorus availability decreases due to higher retention of soluble phosphate, antagonistic effect of chloride and sulfate on plant absorption of P and restricted root growth. Though saline soils are often medium to high in K, during reclamation, losses of K due to leaching may take place leading to its low availability. Excess salts may also interfere with plant nutrition by affecting nutrient availability, uptake, or their physiological role within the plant. Due to considerable variations in the chemical composition, precipitation-dissolution reactions, adsorption-desorption kinetics and transformation of nutrients, crop responses to applied nutrients vary greatly in saline and sodic soils. In saline soils, the solubility of these cation nutrients does not decrease and they remain in available forms. As it was stated earlier, the degree of salinity also influences crop response to fertilizer; if the degree of salinity is initially high, crop response to fertilizer is limited. Through integrated plant nutrient supplies (IPNS), diverse nutrient sources can be deployed to sustain crop yields, and reclaim the soil, provided crop residues are available at the farm level. In short, special plant nutrient management strategies are needed for salt-affected soils. Also, as it would be seen in later part of this write up, salt-tolerant crops/cultivars and cultivation techniques are required for saline agriculture.
6.5. Morphological and anatomical changes in plants due to salinity
As mentioned earlier, salinity often alters the morphology and anatomy of plants. Chloride ions from sea salt altered the growth form and injured leaves of Quercus lobata and Q. agrifolia up to 60 km inland. Leaves of plants that grow on saline soils often are thicker and more succulent than those of growing on salt-free soils. The epidermal cell walls and cuticles of leaves of salinized plants also are thicker. By increasing the internal surface area per unit of leaf surface, leaf succulence may increase CO2 absorption per unit of leaf area.
Increase in leaf thickness in response to salinity has been attributed to an increase in number of mesophyll cell layers or cell size, or both. In salinized Gossypium hirsutum plants, the leaves were thicker because the number of cell layers increased and the mesophyll cells were larger, whereas in Citrus, increase
in the size of spongy mesophyll cells, rather than an increase in cell layers, accounted for thicker leaves. The large cells of leaves of salinized plants result from increased cell wall extensibility together with higher turgor pressures.
Salinity not only inhibits the rate of cambial growth but also influences the anatomy of cambial derivatives. For example, the xylem of salt-treated Populus euphratica trees was ring-porous rather than typically diffuse-porous. The xylem vessels of halophytic trees are more numerous and narrower than those in mesophytic trees. Following exposure of Aesculus hippocastanum trees to salinity, the number of xylem vessels increases and their size decreases. Salinity also increases production of fibers and calcium oxalate crystals in the bark. After exposure to salinity, the xylem increments in salt-tolerant Salix clones are wider with fewer vessels and more fibers and rays per unit area than in salt-sensitive clones.
Salinity often promotes suberization of the hypodermis and endodermis in roots, with formation of a well-developed Casparian strip closer to the root apex than is found in nonsalinized roots. The walls of root cells of salinized plants often are unevenly thickened and convoluted.
6.6. Physiological effects of salinity on plants
6.6.1. General effects
As discussed above in section 6.5.5, salt-induced slowing of plant growth is accompanied by a variety of metabolic dysfunctions in nonhalophytes, including inhibition of enzymatic activity, absorption of minerals, protein and nucleic metabolism, and respiration. Addition of NaCl to mitochondria isolated from leaves of a nonhalophyte (Pisum sativum) and a halophyte (Suaeda maritima) appreciably reduced the rate of O2 uptake by both species. Salinity affects synthesis of carbohydrates as well as transport of photosynthetic products and their utilization in production of new tissues.
In many halophytes, important physiological processes are stimulated or not appreciably altered by salt concentrations that inhibit these processes in nonhalophytes.
6.6.2. Effects on enzymatic activity
Salinity inhibits the in vitro activity of many enzymes. Most enzymes of halophytes (except membrane-bound ATPase) are as sensitive to salinity as the enzymes of nonhalophytes. Isolated enzymes from many halophytes lose approximately half of their activity at salt concentrations approximating those in leaf cells.
6.6.3. Effects on gas exchange (photosynthesis)
Salinity reduces the rate of photosynthesis of both nonhalophytes and halophytes. Although both stomatal and nonstomatal factors have been implicated in the reduction of photosynthesis following flooding with saline water, most of the reduction in photosynthetic rates is the result of nonstomatal effects. In the long term, total photosynthesis is reduced as a result of inhibition of leaf formation and expansion as well as early leaf abscission. Following irrigation of Fraxinus pennsylvanica seedlings with low concentrations of salt solution, the leaves progressively dehydrated, causing partial stomatal closure and decreased CO2 absorption. However, after plants were flooded with high concentrations of salt solution, photosynthetic inhibition was attributed to ion toxicity, membrane disruption, and complete stomatal closure. For the first few days after flooding of Ficus carica plants with salt solution, stomatal conductance was reduced but the rate of photosynthesis was not appreciably altered; however, longer-term salinity treatment greatly inhibited the rate of photosynthesis by nonstomatal effects.
Despite parallel reductions in stomatal conductance and photosynthesis in flooded Taxodium distichum seedlings, the leaf internal CO2 concentrations were relatively stable over a range of salt concentrations in the floodwater. The salinity of floodwater caused excessive accumulation of several ions (Na, K, Ca, and Mg) in the leaves and this increase in leaf ionic content was considered to be the primary cause of the saline-induced reduction in photosynthetic rates. A decline in photosynthetic rate of Prunus salicina flooded with saline water was associated with increases in leaf chloride content and declines in 1,5-bisphosphate carboxylase activity and the pool sizes of triosephosphate, ribulose 1,5-bisphosphate and phosphoglycerate. In many plants, salinity lowers the efficiency of the electron transport chain and injures the light harvesting complex.
6.6.4. Effects on protein metabolism
Salinity decreases protein synthesis and increases its hydrolysis in some plants, resulting in production of amino acids. Salts have antagonistic effects on proteins: (1) breaking of electrostatic bonds and (2) increasing hydrophobic interactions. It has been reported that amino acids that accumulate in response to salinity are toxic in the following order: serine and valine > tyrosine > isoleucine > leucine > threonine > lysine > and proline.
6.6.5. Effects on mineral nutrition
Effects stated earlier, salinity often upsets the nutritional balance of plants by one or more mechanisms including osmotic effects of salts, competitive interactions among ions in the substrate, and effects on membrane selectivity. As root elongation slows, the amount of ions reaching the roots by diffusion decreases. High concentrations of Cl reduce NO3 uptake by plants, and high concentrations of NO3 inhibit phosphate uptake. Salinity decreases uptake of K, Ca, and Mg in phylloclades of Casuarina equisetifolia.
6.6.6. Effects on hormones
Salinity promotes senescence of plant tissues by increasing the production of ABA and ethylene. Salt decreases the cytokinin concentration in roots and shoots of salt-resistant plants but not of salt-sensitive plants. However, the effects of NaCl on salt-sensitive plants do not appear to be mediated by cytokinins because growth reduction precedes the change in cytokinins.
6.7. Mechanisms of growth inhibition by salinity
The mechanisms by which salinity inhibits plant growth have eluded precise characterization, although there has been considerable success in describing their physiological manifestations as discussed above. Over the years emphasis has been placed on three aspects of the physiological effects of salinity on plant growth: (1) turgor regulates stomatal conductance and cell expansion, thereby affecting growth of plants in soils of low water potential, (2) plant growth is limited by a lowered rate of photosynthesis, and (3) excessive uptake of salts affects production of a specific metabolite that directly inhibits growth. The mechanisms of short- and long-term inhibition of shoot growth by salinity may vary. It has been suggested that early growth inhibition was traceable to water stress rather than a specific toxic effect of salt. Hence the water status of roots might regulate shoot growth through a hormonal system, especially one involving ABA. Support for this view comes from the observation that inhibition of leaf expansion by salt occurred after 1 min. An opposing view is that nutrition of the shoot apical meristem may be disturbed and the shoot meristem may provide the inhibitory signal to expanding leaves. Inhibition of shoot growth after weeks to months of salinization has been attributed to excessive salt accumulation in leaves, resulting in a water deficit in the symplast, and to toxic ionic effects. It has been concluded by some workers that the absorbed salts do not directly control growth by influencing turgor, photosynthesis, or activity of a specific enzyme. While emphasizing the complexity of salinity effects, they developed a model that incorporates a two-phased plant growth response to salinity: growth is first reduced by a decrease in soil water potential (a water stress effect), and later a specific effect appears as salt injury in the old leaves, which die because of a rapid increase of salt in cell walls or cytoplasm when vacuoles can no longer sequester incoming salts. Others proposed that accumulation of salt in the old leaves accelerated their death, and loss of these leaves decreased the supply of carbohydrates or growth hormones to meristematic regions, thereby inhibiting growth.
Figure 12. Schematic illustration of the two-phase response to salinity for genotypes that differ in the rate at which salt reaches toxic levels.
The concept of the “two phase growth response to salinity” is shown in Figure 12. In this model, as stated in the foregoing paragraph, the first phase growth reduction happens quickly (within minutes) after exposure to salinity. This response is due to the osmotic changes out side the root causing changes in cell-water relations (osmotic effect). The osmotic effect is similar to water stress and shows little genotypic differences. Several minutes after the initial decrease in leaf growth, there is a gradual recovery of the growth rate until a new study state is reached, dependent upon the salt concentration outside the root. The second much slower effect, taking days, weeks or months is the result of salt accumulation in leaves, leading to salt toxicity in the plant, primarily in the older leaves (i.e. salt-specific effect). This salt toxicity can result in the death of leaves and reduce the total photosynthetic leaf area. As a result, there is a reduction in the supply of photosynthate to the plant, affecting the overall carbon balance necessary to sustain growth. Salt toxicity primarily occurs in the older leaves, where sodium and chloride build up in the transpiring leaves over a long period of time, resulting in high salt concentration and leaf death. Leaf injury and death is probably due high salt load in the leaf that exceeds the capacity of salt compartmentation in the vacuoles, causing salt to build up in the cytoplasm to toxic levels. The rate at which leaves die and thud reduce the total photosynthetic leaf area determines the survival of the plant. If new leaves are produced leaves are produced at a rate grater than the rate at which older leaves die, there are enough photosynthesizing leaves for the plant to flower and produce seeds, although at reduced numbers. If, however, old leaves die faster than the new leaves develop, the plant may not survive long enough to supply sufficient photosynthate to the reproductive organs and produce viable seeds. Based on this two-phase concept, the initial growth reduction both for salt sensitive and salt tolerant plants is caused by an osmotic effect of the salts in the medium outside the roots. In contrast, in the second phase, a salt sensitive species or genotype differs from a salt tolerant one by its inability to prevent salt from accumulating in the transpiring leaves to toxic levels.
6.8. An example showing the combined effects of salinity hazards (osmotic, toxic and nutritional) on rice crop
6.8.1. Effects on plants
1) Affects respiration and photosynthesis processes.
2) Decreased biological N2 fixation and soil N mineralization.
3) Reduced germination rate
4) Reduced plant height and tillering
5) Poor root growth
6) Increased spikelet sterility
7) Excess Na uptake decreases 1,000-grain weight and total protein content in grain, but does not alter major cooking qualities of rice
6.8.2. Symptoms
1) Chlorotic patches appear on some leaves
2) Plant stunting and reduced tillering
3) Patchy field growth
4) Symptoms first manifest themselves in the first leaf, followed by the second, and then in the growing leaf
5) Salinity or sodicity may be accompanied by P deficiency, Zn deficiency, Fe deficiency, or B toxicity
Figure 3b, c, and d show the injury symptoms in rice crop.
6.8.3. How to confirm?
Plant and soil can be tested to confirm salinity. Increased Na content in rice plants may indicate salinity injury, which may lead to yield loss. The critical concentration of salt (NaCl) in leaf tissue, at which toxicity symptoms appear, however, differs widely between varieties. Varieties showing the greatest tolerance for salt within plant tissues are not necessarily those showing the greatest overall phenotypic resistance to salinity.
The correlation between Na:K ratio and salinity tolerance has been established; however, no absolute critical levels in plant tissue are known. A Na:K ratio of <2:1>4 dS m-1 slight yield reduction (10-15%)
3) ECe >6 dS m-1 moderate reduction in growth and yield (20-50%)
4) ECe >10 dS m-1 >50% yield reduction in susceptible cultivars
Exchangeable Na percentage (ESP):
1) ESP <20%>20-40% slight yield reduction (10%)
3) ESP >80% 50% yield reduction
Sodium adsorption ratio (SAR):
SAR >15 sodic soil (measured as cations in saturation extract)
Irrigation water has:
1) pH 6.5-8, EC <0.5>8.4, EC >2 dS m-1 unsuitable for irrigation
4) SAR <15>25 unsuitable for irrigation, very high Na
Note: Measurement of EC as an indicator of salinity is rapid and simple. EC alone, however, is insufficient to assess the effects of salinity on plant growth because salt concentrations at the root surface can be much greater than in the bulk soil. In addition, EC only measures the total salt content, not its composition. Na and B must be considered as well. Salinity is highly variable in the field, both between seasons and within individual fields. Individual EC values must be treated with caution unless they are based on representative soil samples.
6.8.4. Problem with similar symptoms
No other deficiency exhibits these symptoms but salinity.
6.8.5. Mechanism of damage
AS told earlier, salinity is defined as the presence of excessive amounts of soluble salts in the soil (usually measured as electrical conductivity, EC). Na, Ca, Mg, Cl, and SO4 are the major ions involved. Effects of salinity on rice growth are as follows:
1) Osmotic effects (water stress)
2) Toxic ionic effects of excess Na and Cl uptake
3) Reduction in nutrient uptake (K, Ca) because of antagonistic effects
The primary cause of salt injury in rice is excessive Na uptake (toxicity) rather than water stress, but water uptake (transpiration) is reduced under high salinity. Plants adapt to saline conditions and avoid dehydration by reducing the osmotic potential of plant cells. Growth rate, however, is reduced. Antagonistic effects on nutrient uptake may occur, causing deficiencies, particularly of K and Ca under conditions of excessive Na content. For example, Na is antagonistic to K uptake in sodic soils with moderate to high available K, resulting in high Na:K ratios in the rice plant and reduced K transport rates.
Sodium-induced inhibition of Ca uptake and transport limits shoot growth. Increasing salinity inhibits nitrate reductase activity, decreases chlorophyll content and photosynthetic rate, and increases the respiration rate and N content in the plant. Plant K and Ca contents decrease but the concentrations of NO3-N, Na, S, and Cl in shoot tissue increase. Rice tolerates salinity during germination, is very sensitive during early growth (1-2-leaf stage), regains tolerance during tillering and elongation, but becomes sensitive again at flowering.
Several factors affect the tolerance of different rice varieties to salinity:
1) Transpiration rate and potential for osmotic adjustment.
2) Differences in nutrient uptake under Na stress. Tolerant cultivars have a narrower Na:K ratio (higher K uptake) and greater leaf Ca2+ content than susceptible cultivars.
3) Efficient exclusion of Na+ and Cl-. Salt-tolerant rice varieties have a reduced Na+ and Cl- uptake compared with less tolerant cultivars.
4) Rapid vegetative growth results in salt dilution in plant tissue.
7. EXCHANGEABLE SODIUM (SODICITY) EFFECTS ON CROP GROWTH AND SOIL PROPERTIES
The response of plants to excess sodicity may be complicated by a number of factors. Indirect effects on plants produced by structural deterioration of sodic soils, direct toxic effects in case of sodium-sensitive species, and nutritional effects may all be involved
7.1. Indirect effects
7.1.1. Effects of sodicity on soil physical properties through dispersion, puddling and pore clogging
Sodic soils readily become dispersed, and puddled due to the presence of high ES on the exchange complex, this result in smaller pores, pore-clogging by dispersed soil particles, and hard crust formation on soil surface upon drying. Plant growth on these soils is, therefore, inhibited primarily by this poor physical condition of the soil which restricts moisture transmission and aeration and may physically impede root elongation and seedling emergence. Crops which may be primarily affected by poor soil structure include beets, cotton, tomato, and some of the grain crops.
Sodium has the opposite effect of salinity on soils. The primary physical processes associated with high sodium concentrations are soil dispersion and clay platelet and aggregate swelling. The forces that bind clay particles together are disrupted when too many large sodium ions come between them (Figs. 12a and b). When this separation occurs, the clay particles expand, causing swelling and soil dispersion.
Soil dispersion causes clay particles to plug soil pores, resulting in reduced soil permeability. When soil is repeatedly wetted and dried and clay dispersion occurs, it then reforms and solidifies into almost cement-like soil with little or no structure. The three main problems caused by sodium-induced dispersion are reduced infiltration, reduced hydraulic conductivity, and surface crusting.
Salts that contribute to salinity, such as calcium and magnesium, do not have this effect because they are smaller and tend to cluster closer to clay particles (Fig.13a, 13b and 14). Calcium and magnesium will generally keep soil flocculated because they compete for the same spaces as sodium to bind to clay particles. Thus, increased amounts of calcium and magnesium can reduce the amount of sodium-induced dispersion (Figs. 13a, 13b and 14).
Figure 13a. Behaviour of sodium and calcium attached to clay particles.
7.1.2. Moisture transmission
Since sodic soils have poor moisture transmission characteristics like water infiltration into the soil and soil hydraulic conductivity (see Table 10 and Fig. 15), water availability to most crops is lowered in these soils to restrict normal plant growth.
Soil dispersion not only reduces the amount of water entering the soil, but also affects hydraulic conductivity of soil. Hydraulic conductivity refers to the rate at which water flows through soil. For instance, soils with well-defined structure will contain a large number of macro pores, cracks, and fissures which allow for relatively rapid flow of water through the soil. When sodium-induced soil dispersion causes loss of soil structure, the hydraulic conductivity is also reduced. Thus, as the proportion of exchangeable sodium increases, the soil tends to become more dispersed which results in the breakdown of soil aggregates and lowers the hydraulic conductivity of the soil (Fig. 15).
7.1.3. Surface crusting and cracking
The hard crust that frequently forms on the surface of a sodic soil as it dries out often hinders seed emergence, root elongation and development of young seedlings.
Surface crusting is a characteristic of sodium affected soils. The primary causes of surface crusting are 1) physical dispersion caused by impact of raindrops or irrigation water, and 2) chemical dispersion, which depends on the ratio of salinity and sodicity of the applied water.
Surface crusting due to rainfall is greatly enhanced by sodium induced clay dispersion. When clay particles disperse within soil water, they plug macro pores in surface soil by two means. First, they block avenues for water and roots to move through the soil. Second, they form cement like surface layer when the soil dries. The hardened upper layer, or surface crust, restricts water infiltration and plant emergence.
Under field conditions after an irrigation or rainfall, sodic soils typically have convex surfaces. The soil a few centimeters below the surface may be saturated with water while at the same time the surface is dry and hard. Upon dehydration cracks 1-2 cm across and several centimeters deep form Fig. 15) and close when wetted. The cracks, generally, appear at the same place on the surface each time the soil dries unless it has been disturbed mechanically
7.1.4. Aeration
Again, puddled and dispersed soil has a lowered capacity for gaseous exchange with the atmosphere which may result in oxygen deficiency at the absorbing surfaces of the roots, and lack of oxygen may inhibit plant growth. Most plants are unable to take in adequate quantities of water at low oxygen tensions.
7.2. Direct effects
7.2.1. Effect of exchangeable sodium on absorption of nutrients
Experimental evidences reveal that increasing levels of ES in the soil may prevent roots from obtaining an adequate supply of calcium. For most crop plants, calcium becomes unavailable when ES approaches 50 %. Under these conditions, the exchange complex may actually remove calcium from the root tissues and death may ensue because of calcium deficiency. Plants with a high calcium requirement, such as alfalfa, clover, and the majority of other crops, may be included in this category. In general, increasing ESP of the substrate results in a decreased absorption of calcium, magnesium and potassium by plants.
7.2.2. Effect of high pH on solubility, availability and uptake of nutrients
A second effect of excess exchangeable sodium on plant growth is through its effect on soil pH. Although high pH of sodic soils has no direct adverse effect on plant growth per se, it frequently results in lowering the availability of some essential plant nutrients leading to far-reaching effects on the growth and nutrient uptake by crop plants. For example, the concentration of the elements calcium and magnesium in the soil solution is reduced as the pH increases (Table 11) due to formation of relatively insoluble calcium and magnesium carbonates by reaction with soluble carbonate of sodium, etc. and results in their deficiency for plant growth. Similarly, the solubility in soils and availability to plants of several other essential nutrient elements, e.g. P, Fe, Mn and Zn, are likely to be affected.
Table 11. Effect of pH on the solubility of calcium carbonate in water.
pH-----------Solubility of calcium carbonate meq/l
6.21 19.3
6.50 14.4
7.12 7.1
7.85 2.7
8.60 1.1
9.2 0.8
10.12 0.4
7.2.3. Fertility status of sodic soils
We have seen that In addition to several indirect effects which decrease crop yields, sodicity also adversely affect soil fertility. Salt-affected soils generally exist in the hyperthermic temperature regime, conducive for rapid decomposition of organic matter; thus, contain very low organic matter and are poor in fertility. Most sodic soils are deficient in nitrogen, phosphorus, and are medium to high in potassium. Zinc deficiency is widespread and sulfur and boron deficiencies are becoming important in several areas. Also sodium, boron, lithium, fluorine, selenium and molybdenum could be toxic. Boron, lithium and fluorine are phyto-toxic, whereas selenium and molybdenum may not adversely affect plant growth but the crops grown on them, particularly forages, may contain excessive concentration of these elements causing health hazard to grazing animals. High alkalinity and/or salinity decreases microbial activity and hence the rate of N mineralization and, thus, necessitate higher rates of N fertilizers. Also, due to the adverse effects of sodicity on transformations of soil and applied fertilizer N, crops respond to much higher levels of N in these soils compared with normal soils. Green manuring can improve fertilizer N use efficiency by crops. The transformations and availability of applied and soil P and crop responses to P application greatly differ in sodic and saline soils. Available P in the soil increases with increase in EC and pH. During reclamation of sodic soils with gypsum, extractable P declines due to its conversion to less soluble Ca-P compounds. In sodic soils, high Na and low calcium (Ca) result in decreased K uptake by plants. Due to considerable variations in the chemical composition, precipitation-dissolution reactions, adsorption-desorption kinetics and transformation of nutrients, crop responses to applied nutrients vary greatly in saline and sodic soils. In sodic soils, presence of CaCO3 and poor air-relations affect the availability of nutrients. The solubility of nutrient cations decreases due to the dominance of carbonate equilibriums.
7.3. Extent of sodicity hazard in relation to ESP and crops
Under field conditions plant growth is adversely affected due to a combination of two or more of the above discussed factors, depending on the level of exchangeable sodium, nature of the crops and the overall level of management. Table 12 gives the approximate extent of hazards in relation to ESP and crops.
Table 12. Exchangeable sodium percentage (ESP) and sodicity hazard
Approx. ESP--------- Sodicity hazard
<15-------------------->70--------------------Extremely high
Remarks: The adverse effect of exchangeable sodium on the growth and yield of crops in various classes occurs according to the relative crop tolerance to excess sodicity. Whereas the growth and yield of only sensitive crops are effected at ESP levels below 15, only extremely tolerant native grasses grow at ESP above 70- to 80.
7.4. Relative risk of sodium and other salt problems as determined by soil testing
Table 13 presents the relative risk (low, medium and high) of soil and crop damage from sodium and other salts based on soil analyses. Soils that test “high” have a severe salt and/or sodium problem. When pH is above 8.5, a soil test should be monitored for sodium and other ion accumulations, since a pH above 8.5 indicates sodium problems.
Table 13. Relative risk of sodium and other salt problems as determined by soil testing.
7.5.1. Effects on plants
1) Impairs plant growth.
2) Obstructs root development.
3) Restricts water supply to the roots.
4) Results in deficiencies in phosphorus and zinc.
5) Iron deficiency and boron toxicity may also occur.
7.5.2. Symptoms
1) Discoloration of leaves occurs ranging from white to reddish brown starting from the leaf tips.
2) Discoloration spreads down the leaf giving the plant a scorched appearance in more susceptible plants and in severe alkaline conditions (Fig. 17a).
3) Growth and tillering are depressed Fig. 17b).
4) Pattern of damage is patchy Fig. 17b).
Figure 17a. Discoloration spreads down the rice leaf due to sodicity toxicity
Figure 17b. Rice plant stand is patchy and has a poor growth due to sodicity toxicity
7.5.3. How to confirm?
Soil and plant tests can be used to detect sodicity. However, there is no direct test available for plants. The soil can be checked for potential sodicity if the exchangeable sodium > 15% and a soil pH > 8.
7.5.4. Problems with similar symptoms
Strongly sodic soils can also be phosphorus and zinc deficient.
7.5.5. Why and where it occur?
Sodicity is relatively rare especially in irrigated rice systems. Sodic soils have high levels of exchangeable sodium usually more than 15%. Sodicity occurs in semiarid region soils with pH in most cases >8.5 and is often associated with salinity.
7.5.6. Mechanism of damage
The high percentage of sodium in sodic soils usually causes soil structural problems, which can be a problem in aerobic or upland crop systems. The high percentage of sodium can also have a direct effect on some cultivars. Sodicity impairs plant growth and obstructs root development. It also restricts water supply to the roots. The strong basic properties of sodic soils result in deficiencies in phosphorus and zinc. Iron deficiency and boron toxicity may also occur in these soils.
7.5.7. When damage is important?
The damage caused by sodicity occurs throughout the growth cycle of the rice crop.
7.5.8. Economic importance
Sodicity is relatively rare, especially in irrigated rice systems.
7.6. An example of calcium deficiency in sodic rice fields
7.6.1. Effects on plants
1) Affects cell wall constituents in biomembrane maintenance
2) Impairs root function
3) Predisposes the rice plant to Fe
4) May resemble B deficiency
7.6.2. Symptoms
1) Tips of youngest leaves become white or bleached, rolled, and curled
2) Necrotic tissue may develop along the lateral margins of leaves and old leaves eventually turn brown and die (Fig. 18a)
3) Stunting and death of growing point during extreme deficiency (Fig. 18b)
Figure 18a. Leaf discoloration in rice due to calcium deficiency
7.6.3. How to confirm?
Both the soil and plant can be tested for Ca deficiency. The optimal ranges and critical levels of Ca in plant tissue are:
Growth stage - Plant part – Optimum (%) – Critical level for deficiency (%)
Tillering ----Y leaf, shoot-------0.2-0.6------------------<>20%.
For optimum growth, the ratio of Ca:Mg should be > 3-4:1 for exchangeable soil forms and 1:1 in soil solution.
7.6.4. Problems with similar symptoms
Ca deficiency may resemble B deficiency, and plant tissue analysis may be required to distinguish the cause of symptoms.
7.6.5. Why and where it occurs?
Ca deficiency is relatively rare especially in irrigated rice systems. It can be caused by one or more of the following:
1) Small amounts of available Ca in soil (degraded sodic soils, acid, sandy soils)
2) Alkaline pH with a wide exchangeable Na:Ca ratio resulting in reduced Ca uptake. Use of irrigation water rich in NaHCO3.
3) Wide soil Fe:Ca or Mg:Ca ratios resulting in reduced Ca uptake. Long-term irrigated rice cultivation may lead to higher Mg:Ca and Fe:Ca ratios.
4) Excessive N or K fertilizer application resulting in wide NH4:Ca or K:Ca ratios and reduced Ca uptake.
5) Excessive P fertilizer application, which may depress the availability of Ca (due to formation of Ca phosphates in sodic soils).
Ca deficiency is very uncommon in lowland rice soils because there is usually sufficient Ca in the soil, from mineral fertilizers, and irrigation water.
Soils particularly prone to Ca deficiency occur on the following soil types:
1) Sodic, acid, strongly leached, low-CEC soils in uplands and lowlands
2) Soils derived from serpentine rocks
3) Coarse-textured sandy soils with high percolation rates and leaching
4) Leached, old sulfate soils with low base content
5) Sodic soils in which native calcium carbonate solubility is reduced considerable at high pH (Table 11) and in sodic soils with very high ESP which causes depletion of exchangeable Ca.
7.7. An example of iron deficiency in sodic rice fields
7.7.1. Effects on plants
1) Affects photosynthesis
2) Decreased dry matter production
7.7.2. Symptoms
1) Interveinal yellowing and chlorosis of emerging leaves (Fig. 19a)
2) Whole leaves become chlorotic and then very pale (Fig. 19b)
3) Entire plant becomes chlorotic and dies if deficiency is very severe
4) Decreased dry matter production
7.7.3. How to confirm?
The plant and the soil can be tested for Fe deficiency. The optimal ranges and critical levels of Fe in plant tissue are:
Growth stage-Plant part- Optimum (mg/kg)- Critical level for deficiency (mg/kg)
Tillering---------Y leaf-------75-150-------------------- < name="Problems">Problems with similar symptoms
No other rice crop exhibits these symptoms except for a Fe deficient plant.
7.7.5. Why and where it occurs?
Fe deficiency is relatively rare, especially in irrigated rice systems, but may occur in upland sodic soils. One or more of the following can cause Fe deficiency in rice:
1) Low concentration of soluble Fe2+ in upland sodic soils.
2) Inadequate soil reduction under submerged conditions like in sodic soils (e.g., low organic matter status soils).
3) High pH of sodic or calcareous soils following submergence (i.e., decreased solubility and uptake of Fe because of large bicarbonate concentration).
4) Wide P:Fe ratio in the soil (i.e., Fe bound in Fe phosphates, possibly because of excess application of P fertilizer).
5) Excessive concentrations of Mn, Cu, Zn, Mo, Ni, and Al.
6) In upland soils, cultivars with low potential for excretion of organic acids to solubilize Fe.
7) Increased rhizosphere pH after the application of large amounts of NO3-N fertilizer (rare case and is relevant for upland crops only).
The soils, which are particularly prone to Fe deficiency, include the following types:
1) Neutral, calcareous, and sodic upland soils
2) Sodic and calcareous lowland soils with low organic matter status
3) Lowland soils irrigated with sodic irrigation water
4) Coarse-textured soils derived from granite.
7.7.6. Mechanism of damage
Iron is required for electron transport in photosynthesis and is a constituent of iron porphyrins and ferredoxins, both of which are essential components in the light phase of photosynthesis. Fe is an important electron acceptor in redox reactions and an activator for several enzymes (e.g., catalase, succinic dehydrogenase, and aconitase), but inhibits K absorption. On sodic soils, immobilization of Fe in plant roots occurs because of Fe precipitation. Because Fe is not mobile within rice plants, young leaves are affected first.
7.7.7. When is damage important?
This deficiency is important throughout the growth cycle of the rice crop.
7.7.8. Economic importance
Fe deficiency is relatively rare - especially in irrigated rice systems. It can be a source of yield loss in sodic or calcareous soils (especially in the Uplands
7.8. An example of phosphorus deficiency in sodic rice fields
7.8.1. Effects on plants
1) Affects the major functions in energy storage and transfer and membrane integrity
2) Affects tillering, root development, early flowering, and ripening
7.8.2. Symptoms
1) Stunted plants
2) Reduced tillering
3) Leaves, particularly older ones, are narrow, short, very erect, and "dirty" dark green
4) Stems are thin and spindly and plant development is retarded
5) The number of leaves, panicles, and grains per panicle is also reduced
6) Young leaves appear to be healthy but older leaves turn brown and die
7) Red and purple colors may develop in leaves if the variety has a tendency to produce anthocyanin
8) Leaves appear pale green when P and N deficiency occur simultaneously
9) Mild to moderate P deficiency is difficult to recognize in the field
10) P deficiency is often associated with other nutrient disorders such as Fe toxicity at low pH, Zn deficiency, Fe deficiency, and salinity in sodic soils
Other effects of P deficiency include
1) Delayed maturity (often by 1 week or more). When P deficiency is severe, plants may not flower at all.
2) Large proportion of empty grains. When P deficiency is very severe, grain formation may not occur.
3) Low 1,000-grain weight and poor grain quality.
4) No response to mineral N fertilizer application.
5) Low tolerance for cold water.
6) Absence of algae in floodwater.
7) Poor growth (small leaves, slow establishment) of green manure crops.
Figures 20a, b and c show t P-deficiency symptoms in rice crop.
Figure 20c. Leaf discoloration.
7.8.3. How to confirm?
Both plant and soil can be tested for P deficiency. The optimal ranges and critical levels of P in plant tissue are:
Growth stage-Plant part-Optimum (%)-Critical level for deficiency (%)
Tillering--------Y leaf-------0.20-o.40---------------<>0.06% P in the straw at harvest and >0.18% P in the flag leaf at flowering.
For soil, numerous soil P tests are in use and critical levels generally depend on soil type and targeted yield level. Olsen-P (0.5 M NaHCO3 at pH 8.5) and, to a lesser extent, Bray-1 P (0.03 M NH4F + 0.025 M HCl) are used as indicators of available P in flooded rice soils. Critical levels for Olsen-P reported for rice range from 5 mg P kg-1 in acid soils to >25 mg P kg-1 in calcareous and sodic soils.
For lowland rice soils with little or no free CaCO3, Olsen-P test results can be classified as follows:
<5> response to P fertilizer certain
5-10 mg P kg-1
(medium P status) > response to P fertilizer probable
>10 mg P kg-1
(high P status) > response to P fertilizer only at very high yield levels (>8 t ha-1)
For lowland rice soils with little or no free CaCO3, Bray-1 P test results can be classified as follows:
<7> response to P fertilizer certain
7-20 mg P kg-1
(medium P status) > response to P fertilizer probable
>20 mg P kg-1
(high P status) > response to P fertilizer only at very high yield levels (>8 t ha-1)
7.8.4. Why and where it occurs
The common causes of P deficiency are as follows:
1) Low indigenous soil P-supplying power like in sodic soils.
2) Insufficient application of mineral P fertilizer in all soils including sodic soils.
3) Low efficiency of applied P fertilizer use due to high P-fixation capacity or water erosion losses (in upland rice sodic fields only).
4) P-immobilization in Ca phosphates in sodic and calcareous soils and due to excessive liming.
5) Excessive use of N fertilizer with insufficient P application.
6) Cultivar differences in susceptibility to P deficiency and response to P fertilizer.
7) Crop establishment method (P deficiency is more likely in direct-seeded rice due to high plant densities and shallow root systems)
P deficiency is widespread in all major rice ecosystems and is the major growth-limiting factor in upland soils where soil P-fixation capacity is often large.
Soils particularly prone to P deficiency include the following types:
1) Calcareous, saline, and sodic soils
2) Coarse-textured soils containing small amounts of organic matter and small P reserves (e.g., sandy soils in northeast Thailand, Cambodia)
3) Highly weathered, clayey, acid upland soils with high P-fixation capacity (e.g., Ultisols and Oxisols in many countries)
4) Degraded lowland sodic soils (e.g., North Vietnam)
5) Volcanic soils with high P-sorption capacity (e.g., Andisols in Japan and parts of Sumatra and Java)
6) Peat soils (Histosols)
7) Acid sulfate soils in which large amounts of active Al and Fe result in the formation of insoluble P compounds at low pH
7.8.5. Mechanism of damage
Phosphorus is an essential constituent of adenosine triphosphate (ATP), nucleotides, nucleic acids, and phospholipids. Its major functions are in energy storage and transfer and membrane integrity. It is mobile within the plant and promotes tillering, root development, early flowering, and ripening (especially where the temperature is low). It is particularly important in early growth stages. The addition of mineral P fertilizer is required when the rice plant’s root system is not yet fully developed and the native soil P supply is small. P is remobilized within the plant during later growth stages if sufficient P has been absorbed during early growth.
7.8.6. When damage is important?
The damage caused by P deficiency occurs throughout the growth cycle of the crop
7.8.7. Economic importance
P deficiency is fairly common in irrigated rice leading to substantial economic loss.
7.9. An example of zinc deficiency in sodic rice fields
7.9.1. Effects on plants
1) Affects several biochemical processes in the rice plant, such as Cytochrome and nucleotide synthesis, auxin metabolism, chlorophyll production, enzyme activation and membrane integrity
2) Growth is severely affected
7.9.2. Symptoms
1) Symptoms appear between two to four weeks after transplanting
2) Dusty brown spots on upper leaves of stunted plants
3) Uneven plant growth and patches of poorly established hills in the field, but the crop may recover without intervention
4) Tillering decreases and can stop completely and time to crop maturity increases under severe Zn deficiency
5) Increase spikelet sterility in rice
6) Chlorotic midribs, particularly near the leaf base of younger leaves
7) Leaves lose turgor and turn brown as brown blotches and streaks appear on lower leaves, enlarge, and coalesce
8) White line sometimes appears along the leaf midrib
9) Leaf blade size is reduced
Other effects on growth include the following:
1) Symptoms may be more pronounced during early growth stages because of Zn immobilization (due to increased bicarbonate concentration in the soil under strongly reducing conditions following flooding).
2) If the deficiency is not severe, plants can recover after 4-6 weeks, but maturity is delayed and yield reduced.
Figure 21 shows a zinc deficient in rice crop.
Figure 21. Crop yellowing due to zinc deficiency
7.9.3. How to confirm?
There are plant or soil tests to show Zinc deficiency. The optimal ranges and critical levels of Zn in plant tissue are:
Growth stage
Plant part
Optimum
Critical level for deficiency
Tillering
Y leaf
25-50
<20>20 mg kg-1 unlikely (sufficient)
The ratios of P:Zn and Fe:Zn in the shoot at tillering to the PI stage are good indicators of Zn deficiency. Values should not exceed:
P:Zn20-60:1in shoots 6 wk after planting
Fe:Zn5-7:1in shoots 6 wk after planting
Leaf Zn concentration is a less reliable indicator of Zn deficiency, except in extreme cases (leaf Zn <15>7) with moderate to high organic matter content (>1.5% organic C) are likely to be Zn-deficient due to high HCO3- in solution.
A ratio of >1 for exchangeable Mg:Ca in soil may indicate Zn deficiency.
7.9.4. Problems with similar symptoms
The symptoms of Zinc deficiency may resemble those of Fe deficiency, which also occurs on sodic soils. On sodic soils, Zn deficiency is often associated with S deficiency. They may also resemble Mn deficiency and Mg deficiency.
Leaf spots may resemble Fe toxicity in appearance but the latter occurs on high organic status soils with low pH.
7.9.5. Why and where it occurs
Zn deficiency can be caused by one or more of the following factors:
1) Small amount of available Zn in the soil as in sodic soils.
2) Planted varieties are susceptible to Zn deficiency (i.e., Zn-inefficient cultivars).
3) High pH (close to 7 or alkaline under anaerobic conditions). Solubility of Zn decreases by two orders of magnitude for each unit increase in pH in sodic soils. Zn is precipitated as sparingly soluble Zn(OH)2 when pH increases in soil following flooding.
4) High HCO3- concentration because of reducing conditions in calcareous and sodic soils with high organic matter content or because of large concentrations of HCO3- in irrigation water.
5) Depressed Zn uptake because of an increase in Fe, Ca, Mg, Cu, Mn, and P after flooding.
6) Formation of Zn-phosphates following large applications of P fertilizer. High P content in irrigation water (only in areas with polluted water).
7) Formation of complexes between Zn and organic matter in soils with high pH in sodic soil and high organic matter content or because of large applications of organic manures and crop residues.
8) Precipitation of Zn as ZnS when pH decreases in sodic soil following flooding.
9) Excessive liming.
10) Wide Mg:Ca ratio (i.e., >1) and adsorption of Zn by CaCO3 and MgCO3. Excess Mg in soils derived from ultra basic rocks.
Zn deficiency is the most widespread micronutrient disorder in rice. Its occurrence has increased with the introduction of modern varieties, crop intensification, and increased Zn removal. Soils particularly prone to Zn deficiency include the following types:
1) Neutral, sodic and calcareous soils containing a large amount of bicarbonate. On these soils, Zn deficiency often occurs simultaneously with S deficiency (widespread in India and Bangladesh).
2) Intensively cropped soils where large amounts of N, P, and K fertilizers (which do not contain Zn) have been applied in the past
3) Paddy soils under prolonged inundation (e.g., when three crops of rice are grown in one yr) and very poorly drained soils with moderate to high organic matter content
4) Sodic and saline soils
5) Peat soils
6) Soils with high available P and Si status
7) Sandy soils
8) Highly weathered, acid, and coarse-textured soils containing small amounts of available Zn. Soils derived from serpentine (low Zn content in parent material) and laterite.
9) Leached, old acid sulfate soils with a small concentration of K, Mg, and Ca.
7.9.6. Mechanism of damage
Zinc is essential for several biochemical processes in the rice plant, such as:
1) Cytochrome and nucleotide synthesis
2) Auxin metabolism
3) Chlorophyll production
4) Enzyme activation
5) Membrane integrity
Zn accumulates in roots and can be translocated from roots to developing plant parts. Because little retranslocation of Zn occurs within the leaf canopy, particularly in N-deficient plants, Zn deficiency symptoms are more common on young or middle-aged leaves.
7.9.7. When damage is important?
The damage brought about by Zn deficiency is important throughout the growth cycle of the rice crop.
7.9.8. Economic importance
Zinc deficiency is a serious economic problem. For examples, in Japan, Zn deficiency is the cause of the "Akagare Type II" disorder in rice.
8. COMBINED EFFECTS OF SALINITY AND SODICITY
Often saline and sodic conditions, particularly in the Indo-Gangetic plains and several other regions of the world, occur together, and hazards of the combined condition are, therefore, of considerable interest, Based on a study on a large number of soil-tests against crop response, some investigators have tried to correlate the combined effects of salinity and sodicity on crop growth (see Table 14). It is seen from Table 14 that the higher the pH, the narrower the range of tolerable salinity. This suggests an additive effect of salinity and sodicity.
Table 14. Combined crop response to salinity and sodicity.
9. HOW SALT-AFFECTED SOILS AFFECT CROP QUALITY
9.1. Effects on quality of vegetable Crops
1) Salinity has various effects on quality of vegetable crops. It generally causes decrease in the size, quality and market value of fruits (tomatoes, cucumbers, and others), marketable heads (cabbage, heads (cabbage, cauliflower, lettuce), and roots (carrots, radish). Yields of such crops as tomatoes and peppers are reduced partly because of fewer fruits per plant but also because of a marked decrease in n fruit size. As all parts of the plant are stunted, roots, leafy or flowering heads, and other harvested plant parts all exhibit characteristic size decrease. Sweet corn is an exception in that; ear size remains unaffected at moderate salinity levels, although the number of marketable ears per plant decreases appreciably. Stunted growth may also be expected to delay flowering and maturation of some crops like tomatoes, sweet corn, eggplant, peppers, beans, peas, okra, and some others.
2) Salinity, by checking the growth, may produce some favourable effects on some vegetable crops. It increases the sugar content quite markedly in some crops; for example, it contributes to the production of sweeter carrots. But this gain in quality is usually associated with some decrease in yield. Cabbage heads from salty field are generally more solid and firm than that from normal fields, but lowered yields again offset this favourable, effect. Chloride fertilizers are generally believed to make potatoes more starchy and watery; but on saline soils the decreased water availability counteracts this effect so that the tubers produced are of normal starch and water contents. The checking of vegetative growth by salinity may hasten maturation and final harvest dates, especially of crops having intermediate habits, such as potato, melons, etc. When overgrowth is a potential cause of impaired quality, as with split or divided onion bulbs, low salinity levels will check the growth slightly and may prevent the damage.
3) Caution must be exercised in any attempts to use salinity to improve quality, because severe losses can result if salinity becomes excessive.
4) Quality of vegetable crops is also affected by some diseases caused by salinity. For example, blossom-end rot of tomato and pepper, blackheart of celery, and internal browning of lettuce are all symptoms of calcium deficiency which may occur in saline soils characterized by high sulfate and low calcium levels.
9.2. Effects on quality of field crops
Among field crops, quality effects are less conspicuous than among vegetable crops. Salt accumulation in sugarcane complicates the sugar refining process - sugarcane from salty soils produces inferior quality of gur (raw sugar) possessing more salts, and creates crushing trouble, on account of which sugar factories refuse to accept cane grown on salty lands. Grain quality of barley, wheat and other crops seems to be unaffected by salinity. However, grain yields of rice and corn may be reduced without appreciably affecting straw yield. With some other crops like barley, wheat, cotton and some tolerant grasses, seed or fiber production are decreased much less than vegetative growth.
9.3. Effects on quality of forage crops
1) The quality of forage crops is sometimes strongly affected by salinity. Fruit and seed normally accumulate very little salt, but leaves and stems often do, due to which the forage may become unfit for feeding. Rhodesgrass, for example, may cause scouring in cattle because the hay produced on salty lands contains too much salt.
2) Since salinity checks vegetative growth more than the root growth, the forage may actually be richer in certain vitamins and nutrients than forage from non-saline land. The decrease in yield, however, offsets the enrichment in nutrients. Furthermore, the checking of growth may cause the forage to become tougher than the one on non-salty land. Increased toughness, thus, decreases palatability of forage for livestock.
9.4. Effects on quality of fruit crops
1) Relatively little is known about the effects of salinity on fruit crop quality. Excessive leaf burn and leaf drop expose fruit to the sun, and sunscald markedly lowers fruit quality. This is an important factor for grapes. Magnesium deficiency caused by high sulfate levels has been observed on several varieties of grapes, causing damage to fruit quality. Even in the absence of appreciable leaf burn, the checking of growth by salinity may still affect quality by decreasing fruit size, as it happens in musk melons. Muskmelons and some other melons and fleshy fruits grown on salt-affected soils are markedly softer and juicier at the normal harvest stage than from non-saline soils. This softer fruit may not store or ship as well as the former fruit.
2) The check on vegetative growth may result in increased sugar content of fruit and improved flavour. Muskmelons show this effect, but at the cost of smaller fruit and decreased yields.
3) For the more sensitive fruit crops, salinity levels for satisfactory growth are so strongly restricted that salinity effects on fruit quality are less pronounced.
10. CONCLUSIONS
1) The problem of salt-affected soils is of very great magnitude throughout the world considering the vastness of these soils. The situation is further alarming because large areas are still going out of cultivation very rapidly at the rate of about 3 ha per minute.
2) Many of the bio-physical, socio-economical and environmental impacts of salt affected soils in different countries and situations have been enumerated and described. Condensed in so brief a space, the problem may appear exceedingly complex and not too well defined.
3) Some characteristic visual symptoms and indicator plants may be useful in diagnosing hazards of salt-affected soils. Reliable diagnosis of these hazards, however, requires the right kind of laboratory and field tests on soil and plant samples.
4) The excessive concentrations of soluble salts in the root medium inhibit plant growth primarily as a consequence of the decrease in the physiological availability of water to plant as a result of an increase in osmotic pressure of the soil solution to which the EC of soils is closely related to provide a useful index of salinity.
5) The constituent ions present in salt-affected soil may have specific nutritional, or toxic, effects on crop plants.
6) The reduced growth of crops grown on sodic soils may be due to adverse nutritional factors, toxic factors, adverse physical condition of soil, or a combination of all. In a saline-sodic soil" ES and salinity have additive effects on crop plants.
7) Several examples of toxic effects of specific ions and deficiency of several nutrients in salty lands have been discussed in more details to give insight of problems faced by plants in salt affected soils.
8) Various morphological and anatomical changes in plants due to salinity and sodicity; physiological effects (i.e. effects on enzymatic activity, photosynthesis, protein metabolism, mineral nutrition, hormones) of salinity and sodicity on plants, and mechanisms of growth inhibition by salinity and sodicity have been discussed.
9) Because of a variety of detrimental effects on plant development most crops produced on salty lands are of poor quality and composition. Besides its harmful effects, salt affected soils improve quality of certain crops but at the cost of decreased yield.
10) The salt tolerance of plants varies from crop to crop and variety to variety. Several environmental factors may modify the hazards of salty lands to plants. Salinity and sodicity effects on a crop plant may vary depending on the stage of its development.
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