Categories of Salt-Affected Soils

As sodium salts dominate in naturally occurring salt- affected soils, the soils are generally “saline- sodic,” which become “sodic” when soluble salts are leached. Similarly, a sodic soil becomes saline-sodic when salts accumulate in soil layers because of restricted drainage imposed by a degraded soil structure. When sodium salts are not dominant, the soils are referred to as “saline” soils. Generally, saline, sodic, and saline-sodic soils have a spectrum of disorders and the soil solutions have a range of values of cations concentrations and electrical conductivity. Sodic soils are considered191 to have pH >8.5 because of the predominance of carbonate ions in arid soils. However, sodic soils with neutral and acidic pH with the predominance of anions Cland SO ,2 have been reported to be widely present, particularly in Australia.1101 Soil pH also controls the presence of a variety of ionic species, in addition to influencing soil structure. For example, chlorides and sulfates dominate in neutral and acidic soils, whereas in alkaline soils bicarbonate and carbonate ions are prevalent. Microelements such as Fe, Al, Zn, Mn, Mo, Cr, B, Cu, As, Cd, and Se occur in soil solutions as differently charged ionic species ranging from cationic to anionic forms dictated by equilibrium pH.1'11

Thus, the SAR, EC, and pH of soil solutions are determined by the ionic composition and hence, the impact on crop growth and yield, and as well as on soil structure. Table 2 gives different categories of salt-affected soils on the basis of SARC and ECe of soil saturation paste extracts (subscript e denoting saturation paste extract) and pH measured in 1:5 soil-water suspensions. Measurements of ions in extracts from saturated soil paste are common in many parts of the world. If measurements are made in soil solutions from different soil-to-water ratios, the values will be different and appropriate conversion factors have to be used to derive SARC and ECc.131 Table 2 also gives the possible mechanisms through which plant growth is affected in each category. Measurements of all ions in soil solution are necessary to identify toxicity, deficiency, or ion imbalance of various nutrients and microelements in relation to crop productivity.

Toxic Elements in Salt-Affected Soils

The accumulation of toxic elements including heavy metal ions in the form of soluble salts in agricultural soils, in recent years, is caused by the long-term application of wastewater, industrial effluents, mining wastes, sewage sludge (or biosolids), and compost. As a result of global industry outputs, atmospheric deposition (such as acid rains) is another cause of toxic ion dynamics in productive soils. Landfill leachates also pollute nearby farming lands. Although the amounts of these elements are usually much smaller than the major elements (considered to be useful in crop production), their availability for plant uptake or being leached into groundwater is an important environmental issue. Plant uptake of toxic ions, in addition to causing decline in crop production, can affect human health by entering into the food chain. More details on the distribution and properties of heavy metals (or trace elements) in soils can be found in many publications.

Soil Water Dynamics and Salinity Stress

Salt concentration in agricultural soils is transient and varies with depth and changes throughout the growing season in response to rainfall and irrigation, surface evaporation, water use by vegetation, and the hydraulic conductivity (hence the leaching fraction) of soil layers.1121 As the soil dries because of evapo- transpiration, the salt concentration increases, as does the osmotic pressure of soil water. Concomitant changes in matric and osmotic potentials (Table 3) determine plant water uptake in the field. The influence of soil texture and type of clay on plant-available water compounds the effects of matric plus osmotic potentials. As the total water potential decreases below -900 kPa, plant water uptake is greatly reduced, and at -1500 kPa plant wilts completely.131 The two plant responses to salinity, viz. osmotic stress and ion- specific stress, can occur sequentially, giving rise to a two-phase growth response.121 Our experiments1131

TABLE 2 Categories of Salt-Affected Soils Based on ECc (dSm '). SARC, and pHl:5 of Soil Solutions, and Possible Mechanisms of Impact on Plants

No.

Category of Saline Soil

Criteria

Possible Mechanisms of Impact on Plants

1

Acidic-saline soil

ECe >4; SARe <6; pH <6

Osmotic effect; microelement (Fe, Al, Mn, etc.), toxicity; S042- toxicity in very low pH

2

Neutral saline soil

ECe >4; SARe <6; pH 6-8

Osmotic effect; toxicity of dominant anion or cation other than Na-i-

3

Alkaline-saline soil

ECe >4; SARe <6; pH 8-9

Osmotic effect; НСОЗ- and C032- toxicity

4

Highly alkaline-saline soil

ECe >4; SARe <6; pH >9

Osmotic effect; НСОЗ- and C032- toxicity; microelement (Fe, Al, Mn, etc.) toxicity

5

Acidic-saline- sodic soil

ECe >4; SARe >6; pH <6

Osmotic effect; Na-i- and microelement (Fe, Al, Mn, etc.), toxicity

6

Neutral saline- sodic soil

ECe >4; SARe >6; pH 6-8

Osmotic effect; Na+ toxicity; toxicity of dominant anion (Cl- or S042-)

7

Alkaline-saline- sodic soil

ECe >4; SARe >6; pH 8-9

Osmotic effect; Na+ toxicity; НСОЗ- and C032- toxicity

8

Highly alkaline-saline- sodic soil

ECe >4; SARe >6; pH >9

Osmotic effect; Na+ toxicity; НСОЗ- and C032- toxicity; microelement (Fe, Al, Mn, etc.), toxicity

9

Acidic-sodic soil

ECe <4; SARe >6; pH <6

Indirect effect due to soil structural problems; seasonal waterlogging can induce microelement (Fe, Al, Mn, etc.), toxicity

10

Neutral sodic soil

ECe <4; SARe >6; pH 6-8

Indirect effect due to soil structural problems; seasonal waterlogging; Na-i- toxicity at high SARe

11

Alkaline-sodic soil

ECe <4; SARe >6; pH 8-9

Indirect effect due to soil structural problems; seasonal waterlogging; Na-i- toxicity at high SARe; НСОЗ- and C032- toxicity

12

Highly alkaline- sodic soil

ECe <4; SARe >6; pH >9

Indirect effect due to soil structural problems; seasonal waterlogging; Na-i- toxicity at high SARe; НСОЗ- and C032- toxicity; microelement (Fe, Al, Mn, etc.), toxicity

13

Non-salt-affected soil

ECe <4; SARe <6; pH 6-8

Can have problems due to factors other than salts

Note: Toxicity, deficiency, or ion imbalance due to various ions will depend on the ionic composition of soil solution. Source: Data from Rengasamy.131

using soils in pot experiments and also field observations have shown that when the osmotic pressure of soil water is >900 kPa (EC of soil water >25 dSm '), the osmotic effect is predominant and specific ion (such as Na or Cl) effect may be significant at lower levels of salinity (Figure 3). In moderately saline soils when soil fertility level is low and nutrient deficiency is an issue, application of fertilizer at an appropriate level alleviates the salinity stress on plants.1141 Similarly, adding a small amount of calcium has been reported to enhance the salt tolerance of plants at moderate levels of NaCl salinity.1151

Different levels of salinity in the field during the crop growing season induced by the changing soil water content (Table 3) will affect crops differently, either by a predominant osmotic effect or an ion- specific effect. In dryland cropping, a high concentration of salt combined with a low rainfall in the start of the season severely affects the germination of seeds. Similarly, dry spells, enhancing salt concentration, during critical physiological periods of crop growth such as flowering and grain filling, can result in the significant reduction in yield.131 Apart from vertical and temporal variations, spatial variation in salinity across a field can also be large in both dryland and irrigated cropping regions. These factors have to be considered in choosing appropriate crop species relevant to different sections in the field and seasonal weather pattern in farming systems.

TABLE 3 Changes in Matric, Osmotic, and Total Water Potentials of an Alfisol (Loam) and a Vertisol (Clay) with Changes in Soil Water Content

Soil Water Status

Water Content

EC

Matric

Soil Water Potential (kPa)

Total

(g g-D

(dS m-1)

Osmotic

Alfisol (loamy soil) 1:5 Soil water

5.00

0.64

+0.1

-23.0

-22.9

Saturation paste

0.35

9.14

-0.1

-329.1

-329.2

Field capacity (FC)

0.20

16.00

-10.0

-576.0

-586.0

Drier than FC

0.14

22.86

-680.0

-823.0

-1503.0

Permanent wilting point Vertisol (clayey soil)

0.05

64.00

-1500.0

-2304.0

-3804.0

1:5 Soil-water

5.00

0.64

+0.1

-23.0

-22.9

Saturation paste

0.80

4.00

-0.1

-150.0

-150.1

Field capacity (FC)

0.40

8.00

-10.0

-288.0

-298.0

Drier than FC

0.16

20.00

-780.0

-720.0

-1500.0

Permanent wilting point

0.08

40.00

-1500.0

-1440.0

-2940.0

Source: Data from Rengasamy et al.1121

Schematic diagram showing the effect of osmotic pressure of soil solution on yield of crops and ion- specific effect at low levels of salinity

FIGURE 3 Schematic diagram showing the effect of osmotic pressure of soil solution on yield of crops and ion- specific effect at low levels of salinity.

Source: Rengasamy, P.|13)

Hypoxia and Salinity Effects

During high rainfall seasons, soils with restricted drainage become waterlogged and air-filled soil pores are eliminated with severely reduced oxygen availability (hypoxia). Sodic soils with limited porosity become saturated with water in wet conditions, and hypoxia is common in these soils. Hypoxia may lead to many problems that plants may experience such as shortage of ATP in anoxic root cells, reduced carbohydrate production, production of reactive oxygen species, toxic reduction products and raised C02 and organic acids, and shoot wilting from decreased hydraulic conductivity.1161 Hypoxia can worsen the growth of plants exposed to salinity.1171 Because of increased solubilities due to changes in pH and pE associated with hypoxia,11'1 toxicity of Mn, Fe, Al, B, and reduction products such as H2S and methane is common in waterlogged salt-affected soils.

Management of Salt-Affected Soils in Relation to Agricultural Production and Environmental Protection

The solution to improving agricultural production in salt-affected soils without degrading the environments is to link the plant improvement to tolerate salinity and other associated abiotic stress with appropriate soil management and cropping practices. As mentioned earlier, there are different categories of salt-affected soils, each one requiring a specific strategy depending on how root zone processes and plant interactions are affected by soil solution composition. Broadly, there are three ways with some possible actions to resolve the problems in salt-affected soils, based on the schematic presentation in Figure 1:1) prevention of salt accumulation; 2) removal of accumulated salts; and 3) adaptation to saline environment.

Prevention of Salt Accumulation

Natural processes responsible for salt input, such as mineral weathering and deposits from rain and wind, are difficult to prevent or control. However, salt input through irrigation can be reduced by managing irrigation water quantity and quality. Adding salts and toxic elements by adding composts, industrial effluents, mining wastes, sewage sludge, biosolids, and wastewater can be prevented by environmental regulations and guidelines for safe application of these materials. Salt input through groundwater intrusion in soils with shallow water tables can be minimized by growing deep-rooted perennial plants with high transpiration rates to use groundwater and reduce recharge, thus controlling groundwater levels. However, this strategy will concentrate more salt in the root zone in soils where the water table is deep.1121

Removal of Accumulated Salts

Accumulated salts in the root zone layers can be removed by leaching. In dryland cropping regions, good infiltration of rainfall and water movement through the soil profile by improving soil structure can effectively reduce salt load. In irrigation agriculture, the quantity and quality of irrigation water should be evaluated for providing leaching fraction to maintain salt balance. In both cases, care should be taken to avoid leachates from the soil contaminating the deeper soils or the surroundings in the drainage disposal region (Figure 1). Contamination due to As and Se from the drainage water has been reported in different parts of the world.

Adaptation to Saline Environment

Agricultural production in the saline environments can be enhanced by using plants that tolerate salt in the root zone. High genetic variations exist among plant species and crop cultivars that are thriving under different saline environments (or categories of salt-affected soils, Table 2). This can be exploited to develop better-adapted salt-land plants. Selection, breeding, and genetic modification to improve crop performance should focus on the many soil processes and soil solution compositions discussed above to find the new cultivars that are effective in the field so that farmers can adopt them.1181 Plant improvement must also be linked to appropriate soil management and agronomic practices, taking into account of temporal, vertical, and spatial variations in salinity in the field dictated by soil water dynamics.

Conclusions

The constituent cations and anions in solutions of salt- affected soils affect both soil properties and crop productivity. The interactions between root zone environments and plant response to increased osmotic pressure or specific ion concentrations are also influenced by soil processes such as soil water dynamics, soil structural stability, and soil pH and pE. Soil structural stability affects plant performance and also the environment by way of soil erosion. There are 12 categories of salt-affected soils based on the SARC, ECe, and pH of soil extracts. The mechanisms by which plant growth is affected are specific to each category. Elements toxic to plant growth can occur naturally in these soils and also owing to use of wastewater and waste materials such as biosolids, sewage sludge, and compost. The solubilities of these elements are influenced by several factors, but mainly by soil pH and pE. Management of salt- affected soils in relation to agricultural production and environmental protection can be achieved by three ways, viz. prevention of salt accumulation, removal of accumulated salts, and adapting to saline environment by growing appropriate salt tolerant crops. While minimizing salt load in the root zone, care should be taken to avoid leachates from the soil contaminating the deeper soils or the surroundings in the drainage disposal region.

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