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date: 28 February 2024

Soil Salinizationfree

Soil Salinizationfree

  • Pichu RengasamyPichu RengasamySchool of Agriculture, Food, and Wine, University of Adelaide

Summary

Salt accumulation in soils, affecting agricultural productivity, environmental health, and the economy of the community, is a global phenomenon since the decline of ancient Mesopotamian civilization by salinity. The global distribution of salt-affected soils is estimated to be around 830 million hectares extending over all the continents, including Africa, Asia, Australasia, and the Americas. The concentration and composition of salts depend on several resources and processes of salt accumulation in soil layers. Major types of soil salinization include groundwater associated salinity, non–groundwater-associated salinity, and irrigation-induced salinity. There are several soil processes which lead to salt build-up in the root zone interfering with the growth and physiological functions of plants.

Salts, depending on the ionic composition and concentration, can also affect many soil processes, such as soil water dynamics, soil structural stability, solubility of essential nutrients, and pH and pE of soil water—all indirectly hindering plant growth. The direct effect of salinity includes the osmotic effect affecting water and nutrient uptake and the toxicity or deficiency due to high concentration of certain ions. The plan of action to resolve the problems associated with soil salinization should focus on prevention of salt accumulation, removal of accumulated salts, and adaptation to a saline environment. Successful utilization of salinized soils needs appropriate soil and irrigation management and improvement of plants by breeding and genetic engineering techniques to tolerate different levels of salinity and associated abiotic stress.

Subjects

  • Environmental Issues and Problems
  • Agriculture and the Environment

Introduction

Soil salinization occurs through natural or human-induced processes resulting in the accumulation of soluble salts in soil layers. Soil salinity is described and characterized in terms of the concentration and composition of the soluble salts. Dissolved ions in soil water can directly affect crop growth, and the cations adsorbed by soil particles can affect soil structure and indirectly affect crop performance. A report by FAO in 2000 estimated that globally the area of salt-affected soils including saline and sodic soils was 831million ha (Martinez-Beltran & Manzur, 2005). Salt-affected soils extend over all continents including Africa, Asia, Australasia, and the Americas. It has a major impact on agricultural productivity, environmental health, and economic welfare. All soil types with diverse morphological, physical, chemical, and biological properties may be affected by salinization. These salt stores in the soil profile can also cause increases in the salinity of groundwater, as salts can be mobilized through irrigation, deep drainage, and recharge events.

Soil salinization has threatened civilizations in ancient and modern times. Salinity in southern Mesopotamia and in several parts of the Tigris-Euphrates valley destroyed the ancient societies that had successfully thrived for several centuries (Hillel, 2005; Jacobsen & Adams, 1958). Salt-affected soils are naturally present in several countries where many regions are also affected by irrigation-induced salinization. Dry-land salinity is becoming a major issue in many countries, particularly Australia (Rengasamy, 2006). Recent papers (Ganjegunte, Sheng, & Clark, 2014; Young et al., 2015) also highlight the growing problem of soil salinization.

This article presents an overview of soil salinization dealing with different aspects such as processes and assessment of salinization, distribution and extent of salt-affected soils, soil behavior in relation to the composition of salts, direct and indirect constraints to crop production in salinized soils, plant factors in adapting to salinized soils, and managing salinized soils for improving productivity and economy.

Processes of Soil Salinization

The term “primary salinity” refers to salt accumulation over long periods of time through natural processes, whereas “secondary salinity” refers to salt accumulation as a consequence of mismanagement of natural resources. Soluble salts in soils can be from different sources. Weathering of rocks and minerals contribute to soluble salts. Salts can also be deposited by rainfall, are usually low, but, over time, can accumulate in soils. Wind-transported aeolian dust can be a source of soluble salts. Other sources include discharge of saline groundwater and seawater intrusion. Agronomic practices such as fertilizer and pesticide application, irrigation using saline bore water or wastewater, and dumping of waste materials in soils will also contribute to salts in the soil. The salts deposited can be mobilized to different soil layers depending on the rainfall and the water movement influenced by soil hydraulic properties and landscape features. 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 soil layers.

Three major types of salinity based on soil and groundwater processes are found globally (Rengasamy, 2006).

Groundwater-Associated Salinity

In discharge areas of the landscape, groundwater exits at the surface, bringing the dissolved salts. Evapotranspiration increases the potential for the upward movement of water and salts. When the water table is close to the soil surface, maximum amount of salts are stored in the top soil layers. Salt accumulation is high when the water table is less than 1.5 m below the surface, when capillary rise of water is very effective in depositing salts in surface layers (Talsma, 1963). However, this threshold depth may vary depending on soil hydraulic properties and climatic conditions.

Groundwater-associated salinity (also referred as “seepage salinity”) is visible when scalding of soil surfaces occurs, which is associated with a rising saline water table. Generally the water table is closer to the surface at the foot of slopes, break in slopes, and on valley floors than in the higher regions of the landscape. In some instances, groundwater is forced to the surface in upper catchments due to barriers, such as geological structures, to flow before deep valley sediments have filled with water.

Rengasamy (2002a) has reviewed the salinization induced by groundwater processes in Australian landscapes, which can also be applied in many landscapes in the world. Under native vegetation, leaching of salts from the permeable soil layers led to salt storage in deep layers and also in the water table over geological time scales. Under these conditions, the salinity of groundwater was often very high, with an electrical conductivity (EC) ranging from15 to 150 dS/m. When the water table was 4 m below the surface, saline groundwater did not affect the native vegetation. With the clearance of perennial native vegetation and the introduction of agriculture, the equilibrium levels of the water table have changed. In low-lying regions water with dissolved salts has leaked to the groundwater from upper soil layers, and as result, groundwater levels have risen. The salinization process due to water table fluctuations can be summarized as follows: introduction of crops and pasture led to a lower evapotranspiration of water captured from rainfall than occurred under the natural vegetation, leading to the further rise in groundwater levels due to lower water use. As the saline groundwater approached the surface, soil layers in the top 1 m were salinized and waterlogged. Capillary rise of saltwater plays a vital role in the salinization of surface soil. Further, on valley sides of the landscape, saline groundwater can seep to the soil surface.

Non–Groundwater-Associated Salinity

Even in the landscapes where the water table is very deep (below 10 m), salt has been accumulating in soil layers over thousands of years, delivered by many sources, including wind and rain. Under semi-arid conditions, the rainfall has not been sufficient to leach all the salts accumulated below the root zones of native vegetation to the deeper groundwater. The clay layers in deeper subsoils have hindered the movement of water and salt. As a result, a “bulge” of salt has accumulated in the soil layers approximately 4–10 m from the surface. Occurrence of salt bulges in deeper layers in many landscapes in Australia was first shown by Holmes (1960) and recently confirmed by geophysical studies using modern techniques such as airborne electromagnetics (Lawrie, 2005). In salinized soils dominated by sodium salts, soil layers become sodic and highly dispersive. Sodic subsoils with high bulk density can also prevent water transmission and restrict leaching following infiltration, a process that has led to salt accumulation in root zone layers in amounts detrimental to plant growth. This “transient salinity” (Rengasamy, 2002a) fluctuates with depth and its concentration and effect on plant growth change with season and rainfall. Significantly, groundwater processes do not influence this form of salinity. Figure 1 schematically explains the soil processes leading to transient salinity in root zone layers of dispersive soils. Transient salinity is widespread in dryland cropping regions in Australia (Barrett-Lennard, Anderson, Holmes, & Sinnott, 2015; Rengasamy, 2002a). It is estimated that about 67% of cropping area being affected by this form of salinity.

Figure 1. Schematic presentation of transient salinity in dispersive soils (adapted from Rengasamy, 2002a).

The different forms of dryland salinity found in the Australian landscape are illustrated in Figure 2. The problem of transient salinity is not, however, confined to Australia. About 5.8 × 106 km2 of soils around the world are sodic with dispersive soil layers (Bui et al., 1998) and have the potential for transient salinity. The salt accumulation, in addition to increasing the osmotic potential of soil water, can also introduce high concentrations of boron, carbonate species, and microelements such as Al, Mn, and Fe, which can be toxic to plants. As a result of climate changes predicted to occur in the future, transient salinity may increase in regions where rainfall will be lower and temperature higher, when leaching of salts is reduced and concentration of salts is enhanced by high evaporation. In regions where rainfall levels increase, water table–induced salinity can increase.

Figure 2. Different types of dryland salinity in Australian landscapes (adapted from Rengasamy, 2002a).

Irrigation-Induced Salinity

Agricultural production in many parts of the world has increased in the last century because of irrigation. However, irrigation-induced salinity is a growing problem in these regions (Umali, 1993). FAO (1991) reported that about 20–30 million hectares of irrigated lands were severely affected in crop productivity by salinity, and about 80 million hectares were affected moderately. Salts introduced by irrigation water are stored within the root zone because of insufficient leaching. Saline irrigation water, low hydraulic conductivity of soil layers usually found in clayey dispersive (sodic) soils, and high evaporative conditions accelerate salinization of soil layers. The efforts to pump the saline groundwater to reduce water table levels and the re-use of it for irrigation have resulted in salinization of soil layers in Australian irrigation regions (Rengasamy & Olsson, 1993). Recent trends in using industrial effluents and recycled water for irrigation also promote salinization and soil structural deterioration. Improper drainage and methods of application of water play important roles in irrigation-induced salinity. Several reports (e.g., Reitz & Haynes, 2003; Rengasamy & Olsson, 1993; Shainberg & Shalhevet, 1984; Smedema & Shiati, 2002) are available on the quality of irrigation water, its reaction with soil components affecting soil behavior, salt balance and changes in ionic composition, proper irrigation methods, appropriate drainage provision, and general influence on agricultural production.

Assessment of Soil Salinity

The impact of soil salinity on plants and soils depends on soluble salts. Dissolved ions from these salts contribute to the EC of soil water. Hence, measuring the EC of soil water directly in the field or soil water extracts in the laboratory enables quantifying soil salinity (Rhoades, 1993; Tanji & Wallender, 2012). Measuring EC in extracts from water-saturated soil paste or by mixing soils in different proportions with water is a common practice. The extraction of soil water under field conditions using vacuum pumps can also measure soil EC at a given soil water content in the field.

Because of the heterogeneous nature of salt distribution in the landscape in surface soils and subsoils, the number of locations for soil sampling will be large, and EC characterization by laboratory methods is both expensive and time consuming. Proximal sensing by electromagnetic (EM) induction methods has been used for efficient and accurate mapping of saline soils in conjunction with limited soil sampling (e.g., Doolittle & Brevik, 2014; Goff et al., 2014). In addition to EM techniques, aerial photographs, satellite and airborne multiple sensors, microwave sensors, video imagery, airborne geophysics, and hyperspectral sensors have been tried for surveying and mapping salinity in large areas or regions (Metternicht & Zinck, 2009). Calibration based on the calculated EC by these methods and measured EC of soil extracts is necessary to provide reliable estimates of EC.

The unit for EC measurement is dS/m, which is the same as mS/cm or mmho/cm. An EC of 1 dS/m corresponds to approximately 640 mg/L of total dissolved salts assuming a composition of salts commonly found in groundwater across the world, but it may vary between 530 (if the salt is predominantly NaCl) and 900 (if the salts mainly consist divalent ions). Approximately 1 dS/m will correspond to 10 mmolc/L of cations or anions in waters and soil solutions and can vary depending on the cationic and anionic composition of the salt.

The total concentration of electrolytes identified by EC will indicate the severity of salinity and the osmotic effects on plants. However, the composition of soluble salts is important in identifying the effects of specific ions on plant productivity and soil structural health. Although NaCl is the dominant salt in many saline lands, carbonates (HCO3 and CO32−), sulfates, and chlorides of Na, K, Mg, and Ca are also present in saline soils (Rengasamy, 2010), and different combinations of these species can have different effects on soils and plants. For example, Na and K salts can affect soil structural stability and carbonate ions can induce higher toxicity and higher pH compared to chloride ions. The composition of soluble salts in soil-water extracts are generally estimated by using chemical and instrumentation techniques such as atomic absorption spectroscopy or inductively coupled plasma emission spectroscopy. In agricultural regions, individual paddock assessment of heterogeneity of salinity and salt-affected locations will require the use of modern instruments such as EM methods in conjunction with soil chemical analysis in selected spots to identify other soil constraints in addition to salinity.

Distribution and Extent of Salt-Affected Soils

Martinez-Beltran and Manzur (2005) of FAO estimate that around 830 million hectares of global land are affected by salts. Varying degrees of detail and different systems of classification and grouping have been employed in soil surveys in different countries regarding the location and distribution of salinized soils. The available maps have not been prepared on a uniform scale. In many cases there is no clear distinction between “saline” and “sodic” soils, both of which are affected by soil salinization but which have different soil processes and mechanisms that affect plant growth. According to the map of salt-affected soils in Australia by Northcote and Skene (1972), sodic soils are predominant (1,997,000 km2) in Australia compared to saline soils (386,300 km2). Global distribution of salt-affected soils has been presented by several authors, including Szabolcs (1989); Abrol, Dahiva, and Massoud (1988); Ghassemi, Jakeman, and Nix (1995); and Bui et al. (1998). Based on Szabolcs (1989), estimates of the salt-affected soils in the world are given in Table 1. The data presented may be misleading because dissimilar criteria for classifying these soils were used in different geographic regions. For example, sodic soils are defined in Australia as having an ESP (exchangeable sodium percentage) > 6, while in the United States and other parts of the world, ESP > 15 is the criterion.

Table 1. Global Distribution of Salt-Affected Soils

Area in Millions of Hectares

Continent

Saline

Sodic

Total

North America

6.2

9.6

15.8

Central America

2.0

2.0

South America

69.4

59.6

129.0

Africa

53.5

27.0

80.5

South Asia

83.3

1.8

85.1

North and Central Asia

91.6

120.1

211.7

Southeast Asia

20.0

20.0

Australasia

17.4

340.0

357.4

Europe

7.8

22.9

30.7

TOTAL

351.5

581.0

932.2

Plant growth and productivity can be directly affected by the total dissolved salts and the concentration of ions. However, accumulated salts can also change the chemistry of soils and alter their physical conditions and biological functions, indirectly affecting plant growth and performance and leading to environmental degradation. Changes in soil behavior depend on the composition of dissolved ions and their proportions in addition to their total concentration. Cations from the soil solutions are adsorbed by negatively charged soil particles, and anions are adsorbed by the positively charged sites. In addition, anions in soil solutions can also influence soil pH (Tavakkoli, Rengasamy, Smith, & McDonald, 2015). The role of cations and anions in soil behavior is discussed below.

Soil Structural Stability in Relation to Cations and Anions

Different soil constituents such as clay, silt, sand, and organic matter are linked by different types of chemical bonding (Rengasamy & Olsson, 1991) to form soil aggregates of different sizes. Stability of these aggregates during rain or irrigation is necessary to maintain optimum porosity for air, water, and nutrient retention and movement within a soil profile for healthy plant production. Water molecules, which are polar, will react with charged soil particles, resulting in swelling of aggregates and their disintegration into microaggregates. Further reactions lead to separation of clay particles from aggregates, known as “clay dispersion.” In natural soils, clays occur as complex heterogeneous intergrowths of different clay minerals associated with inorganic and organic molecules and biopolymers (Rengasamy & Sumner, 1998). A unique combination of these materials alters the charge contributed by the individual clay minerals and organic materials and results in a specific “charge,” which can be further changed by soil pH. This charge, when negative, is usually balanced by adsorbing cations and, when positive, by adsorbing anions. These adsorbed ions are commonly known as exchangeable cations or anions.

Most salt-affected soils have a net negative charge balanced predominantly by the cations Ca2+, Mg2+, K+, and Na+. These cations are bonded to clays with different degrees of covalent bonding and ionic bonding. The ionic bonding leads to water interaction, whereas covalent bonding does not. For example, Na+ and Cl in NaCl molecule are bonded by electrostatic attraction (ionic bonding) and hence are easily solvated and separated by water molecules. But in CaCO3 molecules, Ca2+ and CO32− are bonded mostly by covalency and water molecules cannot separate them easily. The degree of ionicity of the cation bonding with soil particles (Marchuk & Rengasamy, 2011) determines the extent of hydration reactions. In tropical soils dominated by iron and aluminum oxides, the net charge can be positive and the nature of anion bonding with soil particles determines the hydration reactions. The processes leading to structural changes on wetting of dry aggregates have been described by Rengasamy and Sumner (1998), with details on swelling and dispersion caused by exchangeable cations and the suppression of these phenomena by the flocculating effect of electrolytes present.

Increasing levels of exchangeable sodium in “sodic soils,” developed during soil salinization, have been found to influence clay dispersion and associated poor soil physical conditions such as hard-setting, high soil strength, low porosity, and restrictive hydraulic properties (Qadir & Schubert, 2002; Rengasamy & Olsson, 1991; Shainberg & Letey, 1984; Sumner & Naidu, 1998). Thus, the (ESP) of soils and sodium adsorption ratio (SAR) of soil solutions have been correlated with clay dispersion, soil strength, porosity, hydraulic conductivity, and infiltration rate, which eventually influence seedling emergence, waterlogging, water availability, and crop productivity. However, the critical values of these parameters (e.g., ESP 6 in Australia and ESP 15 in the United States) in identifying soil constraints depend on several soil factors and vary widely in many regions (Sumner, Rengasamy, & Naidu, 1998). For example, subplastic soils in Australia do not disperse even with an ESP of 25–30 (Isbell, 1996). Many authors have concluded that categorization of sodic soils must be developed based on soil behavior, such as clay dispersion, rather than arbitrary threshold ESP or SAR criteria.

It should be noted that ESP or SAR measures do not account for the effects of other exchangeable cations. Many reports have concluded that exchangeable potassium can also cause clay dispersion and associated decline in soil physical conditions (e.g., Chen, Banin, & Borovitch, 1983; Jayawardane, Christen, Arienzo, & Quayle, 2011; Paradelo, van Oort, & Chenu, 2013; Rengasamy & Marchuk, 2011; Smiles & Smith, 2004). Exchangeable magnesium, being divalent, has been customarily considered as equal to calcium in improving soil structural stability. However, Emerson and Bakker (1973) demonstrated that Na-Mg soils disperse more than the corresponding Na-Ca soils. Many others, including Alperovich, Shainberg, and Keren (1981); Keren (1991); Curtin, Steppuhn, and Selles (1994); Zhang and Norton (2002); and Rengasamy and Marchuk (2011) have reported the deleterious effect of exchangeable Mg, albeit small, on soil structural stability. Curtin et al. (1994) concluded that dispersion caused by exchangeable magnesium was about 4–5% of that caused by sodium. This is due to the higher ionicity of clay-Mg bonds compared to clay-Ca bonds. Even exchangeable calcium can lead to hydration of water molecules resulting in limited swelling (Slade, Quirk, & Norish, 1991). A new hypothesis to explain the different effects of exchangeable cations on soil dispersivity is discussed below.

New Hypothesis on Dispersive Soils

Clay dispersion from soils is a good indicator of the effects of cations and anions on soil structural degradation, and many previous studies have shown highly significant correlations between the amount of clay dispersed and soil physical properties (e.g., Levy, Eisenberg, & Shainberg, 1993; So & Aylmore, 1993). Because of the roles played by different cations in clay dispersion, it is better to identify these soils as “dispersive soils” rather than as “sodic” or “potassic” or “magnesic” soils. Anions such as sulfate, chloride, bicarbonate, and carbonate influence soil pH in neutral and alkaline soils (Tavakkoli et al., 2015), and changes in soil pH can alter the hydration charge of clay and organic materials, thereby affecting clay dispersion (Chorom & Rengasamy, 1995; Suarez et al., 1984). At a given soil pH, the net charge is balanced by exchangeable cations. Further, the degree of ionicity of individual cations involved in clay-cation bonds determines the extent of hydration (Marchuk & Rengasamy, 2011). In addition to the influence of ionicity, valency of these cations plays a vital role in the dispersion-flocculation phenomenon (Rengasamy & Olsson, 1991). Based on the polarizability of a cation derived from the ionic potentials and valency (Sposito, 2008), Rengasamy and Sumner (1998) derived the flocculating powers of Ca, Mg, K, and Na and found that the experimental values were close to the theoretical values. Their dispersive powers are the inverse of flocculation potential (Table 2).

Table 2. Flocculating and Dispersive Powers of Cations in Relation to Na and Their Ionicity Indices

Cation

Flocculating Power

Dispersive Power

Ionicity Index

Na

1.0

45.0

0.89

K

1.8

25.0

0.86

Mg

27.0

1.7

0.73

Ca

45.0

1.0

0.67

The “dispersive charge” of a soil is the summation of the amounts of individual exchangeable cations at a given soil pH multiplied by their respective dispersive power and is defined as:

where the concentration of the exchangeable cations is expressed as mmolc/100 g.

Dispersive charge ( mmolc / 100 g ) = [ ( Ca ) + ( 1.7 Mg ) + ( 25 K ) + ( 45 Na ) ]

Dispersive charge is the cause of dispersive forces in hydration reactions. However, these are reduced by the flocculative forces due to the flocculating powers of the cations in the dispersed suspension. The well-known effect of “electrolyte concentration” on dispersion is due to the “flocculating charge” derived from the concentration and flocculating power of the cations involved (Rengasamy, 2002b):

Flocculating charge ( mmol c / L ) = [ ( 45 Ca ) + ( 27 Mg ) + ( 1.8 K ) + ( Na ) ]

where the concentrations of soluble cations in the dispersed suspension are expressed as mmolc/L.

When “net dispersive charge” (dispersive charge minus flocculating charge) is positive, clay dispersion occurs and it is related to the amount of dispersed clay. “Zero point of dispersion” is obtained when dispersive charge equals the flocculating charge and the dispersed clay totally flocculates.

This hypothesis focuses on dispersive and flocculating charges in salt-affected soils to identify soil structural degradation without confounding soil factors like clay mineralogy, organic matter, iron and aluminum oxides, other cementing agents, EC, and pH. Our unpublished results on 96 soil samples collected from various locations in southern Australia with varying clay mineralogy, clay content, organic C content, and pH and electrolyte concentrations have shown a highly significant correlation between net dispersive charge and the amount of dispersed clay (Figure 3).

Figure 3. Relationship between net dispersive charge and the amount of clay dispersed in southern Australian soils.

Dispersive soils can be identified by dispersion test (e.g., Emerson, 2002; Rengasamy, 2002b). Several versions of this test are found in the literature. Measurements of pH, EC, and the cationic composition in the dispersed suspension are necessary to identify problems other than those associated with soil structure. Different categories of dispersive soils, saline-dispersive soils, and saline soils based on soil pH are given in Figure 4 following the discussion by Rengasamy (2010). “Dispersive soils” are a major constraint for plant growth due to soil structural degradation, while “saline soils” induce osmotic and ion specific effects on plants. “Saline-dispersive soils” may have both soil structural and osmotic effects on plants. The different categories based on pH have additional problems such as ion toxicity and deficiency. Future research is needed to establish threshold values for all of these parameters.

Figure 4. Categories and dynamics of dispersive soils as related to dispersive charge, flocculating charge, and pH.

Salinity Effects on Plants

The osmotic effect of salinity is due to the dissolved salts in soil decreasing the osmotic potential of soil water and thereby reducing the ability of the plant to take up water and shrinking its growth rate. The salt-specific or ion-excess effect of salinity occurs when excessive amounts of salts enter the plant in the transpiration stream, causing injury to the leaves and further reductions in growth (Greenway & Munns, 1980). These effects can operate simultaneously or at different stages of plant growth. Plant species vary widely in their tolerance to salt, which is measured by the comparison of biomass production in a salinized soil with that produced in a non-saline soil. Many reports of salt tolerance of crops, vegetables, and fruit trees are available in the literature, including the details of a survey made by the USDA Salinity Laboratory and a comprehensive list of salt-tolerant plants provided by Maas (1986). The data in many cases are for a single cultivar of the species or a limited number of cultivars at a single site, so they are not necessarily representative of the species (Shabala, 2013). Salt tolerance data are mainly based on the EC of the saturation extracts of soils (ECe), but under field conditions soil water content changes with seasonal climatic conditions and the salt concentration experienced by plants can vary widely (Rengasamy, 2010). Further, in addition to salt concentration, the presence of toxic ions such as borate, pH-related nutrient constraints, chemical changes related to waterlogging, and other factors such as soil texture can interfere with salt tolerance of plants. However, these data can be useful in showing variations among plant species in response to soil salinity. Soil salinity ratings for EC1:5 measured in 1:5 soil:water extracts of soils with varying clay content as related to the effects on crops (Hazelton & Murphy, 2007) are given in Table 3 (http://www.ussl.ars.usda.gov/saltoler.htm).

Table 3. Soil Salinity Ratings for EC1:5 of Soils with Different Clay% as Related to the Effects on Crops

Salinity rating

EC1:5 (dS/m) for a range of soil clay%

Plant’s tolerance

10–20%

20–40%

40–60%

60–80%

Very low

<0.07

<0.09

<0.12

<0.15

No effect on crops

Low

0.07–0.15

0.09–0.1

0.12–0.24

0.15–0.30

Sensitive crops affected

Moderate

0.15–0.34

0.19–0.45

0.24–0.56

0.30–0.70

Moderately tolerant crops affected

High

0.34–0.63

0.45–0.76

0.56–0.96

0.70–1.18

Tolerant crops affected

Very high

0.63–0.93

0.76–1.21

0.96–1.53

1.18–1.87

High tolerant crops affected

Extreme

>0.93

>1.21

>1.53

>1.87

Too saline for crop growth

Source: Hazelton and Murphy (2007).

Many authors have also given comprehensive reviews with details on the rationale for breeding salt tolerance, issues related to selection, stress physiology, salt-tolerance mechanisms, genetic variability, breeding methods, and novel concepts such as tissue culture, molecular biology, and modeling (e.g., Munns & Gilliham, 2015; Munns & Tester, 2008; Pessarakli, 1999; Shannon, 1997).

Soil Processes Affecting Crop Production in Salt-Affected Soils

The interactions between root-zone environments in the field and plant responses to salt concentrations are complicated by many soil processes such as soil water dynamics, soil structural changes, solubility of minerals affected by pH and pE (redox potential), and mobility of nutrients and water within a soil profile. These processes may be different in each category of saline, saline-dispersive, and dispersive soils (see Figure 4).

Dispersive Soils

Irrespective of the dominant exchangeable cations or the combination of them, dispersive soils have degraded soil structure. Generally, these soils are low in electrolyte concentration and, hence, osmotic effect of salinity is low. However, depending on the concentration of nutrient ions, nutrient deficiency can occur. Degraded soil structure affects water retention and movement, soil aeration, temperature, and mechanical strength—all of which impair root growth, seedling emergence, and water and nutrient uptake (Bernstein, 1975; Qadir, Oster, Schubert, Noble, & Sahrawat, 2007; So & Aylmore, 1993). Top soil layers with dispersive character are affected by waterlogging, even during moderate rainfall events. When soil dries, it sets hard with high bulk density with crust formation impeding germination and seedling emergence. Dispersive subsoil layers experience compaction and reduction in porosity, which hinders water storage, movement of oxygen and water, the ability of roots to penetrate the soil, and the uptake of water and nutrients. The utilization of “green water” (the hidden water stored in soils) in dryland cropping is severely affected, resulting in limited yield.

The schematic diagram in Figure 5, using the data of Rengasamy, Olsson, and Kirby (1992), shows that the “non-limiting water range” (Letey, 1985) in a dispersive soil is very low compared to a non-dispersive normal soil. Aeration porosity of >15% and mechanical strength of <1 MPa are considered to be conducive to good crop performance, and both are related to soil water content. In this diagram, point B is the water content of a dispersive soil where aeration porosity starts to increase above 15%, and point B where the mechanical strength starts to decrease below 1 MPa. The water contents between B and A are the non-limiting water range of the dispersive soil, which is very narrow compared to the wide range between points D and C of the non-dispersive soil.

Figure 5. Nonlimiting water range in a dispersive subsoil compared with a nondispersive soil in relation to mechanical strength and aeration porosity. The points A and B denote the mechanical resistance and aeration porosity for dispersive subsoil, and C and D are for nondispersive subsoil, respectively.

Even though the general focus on these soils is related to the crop productivity in agricultural soils, dispersive soils can also lead to different types of soil erosion, such as gully erosion and tunnel erosion, affect the geotechnical behavior of foundation soils leading to the instability of buildings, and induce landslides (Torrance, 1999).

Saline-Dispersive and Saline Soils

Dispersive soils can accumulate salts. The salt concentration in soil layers can change with seasonal rainfall and evaporation, and hence termed as “transient salinity.” If the flocculating charge of the salt is lower than the dispersive charge, the soil can be still dispersive, albeit less so. These saline-dispersive soils can have both soil structural degradation and the osmotic effects of salinity. When the flocculating charge is higher than the dispersive charge in saline soils, clays do not disperse, but the salt concentration affects crop growth.

Soil salinity measured in soil-water suspensions in the laboratory is always lower than in the field, where soil water content is generally lower than the water contents used in laboratory tests. In the field, soil water is near field capacity after rain or irrigation events. As the soil dries due to evapotranspiration, salt concentration increases, as does the osmotic pressure of soil water. Concomitant changes in matric and osmotic potentials determine plant water uptake in the field. The influence of soil texture and type of clay on plant-available water compounds the effect of matric plus osmotic potentials (Rengasamy, 2006, 2010).

Nutrient Deficiency and Ion Toxicity

Nutrient deficiency and ion toxicity may occur both in saline and dispersive soils. The mechanisms for these constraints in dispersive soils are different from those in saline soils. Fertility of dispersive soils with low nutrient reserves is compounded by the low supply of water and oxygen to roots in dispersive soil layers. In dispersive soils, when waterlogged or when aeration porosity is reduced, ion toxicity and deficiency occur due to changes in electron and proton activities (pE and pH) in an environment of degraded soil structure (Setter et al., 2009). The interaction between waterlogging and salinity increases the abiotic stress experienced by plants (Barrett-Lennard, 2003). High dispersivity, high pH, and low biological activity encountered in alkaline dispersive soils are not conducive for both the accumulation of organic matter and its mineralization (Tavakkoli et al., 2015). Poor leaching conditions lead to the accumulation of toxic elements such as boron (Barrett-Lennard et al., 2015). Deficiency or toxicity of other ions such as calcium and zinc will be controlled by soil pH (Figure 6).

Figure 6. Abiotic stress and plant growth in alkaline soils in relation to soil pH.

Managing Salt-Affected Soils

The management practices to improve dispersive soils should aim to rectify the soil physical problems associated with swelling and dispersion of soil particles. Thus application of gypsum is widely practiced in sodic soil reclamation. Gypsum, being a calcium source, can reduce the levels of not only exchangeable sodium but also other cations, potassium, and magnesium. Therefore, calcium provided by any amendment can improve dispersive soils. Organic amendments have been used to increase biological activity and soil fertility in addition to improve soil structure. Synthetic organic polymers such as polyvinyl alcohol (PVA) and polyacrylamide (PAM) have been also used to improve soil structure.

Leaching of salts to the level tolerated by crop species is generally practiced in saline soils. Once the salts are leached, the soils may become dispersive or saline-dispersive soils, which may need the reclamation of soil dispersivity. Use of salt-tolerant crops, specific to each site, can increase the productivity in saline soils. Table 4 summarizes the principles involved in the management of salt-affected soil.

Table 4. Principles Involved in the Management of Salt-Affected Soils

Soil Category

Problems Encountered

Principles of Management

Dispersive soils

1. Acidic (pH < 6)

Swelling and clay dispersion affecting soil physical conditions (including surface hard-setting and waterlogging).

Reduction of dispersive charge and increasing flocculation by cations.

Acidic pH affecting crops via element toxicity and/or deficiency.

Increasing soil pH (e.g., lime addition).

2. Neutral (pH 6–8)

Swelling and clay dispersion affecting soil physical conditions (including surface hard-setting and waterlogging).

Reduction of dispersive charge and increasing flocculation by cations.

3. Alkaline (pH 8–9) and highly alkaline (pH > 9)

Swelling and clay dispersion affecting soil physical conditions (including surface hard-setting and waterlogging).

Reduction of dispersive charge and increasing flocculation by cations.

Alkaline pH affecting crops via element toxicity and/or deficiency.

Reduction of pH (<8.2) (e.g., phytoreduction, gypsum application).

Saline-dispersive soils

4. Acidic (pH < 6)

The problems identified in corresponding dispersive soils will be encountered.

Leaching of salts to reduce osmotic effect (crop-specific).

5. Neutral (pH 6–8)

6. Alkaline (pH 8–9) and highly alkaline (pH > 9)

In addition, osmotic effect of salt concentration can affect crop species.

Once salts are reduced, the soils become dispersive soils and the principles involved in corresponding dispersive soils will apply.

Saline soils

7. Acidic (pH < 6)

The major impediment is the salt concentration affecting via osmotic effect.

Leaching of salt to the level tolerated by crop species

8. Neutral (pH 6–8)

9. Alkaline (pH 8–9) and highly alkaline (pH > 9)

Once the salts are leached, the soils may become dispersive or saline-dispersive soils.

Then, the principles of management will be similar to the corresponding dispersive or saline-dispersive soils.

Many amendments have been used to reclaim salt-affected soils, particularly dispersive soils (Oster & Jayawardane, 1998). Gypsum (CaSO4•2H2O), being cheaper compared to other amendments, is commonly used to reduce exchangeable sodium percentage in order to improve soil structural stability. Gypsum is a sparingly soluble salt, and its efficiency in reclamation depends on the amount of water addition by either rainfall or irrigation. Calcium ions from gypsum can be exchanged by soil clays to replace sodium, potassium, and magnesium, thereby reducing the dispersive charge. Simultaneously, these ions can also increase the flocculating charge of soil water. Application of gypsum on the basis of the flocculating effect is considered economical in agricultural lands (Loveday, 1976).

Improvement of soil organic matter helps in stabilizing soil structure. The reclamation of soil dispersivity by organic matter addition depends on its nature and how organic molecules are bonded to clay so that dispersive charge is reduced. Charged organic molecules, when bonded to clay by ionic bonding, will increase clay dispersion. Hydrophobic organic matter will prevent water interaction and, hence, clay dispersion. However, when organic matter is decomposed, products of microbial decomposition of organic matter can reduce dispersive charge and also improve soil fertility.

Combined application of lime (CaCO3) and gypsum (Valzano, Murphy, & Greene, 2001), practiced in acidic dispersive soils, is useful in increasing soil pH and also reducing soil dispersivity. In alkaline soils, phytoremediation of alkalinity by growing certain plants, such as legumes, which can introduce protons in the rhizosphere, is used to reduce soil pH (Qadir et al., 2007). In calcareous soils with calcium carbonate dominance, the protons released by these plants can dissolve carbonate minerals and increase calcium ion concentration in soil water.

Drainage and Salt Removal

Removal of salts from highly salinized soils requires leaching of salts by applying water to the level of leaching fraction, and this process involves proper drainage. To maintain favorable soil water for optimal plant growth and to maintain salinity at the levels tolerated by plants, drainage development is indispensable, mostly in irrigated lands. Drainage provisions are also needed in dryland regions where rainfall controls the mechanisms of drainage. On-farm water management and collection and disposal of drainage water need proper planning. These details are given by Luthin (1957) and Tanji and Wallender (2012).

Salt-Tolerant Crops and Plant Development

The preferred option to improve agricultural production in salinized soils is through growing salt-tolerant plants. Halophytes are salt-tolerant and glycophytes are salt-sensitive. All of the agricultural crops are glycophytes, but some have varying degrees of salt tolerance. These variations occur between species and within species and have been quantified for a range of crops (e.g., Francois & Mass, 1994). Due to the complexity of the trait, both genetically and physiologically, attempts to improve the salt tolerance of crops have met with limited success (Flowers & Flowers, 2005). Furthermore, the development of salt-tolerant plants by conventional breeding is based on the evaluation of genetic materials in simplified conditions such as solution culture, hydroponics, or sand culture, which do not reflect the complex root-zone environments in the field (Tavakkoli et al., 2010). Roy, Negrao, and Tester (2014) have reviewed the roles of a range of genes involved in salt-tolerance traits. They suggest that modern biotechnology through marker-assisted selection or genetic engineering needs to be used at an increased level to introduce the correct combination of genes into crop cultivars. According to Flowers and Flowers (2005), the use of physiological traits in conventional breeding programs and the domestication of halophytes offer viable alternatives to the use of transgenic technologies.

Rozema and Flowers (2008) and Flowers et al. (2010) advocate the concept of “saline agriculture” with initiatives such as developing saline vegetable crops, and crops for fuel and fiber, using saline water for irrigation, using halophytes to develop salt-resistant varieties as well as green manure crops, and technologies that combine saline agriculture with aquaculture. They conclude that such efforts are essential as climate change and population growth combine to challenge the task of feeding the growing global population.

The Effects of Revegetation and Biology in Salt-Affected Soils

Soil biota regulate a number of biological functions that directly affect above- and below-ground plant growth and indirectly through the effects on physical and chemical properties of surface soils. Biological activities in the soil rhizosphere can have negative effects on soilborne plant diseases and positive effects on plant growth through mineralization reactions. In general, increasing levels of salts adversely affect microbiological processes in soils because of low input of organic substrates, through low plant productivity, directly affecting microbial activity. These include effects on carbon and nitrogen mineralization and soil enzyme activities which are important for the decomposition of organic matter and release of nutrients to plants (Pathak & Rao, 1998). Increases in soil salinity have been shown to decrease soil respiration rates and the soil microbial biomass. While osmotic stress usually limits microbial growth in saline soils, ion toxicities associated with pH inhibit microbial growth in alkaline dispersive soils. The ability to form and maintain nitrogen-fixing nodules in plant roots is severely impaired both in saline and dispersive soils (Rao, Giller, Yeo, & Flowers, 2002). Alkaline pH and high carbonate concentrations can also adversely affect metabolic potential and the catabolic diversity of bacterial communities. However, recent reports suggest that rhizospheric fungi and plant growth-promoting rhizobacteria can increase plant yield under stressed and non-stressed conditions (De-la-Pena & Loyala-Vargas, 2014; Nadeem, Ahmad, Zahir, Javid, & Ashraf, 2014). The report (Auge, Toler, & Saxton, 2014) that arbuscular mycorrhizal fungal colonization of roots can also improve plant salt tolerance provides an option of using rhizospheric organisms to improve productivity in saline soils.

Heterogeneity of Salt-Affected Soils

Under field conditions it is common to observe vertical variations of different categories of salt-affected soils (Figure 4) within a given profile located in a site, in addition to the horizontal variations in a farm. For example, the top soil layer may be an acidic dispersive soil, while subsoil can be alkaline dispersive soil or saline dispersive soil. Where different combinations of top soil and subsoil have different problems, different strategies are required for soil management. In agricultural enterprises, high spatial variability within a paddock necessitates the identification of problems on a paddock-by-paddock basis. Different degrees of hazard posed by different levels of salinity and soil structural degradation, in addition to soil textural variations, have to be considered when devising management strategies.

Salt-affected soils can have multiple constraints to crop production, as noted earlier (Figure 4). There is a gap in our knowledge in identifying the predominant, or a common, factor and the relative importance of those constraints in improving yield when different issues in different soil layers cause constraints to plant growth.

Impact of Soil Salinization on Environmental Quality

Soil salinization, distinct from its effects on agricultural productivity, has a number of impacts on environmental quality such as anthropogenic impacts on soil, water, and air that have a negative effect on human and ecological health. Human activities involved in agriculture, forestry, urbanization, and waste disposal have enhanced the potential of environmental degradation due to soil salinization (Sumner & Naidu, 1998).

Dispersive soils generate colloidal suspensions, which become mobilized in the environment, posing a major threat to water quality, in streams and drainage waters. Human activities, including rapid and extensive clearing of native vegetation in many countries, have resulted in changes in local hydrologic balance. As a result, salt mobilization and the formation of dispersive soils increased runoff and erosion. Gully erosion and tunnel erosion are common in dispersive soils. Groundwater has become more saline due to the leaching of salts to the water table. Disposal of drainage waters from salinized lands led to the concentration of heavy metals such as Se and Cd, threatening the wildlife population (Sumner & Naidu, 1998).

With increasing dispersive charge, the concentration of colloid increase in runoff, and the adsorbed pesticides, heavy metal ions, anthropogenic organic compounds, radionuclides, and nutrient ions such as N and P are transported to water bodies (surface waters and ground waters), contaminating the water therein. Suspended colloid particles and the resultant turbidity affect fish populations by reducing feeding efficiency as well as habitat.

Dispersive soils have low strength and bearing capacity, leading to the settling and cracking of foundations and failures of drainage infrastructure within dams and water impoundments. Landslides can occur in dispersive soils depending on the nature of salts that are present. Salt accumulation can also damage transport-related infrastructures such as roads.

Conclusions

Even though soluble salts are inherent in all soils, many processes contributing to salt accumulation in soil layers have been identified. Groundwater-associated salinity, transient salinity, and irrigation-induced salinity are the major processes causing soil salinization in many parts of the world. The particular processes contributing salt, combined with the influence of other climatic, hydrological, and landscape features and the effects of human activities, farming practices, and plant interactions, determine where salinization occurs. Worldwide, more than 800 million hectares of land are estimated to be salt-affected in every climatic zone in every continent except Antarctica.

In agricultural lands, plant growth and productivity are affected directly by soluble salts (by decreasing the osmotic potential of soil water) and specific ion effects. However, the nature of cations and anions influence soil structural stability, affecting soil physical conditions, and indirectly affect plants. Cations adsorbed by soil particles determine the clay dispersion and flocculation. Anions influence soil pH, which affects nutrient availability to plants and can also influence soil dispersivity. Deviating from the traditional importance given to sodium ions alone in explaining the soil structural problems, a new hypothesis is proposed to explain the roles of common cations in causing soil dispersivity by defining dispersive charge and flocculating charge contributed by these cations. New categories of salt-affected soils have been proposed on the basis of clay dispersion and soil pH.

Successful utilization of salinized lands involves appropriate soil management and increasing agricultural production using salt-tolerant crops. Different techniques, including genetic engineering, are being used in developing plants to tolerate different levels of salinity. Each category of salt-affected soil will require a specific approach, taking into account how root zone processes and plant interactions are affected by soil water composition. These strategies should focus on prevention of salt accumulation and its removal from soil layers, particularly in irrigation regions, as well as selection of crops to adapt to saline environment. Saline agriculture, a combination of several methods, aims at using salinized soils for improved economic returns.

Soil salinization, distinct from its effects on agricultural productivity, has a number of impacts on environmental quality, such as anthropogenic impacts on soil, water, and air that have a negative effect on human and ecological health. Hence, management of soil salinity is critical not only to improve the economy of agricultural industry but also to preserve natural ecosystems and avoid environmental hazards.

Acknowledgment

I wish to acknowledge the financial support by the Grain Research and Development Corporation (GRDC) of Australia (project DA00200).

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