Sodic below 4 above 13 above 15 dispersed
Saline-sodic above 4 above 13 above 15 flocculated
Table 2.1 The physicochemical properties of salt-affected soil
2.4 Salt Ions and Compounds
Soluble salt ions most common in salt-affected soils are Ca+2, Mg+2, Na+, K+, Cl-, SO4-2, HCO3-,NO3-, and CO3-2 (this last one at pH 9.0). It is the magnitude and balance of these ions (especially imbalances and dominance on the CEC sites or in the soil solution) that are the basis for the various salinity stresses in a particular landscape. These ions arise from the following:
• Dissolution of minerals in the weathering process
• Salt additions by saline irrigation water
• Salts in standard application of fertilizers and soil amendments
• Salts transported into the root zone by a rising water table (e.g., saltwater intrusion)
• Capillary rise from deeper in the soil
• Seepage zones where saline water moves to another site
• Subsoil migration to lower-topography areas due to gravity
• Flooding, such as in coastal areas
• Saltwater spray
• Use of primary or secondary salinized dredged soils on a site (Robert et al.2012).
Inorganic salts can be present in the soil not just as soluble ions but also as compounds that vary in solubility from relatively insoluble (lime) to moderately soluble mineral forms (gypsum dihydrate). Relatively soluble minerals that can dissolve to release soluble salt ions include (a) various sulfate compounds such as Na2SO4, K2SO4, CaSO4?2H2O (gypsum dihydrate), and MgSO4; (b) chloride compounds, such as KCl, NaCl, and CaCl2; and (c) carbonate or bicarbonate compounds with high solubility such as Na2CO3 and NaHCO3. Examples of insoluble salts would be CaCO3, MgCO3, dolomite (CaCO3?MgCO3), anhydrite of gypsum (CaSO4-i.e., without the hydrated water), and soil minerals such as apatites, while dihydrate gypsum (CaSO4?2H20) is a moderately soluble salt. While the insoluble and moderately soluble mineral forms can influence soils and plants over time, it is the soluble salts that have the most direct and rapid impact on soils and plants, due to their mobility and innate ability to accumulate in soils and plants. When comparing various salt ions versus salt compounds in the soil, individual salt ions can react with other ions to produce salt compounds varying in solubility. (Robert et al., 2012)
The degree of mobility of individual salt ions in response to water movement in the soil is based on whether they reside in the soil solution, as a component of a soluble compound, or on the soil CEC sites that are associated with negative charges on clay and organic matter. Highly mobile ions are Cl-1, SO4, and K+1, while Na+1, Ca+2, and Mg+2 are less mobile in the soil. Ion and nutrient concentrations and mobility in plants are also important in salt-challenged sites. For example, an immobile ion when applied to the turf grass foliage will not move to lower tissues, including the root system. Thus, if Ca is required in the root zone of a sodic soil to maintain root viability (to limit Na displacing Ca in root cell walls and causing roots to deteriorate), application to the shoot tissues would not have any effect. Carrow et al. (2001) reported ion mobility within plants as (a) mobile (N, P, K, Mg, and Na), (b) somewhat mobile (S, Zn Cu, Mo, and B), and (c) immobile (Ca, Fe, Mn, and Si).
2.5 Extent of salinity problem
Globally, it is estimated that there are 76 million hectares (Mha) of human-induced salt affected land, representing 5% of the world’s cultivated land, (Ghassemi el aI., 1995). Salt affected lands are those where crop yields are reduced or where less desirable crops must be grown because of the salinity. This human induced salinization is termed secondary salinization, in contrast to regions that were saline in their native condition. This value underestimated the extent of salinity because it does not include large areas where land could be potentially cultivated if not for the native salinity.
Salinity problems are more prevalent in irrigated lands relative to the total cultivated acreages. This is not surprising as irrigated lands are concentrated in more arid regions, where salinity is more prevalent. Also, irrigation results in land application of more water, thus imposing additional drainage needs to the natural hydrologic system. Of the world’s 227 Mha of irrigated lands, it is estimated that 45.4 Mha, ur 20% are adversely impacted by secondary salinization (Ghassemi el al., 1995).
Salinity is a major threat to current irrigation projects and to the remaining near-surface fresh water supplies in arid regions. The extent of the secondary salinization problem has not stabilized; instead, it is estimated that as much as 2 Mha of irrigated land, representing approximately 1% of the total, is lost from production due to salinity each year (Umali, 1993, in Postel, 1997). Most of the world’s salt affected, cultivated lands are in Asia and Africa, where population densities and economic conditions make the problem proportionately more severe, it is estimated that Egypt, Iran and Pakistan had 33, 30 and 26% respectively of their irrigated land impacted by secondary salinizatiun (Ghassemi el al., 1995). However more developed countries are not immune to these salinity problems, it is also estimated that over 20% of irrigated land in the U.S. is salt-affected (Postel, 1999), a value comparable to the global average.
2.6 Chemical Properties
2.6.1 Soil Reaction-pH
The pH value of an aqueous solution is the negative logarithm of the hydrogen-ion activity. The value may be determined potentiometrically, using various electrodes, or calorimetrically, by indicators whose colors vary with the hydrogen-ion activity. Soil characteristics that are known to influence pH readings include: the composition of the exchangeable cations, the nature of the cation-exchange materials, the composition and concentration of soluble salts, and the presence or absence of gypsum and alkaline-earth carbonates.
A statistical study of the relation of pH readings to the exchangeable-sodium-percentages of soils of arid regions have been made by Fireman and Wadleigh (1951).
(1) Experience and the statistical study of Fireman and Wadleigh (1951) permit the following general .statements regarding the interpretation of pH readings of saturated soil paste pH values of 8.5 or greater almost invariably indicate an exchangeable-sodium-percentage of 15 or more and the presence of alkaline-earth carbonates;
(2) The exchangeable-sodium-percentage of soils having pH values of less than 8.5 may or may not exceed 15;
(3) Soils having pH values of less than 7.5 almost always contain no alkaline-earth carbonates and those having values of less than 7.0 contain significant amounts of exchangeable hydrogen.
2.6.2 Soluble Cations and Anions
Analyses of saline and alkali soils for soluble cations and anions are usually made to determine the composition of the salts present. Complete analyses for soluble ions provide an accurate determination of total salt content. Determinations of soluble cations are used to obtain the relations between total cation concentration and other properties of saline solutions, such as electrical conductivity and osmotic pressure. The relative concentrations of the various cations in soil-water extracts also give information on the composition of the exchangeable cations in the soil.
The soluble cations and anions commonly determined in saline and alkali soils are calcium, magnesium, sodium, potassium, carbonate, bicarbonate, sulfate, and chloride. Occasionally nitrate and soluble silicate also are determined. In making complete analyses, a determination of nitrate is indicated if the sum of cations expressed on an equivalent basis significantly exceeds that of the commonly determined anions. Appreciable amounts of soluble silicate occur only in alkali soils having high pH values.
2.6.3 Exchangeable Cations
When a sample of soil is placed in a solution of a salt, such as ammonium acetate, ammonium ions are adsorbed by the soil and an equivalent amount of cations is displaced from the soil into the solution. This reaction is termed “cation exchange,” and the cations displaced from the soil are referred to as “exchangeable.” The surface-active constituents of soils that have cation-exchange properties are collectively termed the “exchange complex” and consist for the most part of various clay minerals and organic matter. The total amount of exchangeable cations that a soil can retain is designated the “cation-exchange-capacity,” and is usually expressed in meq/100 g.soil. It is often convenient to express the relative amounts of various exchangeable cations present in a soil as a percentage of the cation-exchange-capacity. For example, the exchangeable-sodium-percentage (ESP) is equal to 100 times the exchangeable-sodium content divided by the cation-exchange-capacity, both expressed in the same units.
Determinations of the amounts and proportions of the various exchangeable cations present in soils are useful, because exchangeable cations markedly influence the physical and chemical properties of soils. (Richards, 1954).
2.7 Salinity and Sodicity Problems in Irrigation
The term salinity refers to the concentration of ions in water (Burger and Celkova, 2003). The salinity level for water to be considered as saline depends on the purpose of the water use. Guidelines have been provided for different water uses, including drinking, agriculture and industry.