A soil is salineif the electrical conductivity of its soil solution as obtained by extraction from a water-saturated soil paste (ECe) exceeds 4 dS m-1. (The measurement of electrical conductivity for a soil saturation extract is discussed in Methods of Soil Analysis,listed under For Further Reading at the end of this chapter.) According to this definition, about a quarter of the agricultural soils worldwide are saline, but values of ECe > 1 dS m-1 are encountered typically in arid-zone soils, which cover almost one-third of the global ice-free land area. Ions released into the soil solution by mineral weathering, or introduced there by the intrusion of saline surface water or groundwater, tend to accumulate in the secondary minerals formed as the soils dry. These secondary minerals typically include clay minerals (Section 2.3), carbonates and sulfates (Section 2.5), and chlorides. Because Na, K, Ca, and Mg are brought into the soil solution relatively easily—either as displaced exchangeable cations or as cations dissolved from carbonates, sulfates, and chlorides—it is this set of four metals that contributes most to soil salinity. The corresponding set of anions that contributes to salinity is CO3, SO4, and Cl. Thus, arid-zone soil solutions are essentially electrolyte solutions containing chloride, sulfate, and carbonate salts of four metal cations. According to Eq. 4.21, an electrical conductivity of 4 dS m-1 corresponds to an ionic strength of 58 mM (log I = -1.841 + 1.009 log4 = 0.0584). This level of salinity is less than 10% of that of seawater (EC = 46.21 dS m-1), but high enough that only crops that are relatively salt tolerant can withstand it. Moderately salt-sensitive crops are affected when the electrical conductivity of a soil saturation extract approaches 2 dS m-1, corresponding to an ionic strength of 29 mM, and salt-sensitive crops are affected at 1 dS m-1 (I = 14 mM). Thus, with respect to crop salinity tolerance, a soil can be judged saline at any saturation extract ionic strength greater than 15 mM if crops are stressed.
Keywords: Quirk–Schofield diagram, Suarez sodium adsorption ratio, Vanselow model, alkalinity, borate minerals, calcareous soil, exchangeable sodium percentage, irrigation water quality, leaching fraction, saline soil
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