SALINITY IN IRRIGATION Victor M. Ponce Source: ASCE Manual 71, Second Edition, 2012. [210505] 1. MEASUREMENT OF SALINITY The chemical and physical properties of a soil affected by salts reflects the amount and type of salt present. The use of salinity meters in the field is becoming more and more common. Laboratory analysis of aqueous soil extracts remains the most common technique for assessing salinity. The electrical current carrying capacity of a solution is proportional to the ion concentration in the aqueous solution. Electrical conductivity is measured in ohm-1 or mho. Conductivity is the reciprocal of resistance. In SI units, one mho is designated as Siemens (S). Electrical conductivity is measured in S/cm or, alternatively, in milliS/cm (1 milliS/cm = 0.001 S/cm). One milliS/cm is equal to one deciS/m, or, for short, dS/m. Calculation of total dissolved solids TDS (mg/L) based on electrical conductivity EC (dS/m) 2. SALINITY AND SODICITY A soil is considered saline if the electrical conductivity of a saturation extract exceeds the value of 4 dS/m at 25°C and the percentage of the cation exchange capacity of the soil occupied by sodium is less than 15. The value of 4 dS/m corresponds to approximately 40 meq/L of salt. Sodicity is estimated from the exchangeable sodium percentage (ESP) and the sodium absorption ratio (SAR). ESP is the percentage of the cation exchange complex occupied by sodium ions. Soils with ESP greater than 6 are sodic. Soils with ESP greater than 15 are strongly sodic. The SAR is the ratio of the Na concentration to the square root of the average Ca and Mg concentrations. SAR values greater than 15 indicate sodicity. 3. INSTRUMENTS TO MEASURE SALINITY The four-electrode salinity probe is a probe of small diameter with four electrodes spaced a few centimeters apart along its length of about 150 cm (Fig. 1). Fig. 1   La sonda de salinidad de cuatro electrodos. Two of the electrodes create an electric field. The other two measure the electrical resistance of the soil. To use it, the probe is inserted into the soil to the desired depth. The volume sampled is about 90 cm3. 4. WEATHERING Soils in arid and semiarid regions are relatively unweathered. Unweathered minerals provide plant nutrients, but are also a source of soil salinity. Increases in salt content of 200 to 300 mg/L are common when arid-land soil solutions remain in contact with relatively unweathered soil minerals for substantial periods of time. The amount of salt dissolved under such conditions depends on the quantity of carbon dioxide. The partial pressure of carbon dioxide can reach 100% or more when oxygen is consumed and carbon dioxide released during soil respiration. Dissolution of primary minerals is most important when the irrigation water's salt content is low, less than 100 to 200 mg/L, or when the leaching fraction is at least 0.25. Irrigation with water from California's Feather river, which has a salt content of 60 mg/L, results in more salt in the drain water due to weathering than due to the salt content of the irrigation water. For salt-affected soils, mineral weathering is seldom a significant part of salt balance computations. When irrigation waters have a moderate amount of salt, such as the 800 mg/L that occurs in the Colorado river lower reaches, and leaching fractions are below 0.25, salts precipitated in the soil profile exceed the amount weathered. At low leaching fractions, LF = 0.1, 20% or more of the salt in irrigation water precipitates and is not contained in the drainage water. Salt removal by crops (crops taking up salt) is too small to affect the salt balance. The deeper the soil, the greater the capacity to store salt with minimal yield reduction. 5. LEACHING REQUIREMENTS The amount of leaching needed to maintain a viable irrigated agriculture depends on the salt content of the irrigation water, soil, and groundwater; the salt tolerance of the crop; the climate, and soil and water management. The only economical way to control soil salinity is leaching. Definitions of leaching fraction: Ratio of depth of drainage to depth of applied water (irrigation plus rainfall). Ratio of salt content of applied water to salt content of drain water. Ratio of electrical conductivity of applied water to electrical conductivity of drain water. 6. SALT TOLERANCE The salt tolerance of a crop can be described by plotting the relative yield as a continuous function of soil salinity. For each crop, there is a maximum soil salinity, the threshold dS/m, that does not reduce yield (for example, a value of ECss = 3.6 for corn, shown in Fig. 3). Fig. 2   Variation of relative yield of corn with soil salinity. The threshold dS/m varies from 1 to 8 dS/m. Sodium is not considered an essential element for most plants, but it beneficially affects the growth of some plants at concentrations below threshold dS/m. Injury by sodium to avocado in avocado and citrus is widespread; it causes leaf burn (Fig. 3). Chloride is an essential micronutrient for plants, but, unlike most macronutrients, it is relatively nontoxic. Chloride contributes to osmotic stress. Fig. 3   Leaf burned due to excess of sodium. 7. LEACHING FOR SALINITY CONTROL The volume of water needed for a given degree of leaching may be greater for surface irrigation that for sprinkler irrigation. The minimum depth of water that can be applied uniformly by surface methods is several times greater that the minimum for sprinkler irrigation. Salts should be carefully monitored by direct soil measurements or, frequent, careful observations of crop conditions. Tolerance to salinity depends on each plant (Fig. 4). Fig. 4   Plant tolerance to salinity. For example, given a value of ECss = 10 dS/m: A sensitive crop would have Yr = 0%; A moderately sensitive crop would have Yr = 47%; A moderately tolerant crop would have Yr = 78%; and A tolerant crop would have Yr = 100%.

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