• A soil is considered saline if the electrical conductivity of a saturation extract exceeds 4 dS/m at 25^oC 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 about 40 meq/L of salt.

• Sodicity is estimated with the exchangeable sodium percentage ESP and the sodium absortion ratio SAR.

• ESP is the percentage of the cation exchange complex occupied by sodium ions.

• If ESP exceeds 15, the soil is considered sodic.

• The SAR is the ratio of Na concentration to the square root of the sum of the concentrations of Ca and Mg.

• Values of SAR greater than 15 indicate sodicity.

• Soils with ESP greater than 6 are sodic.

• Soils with ESP greater than 15 are strongly sodic.

• 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.

• 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.

• 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, L = 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.

• 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.

• The leaching fraction is the ratio of depth of drainage to depth of applied water (irrigation plus rainfall).

• The leaching fraction is the ratio of salt content of applied water to salt content of drain water.

• The leaching fraction is the ratio of electrical conductivity of applied water to electrical conductivity of drain water.

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.

• 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.

• Chloride is an essential micronutrient for plants, but, unlike most macronutrients, it is relatively nontoxic.

• Chloride contributes to osmotic stress.

• 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.