Author: Mr. Guy Sela, CEO of SMART! Fertilizer Management software and an international expert in plant nutrition and irrigation.
Several management practices affect the use of marginal-quality irrigation water.
SAR (Sodium Adsorption Ratio) is an irrigation water parameter, which is used to predict problems of water infiltration in soil.
SAR is determined as:
Apart from water shortage, which is a result of of water infiltration problems, some other related problems might occur as well. For example weed growth, diseases, poor aeration, poor germination of seeds, root rot etc.
Various measures can be taken to overcome water infiltration problems, which are water-quality related. Such measures include soil or water amendments, reducing the SAR of the water supply, cultivation and tillage, addition of organic residues and irrigation management.
The purpose of some soil amendments is to counter the effect of sodium, by increasing the soluble calcium content or by increasing the salinity of the irrigation water.
Gypsum and other calcium supplying materials - Gypsum is the most commonly used soil amendment. Since water infiltration problems, caused by sodium, affect mainly the upper few centimeters of soil, repeated small applications of gypsum, incorporated at lower rates into a shallow depth, are preferred over a single large application.
If the salinity of the irrigation water is low (EC<0.5 ds/m), gypsum can be added to the irrigation water at rates of 1-4 meq/l of dissolved calcium.
Other amendments - when lime (CaCO3) is present in soil, some acids or acid-forming amendments can be used. These amendments cause calcium to be released to soil solution. Examples for such amendments are elemental sulfur, sulfuric acid and ferric sulfate.
Organic residues - these amendments improve soil structure and water infiltration, by keeping the soil porous.
Water infiltration can be improved either by increasing the irrigation water salinity or reducing the SAR.
By diluting the irrigation water source with water of lower sodium concentration, the SAR of the irrigation water is reduced, even if calcium and magnesium concentrations are higher.