SOIL AND PLANT ANALYSIS (1+1)

Parameters used to judge the quality of irrigation waters

a. Electrical conductivity (EC)
It is a measure of total soluble salts in water. Since there exist proportionality between concentration of salts in water and electrical conductance the EC is commonly employed as one of the important parameter to judge water quality with respect to salinity. There are many classification available based on EC namely USDA, USSR and Tamil Nadu STL etc. But most widely used one is USDA classification.

b. Potential salinity (PS)
This considers the concentrations of chloride and sulphate in irrigation water. The potential salinity of irrigation water can be derived using the formula


PS = ½ SO^2-₄+ Cl^-


c. Puri’s salt index (PSI)
Puri’s salt index accounts relationship between concentrations of Na and Ca ions in a given irrigation water. PSI can be arrived by applying the concentration values of water in the following formula

PSI = (Total Na⁺ - 24.5) - (Total Ca²⁺ - Ca as CaCO₃) x 4.85


d. Sodium Adsorption Ration (SAR)
This explains the relationship between concentrations of sodium and calcium & magnesium of irrigation water. SAR of given water can be worked out by using the formula

Sodium Adsorption Ratio (SAR) =Na / √Ca + Mg
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e. Soluble sodium percentage (SSP)
The proportion of sodium with that of total cations in a given irrigation water is considered to judge the quality under soluble sodium percentage. in relation to total cations in water


Na
SSP = ---------------------------------------------------x 100
Total Cations (Ca + Mg + K + Na)


f. Residual sodium carbonate (RSC)
In this parameter the proportion of carbonates & bicarbonates in relation calcium & magnesium in irrigation water is considered. Applying concentration values of carbonate, bicarbonate, calcium and magnesium in the following formula the RSC of water is arrived


RSC = (CO_3^2- + HCO₃-) - (Ca + Mg)


g. Permeability Index (PI)
This parameter explains ratio between Na & HCO3 and cations like calcium, magnesium and sodium in irrigation water.
Permeability Index (PI) = Na +HCO₃ x 100 / Ca + Mg + Na

h. Boron concentration
The specific ion effect is taken here to judge the quality of irrigation water. Any ion for that matter exceeds certain limit considered poor quality and may not be suitable for irrigation. Boron being highly soluble in water the concentration of boron is often found to be high in many locations. Hence B concentration in water is commonly considered as one of the parameter to judge the quality of irrigation water.

A part from boron, excess concentrations of ions like carbonate, bicarbonate, chloride, sulphate, nitrate, fluorine and heavy metals including nickel, lead, cadmium etc., are also injurious to growing plants in specific locations. Boron is excessively available in some locations which makes the water quality poor. Further, the critical limits are varying with ions, soil type and crops.