Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 283

Sodicity : The presence of a high proportion of sodium ions relative to other
cations bound to clay particles.
Calcareous : A soil containing significant amounts of naturally occurring
calcium carbonate, which fizzes when dilute acid is added.
Alkalinity : Soils having alkaline reaction, pH >7.0. Problems usually appear at
pH >7.8 as nutrient deficiencies.
Gypsum : Calcium sulfate (CaSO 4 . 2H 2 O) used to supply calcium and sulfur to
improve sodic and saline-sodic soils.
Leaching : The flushing of salts from soil by the downward movement of water.
Leaching fraction : The proportion of the water entering the soil that soil
conditions will allow to pass below the root-zone.

8.3.2.1 Electrical Conductivity (EC)

Electrical conductivity measures the ability of a water sample to conduct electric-
ity. This relates to the amount of total soluble salts (TSS) in the water sample.
Pure water has very low conductivity. As TSS increases, water becomes more con-
ductive. Although different dissolved substances affect conductivity differently, the
average TSS

EC. The SI unit for EC is deci-Siemens per meter (dS/m).
However, milli-mhos per centimeter (mmhos/cm) and micro-mhos per centimeter
(

= 0.66 ×

mhos/cm) are frequently used.
EC is a measure of salinity resulting from all the ions dissolved in a water sam-
ple or saturated paste. This includes negatively charged ions (e.g., Cl - ,NO 3 - ) and
positively charged ions (e.g., Ca 2+ ,Na + ).

μ

8.3.2.2 Total Soluble Salts (TSS) or Total Dissolved Salts (TDS)

TSS refers to the total amount of salt dissolved in the soil extract expressed in
parts per million (ppm). The salts include substances that form common table salt
(sodium and chloride) as well as calcium, magnesium, potassium, nitrate, sulfate,
and carbonates.
Cation Exchange Capacity (CEC) : The amount of exchangeable cations that a
soil can adsorb at a specific pH, expressed as milli-equivalents per 100 g of soil.

8.3.2.3 Exchangeable Sodium Percentage (ESP)

ESP is a measure of soil sodicity or sodium hazard. ESP is the sodium fraction
adsorbed on soil particles expressed as a percentage of cation exchange capacity. It is
calculated by dividing the exchangeable sodium over the Cation Exchange Capacity
(or CEC) and multiplying the product by 100, i.e.,

ESP = (exchangeable sodium/cation exchange capacity) × 100