Exchangeable Sodium Percentage (ESP) ​

(see also SAR, CEC and dispersion)

When a high proportion of the exchange sites on clays or colloid particles is occupied by sodium ions, the soil is said to have dispersive qualities. Sodium can be released to the soil water, and lost through leaching. The ESP of a soil solution is approximately equal to the SAR, and is the amount of exchangeable sodium divided by the cation exchange capacity:

where Na is sodium, Ca is calcium, Mg is magnesium, K is potassium, H is hydrogen and Al is aluminium, and all constituent concentrations are expressed as milliequivalents per lite. The H and Al concentrations can be, and usually are, neglected when calculating ESP.

Generally, the higher the ESP and SAR, the greater is the tendency for soils to disperse and the greater is the concentration of salts required to prevent dispersion. Clays with ESP in the range 6% to 14% are generally moderately dispersive, and have been associated with tunnel erosion. Soils with ESP's of 6% or more should be regarded as potential problem soils.