Determination of Lime Potential

Soil pH measurement is usually affected by a number of factors viz. salt concentration, soil-water ratios, suspension effect etc. The use of O.Ol(M) CaClg solution yields stable readings in pH-measurements. The hydrogen ions in the soil system are distributed between solid and liquid phases as follows. The dynamic equilibrium may be represented as follows. 

The ions in the soil solution constitutes the active acidity and are measured directly as soil pH values. On the other hand the adsorbed H+ ions held on exchange sites are not subject to pH measurements are termed as reserve acidity , both the forms contribute to soil acidity. Thus soil pH does not reflect the total acidity. However a suitable index which takes into account the reserve acidity of soil is the lime potential, which is calculated as follows  

The lime potential is a very reliable estimate to predict the buffering capacity of soils. Smaller the value of lime potential, greater will be the buffering capacity of the soil. 

Schofield and Taylor (1955) suggested the use of ion acitivity ratios for determination of soil acidity. Consider a cation C with valency v. In dilute solution in equilibrium with a soil the acitivity of hydrogen ions divided by the activity of C ions will be constant. The activity function depends on the valency of ions concerned. 

If the soil exchange complex is saturated with both H+ and Ca + ions at equilibrium Schoefield s ratio law states  

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