Principle of Soil Testing for Lime Requirement
(SMP Buffer Method)
To arrive at satisfactory solution to the problems of how much lime to apply , the lime requirement of the proposed crop as well as the actual PH of untreated soil must be considered. Soil acidity and lime have a good correlation in humid region. So the soil PH can be used as an index of the different lime need of various crops. However, the soluble and exchangeable hydrogen must be partly neutralized. Buffer solutions are used in soil test to estimate the lime needed to neutralize the potions of the exchangeable hydrogen ions.
The lime requirement of soil (the amount of lime required to bring about a desired a change in soil PH) is determined by measuring the total soluble and exchangeable hydrogen and aluminum. This may be accomplished by lime requirement method developed by Schoemake MacLean and Pratt (SMP buffer method)
Composition of Buffer:
1.8 g nitrophenol + 2.5 ml triethanol amine 3g CrO4 2g calcium acetate + 53.1 g calcium chloride. Volume made to 1 L and PH adjusted to 7.0.
To determine a lime requirement of a soil the PH of the soil is determined. If the soil is acid and show a Need of lime. Lime requirement is subsequently determined on the same sample by adding specific quantity of SMP buffer to the soil water mixture, determining the PH on PH meter after a specific time period and finding the lime requirement from the table value. Lime stone recommendation recorded in table are based on 8 inch plough depth (20cm).
Soil Buffer PH
Desired PH t/ha