Alternative use of the SMP‐buffer solution to determine lime requirement of soils

A close relationship was found between the pH of soil suspensions in the SMP buffer solution (pHsmp) and the potential acidity of soils (H + Al) extracted by a neutral calcium acetate solution (r = 0.98), for twenty six soil samples of the State of Sao Paulo, Brazil, This relationship was represented by the equation lnY = 7.76 ‐ 1.053X, which allowed for the calculation of H + Al directly from the values of pHsmp. With the values of H + Al and the sum of bases, calcium, magnesium and potassium, the cation exchange capacity (CEC), and the base saturation (V) were calculated. Relationships between the base saturation of the soils and the active acidity of soil suspensions were close, both for pH determined in water (r=0.94) and pH determined in 0.01M CaCl2 solution (r ‐ 0.97). Thus the lime requirement (LR) of soils could be calculated, for given values of pH or base saturation, using the equation LR = CEC (V2 ‐ V1)/100, in which V1 is the base saturation of the soil and V2 is the expected value upon liming. The predicted values for lime required to increase the soil pH in water to either 5.5 or 6.0 were comparable to those obtained by the direct use of the SMP buffer method, and were, respectively, two and four times higher than the amounts required to neutralize exchangeable aluminum, considering the criterion LR = Al × 1.5. The proposed method to determine lime requirement of soils is described in detail and the advantages of its use are discussed.