FIELD CALIBRATION OF LIME REQUIREMENT SOIL TESTS FOR ACID NITISOLS AND FERRALSOLS OF KENYA FOR THE DEVELOPMENT OF A QUICK LIME ESTIMATION TEST KIT

Abstract

The beneficial effects of liming acid soils (pH< 5.5) have been proven beyond doubt. However the problem of how much lime to apply has always faced farmers. With the main objective to assess the lime estimation methods currently in use in the world and select one that would be suitable in Kenya for development of a quick lime estimation kit lime rates of 0, 0.5, 1.0,2.4,  6, and 8 t ha^-1 were incorporated to 0-15cm soil depths at various field experimental 8 sites in the tea growing acid Nitisols of Central Kenya and in the acid Ferralsols of Western Kenya using maize as a test crop. Soil samples were collected for each liming level after 3 months for laboratory lime estimations using Ca(OH)₂ titration, dolomitic lime - soil incubation for 7 days, and with a range of buffer methods that included SMP single and double-SMP buffer methods, Yuan double-buffer method, Mehlich single buffer method, modified Mehlich single buffer method, and Adams and Evans buffer method. The optimal field lime requirement for maize was 1 t ha^-1 and  7 t ha-1(pH 6.2 and pH 5.5) in Ferralsols and Nitisols respectively, indicating the great need for accurate lime requirement prediction. The Ca(OH)₂ titration gave a  high correlation (r²= 0.87) with field lime requirement. Soil-lime incubation gave a very low correlation (r² = 0.007) with field lime requirement. Among the buffer methods, the modified Mehlich buffer method gave high and consistent correlations for parameters against the field lime requirements to pH 5.5. The SMP double buffer, the Yuan double buffers and the modified Mehlich buffer methods gave the least % deviations from the field lime requirements. However because of the ease of preparation and the absence of harzadous chemicals in the modified Mehlic buffer, the latter is recommended in the preparation of the quick lime estimation test kit. Soil pH gave a high correlation (r² = 0.98 (Nitisols) and 0.7 (Ferralsols) with field lime requirement and is recommended for use in lime estimations especially in Nitisols.

Key words: Buffer pH, Ca(OH)₂ titration_, field trials, lime-soil incubation, lime requirement