The availability of Potassium in some Kenya Soils.

Abstract

The availability of potassium in eleven soil types from various parts of Kenya was tested in a greenhouse experiment. The soils were cropped with sorghum in pots of two different sizes. The concentration and yield of potassium in the above ground 'portions of the sorghum plants were measured after the plants were harvested. The plants were grown for 8 weeks. The intensity of potassium supply (available K) in the soils was measured by the use of four different methods. The K uptake by the sorghum plants was very significantly related (P<O.OO1) with the K-intensity measured according to various methods. Exchangeable K gave the lowest degree of correlation. Free energy of exchange (~G) of K with Ca + Mg in the soils and the equilibrium activity ratios (ARe k) of K and Ca + Mg in the soils, were best correlated with K uptake. At 6G values in excess of -3500 calories, K uptake by the sorghum plants was low. Hi.gh K uptake was observed at ~G values ranging from -2500 to -3000 calories. The method for determining available K currently in use 1n Kenya gave satisfactory results although the correlation obtained with this method was not as high as that obtained with the method in which the free energy of 'exchange of K with Ca + Mg was taken into account. A "correction" of the available K-value determined with the Mehlich method •(as used in Kenya) according to the conversion method used in the Netherlands improved the degree of correlation with K uptake. Of the eleven' soil types, four were found to be low in K and seven were high in K.