Comparison of potassium chloride, bicarbonate, and metaphosphate, and calcined orthoclase, as sources of potassium for white clover

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Abstract
KCl, KHCO3, KPO3, and a calcined potash feldspar, were compared as sources of K in a pot experiment with a K-deficient yellowbrown loam, a soil from andesitic volcanic ash (the New Plymouth black loam), using white clover as the test crop. KHCO3 and KPO3 gave responses similar, in terms of dry matter yield and K content of the clover, to those given by KCl, and uptake of K was just as rapid. Calcined feldspar gave only a very slight response. Losses of cations by leaching from the mineral soil were small for all treatments, particularly for K, and there was some indication that lower losses occurred from the KHCO3, and especially the KPO3, treatments, as compared with the KCl. Exchangeable K values reached very low levels on all treatments during the period of the experiment, and this was reflected in low levels of K in the plant tops. This lowered K content in the plant was to some extent counterbalanced by an increase ill the content of Ca and Mg. In a subsidiary pot experiment calcined feldspar was compared with KCl on a K-deficient acid peat soil; although the response to the feldspar was relatively greater than on the mineral soil, it was still very much less effective than KCl.