Fate of Potassium Applied To Pasture On A Soil Derived From Andesitic Ash

Abstract

Summary A pot experiment was used to study the problem of short‐lived responses to potassium on yellow‐brown loam soils of North Taranaki, derived from fine‐textured andesitic volcanic ash. Undisturbed cores of soil carrying original pasture were treated with sheep or cow urine at rates corresponding to one or two urinations, the potassium added in the urine being approximately equivalent to 8 and 16 cwt/ac of KCl. KCl was applied at 2, 4, and 8 cwt/ac. Results, over a period of six months from spring to autumn, were as follows. The K content of both grass and clover was increased by the treatments, approximately in proportion to the amount added, with both KCl and urine. A large growth response to the urine applications was obtained, attributable mainly to nitrogen. A small growth response to KCl was obtained; this was the same at all rates of KCl and was entirely due to increased growth of clovers. Topdressing with KCl greatly increased the percentage of clovers in the pasture. Because of increased growth and K content, a large proportion of the added K was taken up by the pasture in all treatments. Losses of K by leaching were comparatively small, and practically all occurred within three months. Appreciable amounts of Ca and Mg were found in the leachates from all treatments. The amounts of Mg lost indicated that continued topdressing with potassium could possibly result in a deficiency of magnesium in some soils. The level of exchangeable K in the soil at the beginning of the experiment was slightly above the accepted response level of 0.55 me.%. After six months the control and the 2 and 4 cwt/ac KCl treatments showed deficient levels of exch. K, the single urination treatment was on the borderline, and only the 8 cwt/ac KCl and double urination treatments were still above the response level. At the normal rate of topdressing of 2 cwt/ac of KCl, uptake of available soil and fertiliser K by the pasture was so rapid that a deficiency level of K in the pasture was reached within three months. The effects of a single urination on soil and plant K had completely disappeared at the end of six months. It is concluded that the short effective period of water‐soluble potassium fertilisers on soils of the yellow‐brown loam group under pasture is due principally to luxury uptake of applied potassium by the pasture, and its concentration by the grazing animal within the small areas covered by urine patches.