Effects of withholding phosphate and potash fertilizer on grain yield of cereals and on plant-available phosphorus and potassium in calcareous soils

Abstract

To determine to what extent extractable phosphorus (P) and potassium (K) in calcareous soils can be depleted without yield loss in cereal crops, all combinations of 0 and 22 kg ha−1 of fertilizer P and 0 and 42 kg ha−1 of fertilizer K were applied each year at two sites in south-west England. At a site with a shallow chalk soil in a grass-arable rotation and at a site with a shallow limestone soil in continuous cereals, withholding P fertilizer for more than 3 years was judged to be detrimental to grain yield. A yield benefit to applied K was obtained in the ninth and subsequent years at the chalk site but withholding K fertilizer did not reduce yield at the limestone site over an 8-year period.On non-fertilized plots, extractable P and K concentrations in the soil declined by approximately 50 per cent at both sites but the rate of decline was faster at the limestone site, where the amounts of P and K in the soil were initially greater. Extractable P and K in the soil were also not maintained on fertilized plots even though the amount of fertilizer P (both sites) and fertilizer K (limestone site) applied was equal to or greater than the calculated offtake of P and K in the crop. Extractable K in the soil stabilized towards the end of the run-down period, indicating a release of non-exchangeable K. The results highlight the shortcomings of P and K fertilizer policies which rely solely on soil analysis or which simply match crop offtake.