RESIDUAL VALUE OF PHOSPHATIC FERTILIZER ON CHERNOZEMIC SOILS

Abstract

The residual effects of single large applications of phosphatic fertilizer, up to 400 kg P/ha, were measured on Chernozemic soils in Manitoba and Saskatchewan. The single large applications of P increased the yield of wheat and increased the amount of available P in the soil. Yields were not increased with additional P applied annually with the seed until the available P (NaHCO 3 soluble fraction) was reduced to approximately 10 ppm in the upper 15 cm of soil. Soil from the field test locations was used in the greenhouse to determine the persistence of the residual effects of P. The available P was decreased to approximately 10 ppm by five alternate crops of barley and oats in soils to which 100 kg P/ha had been applied, by 14 crops when 200 kg P/ha had been applied, and did not reach this level with 19 crops when 400 kg P/ha had been applied. The rate of removal of available P from the untreated soils and the treated soils, in which the available P had been reduced to approximately 10 ppm, were similar. When the available P reached this level an equilibrium was reached between rate of removal and rate of replenishment of available P from "fixed" sources of P in the soil. There was a high correlation between field and greenhouse results. On the basis of this, it might be expected that a single large application of phosphatic fertilizer would be as effective for cereal crop production and recovery of applied P as small annual applications of P placed with the seed.