The evaluation of phosphorus sources for nutrient budgeting on organic farms.

Abstract

The use of synthetic P fertilisers such as superphosphate is not permitted by regulation on organic farms. The use of basic slag and rock phosphate is permitted by regulation but the effects of the application of these compounds on the soil and herbage concentrations of phosphorus (P) on organic farms has not been assessed. Experiments were carried out from 1994 to 1997 on the effect of basic slag and rock phosphate on the concentrations of P in four different soil types and on herbage in organic grassland. Superphosphate was included in some of the experiments as a comparison. The investigations showed that: Superphosphate applied at equivalent rates of elemental P gave the largest initial increase in soil and herbage concentrations of P but was no more effective at raising the soil and herbage concentrations of this element than basic slag or rock phosphate in the long term. The application of basic slag and rock phosphate gave increases in soil and herbage concentrations of P which were comparable to those obtained with superphosphate but were more slow acting Basic slag gave an increase in the concentration of P in an organic soil which was high in P (> 10 mg/kg) but gave only a slight increase of P in the herbage. Basic slag gave an increase in soil P concentration in a range of soils and was more persistent in these soils than superphosphate. Basic slag gave an increase in herbage yield which was equivalent to that given by superphosphate in a low P soil. Rock phosphate gave a larger herbage yield increase in a low P soil than superphosphate or basic slag. Rock phosphate is a more persistent source of P in low P soils than superphosphate or basic slag. In soils with a high pH rock phosphate may be less effective as a phosphatic fertilizer than superphosphate or basic slag.