Plant Availability and Form of Residual Phosphorus in Davidson Clay Loam

Abstract

AbstractThe form and availability of residual P in Davidson clay loam, a Reddish Brown Lateritic soil, was investigated. Various levels of residual P had been established in the soil by annual application of superphosphate for 17 years. In a greenhouse experiment, residual P from annual application of 21 lb P/acre was adequate to supply the P requirement of three successive crops of alfalfa (Medicago sativa L. ‘DuPuits’); application of 10.5 lb P/acre was inadequate. Phosphorus fractionation by the Chang and Jackson procedure (5) showed that native and residual P was present in the soil predominantly as iron phosphate; application of superphosphate resulted in small but significant increases in aluminum and calcium phosphates. A comparison of the inorganic fractions in soil before and after growth of the three crops of alfalfa provided evidence that aluminum, iron, and calcium phosphates were sources of P to the first crop and that iron phosphate was the main source to the second and third crops. The availability of the three fractions was explained on the basis of their composition and soil pH and exchangeable Ca2+ content.