Evaluation of the residual effect of P fertilizer's on plant P nutrition using isotopic techniques

The residual effect of P fertilizers previously applied to a soil on plant P nutrition was examined by both the isotopic dilution method (pot experiment) and isotopic exchange method (laboratory test) using 32p as a tracer. The fraction of P derived from a fertilizer in plant (%Pdff in plant) was compared with the fraction of P derived from fertilizer in soil solution (%Pdff in soil solution) which is a new laboratory index proposed by Morel and Fardeau to predict %Pdff in plant. Four soil samples of a Humic Andosol from long-term experimental plots, which received no fertilizer (A1 soil), a readily soluble fertilizer (A2 soil), the same readily soluble fertilizer (RSF) plus a fused magnesium phosphate (A3 soil), and combination of RSF with Florida phosphate rock (A4 soil), were tested. In the pot experiment, maize (Zea mays) was grown during 38 days and dry shoot weight, P uptake and specific radioactivity were measured. Dry shoot weight, P uptake and L-value were the highest in A3 soil, followed by A4, A2 and A1 soils. %Pdff in plant were 71,9%, 51,9% and 15,4% in A3, A4 and A2 soils respectively. A laboratory study using 32p isotopic exchange kinetics was carried out to examine three status parameters of soil P, intensity, quantity and capacity factors. Goof agreement was obtained between quantity factor (E1-value), and the pot experimental data; i.e. P uptake and L-value. %Pdff in soil solution were similar to those %Pdff in plant except for A4 soil. The enhancement of P uptake by the plant from the phosphate rock obtained in A4 soil could be attributed to specific plant factors and soil moisture conditions. (authors)