Uptake of residual phosphate and freshly applied diammonium phosphate by<i>Lolium perenne</i>and<i>Trifolium repens</i>

Abstract

Abstract Residual fertilizer phosphorus (residual P) may significantly contribute to crop P nutrition. To test this hypothesis, a pot experiment was conducted with ryegrass (Lolium perenne) and clover (Trifolium repens) grown separately on three different soils which either had not received P fertilizer for at least nine years ( 0F ) or had received P fertilizer equivalent to crop P off‐take ( F ). Soils in the pot experiment were given either none ( 0F, F ) or a single rate of 15 mg P (kg soil) –1 as diammonium phosphate ( 0F+DAP, F+DAP ). In the treatments 0F+DAP and F+DAP DAP had been labeled with 33 PO 4 while in the treatments 0F and F the pool of available soil P had been labeled with carrier‐free 33 PO 4 . This allowed estimating the quantities of P in plant dry matter that derived from native soil P, residual fertilizer P or fresh fertilizer P. Fourteen to 62 % of the P in the above ground biomass of white clover or perennial ryegrass were derived from residual P whereas 7 to 28 % were derived from freshly applied DAP. The proportion of P derived from residual P was correlated to the total amount of P fertilizer added to the soils, while the proportion of P derived from DAP was correlated to the concentration of P in the soil solution of the 0F and F soils.