Mobilization of residual phosphate of different phosphate fertilizers in relation to pH in the rhizosphere of ryegrass

Abstract

Only 10 to 20% of the P in fertilizers are utilized by crops in the year of application. The value of the remaining 90% to 80% for succeeding crops is uncertain. This paper is aimed at assessing the residual value of several P-fertilizers such as superphosphate (Super-P), Thomasphosphate (Thomas-P), Rhenaniaphosphate (Rhenania-P) and Hyperphos (Hyper-P) a ground rock phosphate. These fertilizers had been applied annually for ten years to supply a total of 520 kg P ha−1 to a silt loam soil derived from loess. Fertilizer P accumulation compared to the unfertilized plots was 520 kg ha−1 for Hyper-P and 410 kg ha−1 for Super-P (4.2 and 3.3 mmol P kg−1 soil). The residual value of this P was assessed by both conventional soil test procedures and P-depletion at the soil-root interface by ryegrass (Lolium perenne L.) supplied with either NO3-N, NH4-N or no N. The different N sources changed soil pH in the rhizosphere and thereby the solubility of P present in this region. To measure P depletion at the soil-root interface, ryegrass seedlings were grown on a soil block covered with a nylon screen, mesh 30µm. A dense root mat developed simulating a plane root surface. After 10 days of growth the soil block was sliced into 0.2 mm layers parallel to the root mat. These soil samples were analysed for P (4N HCl) giving P concentration as a function of distance from the root surface. Phosphorus depletion at the root surface, in mmol kg−1, was 1.7 for the No-P and 3.2 for the Super-P treatment. Thomas-P and Rhenania-P were in between while for Hyper-P the depletion was only 1.0. This gave a residual effect of 47% for Super-P and a negative 15% for Hyper-P. Acidification of the rhizosphere due to NH4-N supply had no effect on the residual effect of Super-P but increased that of Hyper-P to +18%. NH4-N increased the residual effect of Thomas-P from 16 to 28% and of Rhenania-P from 9 to 37%. The supply of NO3-N increased the rhizosphere pH and decreased the residual effect of Super-P but increased slightly the residual effect of the other P-fertilizers as compared to No-N application.