UPTAKE OF PHOSPHATE FROM SOILS AND FERTILIZERS AS AFFECTED BY SOIL-P AVAILABILITY AND SOLUBILITY OF PHOSPHORUS FERTILIZERS

Fertilizers labelled with 32P were used to measure amounts of phosphorus, Ps and PF, taken up by Lolium perenne from available soil P and from P fertilizer respectively, when applied at a rate of 66 mg P·(kg soil-1) in greenhouse experiments. The quantity Ps of phosphorus taken up from soil in the presence of P fertilizer was compared to the quantity Po taken up from soil without P fertilizer. The quantity (Ps-Po) is positive for low Po values, i.e. in soils poor in available phosphorus, but is negative for high Po values indicating that an input of P fertilizer can induce a decrease in the utilization of available soil phosphorus. Moreover, for a given soil, the quantity (Ps-Po) depends on the chemical form of the fertilizer. The standard method of evaluation of P fertilizer efficiency is based on the assumption that Ps=Po, but Ps can differ from Po. This result can explain the contradictory data published from field experiments about the efficiency of the various P fertilizers. © 1990 Kluwer Academic Publishers.