PHOSPHATE-RICH SOILS IN THE EUROPEAN-UNION - ESTIMATING TOTAL PLANT-AVAILABLE PHOSPHORUS

The estimation of plant-available phosphorus (P) has become important for farmers and environmental agencies, because it can help them: (i) use existing soil P reserves more efficiently, and (ii) establish crit eria for reducing or stopping application of P fertilizers, which can cause water eutrophication. In this study, total plant-available P (TP AP) in 12 P-rich soils from major agricultural areas of the European U nion was estimated by successive cropping in pots, and compared to the amount of P extracted by different methods. Plant desorption curves g enerally conformed to a Temkin equation (P uptake=A+B log(P concentrat ion in soil solution)). P uptake was also linearly related to the chan ge in Olsen P of soil. Of the various laboratory methods used to estim ate TPAP, extraction by goethite (viz., a 'near-infinite' Fe oxide sin k) proved the most efficient, followed by repeated extraction with an anion-exchange resin in chloride form. Isotopically exchangeable P (IE P) was found to be useless for estimating TPAP, particularly in acid s oils. However, IEP at 14 days, combined with the P concentration in th e soil solution, provided a good estimate for TPAP. In practice, TPAP can also be predicted from Olsen P, provided the varying efficiency of bicarbonate in extracting P from soils in different pH ranges is cons idered. (C) 1997 Elsevier Science B.V.