European soils overfertilized with phosphorus .1. Basic properties

Soils overfertilized with phosphorus (P) are widespread in the European Union and there is consensus among soil scientists to better explore their potential to release phosphate. In this work we report the principal physical and chemical properties of twelve overfertilized benchmark soils of contrasting agricultural areas in Italy, Germany, Great Britain, and Spain. The criterion used to consider them as overfertilized was that the available P amount, measured by the regional soil P test, was at least twice as large as the accepted critical level for an average crop. The soils could be usefully divided into three groups, calcareous, slightly acidic and acidic based upon their basic chemical properties and reactions and proportion of the major P fractions (NaOH- plus citrate-bicarbonate-, citrate-bicarbonate-dithionite-, and HCl-extractable P). Six extraction procedures commonly used to evaluate potentially plant-available P (Olsen, anion-exchange resin (resin-), anion plus cation-exchange resin (resin+/-), Ca acetate-lactate (GAL), water, and Fe oxide-impregnated paper strips (strip) were compared. The extractable P values by each method were correlated but the amount of P extracted varied and differed in the order water-P<Olsen-P<CAL-P<strip-P<resin(+/-)-P<resin(-)-P. Olsen-P and strip-P extracted a proportion of the more available P fraction (NaOH- and citrate-bicarbonate-extractable P), which increased with increasing pH, and decreasing amounts of active Fe and Al forms in soil. Consequently, pH can be used in conjunction with simple soil P tests to provide a first evaluation of potentially releaseable P.