Changes in bicarbonate-extractable inorganic and organic phosphorus by drying pasture soils

Soils are commonly dried in the laboratory prior to the determination of P fractions, but this can profoundly influence the results. We investigated the impact of soil drying on bicarbonate-extractable inorganic and organic P in 29 permanent lowland pasture soils from England and Wales (total C 29-80 g C kg(-1) soil, clay 219-681 g kg(-1) soil, pH 4.4-6.8) by extracting soils at approximate field moisture capacity and after air-drying at 30degreesC for 7 d. Air-drying increased the mean bicarbonate-extractable inorganic P from 14.8 to 22.5 mg P kg(-1) soil, and the mean bicarbonate-extractable organic P from 17.4 to 25.7 mg P kg(-1) soil. Proportional increases for individual soils following drying were between 11 and 165% for inorganic P, and between -2 and 137% for organic P, being greatest in soils with low P concentrations. The results are unlikely to influence tests for plant-available P, because these are derived from analyses of air-dried samples, but have important implications for attempts to relate bicarbonate-extractable P fractions to processes operating under field conditions.