Soil-available phosphorus as evaluated by desorption techniques and chemical extractions

The P-supplying power of the fine-textured soils from Abitibi-Temiscaming (Quebec, Canada) has never been investigated. Anion exchange membranes (AEM), the electro-ultrafiltration (EUF) technique and chemical extractions were used to describe their P status. Phosphorus was desorbed from four soils that had received 52.4 kg P ha(-1) yr(-1) from 1994 to 1996 and from unfertilized soils. The cumulative amount of P desorbed in 75 min by EUF from unfertilized soils varied from 10 to 69 mg P kg(-1). Averaged over all sites and treatments, the amount of soil P extracted by different methods ranged from 1.0 to 18.9 mg P kg(-1). The cumulative grass P uptake was significantly related to P desorbed by all methods (0.39 < r < 0.84). A close relationship between grass P uptake and P(AEMs) indicated the adequacy of the AEMs to determine plant-available P in Abitibi-Temiscaming soils. A high degree of linearity (0.97 < R(2) < 0.99) Of the EUF-P desorption curves at 200 V and 20 degreesC with time was obtained. This indicates that soils have a high capacity to buffer removal of P from soil solution with reserve P. The EUF technique has shown that these soils hold very large inorganic P reserves and this may explain the lack of grass response to P fertilizers.