EVALUATING PLANT-AVAILABLE PHOSPHORUS WITH THE ELECTROULTRAFILTRATIONTECHNIQUE

The electro-ultrafiltration (EUF) technique successfully evaluated the short-term and long-term supplying power for many essential plant nut rients of European and Asian soils but has not been evaluated for P in North America. The objective of this study was to compare the capacit y of EUF with other extraction techniques to predict the plant availab ility of soil P and P fertilizer responsiveness of oat (Avena sativa L .) and corn (Zea mays L.) grown in the greenhouse. Increasingly higher average concentrations of soil P were extracted by EUF at 50 V and 20 -degrees-C (1.53 mg kg-1),water 2.28 mg kg-1), EUF at 200 V and 20-deg rees-C (4.76 mg kg-1), 0.5 M NaHCO3 (13.0 mg kg-1), HCO3- resin (41 mg kg-1), EUF at 400 V and 80-degrees-C (44.2 mg kg-1), and Sr citrate ( 51 mg kg-1). The P desorbed by EUF at 20-degrees-C was smaller than th e 0.5 M NaHCO3 P in acidic soils while comparable amounts were desorbe d by the two methods from neutral to calcareous soils. The total amoun t of P desorbed by EUF between 30 and 55 min at 400 V and 80-degrees-C was equivalent to chemical methods but less precise than HCO3- resin in predicting the amount of P uptake by oat and corn as well as the fe rtilizer responsiveness of oat grown to Zadoks 71 stage. The predictio n of P availability by the different EUF fractions could not be improv ed by the inclusion of selected additional physical and chemical chara cteristics of the soils studied. The desorption of soil P by EUF provi des an evaluation comparable to chemical methods of the P-supplying po wer of northeastern North American soils. The large amount of time inv olved in the EUF extraction limits its applicability as a soil-testing procedure.