NEW METHOD FOR ASSESSMENT OF LONG-TERM PHOSPHATE DESORPTION FROM SOILS

A new method was developed to study P desorption kinetics. This new technique uses dialysis membrane tubes, filled with hydrous ferric oxide (ferrihydrite) acting as an ''infinite'' P sink. This system is mechanically stable for very long reaction periods, provided that a microbial inhibitor, e.g., chloroform, is added to the soil suspension to prevent hydrolysis of the membrane. The pH of the soil solution during desorption remains almost constant. After the desired time of contact between soil suspension and P sink, the sink can be easily separated from the soil suspension with practically no loss of soil material. As such, the new technique has important advantages to the Fe oxide impregnated filter paper P extraction method. The system is capable of maintaining a constant low P activity in solution, necessary to study long-term P desorption kinetics of soils. This method was tested on six sandy soil samples and a comparison made with the amount of P desorbed by a single Fe oxide impregnated filter paper extraction (P-i). An important finding from this experiment was that P desorption continues for long periods. No desorption maximum was reached within 500 h, as is often suggested by desorption results based on repeated extractions with Fe-impregnated filter paper. Furthermore, relatively large differences were observed between different soils with respect to the quantity of oxalate-extractable phosphate released by the soils after a specified time of desorption.