An investigation of the anion-exchange resin method for soil phosphate extraction

Summary The anion-exchange resin method for soil-phosphate extraction was investigated on 4 different soils under varying experimental conditions. The variables were: (a) the type of anion-exchange resin, (b) the anionic form of the resin, (c) the ratio between the amounts of resin, soil, and water, and (d) the time of shaking. The amount of P extracted was dependent on the anionic form of the resin. For resins in the chloride form both the amount of P extracted psr soil unit and the pH of the soil suspension varied with the type of resin and the soil-water ratio. Resins in the bicarbonate form stabilized the system, so that the amount of P extracted and the suspension pH were almost independent of the type of resin and the soil-water ratio. The results indicated that the rate-determining step in the overall process of P transport from the soil phase through the water phase to the resin phase is the P desorption from the soil phase to the water phase, provided the resin is added in excess. The rate of this P desorption is dependent on the chemical composition of the water phase, which in turn is governed by the type of soil, the soil-water ratio, the time of shaking and the anionic form of the resin. In a final experiment a resin was used in the chloride- and the bicarbonate form, respectively, for extraction of phosphate from 34 soils. The available P of these soils had been determined 15 years before in a pot experiment with ryegrass and by different laboratory methods². The degree of correlation between the ryegrass P uptake and the P. determined by the laboratory methods decreased according to the following order: resin (bicarbonate form), resin (chloride form), 0.5M sodium bicarbonate, L value, E value, ammonium lactate solution, sodium zeolite, 0.01M calcium chloride, phosphate potential, and 0.1M sulphuric acid. It is recommended that resins in the bicarbonate form should be used for both routine as well as more advanced analyses of the ability of soils to supply phosphate to plants. A final procedure for the analysis is given in the paper.