Resin extraction of labile, soil organic phosphorus

In order to develop a method for estimating labile, soil organic phosphorus (P_o), macroporous anion-exchange and adsorbent (XAD) resins were tested for their ability to extract P_o from a permanent pasture soil. Experimental variables included: ionic form, soil-water resin ratio, addition of cation-exchange resin, extraction time, extraction temperature, continuous and stepwise extraction and soil pretreatment (chloroform and microwaves). The amounts of extracted P_o and P_i (inorganic phosphorus) showed little variation between the anion-exchange resins, when used in the bicarbonate form. In the chloride form, the amounts extracted were less and more variable. XAD resins extracted much less P_o and very little P_i. The macroporous anion-exchange resin, Lewatit MP500A in the bicarbonate form, was chosen for further studies. It extracted 21 mg P_o kg^?1 from air-dried soil and 8 mg P_o kg^?1 from moist, incubated soil. The specific amounts of P_o and P_i extracted were little affected by variations in the ratio between soil, water and resin, but increased with extraction time and temperature. Chloroform pretreatment of the soil mainly increased extracted P_i, whereas microwave pretreatment only increased extracted P_o. The magnitude of these increases was approximately constant irrespective of extraction temperature, indicating that the increases came from killed micro organisms.