RESIN EXTRACTION OF LABILE, SOIL ORGANIC PHOSPHORUS

In order to develop a method for estimating labile, soil organic phosp horus (P(o)), macroporous anion-exchange and adsorbent (XAD) resins we re tested for their ability to extract P(o) from a permanent pasture s oil. Experimental variables included: ionic form, soil-water-resin rat io, addition of cation-exchange resin, extraction time, extraction tem perature, continuous and stepwise extraction and soil pretreatment (ch loroform and microwaves). The amounts of extracted P(o) and P(i) (inor ganic phosphorus) showed little variation between the anion-exchange r esins, when used in the bicarbonate form. In the chloride form, the am ounts 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 studie s. 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) ext racted were little affected by variations in the ratio between soil, w ater and resin, but increased with extraction time and temperature. Ch loroform pretreatment of the soil mainly increased extracted P(i), whe reas microwave pretreatment only increased extracted P(o). The magnitu de of these increases was approximately constant irrespective of extra ction temperature, indicating that the increases came from killed micr o-organisms.