Phosphorus

Methods for determining soil phosphorus (P), its various forms and availability to plants, have been essential in developing principles and knowledge of the nature and behavior of P in soils. The selection of a suitable method demands a clear statement of objectives that require the soil P measurement. Other considerations include soil properties, sample condition or environment, accuracy and reproducibility, and the facilities, equipment, and personnel available. This chapter enumerates various procedures in detail, including those for total P, organic P, fractionation of soil P and seven indices of P availability. Indirect procedures are used to estimate the total organic P content of soils and are commonly referred to as either extraction or ignition methods. Knowledge of the distribution of soil P among chemical forms should be useful in studies of soil chemistry, soil genesis, soil fertility, and phosphate reactions in lake and river sediments.