Abstract: As an essential element of plant nutrition, phosphorus plays an import ant role in the agricultural sustainable development and ecosystem balance. Adop ting appropriate soil phosphorus fractionation scheme to study the forms, transf ormation, and availability of soil phosphorous is critical for understanding soi l phosphorus supply and its losses. This paper reviewed the recent researches about the forms of soil inorganic and organic phosphorous, as w ell as their fractionation schemes and limitations. Hedley method, a widely used soil phosphorous fractionation scheme, gives attention to both organic and inor ganic phosphorous forms, being available to understand the bioavailability and d ynamics of soil phosphorus. The process of Hedley method and its application sco pe were described. A detailed discussion of its modification, the Tiessen method , was also presented.

Key words: maize, seasonal drought, crop water deficit index, developmental stage.