Organic phosphorus (Po) accounts for 20 to 60% of total P in most mineral soils. This chapter discusses the short- and long-term dynamics of Po in soils, and in the soil-plant system. It examines experimental methods used for the study of Po in soil and the associated problems. The chapter explores the possibility of using laboratory or other tests as a means of predicting the availability of soil Po to plants. Long-term research sites have been used to assess the contribution of soil organic matter to N and P nutrition of crops. The importance of the rhizosphere and of microbial colonies in the turnover of soil P results in considerable spatial variability at a microscale. The variation of P compositions of different soils of known history allowed an empirical interpretation of the roles of bicarbonate and hydroxide extractable Po in P transformations associated with microbial P uptake, plant roots, cultivation, or soil development.