The present level of understanding of P controls on organic matter accumulation and cycling in a pedological context owes much to soil sequence studies, and the hypotheses that emerged from them to explain the variation of P compounds with soil type and development. It is now widely accepted that nutrient transformations in soil are closely linked through a more dynamic biological cycle in which microorganisms have a central role. Concepts developed to account for the effects of P on N cycling, and for interpreting inter-relationships of C, N, P and S in both a pedological and biological context have yet to be tested adequately across different ecosystems. These concepts are discussed, in relation to some recent supporting evidence.