Biological cycling of phosphorus (P) in soils includes various processes: the release and mineralisation of P from plant residues, microbial immobilisation, re-mineralisation and turnover, mineralisation of soil organic P, and flush effects due to abiotic stress, e.g. drying and rewetting. The influence of these processes on plant available P is largely unknown. We analysed soil samples from several long-term field experiments in order to assess the effect of crop rotation, tillage, P fertilization and residue management on soil P pools, especially available inorganic, microbial and total organic P. Radioisotopes (P-32, P-33) were then used to quantify P fluxes, specifically dynamics of microbial P and basal mineralisation of soil organic P. We will present results in relation to data previously obtained on a tropical Ferralsol from Kenya and to published data from temperate soils.