Phosphorus (P) is one of the most limiting essential nutrients for agricultural crop production. Diminishing global reserves of rock phosphate are expected to reduce supply and increase the cost of mineral P fertilizers, a major concern in regions where low soil available P levels constrain crop production. In other parts of the world, intensive livestock production and agricultural management have resulted in high soil available P concentrations, which contribute to environmental pollution and threaten water quality. The objective of this review was to examine the factors affecting soil available P in agroecosystems. Physicochemical and biological controls on the soil available P, in the context of P biogeochemical cycling, are presented. Agricultural management practices such as crop rotations, tillage, and P fertilizer sources influence the size of the soil available P pool, while environmental conditions such as freezing–thawing and wetting–drying cycles control the temporal dynamics of this pool. Methods to evaluate soil available P in the laboratory and in situ are reviewed. Attention is given to the isotopic dilution method that quantifies fluxes of P ions between soil solid phase and soil solution, which can be combined with the Freundlich kinetic equation to describe diffusive soil P transfer, leading to the development of a process-based mass-balance model to assess soil available P. This model has potential to advance scientific understanding about soil available P dynamics for better decision making about P fertilization and agroenvironmental management in sustainable cropping systems.