Phosphorus (P) is an essential element for all life, serving as a structural component of cells, being integral in energy transfer, and a component of genes and enzymes (Campbell and Reece, 2002; Schachtman et al., 1998). More than other nutrients, P is held tightly by the soil, so only low concentrations relative to plant requirements are in the soil solution (Holford, 1997). In alkaline soils, it is generally accepted that P is retained in the soil as calcium phosphate (CaP) minerals, limiting plant growth and agricultural production (Conyers and Moody, 2009; Holford, 1997). This research investigates factors that influence the supply of P to the soil solution of alkaline Vertosols, focusing on pH and the concentration of P itself and calcium (Ca) in the solution. The soils were taken from the Northern Grains Region (NGR), a productive agricultural region of 4 M ha in northern New South Wales and southern Queensland, Australia. The soils contain a diminishing pool of phosphorus (P) that is rapidly available to plants, and an acid-soluble P fraction that has been considered to slowly replenish the available pool. The process of replenishment is not well understood, and this may lead to inefficient use of fertiliser P. It is possible that modification of soil in the rhizosphere, particularly acidification, enables plants to directly access the acid-soluble P fraction.