Calcium phosphate (CaP) minerals may comprise the main phosphorus (P) reserve in alkaline soils, with solubility dependent on pH and the concentration of Ca and/or P in solution. Combining several techniques in a novel way, we studied these phenomena by progressively depleting P from suspensions of two soils (low P) using an anion-exchange membrane (AEM) and from a third soil (high P) with AEM together with a cation-exchange membrane. Depletions commenced on untreated soil, then continued as pH was manipulated and maintained at three constant pH levels: the initial pH (pHi) and pH 6.5 and 5.5. Bulk P K-edge X-ray absorption near-edge structure (XANES) spectroscopy revealed that the main forms of inorganic P in each soil were apatite, a second more soluble CaP mineral, and smectite-sorbed P. With moderate depletion of P at pHi or pH 6.5, CaP minerals became more prominent in the spectra compared to sorbed species. The more soluble CaP minerals were depleted at pH 6.5, and all CaP minerals were exhausted at pH 5.5, showing that the CaP species present in these alkaline soils are soluble with decreases of pH in the range achievable by rhizosphere acidification.

Alkaline Vertisols contain calcium phosphate (CaP) minerals that dissolve in response to both acidification and the depletion of concentration of phosphorus (P) or calcium (Ca) in the soil solution, conditions commonly observed within the rhizosphere. In these soils, reserve-P is defined as the difference between the concentrations of P extracted by 0.5M sodiumbicarbonate and 0.005M sulfuric acid. Tomimic rhizosphere modification we sequentially extracted P from three alkaline Vertisols that contained concentrations of reserve-P ranging from 300 to 6500 mg kg⁻¹ using an anion sink, and a combined anion and cation sink. The extractions commenced on untreated soil, and then three target pH regimes were imposed: 1) maintain the initial pH; 2) incrementally acidify to pH 6.5 then maintain; and 3) incrementally acidify to pH 5.5 then maintain. Extractable P increased with decreasing solution pH in all soils until the acid soluble P was depleted. In each soil and at each pH level,more P was extracted when the combined sink was used compared with the anion sink alone. The release of acid soluble-P in these soils was indicative of CaP minerals of varying thermodynamic stability. In addition to the relatively constant concentrations of P extracted at the initial pH with the anion sink, moderate acidification to pH 6.5 released 9% of the reserve-P in the high P soil, but this varied from 18 to 33% in the two other soils containing lower soil P. These findings show that the release of P in alkaline soils beyond that measured by a bicarbonate extractant is influenced by the modification of soil pH and by the removal of Ca from the solution, which has implications for plant availability and response to added fertiliser. Further research is needed to identify the individual species of soil P that are involved in buffering the soil solution, and what their potential availability is to plants via rhizosphere modification.