The release of phosphorus in alkaline vertic soils as influenced by pH and by anion and cation sinks

• Acidification of alkaline Vertisols to pH 6.5 and 5.5 released distinct pools of P.
• Release of acid-soluble P was constant under stable pH and ion concentration.
• With an anion sink, acidification to pH 6.5 released 9–33% of the acid-soluble P.
• With a cation and anion sink, all the acid-soluble P was readily depleted.

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 0.5 M sodium bicarbonate and 0.005 M sulfuric acid. To mimic 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.