The Manaaki Whenua – Landcare Research method (number 132) specifically tests the sorption of phosphate. This test is often used to quantify anion storage in general and is thus called anion storage capacity (ASC), particularly in the soil fertility context, where it is used as a measure of the possibility of rapid leaching of mobile anions. However, under some circumstances, such as certain levels of pH, or soil orders, e.g. Pumice soils, the retention of phosphate will differ from that of other anions and should not be relied on as a measurement for all anions, e.g. sulphate.
The extent to which P is adsorbed is influenced by several factors, including type of clay, pH, and soil chemistry. Allophanic soils are dominated by allophane (and also by imogolite or ferrihydrite) minerals. These minerals adsorb phosphate ions. Allophanic soils have high values of P retention as they retain a high proportion of applied P. In contrast, soil such as some Recent, Podzols, Organic, and Semi-arid soils have low values of P retention, and therefore may require lower inputs of P to minimise risks to water quality. P retention in both topsoil and subsoil is a key diagnostic criterion used to separate some soil orders in the New Zealand Soil Classification, which is the basis of mapping different soils in S-map.
MWLR website information on laboratory soil testing methods: https://www.landcareresearch.co.nz/partner-with-us/laboratories-and-diagnostics/environmental-chemistry-laboratory/soil-testing/
Blakemore LC, Searle PL, Daly BK 1987. Methods for chemical analysis of soils New Zealand Soil Bureau Scientific Report 80. 103 p.
Saunders WMH 1965. Phosphate retention by New Zealand soils and its relationship to free sesquioxides, organic matter, and other soil properties. New Zealand Journal of Agricultural Research 8(1): 30–57. https://doi.org/10.1080/00288233.1965.10420021
Marsh KB, Tillman RW, Syers JK 1992. Effect of changes in pH on sulphate and phosphate retention in soils. New Zealand Journal of Agricultural Research 35(1): 93–100. https://doi.org/10.1080/00288233.1992.10417706