Particularly since the findings of Bache and Williams (1971), different ‘P index’ methods have emerged. Regional contributors in addition to the Anion Storage Capacity test of Saunders (1965, 1968) and Blakemore et al. (1987) include Holford and Cullis (1985), Moody et al. (1988), Dear et al. (1992) and Burkitt et al. (2002, 2008).
The main principles for selecting the ‘index’ method to use are:
• effective across a wide range of soils;
• national/international use or acceptance;
• analytical robust method, suited to automation or rapid throughput at low cost, with stable reagents and good within and between laboratory agreement on results, and
• interpretative criteria available or easily developed.
Full analytical details for Method 9I1 (Phosphate Sorption Index) have been available for years (Rayment and Higginson 1992); however, it has not been used widely. The Anion Storage Capacity test (Method 9H1) has greater support, particularly in New Zealand. It has an application in Soil Taxonomy and in New Zealand’s revised soil classification system.
A P index test (termed PBI) was developed, particularly for use in Australia (Burkitt et al. 2002). The expectation was that PBI would better reflect changes in P sorptivity or P ‘fixing’ ability of fertilised soils. That test incorporates a measure of contemporary soil P fertility (either Colwell-P or Olsen-P), although more recent research suggests it may be preferable not to adjust the index for current soil P fertility (Burkitt et al. 2008). Methods with (9I2 and 9I3) and without (9I4) adjustments for current soil P fertility are included, inclusive of alternative analytical ‘finishes’.
The acronym for PBI when it incorporates the Colwell-P test as the measure of current soil P fertility is PBI+ColP. Its companion, based on Olsen-P, is PBI+OlsenP. When neither is used, it is PBIunadj.