Effects of extraction time and phosphorus speciation on soil test phosphorus data: A case study of Illinois agricultural soils

- The results of Soil Test P can be influenced by extraction time and P speciation.
- Bray P-1 and Mehlich 3 methods were tested using batch kinetic experiments.
- Longer extraction time than recommended time will significantly increase inorganic P.
- The contribution of organic- and colloidal-P can be critical to Soil Test P.

Bray P-1 and Mehlich III STP extractions aim to assess the concentrations of labile P in soils. It is unknown, however, how the observed quantity of labile P is affected by the release of variable P species. The kinetics of orthophosphate (Pi), organic-P (Po), and colloidal P were evaluated in manure-amended and chemical fertilizer-amended Central Illinois agricultural soils using Bray-1 and Mehlich III extractants. In both soils, Bray and Mehlich III extractable P increased with increasing time. The extracted P species are not only Pi but also Po and colloidal P. Between 10 and 15% of total Po was extracted from each soil with manure amended soils releasing more total Po than fertilizer amended soils. BrayP-1 and Mehlich III extracted colloidal P from manure-amended and fertilizer-amended soils, but concentrations were only significant after 1-3 h. The results indicate that P speciation and extraction time have major impacts on the results of STP extractions, therefore, to fertilizer recommendation.