Abstract
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 1–3h. The results indicate that P speciation and extraction time have major impacts on the results of STP extractions, therefore, to fertilizer recommendation.
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