Phosphorus saturation of Finnish soils: evaluating an easy oxalate extraction method

Authors

• R. UUSITALO
• H.-R. TUHKANEN

DOI: https://doi.org/10.23986/afsci.5647

Abstract

The aim of this study was to test whether phosphorus saturation of surface sorption sites of (oxyhydr)oxides of aluminium (Al) and iron (Fe) in Finnish soils can be assessed using a single oxalate extraction and, if so, whether the results are closely related to the P forms likely to influence the P concentration in runoff waters. Ten soil samples with varying clay content and P status were studied. Desorption tests were conducted by submitting the soils sequentially to nine anion exchange resin (AER) extractions. Sorption of P was studied by shaking the soils in P standard solutions (0-250 ppm). Soil inorganic P was characterised by sequentially extracting P from the fractions assumed to be connected to Al and Fe compounds and present as the stable apatitic form. The desorption studies and the fractioning of inorganic P suggested that oxalate solution dissolves apatitic P and/or other relatively stable P-bearing compounds, probably referring to the sum of inorganic P fractions rather than labile P. The amount of P desorbed in the nine AER extractions was about 80-280 mg/kg, whereas oxalate extracted about 490-1100 mg P/kg, which approximated the sum of the inorganic P fractions. Therefore, in soils high in apatitic P, oxalate-extractable P does not seem to be a reliable measure of the P saturation of Al and Fe oxide surfaces that regulate the P concentration in soil solution and runoff water.;