Soil Phosphorus Forms Extracted by Soil Tests as a Function of Pedogenesis

One hundred and twenty uncultivated and unfertilized surface soil samples (Al horizon), from around the world, were divided into calcareous, slightly weathered (mainly Mollisols), and highly weathered (mainly Oxisols and Ultisols) soils. Bray 1 (NH₄F + HCl), Mehlich 1 (HCl + H₂SO₄), Olsen (NaHCO₃), and Texas A&M (NH₄Ac + EDTA) soil-P fertility tests and a sequential P fractionation (resin-NaHCO₃-NaOH-HCl) were carried out to determine the relative contribution of resin-extractable labile and more stable inorganic and organic P fractions to soil test P in differently weathered soils. A significantly (at the 5% level) greater amount of Bray 1, Mehlich 1, and Texas A&M extractable P was attributed to dissolution of Ca-phosphates by the acidic extractants in calcareous compared to highly weathered soils. A shift from Ca- to Al- and Fe-phosphates with increased soil weathering was apparent with Ca-phosphates comprising 55, 17, and 4% of the total P in calcareous, slightly, and highly weathered soils, while 4, 12, and 20%, respectively, was associated with Al and Fe in these three soil groups. A significantly larger (at the 5% level) organic P content was observed in highly weathered (112 mg P kg⁻¹, 26% of total P) compared to calcareous soils (36 mg P kg⁻¹, 7% of total P). Correlations between soil test P and P fractions showed that only Olsen P was related (at the 1% level) to resin P for the calcareous soils, although all tests were closely related to resin P for slightly weathered soils. For the highly weathered soils all tests except Olsen were closely related to resin P. A greater proportion of Bray 1, Mehlich 1, and Texas A&M P variability was accounted for by organic P fractions in highly weathered (average 17%) compared to calcareous (average 4%) and slightly weathered soils (average 1%). The relationship between organic P and soil test P in uncultivated highly weathered soils is confirmed and this relationship should be carefully considered when evaluating P fertility in these soils.