Humic (Organic Matter)-Al(Fe)-Phosphate Complexes: An Underestimated Phosphate Form in Soils and Source of Plant-Available Phosphate

The chemical interaction between soil organic matter and the orthophosphate anion (phosphate [P]) is important for reactions of P at the soil solid phase, in soil solution, and in natural waters. Humic substances account for an important or dominant part of soil organic matter and of dissolved organic carbon in rivers and lakes. Orthophosphate is not bound directly to the humic molecules, but metals complexed by the humic substances can bind P anions of which Al(III) and Fe(III) seem to be the most important. Humic-metal-P associations or complexes (HMEP) are not detected by conventional P speciation procedures in natural water and in soil solution. In addition, HMEP cannot be detected at the soil solid phase by traditional P fractionation procedures. The concentration of free orthophosphate in solution is determined spectrophotometrically by the phosphomolybdate method in its many variations, with organic P being calculated as the difference between total P and inorganic P. During the photometric procedure, more than 50%, in many cases, more than 80% of the humic-associated P, is hydrolyzed and therefore considered to be free orthophosphate. This is the main reason why HMEP are often ignored in soil chemistry. Hydrolysis occurs during the analytical speciation of P in soil solution and in soil P fractionation procedures. A step to preseparate according to the molecule mass, for example, by 5,000 to 20,000 Da ultrafiltration or by gel permeation chromatography, is therefore required. In natural water and in soil solution, HMEP can account for more than 50%, in some cases, more than 80% of the P in solution. Free orthophosphate in solution is often in equilibrium with P in HMEP. The potential relevance of HMEP at the soil solid phase can be as follows. If the content of alkaline pyrophosphate-extractable Al + Fe (after McKeague, J. A., J. E. Bryden, and N. N. Miles. 1971. Differentiation of forms of extractable iron and aluminum. Soil Sci. Soc. Am. Proc. 35:33-38) is high compared with that of acid oxalate-extractable Fe + Al (Tamm reagent, after Schwertmann, 1964), then humic-Al(Fe) complexes are important for P sorption and transformation in soil compared with oxides or clay minerals. In soils, evidence exists for the importance of HMEP in Spodosols and Andosols. Humic-P complexes are important with respect to the soil P availability to plant roots. Mobilizing plant species often excrete citrate or oxalate as a result of P deficiency. The P from HMEP is much better mobilized by dibasic and tribasic low-molecular-weight anions than P adsorbed to inorganic surfaces such as goethite or ferrihydrite.