The influence of iron oxides on phosphate adsorption by soil

Abstract

SummarySoils from Denmark and Tanzania were extracted with ammonium acetate (controls), EDTA to dissolve amorphous iron oxides, and dithionite‐EDTA (DE) to dissolve crystalline iron oxides. The phosphate adsorption capacities of the extracted soils were taken as the maximum quantity of phosphate adsorbed computed from the Langmuir equation. The decreases in the phosphate adsorption capacity following EDTA extraction and DE extraction were attributed to the removal of iron oxides. Close correlations (P<0.001) were found (i) between EDTA‐extractable iron (amorphous iron oxides) and the decrease in phosphate adsorption capacity following EDTA extraction, and (ii) between the difference between DE‐extractable iron and EDTA‐extractable iron (crystalline iron oxides) and the further decrease in phosphate adsorption capacity following DE extraction.The phosphate adsorption capacity, estimated to be approximately 2.5 μmol P m−2, was in good agreement with the capacity of various synthetic iron oxides. The calculated phosphate adsorption capacity of soil iron oxides, obtained from the contents and specific surfaces of amorphous and crystalline iron oxides together with the phosphate adsorption capacity per m2 for synthetic iron oxides, compared favourably with the measured phosphate adsorption capacity.