The influence of iron oxides on the surface area of soil

Summary Soils from Denmark and Tanzania have been extracted with ammonium acetate (controls), EDTA to dissolve amorphous iron oxides, and dithionite‐EDTA (DE) to dissolve crystalline iron oxides. The surface areas of the extracted soils have been determined by applying the BET equation to nitrogen adsorption and by water adsorption at 19 percent relative humidity. High correlations ( P < 0.001) were found (i) between EDTA‐extractable iron (amorphous iron oxides) and the decrease in the surface area following EDTA extraction, and (ii) between the difference between DE‐extractable iron and EDTA‐extractable iron (crystalline iron oxides) and the further decrease in the surface area following DE extraction. The calculated specific surfaces of both the amorphous and the crystalline iron oxides varied from soil to soil but without any definite trend. The means of all the soils investigated may therefore serve as reasonable estimates of the specific surfaces of amorphous and crystalline iron oxides in soil.