The Nature of Reductant-Soluble Phosphorus in Soils and Soil Concretions

In preliminary studies on the relative abundance of the various phosphorus fractions in the different mechanical separates of soils, a number of iron-manganese concretions were observed even in the very fine sand fractions. Phosphorus forms insoluble compounds with iron and manganese. It was postulated that the nonextractable phosphorus present in soils might be occluded in the iron coatings of weathered colloidal silicates or in micro-concretions. The objective of this investigation was to determine: (1) if solution of the nonextractable phosphorus could be achieved with suitable iron reduction procedures; (2) the effect of grinding on the solubility of nonextractable phosphorus; (3) the mineralogical nature of nonextractable phosphorus as revealed by X-ray analysis. The Jeffries and Deb iron reduction procedures were combined with the Bray and Kurtz phosphorus fractionation procedures in attempts to extract the total soil phosphorus. Solution of the total soil phosphorus was achieved only when the secondary iron compounds were reduced by the Deb sodium acetate-sodium tartrate buffered sodium hydrosulfite procedure. This established the positional nonextractability of the insoluble phosphorus fraction. The phosphorus brought into solution by the buffered Deb procedure was termed reductant-soluble phosphorus in lieu of the term “nonextractable phosphorus” previously used to describe the insoluble phosphorus fraction. Soils and concretions were ball-milled to increase their specific surfaces. In all cases, prolonged ball milling reduced the phosphorus extracted by the Bray and Kurtz phosphorus fractionation procedures. Increasing the fluoride concentration of the extracting solutions enhanced the solubility of the phosphorus present in the ball-milled products. However, it was not possible to solubilize the total soil phosphorus. In view of the similarities in extraction behavior of ball-milled soils and soil concretions, it was concluded that the reductant-soluble phosphorus present in concretions and soils was similarly retained. Powder diffraction patterns of soil concretions revealed the presence of goethite and silica, but no phosphate minerals. It was concluded that reductant-soluble phosphorus probably exists as chemisorbed phosphorus dispersed at random in the matrix making up soil concretions and the iron coatings of the clay separates.