ASSOCIATION BETWEEN COLLOIDAL IRON, ALUMINUM, PHOSPHORUS, AND HUMIC ACIDS

Abstract

In temperate regions a distinct part of phosphorus (P) in soils is associated with organic matter. Organic macromolecules can desorb from the soil matrix and, consequently, act as carriers for P in environments where it would otherwise be immobile. However, not all organic forms of P in soils have yet been characterized, and there is much debate about the involvement of Fe and Al in the association between organic matter and P. We studied the association between P, dissolved organic carbon (DOC), Al, and Fe in alkaline and aqueous extracts of an agricultural sandy soil. In these extracts, the high molecular weight fraction of the DOC was precipitated by adding CaCl2 or HCl. This selective precipitation of the humic fraction of the DOC resulted in the complete removal of Al and Fe from solution, whereas the removal of total P was only partial. The molar ratio for organic carbon (C) over (Al + Fe) in the precipitate was ∼1. This low ratio makes it highly unlikely that most of the Al and Fe are covalently bound to the humic material; rather, it suggests that Al and Fe oxides are, in colloidal form, associated with the humic macromolecules. Another important conclusion was that Al and Fe and a large fraction (more than 50%) of total P were associated with the humic and not the fulvic fraction of the DOC. The different chemical and physical characteristics of these two DOC fractions give them different transport behavior in soils, and this difference is expected to affect the behavior of at least a part of the "organic" P in the soil environment.