Current uncertainties in assessing the colloidal phosphorus loss from soil

Excessive phosphorus (P) loss from soil to water bodies is known to cause negative impacts in the aquatic ecosystem. Along with dissolved reactive P, colloidal P has been of the interest in the agroecosystem research because of its mobility in surface runoff and subsurface flow in soil. The physicochemical characteristics (i.e., high adsorption capacity and low density) of soil colloids can accelerate the migration of P and its mass. Numerous investigations have reported the occurrence of the release of colloidal P from arable and forest soils. While these studies showed clear evidence of colloidal P in soils, it is difficult to single out a factor influencing the release/formation of colloidal P because of variability in experimental designs, methods, and study sites: (1) land and nutrient management, (2) soil type, (3) climate or irrigation, (4) fractionation methods (e.g., filtration, centrifugation) and (5) experimental designs to isolate colloidal P (e.g., soil extract, column leachate, simulated runoff). This review chapter summarizes the current knowledge of colloidal P in soils, and discuss uncertainties in assessing the colloidal P loss from various soil systems.