Potential Availability of Colloidal Phosphorus From the Soil Solutions of Andisols and Oxisols.

Andisols and Oxisols contain large amounts of minerals (Al/Fe oxyhydroxides and allophane) known to effectively adsorb phosphorus (P). Colloidal P constitutes an important part of water extractable P in these soils; however, its availability is poorly understood. We hypothesized that in Andisols and Oxisols colloidal P may contribute to the pool of plant available phosphorus and that this pool has been previously overlooked. A laboratory incubation was conducted to assess the availability of colloidal P from soil water extracts of two Andisols (from Chile and New Zealand) and an Oxisol (from Australia). Soil samples (8 g) were shaken with deionized water (80 mL) for 24 h. Thereafter, the water extracts were obtained by centrifugation and the supernatants collected. The supernatants were radiolabelled with carrier free ³³P (5 kBq) and left to equilibrate by shaking. At day 1, 3, and 14 after equilibration, subsamples (20 mL) of the supernatant were filtered through 0.45 μm (fraction A) and 3 kDa (fraction B) membrane filters. An unfiltered fraction was also kept (fraction C). All filtered and unfiltered samples were analyzed for concentrations of total P, molybdate-reactive P, soluble Al, Fe, Mn, and dissolved organic carbon. The activity of ³³P was measured by liquid scintillation counting. Preliminary results showed that in Andisols colloidal P was isotopically exchangeable but not in the Oxisol which may be related to the specific chemical properties of the soils.