Influence of Surface Sorbed Organic Matter on Phosphorus Bioavailability.

The concentration of phosphorus in soil solution, the dominant factor controlling the availability of soil phosphorus to plants, is determined in many soils by adsorption and desorption processes of phosphorus onto metal (oxy)hydroxides.  These minerals, with their abundant surface hydroxyl groups, have high affinity sorption sites for both phosphorus and organic molecules.  Adsorption studies have shown that phosphorus bonding can be modelled with two bonding sites: a higher phosphorus affinity site that is relatively uncommon and a more common, but lower affinity, site. We hypothesize that the bioavailability of phosphorus is sensitive to both the level of organic matter in soil solution and to the surface properties of the metal (oxy)hydroxides (i.e. surface coverage with respect to both phosphorus and organic molecules).  We coupled chemical investigation of mineral surface properties with a plant bioassay to determine bioavailability over a range of phosphorus and organic matter concentrations.  The bioassay was conducted with tomato plants.  Seeds were pre-germinated and planted into pots (~300 ml) of agar containing suspended goethite.  The goethite either contained no surface organic matter or was preloaded with organic matter from water extracts of a range of organic materials, including soybean, corn and wheat residues; beef and horse manure; and food compost. The goethite (both with and without surface sorbed organic matter) was loaded with phosphorus to levels representing 0, 10, or 80% coverage of surface exchange sites.  Plants were grown for 5.5 weeks with 16 hours of supplemental light.  Plant shoots and roots were harvested, dried, and phosphorus content determined after digestion using a colorimetric method.