Orthophosphate and Phytate Extraction Differences by Common Tests Used for Soil Phosphorus Evaluation from Soil Components.
Chao Shang, Lucian W. Zelazny, and Duane F. Berry. Dept. of Crop & Soil Env. Sciences, Virginia Tech, Blacksburg, VA 24061
Chemically extractable P has been widely used for P environmental assessment because of its strong relationship with P loss through surface runoff and/or subsurface leaching. In this study, five common extractants (water, 0.01 M calcium chloride, Mehlich I, Mehlich III and acidified ammonium oxalate in darkness) were used to recover two major P forms (orthophosphate and phytate) that were adsorbed at various P saturation levels on Na-saturated goethite, gibbsite, kaolinite and montmorillonite. The purpose of the study was to evaluate how these P forms and mineral types influence the efficiency of these extractants to solubilize P and to examine the inter-correlations between extractable P forms. The results show that phytate has a substantially greater Langmuir adsorption maximum and lower solubility in all extractants than orthophosphate for all minerals. Water and 0.01 M calcium chloride extracted negligible phytate from all minerals while extracted orthophosphate increased with increasing P saturation levels and the following mineral order: goethite < gibbsite < kaolinite < montmorillonite. Mehlich I was less efficient than Mehlich III and ammonium oxalate, which are equivalent in terms of the amount of P extracted. The greatest P adsorption and the lowest P extractability were found with goethite. At similar levels of ammonium oxalate extracted P, extracted orthophosphate by Mehlich I is greater than extracted phytate for all minerals. However at similar levels of ammonium oxalate extracted P, extracted orthophosphate by Mehlich III from goethite is less than extracted phytate from the remaining minerals. We observed that phytate saturation seems to have a great potential to cause colloid dispersion in comparison with orthophosphate for all minerals.