Phosphorus Distribution in Soils Used to Develop Isotherm Curves.

Soil phosphorus (P) isotherm curve has been used for a long time to assess some of the P properties of soils. Some researchers have been able to use the sorption parameters, maximum sorption (b) and sorption strength (k), as indicator of soil that could pose risk to the environment. However, there are still many questions unanswered regarding the significance and usefulness of the parameters b and k. This study was developed to assess what soil fractions are most affected by P addition during a P isotherm study. Soils from Minnesota, Iowa, Wisconsin, Texas, and Brazil were used in this study. Phosphorus isotherm curves were developed by shaking, for 24 h, 1 g of soil with 25 ml of a 0.01 M CaCl₂ solution with P concentrations ranging from 0 to 125 mg P kg^-1. After shaking, the samples were centrifuged and the remaining soil was allowed to dry, and submitted to a sequential fractionation study. The results of this study showed that about 80% of P adsorbed was present in the water and anion exchange resin fractions, which are considered the most soluble forms of soil P. An additional 10% of the adsorbed P was present in the NaHCO₃ fraction, which is also considered a labile and relatively soluble fraction. The remaining 10% was bound, in majority, to Fe and Al (NaOH fraction), with little bound to Ca (HCl fraction). The results of this research suggest that isotherm studies might not reflect the real sorption potential of soils, and rather estimates a loosely bound P that is mostly attached to cations adsorbed on the soil CEC. Further analysis such as Xray should be performed to confirm where P is really adsorbing during P isotherm studies.