Long-term phosphorus desorption using dialysis membrane tubes filled with iron hydroxide and its effect on phosphorus pools

Many agricultural fields that have received long-term applications of phosphorus (P) often contain levels of P exceeding those required for optimal crop production. Knowledge of the effect of the P remaining in the soil (residual effect) is of great importance for fertilization management. Plant P availability of residual P in soils is usually estimated using successive cropping experiments carried out in field or greenhouse studies. As this approach is very expensive and time consuming, more rapid soil test methods that can approximate this biological measure are required. The objective of this paper was to use a different approach to evaluate P availability (desorption) over a long period of time instead of the classical means of extraction. Thus, a modified sequential P extraction procedure using dialysis membrane tube filled with ferric hydrate solution (DMT-HFO) was used on the long-term P fertilized soils that received differential P treatments (PoLo, P1L1 and P2L1) to determine the changes in the different P pools and to relate these P fractions with maize yield. In this study, the contribution of both the labile and non-labile Pi fractions in replenishing the solution Pi was significant where as the organic fractions appeared to have limited contributions in replenishing the solution P. Highly significant correlations were observed between dry matter yield and the P pools extracted by HFO-Pi (0.997*), HCO3-Pi (r = 0.994**), OH-Pi (r = 0.969**), OH-Po (r = 0.944**), D/HCl-Pi (0.991**), and C/HCl-Pi (r = 0.997**). Strongly significant correlations were also observed between the different P fractions and plant P uptake. The C/HCl-Pi was the fraction that decreased most especially for the high P treatments indicating that this fraction contributed significantly to the P extracted by DMT-HFO. This suggested that this fraction might be a buffer to more labile P fractions. The combined method employed here could act as an analytical tool to approximate successive cropping experiments carried out under green house condition. But the applicability of this method at a field level should also be assessed. Data from a wider range of soils is also needed to evaluate the universality of this method.