Rate of soil P decline due to crop uptake. Long term curves of depletion

Two field experiments were established on Ustochrept soils at two locations in Emilia-Romagna, in order to follow and describe the rate of decline of available P due to uptake by crops. P availability was tested by the extraction of P using a 0.5 M Na-bicarbonate solution, following the method proposed by Olsen et al. (1954). Over 15 years, Olsen P values decreased with time, but never fell within the deficiency range. Sometimes, however, control plants (P0) showed slightly lower annual P removal than plants regularly supplied with amounts that just replaced those taken up (P1) or higher (P2). An exponential equation (y = a e^bx) was selected to fit the data of Olsen P decline. The function may be compared to that calculated and published for an Umbraquult soil by McCollum (1991). The yearly decline in Olsen P went smaller and smaller with increasing depletion of soil P reserves, but the ability of plants to satisfy their nutrient requirements remained substantially unaffected. It seems reasonable to suggest that at low Olsen P values, some other processes, mainly biological, take place in the soil, mobilizing low-soluble P reserves relatively unavailable to the NaHCO3 extractant. When reserves of easily exchangeable P become sparse the Olsen method may be less efficient in tracing P bioavailability.