Decrease in soil test phosphorus levels under omitted phosphorus fertilizer application

Many European cropped soils have high soil test P (STP) values in the top soil because of P accumulation over many years of fertilizer application. This should allow to save P fertilizer applications for some years without STP values decreasing to a level that might negatively impact crop yield. However, the way STP develops under omitted P fertilizer application is not well understood. We examined STP development under omitted P fertilizer application for timeframes between 7 and 46 years on 96 unfertilized treatments (P0 treatments) of 43 European long-term P field experiments, using five different STP methods. For comparability, values obtained by different STP methods were converted to Olsen-P concentrations. We fitted exponential decay curves to Olsen-P data of each P0 treatment defined by initial Olsen-P values (Olsen- Pi), rates of decrease (k) and asymptotes(A), reflecting minimum obtainable STP. Subsequently, we analysed whether the variables most commonly recorded in experiments, are sufficient to explain the variation in model parameters, these variables being P export, clay content, C org and pH as well as average annual temperature and precipitation. We found that out of our predictor variables, soil clay content, precipitation and temperature were showing the most prominent effects on the parameters Olsen- Pi, A or k. However, the amount of variation explained by the considered variables was too low to potentially facilitate a prediction of STP decrease, and various P0 treatments showed no clear Olsen-P decrease or unexpectedly high asymptotes. This hints at a strong influence of the P sorption capacity of the soil with often high potential for replenishment from less available P pools. In connection with P introduction from the subsoil or possibly from surrounding plots, the extension timeframes of omitted P fertilizer application without reaching critical STP values for crop production, might be explainable. Corresponding effects could not be analysed because of lack of data for most P0 treatments, calling for the additional determination of, for example, the maximum P sorption capacity, total P and sub-soil P in future experiments