Turnover of P in the Swedish Long-Term Soil Fertility Experiments.

Since 1950, there has been an accumulation of fertilizer P in Swedish arable soils amounting to 700 kg P ha^-1, on average. Ten soil fertility trials run for more than 50 years consisting of two cropping systems including farmyard manure addition and mineral fertilisation were used to address questions about the fate of soil and fertilizer P. How is extractable soil P (0.5 M ammonium acetate lactate) affected by different fertilizer P regimes over the long-term? How much soil P must be present in exchangeable form to ensure optimum yield of different types of crops? To what extend is fertilizer P recovered in crops? At all sites, changes of extractable P were linear over time. One major finding was that replacement of P removed by crops through fertilizer P was not sufficient to maintain concentrations of extractable soil P. Correlations between rates of change in extractable P and different soil properties showed that only soil pH values were significantly correlated (R²=0.97**). The higher the pH value in soil, the more fertilizer P was bound in extractable form. Critical levels of extractable P to maintain high yields differed between crop types being highest for sugar beet and canola and lowest for winter wheat. Efficiency calculations revealed that as much as 80% of fertilizer P was recovered in crops using the balance method whereas less than 50% was utilized applying the difference method.