Changes in Soil P from Long-Term Studies with and without N and P Fertilizer.

It is well-recognized that phosphorus (P) loss from agriculture can contribute to eutrophication of nearby water bodies. It has been suggested that the risk of P loss can be reduced by reducing fertilizer application. However, this may lead to reductions in crop yield if the plants cannot access residual soil P. In Swift Current, SK, a long-term study was initiated in 1967 with various wheat rotations (including continuous wheat and fallow-wheat-wheat), with N and P fertilization or with P fertilization alone. In 1995, these plots were split, and P fertilization ceased on half of each plot. Soil samples were collected each autumn, as were straw and grain. Soils were analyzed for various soil P pools, including total P, total organic P,  and soil test P (STP; Olsen P and Mehlich 3 P. Crop yields were calculated, and straw and grain were analyzed for total P. In samples from 1995 to 2010, plots without N fertilization produced significantly lower yields. However, there were no differences in yield in the plots without P compared to those on which P fertilization continued. There were no significant differences in total soil P or total organic P. However, plots without P fertilizer had drawn down pools of STP significantly in the plots receiving N, compared to plots receiving N and P. Plots receiving P but no N showed significant increases in STP compared to plots with N and P.