Changes in Soil Phosphorus Pools in Long-Term Wheat-Based Rotations in Saskatchewan, Canada With and Without Phosphorus Fertilization

Phosphorus (P) is an essential nutrient for all organisms, and many crops require P fertilization for optimum yield. However, there are concerns about the P in agriculture, including the sustainability of phosphate sources for fertilizers and water quality problems from P loss in runoff from agricultural lands. Most crops do not use all of the P added each year as fertilizer, leaving residual soil P that could potentially be used by subsequent crops, minimizing the need for additional fertilization. However, more information is needed to understand soil residual P pools, and their availability to crops. In Swift Current, SK, Canada, a long-term study was initiated in 1967, with four wheat-based rotations [including continuous wheat (CW), fallow-wheat-wheat (FWW), fallow-wheat (FW) and lentil-wheat (WL), with P fertilization and with or without nitrogen (N) fertilization. In 1995, P fertilization ceased on subplots in the CW and FWW rotations, and in 2008 for the FW and WL rotations. This study examined changes in soil P pools (total P, organic P, and Olsen P) from 1995 to 2015 for CW and FWW rotations and from 2008 to 2016 for FW and WL rotations, plus crop yield and grain and straw N and P concentrations. Long-term P addition increased concentrations of soil total and Olsen P in FWW, CW and FW rotations, particularly in plots without N fertilization. However, calculated P depletions based on fertilizer addition and crop P removal were negative only for plots without N fertilization. Cessation of P fertilization reduced concentrations of soil total and Olsen P, especially in plots with N fertilization. Annual yields were affected more by N fertilization and precipitation than P fertilization. Grain and straw P concentrations were not significantly reduced with short-term P cessation in FW and WL rotations, but were reduced with longer-term P fertilizer cessation in FWW and CW rotations.