Soil P Status in a Long-Term Corn-Soybean Rotation as Affected by Tillage and P Fertilization.

Agricultural management including tillage system and fertilization may influence soil P status and dynamics. Phosphorus has been found to accumulate in the surface soil layer in long-term and fertilized agricultural systems under no till (NT) management. We investigated the effect of tillage system (NT versus mouldboard ploughing (MP) and P fertilization on soil P status. The study was conducted on a clay loam soil in a long term experimental site with corn -soybean rotation established since 1992 in Quebec, Canada. Soils were sampled (0–15 cm and 15–30 cm) during fall 2007, 2008 and 2009 in plots that received 0, 17.5 and 35 kg P ha^–1 and 160 kg N ha^–1 every 2 yr since 1992. Soil P status was determined by Mehlich-3 (P_M3) and water extraction (P_w). Soil P fractionation and soil biomass P and C were also performed on soil sampled in fall 2008 and fall 2009, respectively. Soil P_w and P_M3 varied significantly with tillage, P fertilization and season. In both NT and MP, the P_w and P_M3, increased with increasing fertilization rate, but the rate of increase was greater in the NT than CT. The P_w concentration was in averaged 6.2 mg P kg^–1 in NT and 5.2 mg P kg^–1 in MP. Total soil P was higher by 20% under NT compared to MP as reported by other studies. The P/Al saturation index was 0.03 under CT and 0.04 under NT. Soil biomass P increased with P rates only in the 0-15 cm layer under NT. This study indicated that there is an accumulation of soil P under NT system with low potential environmental problems, considering the critical P_w value of 9.7 mg P kg^–1 and the (P/Al)_M3 saturation rate of 0.05.