Status of soil test P across Agassiz research farm in British Columbia, Canada

In Canada, the Fraser Valley is the most intensively managed agricultural landscape with the highest livestock per unit area. This region is also home to 70% of the dairy cows in British Columbia (BC) and the dairy industry ranks first among the top contributors to the total provincial farm cash receipts with a 19% share out of $3,075.8 million in 2015 (BC Ministry of Agriculture (BCMA), 2016). Past and on-going nutrient management practices particularly P fertilization are placing a burden on the public and agricultural sector in the region. Land applications of farm manure based on crop N needs simultaneously with large inputs of mineral fertilizer P have contributed to over- P fertilization compared with crop removals. The objective of this study is to present a snapshot of soil test P in the long-term managed fields of the Agassiz research station, Agriculture and Agri-Food Canada. The land is used for research and corn and grass production for feed production to support a Dairy Centre. Composite soil samples were collected in fall 2017 across 26 fields (10 under corn, 2 under corn/grass, and 14 under grass) before the first rainfall down the soil profile (0-15, 15-30, and 30-60 cm) and analyzed for Kelowna extractable P, the conventional soil test method in region. The concentration of Kelowna extractable P in the 0-15 cm depth varied between 27 and 94 mg kg–1 under corn fields, 55 to 93 mg kg–1 under corn/grass fields, and 12 to 126 mg kg–1 under grass fields. The concentration of Kelowna extractable P in the 15-30 cm depth varied between 17 and 66 mg kg–1 under corn fields, 61 to 80 mg kg–1 under corn/grass fields, and 12 to 67 mg kg–1 under grass fields. The concentration of Kelowna extractable P in the 30-60 cm depth varied between 9 and 33 mg kg–1 under corn fields, 22 to 28 mg kg–1 under corn/grass fields, and 6 to 37 mg kg–1 under grass fields. On average, six fields including corn (2), grass (3), corn/grass (1) productions showed Kelowna P values above the excess class (> 80 mg kg–1) according to local recommendations. These fields also showed the highest Kelowna P concentrations at depth 15-30 cm indicating potential downward movement of P. We conclude that fertilization practices in corn and grass production systems at Agassiz Research Farm including manure application rates resulted in build-up of legacy soil P with time