Status of legacy soil P in agricultural fields in the Hullcar Valley, Canada

The Hullcar Valley located in the North Okanagan is characterized by an intensive beef and dairy production. The objective of this study is to present the status and distribution of legacy soil P in four Benchmarks sites selected in the Hullcar Valley. Soils in most of the study area are well to rapidly-drained soils, in the Chernozemic or Brunisolic soil orders, and there are small areas of poorly-drained Gleysolic or Organic soils (Poon et al., 2017). Soil samples were collected at 0-30 cm, 30-60 cm and 60-90 cm depth in four sites cropped with silage corn (sites #10 and #34) and alfalfa (sites #23 and #37) in Mid-October 2016 and Mid-April 2017 and analyzed for Mehlich-3 extractable P and other cations. Mehlich-3 P (PM3) and P saturation index [PSI, (P/Al)M3] in the 0-30 cm soil depth were higher in corn sites compared with alfalfa sites. Mehlich-3 extractable P in the 0-30 cm depth was 159 mg kg–1 in site # 10 and 116 mg kg–1 in site #34, but 139 mg kg–1 in site #23 and 96 mg kg–1 in site # 37. The PSI in the 0-30 cm depth was 26% in sites #10 and #34, but 18% in site #23 and 15% in site #37. Mehlich-3 extractable P and PSI decreased with depth at all sites, but #37 where higher concentrations were obtained at 60-90 cm (PM3 = 82 mg kg–1 and PSI = 31%) compared with the layer above (30-60 cm, PM3 = 56 mg kg–1 and PSI = 9.3%) indicating a downward movement of P from the upper layer in this site. Results of the benchmark testing indicate that soil test P values in the Hullcar Valley are above the excess class according to local recommendations suggesting that fertilization practices including manure application rates resulted in build-up of legacy soil P with time.