Horizontal and Vertical Distribution of Soil Phosphorus Forms Following One-Year Farrowing Sows on an Outdoor Hog-Rearing Farm in Eastern Ontario.

“Humanely-raised”, “Pastured-raised”, “Raised without the use of antibiotics” are new marketing niche pork products associated with outdoor hog farming systems. Outdoor hog has the potential for re-using on pasture the nutrients provided in supplemental feeds, but runoff is a concern. Despite the increasing demand by consumers and economic returns, little is known on the spatial distribution of P forms due to the herd behaviour of hogs on the paddock. This knowledge would be essential in sustaining this production system and reducing potential environmental risks. The objective of this study was to characterize the spatial variability of soil P within a paddock in an outdoor hog-rearing farm following one year of farrowing sows. The study was conducted on a commercial outdoor hog-rearing farm at Keene, Ontario. The farm was implemented with six grid samplings [principal area (10 m × 10 m distance between each 64 sampling points), feeding area (4 m × 4 m distance between each 16 sampling points), two wallow areas (1 m × 1 m distance between each 16 sampling points), and two huts (1 m × 1 m distance between each 16 sampling points)]. At each point, soil was collected at 0-15, 15-30, and >30 cm and analysed for Olsen P (P_Ol), total P (P_T), and Organic P (P_O). The soil P status in the paddock was significantly different among the six areas and the extent was affected by soil depth. In the 0-15 cm depth, the highest mean value of P_Ol was found in the feeding (25.6 mg kg^–1) followed by the wallow (19.7 mg kg^–1) and huts (19.4 mg kg^–1) and the lowest in the principal area (12.3 mg kg^–1); in contrast, at depth > 30 cm, P_Ol was greater in the huts (39.2 mg kg^–1) followed by wallow (25.3 mg kg^–1) and feeding (22.8 mg kg^–1) and the lowest value was found in the principal area (5.7 mg kg^–1). Whereas, in the 0-15 cm depth, P_T was more abundant in the wallow (880 mg kg^–1) and principal area (815 mg kg^–1) followed by the feeding (731 mg kg^–1) and huts area (657 mg kg^–1). The P_T decreased with depth in the principal area but, remained constant in the huts and wallow areas. The distribution of P_O among the six areas and down the soil profile paralleled that of P_T with the highest mean value found at depth 0-15 cm in the wallow (718 mg kg^–1) and the lowest at depth > 30 cm in the huts (481 mg kg^–1). The proportion of P_T that was P_O varied between 0.81 in the wallow and 0.73 in the huts. Our results indicate that soil P distribution is not uniform across the studied paddock. Instead, there are hotspots of high soil P concentrations which seems to be influenced by the behaviour of farrowing sows. Our results also indicate that P transfer down the soil profile in the feeding, hut and wallow areas occurs in the more labile form. More frequent repositioning of the feeding area and hut locations across the paddock and alternatively installation of leaching resistance sheets under these areas would be necessary to limit P transfer down the soil profile.