Phosphorus (P) is an essential nutrient for plants, including grasslands. However, continuous applications of P fertilizer result in P accumulations in the soil, increasing the risk of P losses through runoff and erosion. Since 2008, more than 31 million tonnes of organic fertilizers, representing more than 95,000 tonnes of P₂O₅, were applied to agricultural fields in Eastern Canada. Thus, grassland systems were fertilized intensively using organic fertilizers with high P concentrations, increasing the risk of P environmental losses in runoff. The sustainable management of P is based on balanced fertilization between P supply and the real demand for soil P. It is important to well understand the spatial variability of P in grassland soils in order to improve the economic and rational use of P fertilizers and to promote the profitability and sustainability of agricultural companies, while reducing P environmental losses. The objective of this study was to investigate spatial variability of soil available P of two commercial fields under two contrasting grassland systems in Eastern Canada. Two commercial fields, referred as young grassland (YG; 2.4 ha; 2 years) and old grassland (OG; 2.5 ha; 10 years) were selected. Both soils of fields were classified as Humo-Ferric Podzols, and received organic fertilizers (manure, slurry manure) following local recommendations. Descriptive statistics and geostatistical tools were used to define spatial variability of soil P. A 16 m by 16 m triangular grid was performed to sample soils at two depths (0–5 and 5–20 cm). Soils were analyzed for available P and aluminum (Al) by the Mehlich-3 method (M3). The agri-environmental indicator (P/Al)_M3 was calculated. Statistical and geostatistical analyzes were performed using SAS and GS+ software, respectively. Under YG, average P_M3 (52 mg/kg) and (P/Al)_M3 indices (3%) were similar  in both layers.  Under OG, these P indices were higher (125 mg/kg and 7%) in the 0–5 cm layer than 5–20 cm layer (75 mg/kg and 4%). Intensity of variation of P indices was high (CV> 50%) and moderate spatial structure (25–75%) was observed for P_M3 and (P/Al)_M3 on both fields. Repeated application of organic amendments can have a long-term impact on soil P accumulation, thereby reducing the variability and spatial dependence of P in permanent grasslands. A soil sampling strategy based on the 0–5 cm layer should be adopted in permanent grasslands for sustainable P recommendations in Eastern Canada.