Recent studies exploring soil fertility on organic dairy farms in Ontario have reported low soil test phosphorus (STP) values; however, the resultant impact on productivity is not clear. It has been suggested that soil biological activity in organically-managed systems has an increased role in providing the crop with phosphorus (P), compared to conventional systems receiving water-soluble mineral fertilizers. The objective of this research was to explore further the relationships between soil test P, arbuscular mycorrhizal fungi (AMF) root colonization, AMF community structure, phosphatase activity, and productivity of organic perennial forage cropping. Forage fields on three long-term (>20 yr) organically-managed dairy farms with low STP and relatively high yields were selected and compared with adjacent conventional dairy farm fields with a long-term history of water-soluble mineral P fertilizer application. Prior to the first forage cut (June 2009), root, plant, and soil samples were collected from each field. Subsequent cuts of hay were obtained over the season to measure total forage yields and P uptake. The data confirm that forage yields on organic fields were comparable to conventional fields despite having a lower STP. AMF root colonization was greater in organic systems, which likely contributed to ensuring sufficient crop P nutrition. An Analysis of Similarity (ANOSIM) using data generated from Denaturing Gradient Gel Electrophoresis (DGGE) images revealed significant differences in AMF community structure between soils under the two management systems; however, differences due to geographical location were greater. This research demonstrates further the crop production potential in organically-managed systems and contributes to a better understanding of soil P fertility and cycling.