Vegetative Composition, Arbuscular Mycorrhizal Fungi Root Colonization, and Biological Nitrogen Fixation Distinguish Organic and Conventional Perennial Forage Systems

Low concentrations of Olsen soil test phosphorus (STP) on organic dairy farms in Canada have been reported, however, the resulting impact on crop productivity is unclear. Th is study evaluated mixed forage (alfalfa [Medicago sativa L.]/ timothy [Phleum pratense L.]) productivity, nutrient uptake, and related soil biological and biochemical properties on organic and conventional dairy farms in Ontario, Canada. Despite a lower STP concentration, mean organic forage yield (8.0 Mg DM ha–1) did not signifi cantly diff er from conventional (9.5 Mg DM ha–1). Th e proportion of N derived from biological nitrogen fixation (BNF) in alfalfa did not diff er by management system, however the total forage BNF-N was greater in the organic fields due to a twofold increase in legume proportions. Alfalfa tissue P concentrations, above the suggested critical value of 2.0 g kg–1 in all systems, were positively correlated with STP (r = 0.82, P = < 0.0001) and significantly greater in conventional (3.3 g kg–1) than organic (2.4 g kg–1). Mean arbuscular mycorrhizal fungi (AMF) root colonization of alfalfa was greater in organic systems (37 vs. 23%) and was negatively correlated with STP (P = 0.0006). Management systems did not diff er in soil alkaline and acid phosphatase activity, or phytase-labile P. It was concluded that legume-based forages can produce acceptable yields at low STP concentrations (<10 mg kg–1) and supports the importance of legumes, legume BNF, and biological processes linked to soil P cycling in aintaining productivity in organic dairy farming systems.