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We investigated the effects of high biomass yielding winter cover crops, namely grazing vetch (Vicia dasycarpa L.) and oats (Avena sativa L.), on soil phosphorus (P) availability in low fertiliser input maize-based conservation agriculture systems. Soil samples were collected from the 0–50-mm depth of experimental plots after 4 years of maize–winter cover crop rotations. A sequential fractionation scheme was used to separate total soil P into labile, moderately labile and non-labile organic P (Po) and inorganic (Pi) pools. Labile P pools included microbial biomass-P as well as Pi and Po pools extracted using 0.5 M NaHCO3 and 1.0 M HCl. The non-labile P pools were humic-P and 1.0 M H2SO4 extracted P. Soil on the maize–winter cover crop rotations had higher HCl-Pi and total P than the soil on the maize–fallow rotation. The cover crops had no significant (p>0.05) effect on NaHCO3-Po, NaHCO3-Pi, HCl-Po, fulvic acid-P and recalcitrant H2SO4-P fractions. Non-application of fertiliser increased accumulation of humic-P on the maize–oats rotation. Cover crop biomass input explained 73% of the variations in microbial biomass-P and 33% of variations in total labile P. Phosphorus concentration of young maize plants was significantly increased by the cover crops, with a positive correlation to HCl-Pi (rs=0.90). This contribution from winter cover crops to P availability in the surface soil suggests that, in the long term, fertiliser P could be reduced in such systems.