An improved integration of mycorrhizas may increase the sustainability in plant production. Two strategies for increasing the soil inoculum potential of mycorrhizal fungi were investigated in field experiments with leeks: Pre-cropping with mycorrhizal main crops and pre-establishment of mycorrhizal cover crops. Experiments on soils with moderate to high P content (26-50 mg kg(-1) bicarbonate-extractable P) showed that the previous crop influenced mycorrhiza formation, uptake of P, Zn, and Cu, and early growth of leek seedlings. A cover crop of black medic, established the previous autumn, increased the colonization of leek roots by mycorrhizal fungi. During early growth stages, this increase was 45-95% relative to no cover crop. However, cover cropping did not significantly increase nutrient concentration or growth. These variables were not influenced by the time of cover crop incorporation or tillage treatments. Differences in colonization, nutrient uptake and plant growth diminished during the growing period and at the final harvest date, the effects on plant production disappeared. High soil P level or high soil inoculum level was most likely responsible for the limited response of increased mycorrhiza formation on plant growth and nutrient concentrations.