AbstractIncreasing the abundance of legumes in mixed pasture and hayfield stands can improve forage nutritional quality and nutrient cycling; however, establishing legumes can be difficult, especially without tillage or chemical suppression of existing vegetation. Soil amendments that reduce the competitiveness of the existing plant community or improve the vigor of interseeded legumes could reduce the need for more intensive management practices at the time of interseeding. We compared a range of commercially available soil amendments for their effects on the no‐till establishment of hairy vetch (Vicia villosa Roth) interseeded into an existing sod. Amendment treatments were applied in September and included wood ash, biochar, lime, and K applied at different rates, and unamended tilled and untilled treatments as controls. Hairy vetch was no‐till interseeded immediately after treatment application and biomass was measured the subsequent June. Results were consistent across the two site‐years and indicated that an intermediate rate of wood ash (8967 kg ha−1) increased hairy vetch biomass in the spring by 8.5 and 12.4 times compared to the tilled and unamended controls, respectively. We used nonparametric multiplicative regression (NPMR) to explore the relationship between soil physical and chemical properties and hairy vetch biomass across a range of wood ash application rates. We found that soil K alone explained 42% of the variability in hairy vetch biomass. This research suggests that wood ash soil amendments may improve the chances of successfully interseeding legumes into perennial forage crops without the use of herbicide or tillage to suppress the existing plant community.