Agricultural intensification within forage systems has reduced grassland floral diversity by promoting ryegrass (Lolium spp.), damaging soil functionality which underpins critical ecosystem services. Diverse forage mixtures may enhance environmental benefits of pastures by decreasing nutrient leaching, increasing soil carbon storage, and with legume inclusion, reduce nitrogen fertilizer input. This UK study reports on how species-rich forage mixtures affect soil carbon, phosphorus, and nitrogen at dry, medium and wet soil moisture sites, compared to ryegrass monoculture. Increasing forage mixture diversity (from 1 to 17 species) affected soil carbon at the dry site. No effect of forage mixture on soil phosphorus was found, while forage mixture and site did interact to affect soil nitrate/nitrite availability. Results suggest that forage mixtures could be used to improve soil function, but longer-term studies are needed to conclusively demonstrate environmental and production benefits of high-diversity forages.