Root inhabiting arbuscular mycorrhizal (AM) fungi can promote the acquisition of nutrients, particularly phosphorus, as part of their resource-based symbiosis with plants, and in turn, may have an effect on plant fitness and adaptation to nutrient levels. We hypothesized that plant populations adapt to their local soil nutrient environment by adapting to their local AM fungi for the purpose of phosphorus acquisition. We tested this hypothesis by manipulating the presence of AM fungi in soil biota and levels of soil-phosphorus in a reciprocal transplant experiment using two populations of Lobelia siphilitica from nutrient poor soils. We found evidence of adaptation to local AM fungi only when soil-phosphorus was low, suggesting that when soil-phosphorus is limiting, AM fungal communities are the primary cause of plant adaptation to soil biota.