More than 500 years ago, Polynesians began cultivating dryland taro (Colocasia esculenta) and sweet potato (Ipomoea batatas) on the leeward, dry side of Haleakala Volcano on the island of Maui, Hawaii. Their use of digging sticks mixed fine-grained ash with a cinder layer ~50 cm below the surface. By comparing nutrients and carbon in these mixed, cultivated zones with those from adjacent, undisturbed profiles, we quantified nutrient drawdowns and carbon increases associated with ~3 centuries of agriculture. Cultivation was responsible for harvest losses of nutrients but also catalyzed rapid chemical weathering and accelerated leaching losses of nutrients. The depletions of total alkali and alkaline earth elements as well as phosphorus by cultivation were both large and rapid: nearly the same quantity of the potassium lost from soils ranging in age from 20 to 226 ky occurred during the Polynesians' 300-y agricultural tenure. Cultivation also resulted in a three-fold increase in soil organic carbon. Leaching losses of mobile soil nutrients exceeded depletions associated with potato harvest. Assuming leaching of soil P is negligible, differences between cultivated and uncultivated profiles suggest prehistoric sweet potato yields averaged ~4 Mg ha^-1 y^-1.