Velvet beans, fast growing leguminous cover crops used in the humid tropics, are shallow rooted on acid soils. This might be due to an inherent branching pattern, to an intrinsic toxicity of the acid subsoil or to a relative preference for root development in the topsoil. Such preference could be based on soil chemical factors in the subsoil or on physical factors such as penetration resistance or aeration. In a field experiment with two species of velvet bean (Mucuna pruriens var. utilis and M deeringiana all topsoil was removed and plants were sown directly into the acid subsoil (...)