Tree effects on crop growth on a phosphorus-fixing Ferralsol

africa oriental afrique orientale agroforesteria agroforesterie agroforestry contenido de agua en el suelo crop yield east africa ferralsol ferralsoles ferralsols fosforo grevillea robusta kenia kenya phosphore phosphorus plant competition rendement des cultures rendimiento de cultivos rhizosphere rizosfera soil water content teneur en eau du sol zea mays