"trees." . . "maize." . . "fallow." . . "soil fertility." . . "nitrogen fixation." . . "sesbania sesban." . . . . "kenya." . . "agroforestry." . . "tanzania." . . . "Planted tree fallows and their influence on soil fertility and maize production in East Africa : nitrogen fixation and soil nitrogen dynamics" . . . . . . . . . . . . .