INITIAL RESPONSES OF MAIZE AND BEANS TO DECREASED CONCENTRATIONS OF MONOMERIC INORGANIC ALUMINUM WITH APPLICATION OF MANURE OR TREE PRUNINGS TO AN OXISOL IN BURUNDI

Prunings of Calliandra calothyrsus, Grevillea robusta, Leucaena divers ifolia and farm yard manure were applied each cropping season at 3 and 6 t dry matter ha(-1) to an Oxisol in Burundi. The field plots also r eceived basal applications of nitrogen (N), phosphorus (P) and potassi um (K). Application of the tree prunings or farm yard manure decreased the concentration of monomeric inorganic aluminium (Al) in soil solut ion from 2.92 mg Al dm(-3) in the control plots to 0.75 mg Al dm(-3) i n the plots receiving 6 t ha(-1) Calliandra prunings. The other organi c materials also decreased the concentration of monomeric inorganic al uminium in the soil solution. The lowered Al concentration led to a co rresponding decrease in the percentage Al saturation of the 0-10 cm so il layer from 80% to 68%. Grain yields of maize and beans were strongl y inversely related to the percentage Al saturation of the soil. This confirms that soil acidity was the main constraint to maize and beans production. The yield improvement was mainly attributed to the amelior ating effects of the organic matter application on Al toxicity. The nu trient content had less effect presumably because of fertilizer use. I n the best treatments, the yield of maize increased from 0.9 to 2.2 t ha(-1) and the corresponding beans yield increased from 0.2 to 1.2 t h a(-1).