CROP RESIDUE APPLICATION INCREASES NITROGEN-FIXATION AND DRY-MATTER PRODUCTION IN GROUNDNUT (ARACHIS-HYPOGAEA L) GROWN ON AN ACID SANDY SOIL IN NIGER, WEST-AFRICA

Field experiments were conducted during the rainy seasons of 1989, 1990 and 1991 on an acid sandy soil in Niger, West Africa, to assess the effect of millet straw application (+CR) on growth and N2 fixation of groundnut (Arachis hypogaea L.). Three years of +CR (4 t ha-1 yr-1) increased symbiotic N2 fixation, total dry matter production (haulm plus pods) by 83% and total nitrogen (N) accumulation by 100%. Concentration of N in the shoot dry matter and total N in the soil were only slightly affected by the +CR treatment. Crop residue application increased the concentration of potassium (K) and molybdenum (Mo) and decreased the concentrations of aluminium (Al) and manganese (Mn) distinctly, both in the plant (shoot and nodule dry matter) and in the soil. The increase in dry matter production and N uptake was mainly due to improved N2 fixation reflected by enhanced formation and growth of nodules as well as nitrogenase activity. This was attributed to improved chemical soil conditions, particularly to the higher availability of Mo and the lowered content of available Al and Mn. Although with the application of 4 t CR ha-1, 60 kg K were supplied, increased growth could not be attributed to the additional supply of K.