Field experiments were conducted during the rainy seasons of 1990 and 1991 on an acid sandy soil (Luvic Arenosol) in Niger, to assess long-term (since 1986) and short-term (since 1990) effects of millet straw (crop residues) at different amounts (2 t and 6 t ha-I) and modes of application (incorporation, mulching and burning), and of phosphorus (P) fertilization on dry matter yield of pearl millet [Pennisetum g laucum (L.) R. Br.]. Especially long-term, but also short-term application of crop residues increased dry matter yield of pearl millet by more than 60%, whereas their omission decreased yield immediately. Compared to mulching or burning, higher yields were obtained with incorporation of crop residues. When crop residues were applied as mulch, dry matter yield was increased to the same extent by 2 t and 6 t ha-I. Similar dry matter yields were achieved with crop residues (-P) to those with P fertilizer only. An additional yield increase could be obtained by simultaneous application of crop residues and P. Without crop residues, potassium (K) concentrations in the shoot dry matter indicated K deficiency. With crop residues considerable amounts of K (15 kg C I) were provided and raised the K concentrations in the plants to the sufficiency range. Although with crop residues some P was provided• (1.5 kg C 1), the beneficial effects were primarily attributed to increased P acquisition by the millet plants, which was reflected in enhanced root growth in the topsoil (0-10 cm).

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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 N 2 fixation of groundnut (Arachis hypogaea L.).Three years of +CR ( 4 t h a -1 yr -1) increased symbiotic N 2 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 (A1) 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 N 2 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 AI and Mn.Although with the application of 4 t CR ha -I, 60 kg K were supplied, increased growth could not be attributed to the additional supply of K.

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