Saturday, 15 July 2006

Residual Benefits of Two Cowpea Genotypes and Natural Fallow to Subsequent Maize in the Northern Guinea Savannah of Nigeria.

E.N.O. Iwuafor1, Yusuf A.A.1, Olufajo O.O1, R. Abaidoo2, and N. Sanginga3. (1) Soil Science Dept, Faculty of Agriculture/Institute for Agricultural Research, Ahmadu Bello Univ, Samaru, Zaria, Nigeria, (2) International Institute of Tropical Agriculture, Idiose, Ibadan, Nigeria, (3) Tropical Soil Biology and Fertility Institute of CIAT(TSDF-CIAT), Nairobi, Nairobi, Kenya

Increased yields of maize grown after cowpea than maize has been attributed to enhanced N-availability and other rotation effects. Improvement in soil chemical properties has been indicated to be one of the most important rotation effects. Soil properties and grain yield were evaluated where different crop rotations were used. Two promiscuous improved cowpea lines (IT 96D-724 and SAMPEA-7), natural fallow and a hybrid maize (Oba Super 2) grown in 2003 were followed by a test crop of maize in 2004 with and without nitrogen (N) fertilizer (0 and 90 kg N ha-1). Rotation and N fertilizer had significant (P<0.05) influence on the organic carbon, total N content and cation exchange capacity of the soil and unfertilized continuous maize resulted in the lowest values of these important soil properties. Soil total N of maize plot following SAMPEA-7 was slightly higher (0.920 g kg-1) than that following IT 96D-724 (0.870 g kg-1). The main effect of rotation on exchangeable cations was highly significant with all the rotation systems performing better than the continuous maize. Fertilization significantly affected exchangeable Ca and K concentrations only. Unfertilized maize grain yield were significantly higher following the two cowpea varieties than maize and natural fallow. The average increase of both cowpea cultivars were 50% higher than that of maize-maize and 22% higher than that of the fallow-maize system even though the cowpeas were not inoculated with rhizobia. It was clear that N was the major limiting factor in the soil from response to fertilizer N and the residual effects of the cowpea. However, there was no interaction between rotation and fertilizer effects which indicates that other non-N effects were equally important. The study showed that crop rotation involving grain legumes is a viable management option that helps increase maize yield and can substitute the unproductive fallow system traditionally used for soil fertility maintenance. To attain highest productivity level, it is necessary to combine rotation systems with mineral N fertilizer. Key Words: Cation exchange capacity, Crop rotation, Exchangeable bases, Maize, N Fertilizer, Organic carbon, pH, Total nitrogen.

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