Effect of Cowpea Organic Residues and Fertilizer N on Soil Fertility, Growth and Yield of Upland Rice.
Gilbert U. Okereke, Nnandi Azikiwe University, Awka, Nigeria, Samuel E. Egwu, Ebonyi State University, Abakiliki, Nigeria, and Peter Nnabude, NAU, Awka, Nigeria.
Increased pressures on land resulting from rapid population growth in many developing countries of the tropics and sub-tropics have resulted in soil deficient in organic matter and nitrogen. Since farmers in these developing countries, cannot afford costly agricultural inputs such as fertilizer, the use of cowpea residues in cowpea/rice cropping system could be an affordable soil management technology for sustainable upland rice production in such countries. In the first experiment, 30 cultivars of cowpea were screened to select culivars with high grain and dry matter yield. The second experiment was designed to determine the best level of fertilizer N that will cause maximum yield of upland rice, on plots previously incorporated with the selected cowpea cutivars. In the screening study, 30 cowpea cultivars were planted on 30 plots with 4 replications plot-1. Nodule number was counted at the flowering stage while shoot dry matter, total N and grain yield were recorded at harvest. The range in nodule number, dry matter yield, total N and grain yield of the 30 cowpea cultivars were 16-26 plant-1, 2.7- 5.8 t ha-1, 51-126 kg ha-1 and 0.47-1.28 t ha-1 respectively. Cowpea cultivars IT 86D – 440, IT 86D – 1056, IT-86D-1010 IT 85D – 3550 – 1, IT 86D – 2850- 2 and Ife Brown which had the highest nodule number, shoot dry matter, total N and grain yield were selected for subsequent cowpea rice rotation experiment. In the follow up field experiment, three levels of nitrogen (0, 40, and 80 kg N ha-1) were applied on rice plots where residues of the 6 selected cowpea residues were incorporated. Soil analysis after incorporation of cowpea residues and prior to rice planting showed that the incorporated cowpea residues significantly improved the N of the soil compared to the fallow plots probably due to organic matter decomposition and subsequent release of NH+4-N to the soil. The growth performance of rice grown on cowpea residue incorporated plots show that tiller number, shoot dry weight, N uptake and grain yield of rice were significantly improved relative to the fallow treatment. The higher soil N and improved growth performance of rice due to incorporation of cowpea residues, suggests that the yield of rice in marginal soils, can be improved by using cowpea green manuring in cowpea rice cropping system. At 40 kg N ha-1 fertilizer application, grain yield under fallow plots was 2.3 t ha-1 representing an increase of 1.6 t ha-1 which is a substantial increase in grain yield for the fertilizer supplement. These result show that inspite of higher yield resulting from cowpea residue incorporation, maximum yield can only be achieved by the application of 40 kg N ha-1 inorganic fertilizer, as further increase in fertilizer level to 80 kg N ha-1 did not cause a significant increase in grain yield.