Abstract:
The NERICA 1 rice variety was grown in rotation with indeterminate cowpea (Adidome mottled), in the Hohoe and Kwaebibirem Districts of Ghana, from 2007-2010, in a randomised complete block experiment, with three replications. Soil and cowpea samples were analysed for nitrogen (N) and organic carbon (OC) contents. Socioeconomic surveys were also conducted before and after the agronomic experiments. The treatments included: Cowpea + P2O5 + K2O (T1), Cowpea only (T2), Cowpea plus 50 % recommended inorganic N fertilizer, using sulphate of ammonia + P2O5 + K2O (T3), Recommended N, i.e. 90 kg N ha-1 + P2O5 + K2O (T4), 75 % N + P2O5 + K2O (T5), 50 % N + P2O5 + K2O (T6), 25 % N + P2O5 + K2O (T7) and no fertilization, control (T8). The P2O5 and K2O fertilizers (triple super phosphate and potassium sulphate, respectively) were applied basally, at rates of 45 kg P2O5 ha-1 and 35 kg K2O ha-1. At Likpe Todome, total N of cowpea residue was 46.2 kg ha-1. Soil OC under cowpea residue was higher than the control (2.0 % and 1.7 %, respectively); soil bulk density (BD) was lower under cowpea residue than the control (1.2 and 1.4 kg m-3, respectively). Grain yields of rice in the cowpea residue treatments were not significantly different from that of the recommended inorganic N, but they all had higher yields than the control. Grain yields were 3.0, 2.9 and 2.6 t ha-1, in treatments T3, T4 and the control, respectively. At Kukubi, total N of cowpea residue was 75.0 kg N ha-1. Soil OC in the 0-15 cm horizon of the plots with cowpea residue was 1.2 %, compared with 1.1 % in the control. Grain yields in T3 and T4 did not differ significantly from each other, but were both higher than in the control. Grain yields were 1.8, 1.4 and 0.3 t ha-1 in T3, T4 and the control, respectively. Soil OC, and the tillering, TDW and grain yield of rice were higher at Likpe Todome than at Kukubi. The net benefit (financial) of treatment 2 was the highest, followed by treatment 1; treatment 4 had the lowest net benefit the dams are equipped with appropriately designed fish passages.