Browsing by Author "Wopereis, M. C. S."

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  • Long-term fertility experiments for irrigated rice in the West African Sahel: Effect on soil characteristics
    Haefele, S. M.; Wopereis, M. C. S.; Schloebohm, A. M.; Wiechmann, H. (Elsevier B.V., 2003)
    Long-term fertility experiments (LTFEs) are a tool to investigate the sustainability of cropping systems. The present study analyzed two LTFEs for intensive rice-based irrigated systems in the Senegal River valley at Ndiaye and Fanaye (Sahel savanna). The trials were established in 1991, contain six different fertilizer treatments and rice is grown two times per year. Soil types are a typical Orthithionic Gleysol and an Eutric Vertisol at Ndiaye and Fanaye, respectively. The objectives of the presented study were to analyze the effect of intensive irrigated rice cropping on the soil resource base by studying the changes of soil characteristics over time and by comparing soil N, P and K pools in different fertilizer treatments. In the LTFE at Ndiaye, topsoil pH values increased significantly from 5.5 to about 6.5 and electrical conductivity was high but remained stable. Soil organic carbon (SOC) and total soil nitrogen (TSN) dropped slightly after 16 consecutive seasons but the difference was statistically not significant. At both sites, exchangeable N ranged between 1.6 and 2.8% of TSN and fixed N accounted for 5.58.2% of TSN, with slightly higher values in Fanaye. Treatment differences in N dose had no significant effect on these parameters. Results of *13C analysis showed a decrease due to rice cropping at both sites, and the measurements indicate high turnover rates of soil organic matter. Soil analyses of total soil P and K and of different pools indicated only small changes when these elements were applied at medium quantities. In contrast, treatments with N application only showed considerable soil P and K depletion, and rice cultivation without P and/or K application cannot maintain soil fertility. The soil mining process is relatively quick for P due to the naturally low soil P status, whereas the high soil K reserves buffer even important negative K balances for decades. It is concluded that irrigated rice cultivation in the region can maintain soil fertility if at least medium P doses are applied together with nitrogen.
  • Soil fertility management in irrigated rice system in the Sahel and Savana regions of West Africa
    Donovan, C.; Wopereis, M. C. S.; Guindo, D.; Nebie, B. (1999)
    In irrigated rice production in West Africa, nitrogen (N) and phosphorus (P) fertilizers make up about 20% of total production costs. This research seeks to evaluate whether those fertilizers are profitable under current use by farmers and to identify the factors that may improve fertilizer efficiency and profitability. A combination of farmer surveys and on-farm trials were used to determine actual fertilizer use, costs, and net revenues from fertilizer in key irrigated system in Mali (Office du Niger), Burkina Faso (Kou Valley), and Senegal (Thiagar and Guede). A second paper provides an agronomic analysis of soil fertility management at these sites. Net returns to fertilizer use were estimated and value/cost ratios (VCRs) calculated. A value/cost ration of 1.5-2.0 was considered desirable for farmer adoption under West African conditions. Average VCRs for fertilizers ranged from 1.6 in the Kou Valley in Burkina Faso, to 3.6 in the Office du Niger, Mali. In researcher-managed on-farm trails in Thiagar, Senegal, fertilizers had VCRs of 1.5-3.1. Large N does of 180kg N ha -1 still had VCRs of 2.5. Farmers in the region used less than recommended doses of N., with the exception of farmers in the Office du Niger, Mali. With an average application of 89 kg N ha-1, farmers in Thiagar, Senegal were able to gain 54,000 FCFA ha-1(US$110) of net revenues from the fertilizers with fertilizer N recovery rates averaging 34%. Improving that recovery rate by 50% could increase net revenues by 50%. In field trails in Thiagar, increasing N application to 180 kg ha-1 resulted in net returns of near 200,000 FCFA ha-1 for an investment of 118,000 FCFA ha-1. Risk of negative net returns was found in Thiagar and in Kou Valley, and was related to weed infestation and water scarcity, respectively. Farmers in the dry season in Kou Valley had a 33% probability of net losses with fertilizer application because of unreliable water supply. In contrast, no farmers had negative returns in the Office du Niger despite of high N application rates. Farmers can improve profitability and economic efficiency in irrigated rice production in two ways. First, they can modify crop management practices (date of seeding, date and mode of fertilizer application, etc.) in order to improve the recovery rate of applied N, thus relaxing the nitrogen at least to recommended levels in most areas. Credit allocations that restrict fertilizer purchases are counterproductive given the profitability of fertilizers. Negative returns were found only in areas with high risks of water scarcity or weed infestation. Researchers and development agents need to develop more site-specific fertilizer recommendations that correspond to weather, cultivars, prices of inputs and outputs, and fertilizer products.