Productivity and water use efficiency of important crops in the upper oueme catchment: influence of nutrient limitations, nutrient balances and soil fertility.

Abstract

The Upper Oueme catchment in the West African subhumid zone is a region in Northern Benin, which actually experiences major changes in land use, water availability, and population density. In the context of the IMPETUS project, the present work aimed to i) identify nutrients which are limiting productivity on the basis of soil and plant analysis, ii) compare effects of fertilizer application nutrition to current farmer’s practice, iii) determine the water consumption per unit of biomass (maize) and per unit of area, and iv) assess (simplified) nutrient balances to predict long-term trends of nutrient availability and soil productivity. Field experiments were carried out in 2001 and 2002 using a randomized complete block design with four treatments, 2001: n = 80, 2002: n = 109) at three sites: Beterou, Dogue, and Wewe. Soils of the sites had low fertility and were PLENTOSOL and Ferric-Profondic LUVISOL in Beterou, PLENTOSOL and LIXISOL in Dogue and ACRISOL or Plenthic-LIXISOL in Wewe.Treatments were: T0: farmer’s practice or plots without mineral or organic fertilizer applied with exception of cotton, where farmers applied fertilizers as usual; T1M: 10 t ha-1crop residues; T1F: 10 t ha-1 of farmyard manure in 2001; T2: mineral fertilizer at the rates recommended; T3M: mineral fertilizer as applied in T2 + 10 t ha-1 of crop residues for mulch in 2001 and 2002, while for T3F mineral fertilizer was applied as in T2 + 10 t ha-1 of farmyard manure. Residual effects of manure application were evaluated without further OM application. Soil samples were taken before and at the end of the experiment to appreciate the nutritional status of plots. Leaves were sampled at critical stages for plant for nutrient assessment through critical Value Method CVM and Diagnosis and Recommendation Integrated System (DRIS). Yield (DM) of all the crops, their total biomass and harvest index were evaluated at harvest. A partial nutrient balance was calculated on the basis of tissue and product analysis for a high and a low – yielding sub-groups. Actual evapotranspiration was estimated by gravimetry, humidimetry and tensiometry for water use efficiency (WUE) of maize in Dogue in 2002. Rainfall during the crop growth was used to calculate rainfall use efficiency (RUE). Water or rainfall use efficiency was determined as the ratio between above ground biomass and rainfall for RUE or actual evapotranspiration for WUE. Crop productivities were significantly affected by farmer’s practice and the type of organic matter applied. Organic or mineral fertilizer or the combination of both increased crop productivities, RUE and WUE of maize although a relatively high variability was observed between individual plots and farmers. Nitrogen was the most limiting nutrient followed by potassium and phosphorous according to DRIS-Evaluation while the CVM method revealed most of the macronutrients as low or close to the critical level. However, only the nutritional imbalance index of maize decreased from 2001 to 2002. Standard nutrient levels and reasonable DRIS norms were established for N, P, K, Ca, Mg, S, Zn, Mn of maize, sorghum, cotton, yam and groundnut. They can be used to evaluate crop nutritional status, to correct nutritional imbalances and to improve crop productivities. They can also be used as a basis for calibrating the fertilization programs for these crops. Negative nutrient balances were observed, as inputs of nutrients were insufficient to compensate outputs. The strategies to compensate the nutrient gap are to increase the recycling of residues, to increase the application of manure, or introduce fertilizers or a combination of all three. Actual farmers' practices in maize, sorghum, groundnut and yam cropping systems lead to depletion in soil nutrient levels, as there is actually almost no return of nutrients to the fields and mineral fertilizer are only rarely applied. When calculating the balance for a typical yam-cotton-maize-groundnutsorghum rotation, the nutrient balances are negative by 177 kg ha-1 N, 33 kg ha- 1 P and 163 kg ha-1 K. This leads to nutrient depletion (as actually found in the project area) and not sustains adequate yields. The only desirable scenario could be the practice of integrated soil fertility management where mineral and organic fertilizers are combined. Here, one should as well take into account crop rotations with legumes to optimize nitrogen fixation, mineral fertilizer, and efficient management of crop residues. Management methods that limit nutrient losses and increase water use efficiency are some of the approaches that will be used to improve and sustain soil fertility and conversely to enhance crop production and in Upper Oueme Catchment.