The paper provides an insight into the problem of land degradation in Sub-Saharan Africa, with emphasis on soil erosion and its effect on soil quality and productivity, and proposes a lowland-based rice-production technology for coping with the situation. Crop yields are, in addition to the degree of past and current erosion, determined by a number of interacting variables. This, coupled with the generally weak database on erosion-induced losses in crop yield in spite of the region’s high vulnerability to erosion, makes it difficult to attain a reliable inference on the cause-effect relationship between soil loss and productivity. Available data suggest, however, that the region is at risk of not meeting up with the challenges of agriculture in this 21st century. Based on the few studies reviewed, methodology appears to have an overwhelming influence on the erosion-productivity response, whereas issues bordering on physical environment and soil affect the shape of the response curve. We argue that thesawahecotechnology has the potential of countering the negative agronomic and environmental impacts of land degradation in Sub-Saharan Africa. This is a farmer-oriented, low-cost system of managing soil, water, and nutrient resources for enhancing lowland rice productivity and realizing Green Revolution in the region.

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In this paper, we review the soil fertility characteristics and the nature of material in the West African lowlands in comparison with paddy soils in tropical Asia to examine their potential for rice cultivation. Soil samples collected from major lowland ecosystems, i.e., inland valleys (185 locations) and flood plains (62 locations), in 13 countries (Benin, Burkina Faso, Cameroon, Cote d'Ivoire, Ghana, Guinea, Niger, Nigeria, Liberia, Mali, Senegal, Sierra Leone, and Togo) generally show low values of pH, total C and N, available (Bary-2) P, exchangeable Ca and Mg, effective cation exchange capacity (ECEC), and clay content. These properties of the 87 topsoil samples selected from 247 collected samples are well associated with mineralogical composition. The clay and primary minerals predominantly consist of kaolinite and quartz, respectively, which suggests that the lowland soils in the region have low nutrient-holding capacity and a limited potential for inherent nutrient supply. In general, soil pH, available P, exchangeable bases (Ca, Mg, K, and Na), and ECEC decrease while total C, total N and exchange acidity (Al and H) increase with increasing rainfall. This tendency is mostly explained by the enhanced biomass production and soil weathering sequence governed by the climate. In terms of rice production, the lowland soils in West Africa have lower values of general fertility parameters and poorer mineralogical characteristics compared to paddy soils in tropical Asia, which includes Bangladesh, Cambodia, India, Indonesia, Malaysia, Myanmar, Philippines, Sri Lanka, and Thailand. In addition, deficit levels of S and Zn for rice production are widely observed in the lowland soils in West Africa. These findings suggest that soil fertility characteristics show substantially less potential for rice production in West Africa than in tropical Asia.Discipline: Soils, fertilizers and plant nutrition Additional key words: agro-ecological zones, flood plains, inland valleys, mineralogical composition, sulfur and zinc deficiencyThe present paper is a joint contribution from the projects 'Development of Sustainable Rice Farming Systems in Low Activity Clay

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Lowland sawah is viewed as a sustainable alternative to traditional rice culture in West Africa. Sawah (a bund-demarcated, puddled, leveled, and water-regulated rice field) has received growing research attention lately, but no data exist yet on the system's long-term agronomic impact. In a clayey inland-valley soil in southern Ghana, 10-year-old sawah plots (OSP), fresh sawah plots (FSP), and non-sawah plots (NSP) were maintained under both ponded and nonponded conditions in 2007. The OSP enhanced soil status of exchangeable nutrients compared to NSP. There were relative improvements in soil bulk density, total porosity, and field moisture content (OSP ≥ FSP > NSP), with clear benefits of ponding over non-ponding in OSP. The NSP was so unsustainable that it showed less favourable values of these variables than an adjacent fallowed plot. These soil variables deteriorated with time, with significant differences in FSP. Soil moisture retention data for tension range of 0-300 kPa depicted the importance of puddling and ponding. These results highlight the agronomic benefits of continuous sawah-based rice production. Although the positive effects of puddling on the soil hydrophysical properties were largely responsible for the wide margin in yield between sawah and traditional systems, other yield-enhancing factors, particularly bunds for water control, were also lacking in the latter.Additional key words: age of sawah; moisture retention; Oryza sativa; ponded sawah plot; rice grain yield; seasonal puddling; soil bulk density. ResumenEfectos a largo plazo de las tierras bajas del sistema Sawah en la región Ashanti de Ghana sobre las propiedades fisicoquímicas del suelo y el rendimiento del arroz Las tierras bajas del sawah (campos de arroz encharcados y delimitados por lomos de inundación, dotados de una buena nivelación del terreno y regulación del agua) constituyen una alternativa sostenible al sistema tradicional de producción de arroz en África Occidental. El sawah ha despertado una creciente atención, pero aún no existen datos concretos sobre su impacto agronómico a largo plazo. En el año 2007, bajo un suelo arcilloso de valle interior, se llevaron a cabo unos ensayos de cultivo de arroz tanto bajo condiciones de inundación permanente como temporal, para comparar parcelas de sawah de 10 años de antigüedad (OSP) con otras de nueva implantación de sawah (FSP) y con parcelas no sometidas al sawah (NSP). Se observó un incremento de la fertilidad del suelo bajo OSP en comparación con NSP. Otras propiedades del suelo, como densidad aparente, porosidad total y contenido de agua a capacidad de campo mostraron únicamente mejoras relativas (OSP ≥ FSP> NSP), indicando los beneficios derivados del encharcamiento sólo en OSP. En NSP se obtuvieron peores resultados que en el caso de los barbechos adyacentes. En FSP,