with 4 figures and 3 tables Summary. Though gully erosion is often mentioned as a major process of land degradation in South Eastern Tanzania, little information is available on its distribution. The Makonde plateau and adjacent inland plains are of particular concern as they are densely populated and are major areas of cashew nuts production. The occurrence of gully erosion was assessed in 66 villages selected by stratified random sampling in an area of 13,000 km 2 . Difference in susceptibility to gully erosion between landscape units was assess by determining topographic threshold parameters of 22 gullies on the Makonde plateau and 14 in the inland plains. Overall, gullies are common and spread equally over the different landscape units. Their occurrence is positively associated with terrain roughness (Cramér's V = 0.30; P = 0.05) and negatively with population density (V = 0.44; P Ͻ 0.01). On the Makonde plateau occurrence of gully erosion is associated with the presence of roads, while on the inland plains it is predominantly found in fields (V = 0.37; P Ͻ 0.05). This association is explained by the high susceptibility of the Makonde plateau to gully erosion and is due to the particular nature of its deep, highly weathered, sandy soils. Appreciating differences in susceptibility to gully erosion between landscape units is most relevant for targeting soil conservation measures.Résumé. L'érosion en ravine dans le Sud Est de la Tanzanie: répartition spatiale et seuils topographiques. -Bien que l'érosion en ravine soit souvent mentionnée comme processus important de dégradation de la terre dans le Sud Est de la Tanzanie, peu d'information est disponible sur sa répartition. Le plateau Makonde et les plaines intérieures adjacentes sont d'intérêt particulier étant densément peuplé et étant l'aire principale de production des noix d'anacardier. L'occurrence de l'érosion en ravine a été évaluée dans 66 villages répartis sur une superficie de 13.000 km 2 et sélectionnés par échantillonnage aléatoire stratifié. La différence de susceptibilité à l'érosion en ravine entre les unités paysagiques a été évaluée en déterminant les paramètres du seuil topographiques de l'érosion de 22 ravines sur le plateau Makonde et 14 dans les plaines intérieures. En général, l'érosion en ravine est courante et est également répartie dans les diverses unités paysagiques. Leur occurrence est positivement associée à la rugosité du terrain (V = 0.30 de Cramér; P = 0.05) et négativement avec la densité de population (V = 0.44; P Ͻ 0.01). Sur le plateau Makonde l'occurrence de l'érosion en ravine est associée aux routes, alors que sur les plaines intérieures on la trouve principalement dans les champs (V = 0.37; P Ͻ 0.05). Cette association s'explique par la susceptibilité élevée du plateau Makonde à l'érosion en ravine et est principalement due à la nature particulière de ses sols pro-
Land degradation in South Eastern Tanzania, the country's major cashew producing area, has been attributed to deforestation. By comparing land-use/cover maps derived from aerial photographs of 1965 with maps derived from satellite images of 2002, we assessed how land-use changed in six villages, and relate these to local and global drivers. Land-use/cover changes are complex processes, which we analyzed by determining the relative net changes, losses, persistence and gains of each land-use/cover categories. Widespread planting of cashew trees only started in the 1960s; while the 'villagisation' program in the 1970s, altered settlement patterns as centrally planned villages were created. Population growth and rural development policies were major local drivers for land-use/cover change; international trade and technological innovations were principal global drivers. Though population increase led to a reduction of natural vegetation, the spread of cashew trees resulted in a case of 'more people, more trees.' How far the ensuing deforestation affected the biodiversity of the area and how sustainable the production of cashew nuts actually is, remains yet unresolved questions.
RUSLE, the revised universal soil loss equation, is widely used for estimating potential soil erosion by water. Field measured model factors are however scarce for tropical regions. We derived RUSLE factors for several soil conservation measures based on three seasons of field plot measurements (2008-2010), in two contrasting landscape units of South Eastern Tanzania, the country's most important cashew growing area. Whereas the derived factors are useful for applying RUSLE in similar environments, this study points to the importance of understanding differences between soil types and landscape units when assessing potential soil erosion. On the Makonde plateau, rainfall erosivity was higher than on the inland plains (7,130 vs 5,783 MJ mm ha-1 h-1 year-1). The soil erodility K factor was also higher (0.014 t h MJ-1 mm-1) on the sandy Cutanic Acrisols of the Makonde plateau than on the clayey Acric Ferralsols (0.006 t h MJ-1 mm-1) of the inland plains. Likewise, soil loss on the Makonde plateau was much higher than on the inland plain (e. g. for maize 33-127 t ha-1 season-1 vs 3-10 t ha-1 season-1). The differences between the C factor for "maize", and for "maize with crop residues", as well as the differences between the P factors for "lemon grass strips" and "ridges and furrows" indicate that although soils of the Makonde plateau are more susceptible to soil erosion, these soils are also more responsive to soil conservation measures compared to soils of the inland plains. Farmers' local technique of making "ridges and furrows" is particularly effective. Furthermore, the C factors for cashew groves are one order of magnitude lower (0.08-0.09) than for "maize", or for "maize with crop residues" (0.2-0.7), but are still much higher than for "bush fallow" (0.001). Deforestation for agriculture hence bears the risk of increasing soil erosion rates; however, this risk can be minimised by growing cashew trees.
The response of sesame to fertilizer was studied in 11 trials at four different sites, in which phosphorus was applied at rates from 0 to 26 kg ha' 1 to the seed bed and nitrogen at rates from 0 to 60 kg ha" 1 after thinning. Yield responses, found only to the main effects of nitrogen and phosphorus, are discussed in relation to soil type and previous cropping history of the sites and used to calculate gross cash returns from fertilizer use. Seed oil content was affected by environment, but responses to nitrogen and phosphorus were inconsistent. RESUMEN-SUMARIOLa respuesta del sesamo a los fertilizantes, se estudio en 11 pruebas en cuatro sitios diferentes, en las cuales se aplico fosforo a tasas entre O y 26 kg ha" 1 al semillero y posterior al raleo se aplico nitrogeno a tasas entre O a 60 kg ha" 1 . En este trabajo se discuten las respuestas en rendimiento, encontradas solo en relacion a los principales efectos del fosforo y el nitrogeno, en relacion con el tipo de suelp y la historia de cultivos previos en los sitios y se usaron para calcular los retornos brutos en dinero como consecuencia del uso de fertilizantes. £1 contenido de aceite en las semillas fue afectado por el medio ambiente, pero las respuestas al nitrogeno y al fosforo fueron inconsistentes.As in many other countries, sesame [Sesamum indicum L.) grown by smallholders in south-east Tanzania almost never receives artificial fertilizers, but it is known to grow best on fertile soils and in parts of East Africa, including Tanzania, comes early in the crop sequence under shifting cultivation (Acland, 1980).Poor and patchy growth of locally-cultivated sesame can more often than not be traced to low soil fertility but yield responses to fertilizer application
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