A general study was undertaken, within a European Union INCO - DC project on sustainable management of Southern Africa Savannas, i) to identify the major constraints to the sustainable management of soil and water resources under different land use patterns, i.e. traditional subsistence agriculture and agro-forestry, commercial agriculture and forestry, natural state and wildlife utilisation; ii) to select a minimum data set of physical, chemical and biological indicators for screening the quality and health of soil in relation to soil use and environment conditions; iii) to define the most appropriate practices and to suggest alternatives to the current land use for promoting long term sustainable development and the conservation of natural resources. The north-eastern region of Botswana (Chobe District) was selected as a case study because the three main land uses described above coexist, although undeveloped areas still predominate. Two main soil types dominate in the region: Aridisols and Entisols on Kalahari sands, and Vertisols on the lacustrine plains near Pandamantenga village. Sandy soils are wind-deposited, deep, light-textured soils (sands and loamy sands) that extensively display almost uniform physical, chemical and biological properties and are characterised by similar management constraints. They have a combined silt and clay content of less than 10% in the upper two metres. In its natural environment the sustainability of the soil ecosystem is extremely fragile, being maintained mainly by the re-cycling of litter by living organisms, both flora and fauna. When human settlement is attempted or initiated, the soil's inherent chemical fertility decreases rapidly as a consequence of clearing and cultivation. Furthermore, fertilisers have very little mid-term ameliorative effects or none at all, as these soils have a poor nutrient retention capability because of their very low cation exchange capacity. The low content of mineral and organic colloids adversely affects physical processes and properties. Low soil moisture availability, and the related reduction in nutrient dynamics and biological activity are further exacerbated by prolonged dry spells and/or frequent droughts. The above considerations suggest that, in general terms, these soils are not suitable for low input agriculture, even under more favourable circumstances. However, when human settlement is inevitable, irrigation should be encouraged and supported wherever possible to alleviate the lack of available soil moisture. With regard to soil fertility management, regular additions of large quantities of cattle or other manure are necessary to maintain the quantity and quality of the organic fraction at a level similar to that found in the soil under natural conditions. In most cases the only sustainable farming system that can be recommended, both from an environmental and economical point of view, is extensive livestock rearing. Conditions might be improved by introducing drought resistant fodder crops, but the selection of locally adapted cattle, or preferably game rearing for meat production, would probably be more sustainable. However, forestry would be the most suitable and sustainable land use on Kalahari sandy soils in many circumstances. Vertisols dominate in Pandamantenga plains and most of land used for commercial agriculture in southern and central plains belongs to this soil group. The northern plain is still undeveloped. A total of 25.074 ha are at present under cultivation in the other two plains the cultivated crops being sorghum, cotton, sunflower, maize, g/nuts and s/beans. The aim of the Pandamatenga commercial farming project was to achieve food self sufficiency in cereal production in Botswana, but this policy objective has not been realised as a result of a number of constraints such as flooding, drought, and lack of dryland farming experience in Vertisols. Pandamantenga Vertisols present all the physical and chemical features typical of this soil order. On the basis of the physical and chemical analyses available from various sources, these soils have moderate to low inherent fertility. One of the most outstanding features is the low content of OM, which is mostly in the form of recalcitrant and physically protected compounds. However, considering the poor fertility of the Kalahari sandy soils, Pandamatenga Vertisols can be considered to be the most suitable for dryland cropping in Botswana in spite of their serious physical constraints and provided rains are sufficient and well distributed during the wet season. Though the mean annual rainfall is about 600mm, it is concentrated in the3 months of December, January and February and there are large fluctuations within and between the seasons. This often results in droughts or prolonged flooding. The very high smectitic clay content, resulting in a low infiltration rate, largely tends to exacerbate this phenomenon which has apparently been the main cause of crop failure in recent years. Although Sandy soils and Vertisols have many contrasting characteristics, an attempt has been made to identify some indicators of soil quality that can be applied equally to the two soil types when they occur under the same environmental conditions. These indicators have been selected by taking into account only their intrinsic soil properties. On the basis of an extensive literature review of the proposals suggested by several authors for assessing soil quality and sustainability, a preliminary screening was carried out. The screening was designed to identify the most appropriate parameters able to provide a realistic picture of soil resources and their potential response to inputs of different levels of intensity, in the region under study. The most suitable physical, chemical and biological parameters were then validated by comparison with the field data obtained for the two different soil types. The final set of parameters, of general validity, adopted for assessing the quality and predicting the sustainability of these soil types in relation to specific land uses, are applied to representative Vertisols of the Pandamantenga development area to assess the long-term viability and potential sustainability of both small-scale and commercial farming. Data from cultivated soils (commercial farms) and from others without apparent management were selected. The data analyses suggests that some physical soil properties (such as the poor drainage and the difficult workability of dry soil, both depending on the abundance and properties of smectite clay) can seriously hamper the sustainable development of subsistence agriculture. However the comparison of cultivated (commercial farming) and uncultivated soils shows a considerable resilience in the soil's physical condition which remains almost unchanged after cultivation. However, a significant loss of chemical fertility results from intensive high input farming. In fact, the selected indicators emerge as a suitable tool for identifying the main constraints for a specific land use for Pandamantenga's Vertisols because the conclusions that can be drawn on the sustainability of different farming systems closely reflect the problems and the limitations encountered by farmers.
Integrated approach to the assessment of soil quality and environmental constraints for a sustainable land use in southern Africa savannas: a Botswana case study
D'Acqui LP
2000
Abstract
A general study was undertaken, within a European Union INCO - DC project on sustainable management of Southern Africa Savannas, i) to identify the major constraints to the sustainable management of soil and water resources under different land use patterns, i.e. traditional subsistence agriculture and agro-forestry, commercial agriculture and forestry, natural state and wildlife utilisation; ii) to select a minimum data set of physical, chemical and biological indicators for screening the quality and health of soil in relation to soil use and environment conditions; iii) to define the most appropriate practices and to suggest alternatives to the current land use for promoting long term sustainable development and the conservation of natural resources. The north-eastern region of Botswana (Chobe District) was selected as a case study because the three main land uses described above coexist, although undeveloped areas still predominate. Two main soil types dominate in the region: Aridisols and Entisols on Kalahari sands, and Vertisols on the lacustrine plains near Pandamantenga village. Sandy soils are wind-deposited, deep, light-textured soils (sands and loamy sands) that extensively display almost uniform physical, chemical and biological properties and are characterised by similar management constraints. They have a combined silt and clay content of less than 10% in the upper two metres. In its natural environment the sustainability of the soil ecosystem is extremely fragile, being maintained mainly by the re-cycling of litter by living organisms, both flora and fauna. When human settlement is attempted or initiated, the soil's inherent chemical fertility decreases rapidly as a consequence of clearing and cultivation. Furthermore, fertilisers have very little mid-term ameliorative effects or none at all, as these soils have a poor nutrient retention capability because of their very low cation exchange capacity. The low content of mineral and organic colloids adversely affects physical processes and properties. Low soil moisture availability, and the related reduction in nutrient dynamics and biological activity are further exacerbated by prolonged dry spells and/or frequent droughts. The above considerations suggest that, in general terms, these soils are not suitable for low input agriculture, even under more favourable circumstances. However, when human settlement is inevitable, irrigation should be encouraged and supported wherever possible to alleviate the lack of available soil moisture. With regard to soil fertility management, regular additions of large quantities of cattle or other manure are necessary to maintain the quantity and quality of the organic fraction at a level similar to that found in the soil under natural conditions. In most cases the only sustainable farming system that can be recommended, both from an environmental and economical point of view, is extensive livestock rearing. Conditions might be improved by introducing drought resistant fodder crops, but the selection of locally adapted cattle, or preferably game rearing for meat production, would probably be more sustainable. However, forestry would be the most suitable and sustainable land use on Kalahari sandy soils in many circumstances. Vertisols dominate in Pandamantenga plains and most of land used for commercial agriculture in southern and central plains belongs to this soil group. The northern plain is still undeveloped. A total of 25.074 ha are at present under cultivation in the other two plains the cultivated crops being sorghum, cotton, sunflower, maize, g/nuts and s/beans. The aim of the Pandamatenga commercial farming project was to achieve food self sufficiency in cereal production in Botswana, but this policy objective has not been realised as a result of a number of constraints such as flooding, drought, and lack of dryland farming experience in Vertisols. Pandamantenga Vertisols present all the physical and chemical features typical of this soil order. On the basis of the physical and chemical analyses available from various sources, these soils have moderate to low inherent fertility. One of the most outstanding features is the low content of OM, which is mostly in the form of recalcitrant and physically protected compounds. However, considering the poor fertility of the Kalahari sandy soils, Pandamatenga Vertisols can be considered to be the most suitable for dryland cropping in Botswana in spite of their serious physical constraints and provided rains are sufficient and well distributed during the wet season. Though the mean annual rainfall is about 600mm, it is concentrated in the3 months of December, January and February and there are large fluctuations within and between the seasons. This often results in droughts or prolonged flooding. The very high smectitic clay content, resulting in a low infiltration rate, largely tends to exacerbate this phenomenon which has apparently been the main cause of crop failure in recent years. Although Sandy soils and Vertisols have many contrasting characteristics, an attempt has been made to identify some indicators of soil quality that can be applied equally to the two soil types when they occur under the same environmental conditions. These indicators have been selected by taking into account only their intrinsic soil properties. On the basis of an extensive literature review of the proposals suggested by several authors for assessing soil quality and sustainability, a preliminary screening was carried out. The screening was designed to identify the most appropriate parameters able to provide a realistic picture of soil resources and their potential response to inputs of different levels of intensity, in the region under study. The most suitable physical, chemical and biological parameters were then validated by comparison with the field data obtained for the two different soil types. The final set of parameters, of general validity, adopted for assessing the quality and predicting the sustainability of these soil types in relation to specific land uses, are applied to representative Vertisols of the Pandamantenga development area to assess the long-term viability and potential sustainability of both small-scale and commercial farming. Data from cultivated soils (commercial farms) and from others without apparent management were selected. The data analyses suggests that some physical soil properties (such as the poor drainage and the difficult workability of dry soil, both depending on the abundance and properties of smectite clay) can seriously hamper the sustainable development of subsistence agriculture. However the comparison of cultivated (commercial farming) and uncultivated soils shows a considerable resilience in the soil's physical condition which remains almost unchanged after cultivation. However, a significant loss of chemical fertility results from intensive high input farming. In fact, the selected indicators emerge as a suitable tool for identifying the main constraints for a specific land use for Pandamantenga's Vertisols because the conclusions that can be drawn on the sustainability of different farming systems closely reflect the problems and the limitations encountered by farmers.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
