ISRIC Report 2004/02 The Impact of Desertification on Food Security in Southern Africa: A Case Study in Zimbabwe

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VWP van Engelen, S Mantel, JA Dijkshoorn, JRM Huting
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Degradation of the drylands, also known as desertification, of Southern Africa is thought to have a negative impact on the food security of the region. General observations on the degree of the impact have been made over the years but quantification of the process has been lacking. The United Nations Environment Programme, UNEP, requested ISRIC – World Soil Information, to execute a study on Zimbabwe and to quantify the effects of degradation, in particular soil degradation, on the productivity of the country. Zimbabwe has been selected because of its representativeness for the region in respect of natural resources. Also, the country has a long history of research in soil erosion and well-stocked archives of natural resources data.The study consists of a series of modelling exercises using soil and terrain data (SOTER), climate and land use data. The land use under consideration is rain-fed maize cultivation with low inputs and technology; the dominant land use in Zimbabwe. A mixed qualitative-quantitative approach is used, consisting of an assessment of the suitability of the land for the land use under consideration, a soil
loss scenario – 20 years of continuing erosion – and a quantification of the productivity under the current and future soil conditions. Finally a comparison of
the two situations is made.
About 53 % of Zimbabwe is suitable for the specified land use. Constraint-free productivity of maize is from 10 to 15 t.ha-1 for almost half of the country. Modelled
production figures for yields in the suitable area under nutrient limitations are much lower: about 90% of that area has a productivity under 1.5 t.ha-1. The actual yields in the Communal Lands, where the land use type is similar, fall within the lower range. Productivity after 20 years of simulated erosion is lower under the nutrient-limited
modelling conditions: slightly less than half of the suitable land units will lose between 25 and 50% of their, already low, productivity. The possible effects of
climate change in combination with the scenario of erosion will aggravate the food security situation even more if no counter measures are taken. Most soils in Zimbabwe have a low inherent fertility. Yields are low; without appropriate management, soils will be further degraded. Sustainable maize production in Zimbabwe requires nutrient amendments; integrated soil fertility management is the key to sustained crop production.
Keywords: natural resources databases, qualitative land evaluation, crop growth modelling, erosion modelling, food security, land degradation