s:972:"%T Plant nutritional problems in marginal soils of developing countries 
%A Smithson P C 
%A Sanchez P A J 
%X This chapter presents the global distribution of the major problem soils of the world, with special emphasis on soil chemical constraints on agricultural production. The focus is on developing countries particularly in sub-Saharan Africa where food security is endangered by soil constraints. The most widespread constraints, aluminium toxicity and phosphorus deficiency are highlighted. It is argued that the major cause of the decline in food production in Africa is not inherent infertility, but rather soil fertility depletion resulting from years of continuous cropping with few external inputs. Various options for agricultural development of problem soils are presented; highlighting the use of organic inputs. Finally approaches currently under study at ICRAF, in which new techniques suitable for assessment of soil fertility status are detailed. 
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