Plant and Soil 245: 169–180, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 169 Appropriate farm management practices for alleviating N and P deficiencies in low-nutrient soils of the tropics Paul C. Smithson 1,3 & Ken E. Giller 2 1 International Centre for Research in Agroforestry, P.O. Box 30677, Nairobi, Kenya 2 University of Zimbabwe, Box MP 167, Mt. Pleasant, Harare, Zimbabwe. 3 Corresponding author ∗ Key words: agroforestry, nitrogen, organic matter management, phosphorus, rock phosphates Abstract Tropical upland regions have both the world’s highest population growth rates and a preponderance of soils less suitable for agriculture. Nitrogen deficiency is almost universal, while acid infertility and the related problem of P deficiency affect more than 40% of tropical soils. Nutrient depletion, as opposed to inherent infertility, affects large areas due to continuous cropping with few inputs. Nitrogen management requires a continual supply of N that can be achieved through fertilization, green manuring, legume rotations or leguminous tree-shrub fallows. Rotations and fallows usually require foregoing one or more staple crop harvests, which is not feasible for smallholder farmers in land-limited areas. In such areas there are few options for farmers except mineral N fertilizers. Phosphorus availability can be built up in soils, but to achieve this, external inputs of inorganic P are essential. Use of P-efficient crops, or mobilisation of soil organic P by various means, are temporary solutions since they involve only offtake with no addition. Phosphorus additions may be either as soluble processed fertilizers or indigenous phosphate rocks (PRs). Most tropical PR deposits are unreactive and require processing before use. Some lower-cost options include partial acidulation, blending with soluble P sources or microbial solubilization. The relevance of microbial solubilization in particular requires testing at larger scale. There are no magic solutions to soil nutrient deficiencies or toxicities; to maintain productivity mineral fertilizers are necessary. They should be used in judicious amounts and coupled with improved organic matter management. Introduction Increasing world population requires that agricultural production increase so as to keep up with basic food needs. This is especially true in tropical regions, where population growth rates are the highest in the world. At the same time, soils which are less suitable for ag- ricultural production are relatively more abundant in tropical regions compared to the temperate zone, while the chemical inputs needed for high productivity are often in scarce supply in remote areas and beyond the economic grasp of smallholder farmers. Soils of low inherent fertility, particularly Oxisols and Ultisols, cover large areas of the tropics world- wide. Soils with one or more chemical constraints to agricultural development (primarily acidity and re- ∗ FAX No.: +254-2-524001; Tel.: +254-2-524000; E-mail: p.smithson@cgiar.org lated problems) occupy about 47% of South America and 59% of Southeast Asia, compared with 22% of North America and 33% of Europe (Dent, 1980). While Dent (1980) estimates only 18% of African soils to suffer from chemical constraints, this fig- ure rises to 32% when only soils not affected by drought or other non-chemical constraints are con- sidered (Woomer and Muchena, 1996). The situation in sub-Saharan Africa can be con- trasted with much of the tropics, as Africa has large areas of soils which were once moderately fertile, but in which continuous cropping with few external inputs has led to fertility depletion. Recent studies give an estimate of the magnitude of the problem. Smaling et al. (1997) have made estimates of nutrient depletion at farm, national and continental scales in sub-Saharan Africa; estimated annual per-hectare losses in Africa are 22 kg N, 2.5 kg P and 15 kg K. Loss estimates vary widely, but are negative throughout sub-Saharan