Plant Nutritional Problems in Marginal Soils of Developing Countries

Summary

Tropical regions have both the highest population growth rates in the world and a preponderance of soils which are less suitable for agricultural production. The relatively infertile Oxisols and Ultisols occupy about 43% of the tropics but only 7% of the temperate zone, while fertile Mollisols account for 2% of the tropics and 11% of temperate regions. Within the tropics, soils with chemical deficiencies or toxicities total about 47% of South America, 59% of Southeast Asia and 18% of Africa. In Africa, nutrient depletion of originally fertile soils has been caused by decades of continuous cropping with few external inputs, which in turn has resulted in stagnant or declining yields in many areas. Aluminum toxicity is among the most widespread problems in Latin America and Asia. Phosphorus deficiency (often accompanied by high phosphorus fixation) usually accompanies Al toxicity, and also occurs in many otherwise relatively fertile soils. Nitrogen deficiency can be considered as a given in almost all agroecosystems. Appropriate management practices vary according to the major deficiency or toxicity. Nitrogen deficiency may be alleviated in part by improved organic matter management. Leguminous intercrops or fallows, or biomass transfer from off-site, are some options for N management, though it must be recognized that in many situations the loss of time or area to fallows is untenable due to land pressure. For P management, inorganic additions are necessary. Soluble P sources, phosphate rocks and organic amendments should be used in an integrated fashion to recapitalize soil P in nutrient depleted soils. For Al-toxic soils, required lime rates of up to several t ha⁻¹ and high transport costs often preclude liming. Selection and breeding of Al-tolerant and agronomically acceptable varieties are probably the best options for agriculture in acid soils. Along with integrated agricultural systems, newer methods of soil characterization need to be developed and standardized. Technological advances in remote sensing will allow better characterization over large areas in the near future. In the laboratory, newer methods as well as reassessments of older techniques, will make possible better indices of nutrient availability and potential yields.