ABSTRACT
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Phosphorus deficiency is a widespread constraint on agronomic productivity. This
chapter describes biological processes that enhance phosphorus availability in phos-
phorus-deficient soils, which can form the basis for sustainable approaches to maintaining
phosphorus nutrition. We focus on tropical soils, which are notoriously poor in plant-
available phosphorus, with almost half being considered phosphorus-deficient for
agricultural production (Fairhurst et al., 1999). More than 70% of these are highly-
weathered Oxisols and Ultisols (Soil Survey Staff, 1999), termed Ferralsols in the FAO
taxonomy scheme. These acidic soils are low in exchangeable bases, but rich in well-
weathered material such as 1:1 clay minerals (mainly kaolinite) and sesquioxides of iron
and aluminum (Tiessen and Shang, 1998). This confers a considerable capacity to retain
phosphate, but means that the concentrations available to plants are naturally very low.
Increasing phosphorus availability in such soils is therefore essential for improving
agricultural productivity.
