The components of plant root exudates are many and complex (Table 1), and serve not only as a source of carbon substrate for microbial growth, but also con- tain chemical molecules that promote chemotaxis of soil microbes to the rhizosphere. Although the root exudates of N2-fixing legumes are known generally for their capacity to attract rhizobia to root hairs, it is in fact their individual chemical components such as flavonoids (Caetano-Anolles et al., 1988), aro- matic acids (Parke et al., 1985), amino acids and dicarboxylic acids (Barbour et al., 1991) that func- tion as specific chemoattractants for micro-organisms (Table 2).