Potential growth-promoting metabolites were assayed in vitro for each of these strains, and data examined for correlates with growth enhancement. The metabolites evaluated were (i) phosphate solubilizing agents, (ii) siderophores capable of converting ferric iron to soluble forms, and (iii) indole-3-acetic acid (IAA) and auxin analogues. Phosphate solubilizing agents can stimulate plant growth where phosphates are limiting, especially in acid soils in which phosphorus is rapidly immobilized after addition to the soil as a soluble fertilizer becoming unavailable for assimilation by plants