rhizosphere the particular area immediately surrounding the roots of plants in which their exudations affect the surrounding microbial flora. Many exudations are nutrients which the microorganisms utilize and include carbohydrates, vitamins and amino acids. Conversely, the plant roots may take up minerals released by the microorganisms. Bacteria may be present in the rhizosphere in numbers fifty times greater than in surrounding soil. Gram-negative rods predominate whereas Gram-positive forms are less common than in the surrounding soil. Fungi are usually present in the rhizosphere in similar quantities to those in the surrounding soil, though some plants encourage particular forms.