98 ENVIS Introduction Isolation of Potential Phosphate Solubilizing Bacteria from the Rhizosphere of Fast Growing Native Tree Species Ailanthus excelsa Roxb. in Tamil Nadu, India Sangeetha Menon and V. Mohan Institute of Forest Genetics and Tree Breeding, Coimbatore - 641 002 A ilanthus excelsa Roxb. is a large, fast growing, lofty deciduous tree (family: Simaroubaceae), found generally around villages and is widely distributed in Central and South India. It grows well in the semi-arid and semi-moist regions. It is able to adapt itself to a variety of soils including loamy or sandy soils. It prefers well drained soils and avoids clayey and waterlogged soils. It is a strong light demander. It can be effectively used in large-scale afforestation programmes in India. This tree attains a height of 18 to 24m. The leaves, gum and bark are of medicinal value (Dhillon et al., 2007). One of global ecological problems of agriculture and forestry is the problem of ‘over phosphatization’ of soils. Till now process of biological transformation of phosphorus in soil is poorly studied. Several soil bacteria and fungi, notably species of Bacillus, Pseudomonas, Penicillium, Aspergillus, etc. secrete organic acids and lower the pH in their vicinity to bring about dissolution of bound phosphates in soil (Gerretsen, 1948; Sundara Rao and Sinha, 1963; Gaur and Ostwal, 1972). Several reports have examined the ability of different bacterial species to solubilize insoluble inorganic phosphate compounds, such as tricalcium and dicalcium phosphate, hydroxyapatite, and rock phosphate (Goldstein, 1986). Among the bacterial genera with this capacity are Pseudomonas, Bacillus, Rhizobium, Burkholderia, Achromobacter, Agrobacterium, Micrococcus, Aereobacter, Flavobacterium and Erwinia. There are considerable populations of phosphate-solubilizing bacteria in soil and in plant rhizospheres (Sperber, 1958). These include both aerobic and anaerobic strains, with a prevalence of aerobic strains in submerged soils. A considerably higher concentration of phosphate- solubilizing bacteria is commonly found in the rhizosphere in comparison with non-rhizosphere soil (Raghu and MacRae, 1966). Phosphate solubilizing bacteria (PSB) such as Thiobacillus, Bacillus, Pseudomonas, etc. convert non-available inorganic phosphorus present in soil into an available form utilizable by crop plants. These bacteria also produce iron chelating substances, called siderophores, which chelate the iron present in the root zone. Use of these microbial inoculants as bio- fertilizer in nurseries may considerably reduce the requirement for chemical fertilizer as well as save cost, time and labour