ISSN 0367-6315 Korean J. Soil Sci. Fert. 45(6), 998-1003 (2012) http://dx.doi.org/10.7745/KJSSF.2012.45.6.998 Influence of Different pH Conditions and Phosphate Sources on Phosphate Solubilization by Pantoea agglomerans DSM3493 Buddhi Charana Walpola, Mi-Jung Keum, and Min-Ho Yoon* Department of Bio-Environmental Chemistry, College of Agriculture and Life Sciences, Chungnam National University, Daejeon, 305-764, Korea Pantoea agglomerans DSM3493 was isolated from green house soils collected from Chungchugnam-do province, Gongju-Gun area in South Korea and phosphate solubilization and organic acid production of the strain were assessed using three types of insoluble phosphate sources (Ca phosphate, Fe phosphate and Al phosphate) under three different pH conditions (7, 8 and 9). The highest Ca phosphate solubilization (651 µg mL -1 ) was recorded at pH 7 followed by pH 8 and 9 (428 and 424 µg mL -1 respectively). The solubilization rate was found to be 80.4, 98.1 and 88.7 µg mL -1 (for Fe phosphate containing medium) and 9.3, 12.1 and 29.8 µg mL -1 (for the Al phosphate containing medium) respectively at pH 7, 8 and 9. Though increasing pH of the medium caused reduction in the rate of solubilization of Ca phosphate, solubilization of Fe and Al phosphates enhanced with increasing pH. By contrast, the highest amount of organic acid was produced with Ca phosphate while the lowest was recorded with the presence of Al phosphate. Among the organic acids, gluconic acid production was found to be the highest, followed by oxalic acid and citric acid regardless the source of phosphate. Results can thus be concluded that the production of organic acids appears to play a significant role in the inorganic phosphate solubilization. Key words: Organic acids, Phosphate solubilization, Pantoea agglomerans DSM3493 Received : 2012. 9. 10 Accepted : 2012. 11. 22 *Corresponding author : Phone: +82428216733 E-mail: mhyoon@cnu.ac.kr Article Introduction Phosphorous (P), one of the major essential macro- nutrients required by plants, is found in soil as inorganic and organic forms both of which are used as source of fertilizers. However, the majority of applied phosphatic fertilizers are readily fixed in soil and become unavailable to plants. Inorganic phosphates in acidic soils are associated with iron (Fe) and aluminum (Al) compounds; whereas calcium (Ca) phosphates are predominant inorganic phosphate form in neutral or calcareous soils (Gyaneshwar et al., 2002). Therefore, unfavorable pH along with high reactivity of aluminum and iron in soils decrease phosphorous availability as well as phosphatic fertilizer efficiency (Hao et al., 2002). There are some microorganisms reported to be involved in solubilizing insoluble phosphate complexes, and they are called as phosphate solubilizing microor- ganisms (PSMs). The mechanism of phosphate solubili- zation by PSMs is associated with acidification, chelation, exchange reactions and release of low molecular weight organic acids such as gluconic acids, oxalic acids, citric acids, succinic acids etc. (Chaiharn and Lumyong, 2009; Gulati et al., 2010). However, the major microbiological means by which insoluble phosphate compounds are solubilized is through the production of organic acids, accompanied by acidification of the medium. Hydroxyl and carboxyl groups of these acids chelate the cations bound to phosphate, thereby converting it into soluble forms. However phosphate solubilization is a complex phenomenon, which depends on many factors such as nutritional, physiological and growth conditions of the medium. PSMs isolated from different soils have been used in evaluating their mineral phosphate solubilizing activity and organic acid production with various phosphate sources such as Ca phosphate, Fe phosphate and Al phosphates (Fankem et al., 2006). However, information on the effect of pH for organic acid production under different phosphate sources is scanty. Therefore, the present study was conducted to assess the influence of different pH and