MSc.

The study entitled “Development of inoculant cultures of zinc solubilising microorganisms” was conducted at College of Agriculture, Vellayani during the period 2014 - 2016 with the objective of isolation, characterization and evaluation of zinc solubilising microorganisms from soils of Kerala and to develop inoculant culture of the best zinc solubilising isolate. Microorganisms capable of solubilising zinc were isolated from Agroecological units 20, 21 (Wayanad) and 8 (Thiruvananthapuram) by serial dilution technique on Bunt and Rovira medium containing 0.1% insoluble zinc oxide. Ten isolates of bacteria capable of solubilising insoluble forms of zinc (zinc oxide) were obtained and allotted code numbers from ZSB-1 to ZSB-10. These were subjected to plate assay and broth assay in media supplemented with 0.1 per cent insoluble forms of zinc as zinc oxide or zinc phosphate. After three days of incubation of test plates, all the ten isolates solubilised zinc and produced clearing zone around the colonies on solid media. The size of the solubilisation zone ranged from 8.67 mm to13.33 mm in zinc oxide and from 1.00 mm to 5.33 mm in zinc phosphate incorporated medium. In broth culture, maximum solubilisation of zinc in both sources was observed on 30th day in the range of 35.91 ppm to 104.08 ppm in zinc oxide supplemented medium and 1.38 ppm to 4.15 ppm in zinc phosphate supplemented medium. The isolate ZSB – 4 showed maximum solubilisation of zinc in plate assay and broth assay. For soil incubation study, the isolate with maximum zinc solubilisation (ZSB – 4) was inoculated in zinc deficient soils and analysed for the soil chemical parameters and population dynamics of the bacterial isolate for a period of three months. The treatments were designed as T1 and T2 with Zn at two levels as ZnO, T3 with Zn solubilising culture alone @ 2 kg ha-1, T4 and T5 with Zn solubilising culture @ 2 kg ha-1 supplemented with Zn at two levels as ZnO and T6, the absolute control had no insoluble zinc supplementation and inoculation with the bacterial isolate. The isolate ZSB – 4 significantly increased the available zinc content in soil from 0.55 ppm to 9.47 ppm in treatment T4 (ZSB – 4 @ 2 kg ha-1 + zinc oxide @ 1 kg ha-1) during the incubation period. The same treatment registered the highest mean value for available phosphorus content, 12.09 kg ha-1 and 12.26 kg ha-1 respectively for the 2nd and 3rd month. There was an increase in oxidisable organic carbon content in the 3rd month for treatments T4 (1.44%), T5 (1.43%) and T3 (1.38%) and it was maximum in T4 which was statistically on par with T3 and T5. On the 3rd month, there was a decrease in the available boron content in soil for the treatments T3 (0.30 ppm) and T5 (0.34 ppm) when compared with previous months and the treatment T4 (0.33) maintained the level of boron. The total zinc content of the soil ranged from 0.05 % to 0.08 % during the entire three months and the mean values had no significant difference among treatments. The maximum colony count of 9.3 x 103 cfu g-1 of soil was recorded in the treatment T4 during the 3rd month which was on par with T5 (9.1 x 103 cfu g-1) and T3 (8.8 x 103 cfu g-1). The best isolate ZSB – 4 was subjected to molecular characterization and it was revealed that the organism is Bacillus cereus. Different carrier materials like talc, lignite, vermiculite, vermicompost and perlite were used in the present study to assess survival of ZSB – 4. Among the different carriers tested, the talc powder supported the maximum population of 3.9 x 108 cfu g-1 during the 3rd month. Based on the results of present study it can be concluded that application of talc based formulation of the zinc solubilising bacteria ZSB – 4 (Bacillus cereus) @ 2 kg ha-1along with zinc oxide @ 1 kg ha-1 was found to increase zinc content in soil after incubation. Based on the survival of the isolates in different carriers for the development of inoculant cultures, talc was found to be most suitable carrier for the formulated product.