MSc

A study was undertaken on ‘Exploration of native mineral phosphate solubilizing microorganisms as biofertilizer for the acidic soils of Kerala’. The main objective was to
exploit native microorganisms with mineral phosphate solubilization and plant growth
promoting activities for the acidic soils of Kerala.
Two hundred microbial isolates (173 bacteria and 27 fungi) from the repository maintained at Dept. of Agril. Microbiology were screened on Pikovskaya’s agar medium.Phosphate Solubilization Efficiency (PSE) ranged from 106.6% to 555.5%. PSE was higher in bacteria but fungi were faster in solubilization of P. Twenty most efficient isolates were selected for quantitative estimation of P-solubilization , using Mo-blue
method. Two fungal isolates namely PSF-183 and PSF-182 solubilized maximum amount of P (103.5μg/ml and 102.5μg/ml respectively) in 14 days. A corresponding drop in pH
was recorded in all the isolates, which indicated acid production.
The 20 selected isolates were also characterized using cultural, biochemical, physiological and molecular tests. All the isolates tested positive for catalase and oxidase production. PSB-14 ranked first with respect to tolerance to acidity, followed by PSB-22. Bacterial isolates were identified as Acinetobacter sp., Providencia sp., Achromobacter sp., Providencia alcalifaciens, Bacillus cereus, Stenotrophomonas maltophilia, Ochrobactrum sp., Pseudomonas sp., and Burkholderia sp. using 16S rDNA sequencing.Fungal isolates were identified as Aspergillus sp., Penicillium sp., Trichoderma sp. and yeast. Six isolates of Aspergillus niger and A. flavus were discarded and fourteen isolates were further tested for plant growth promoting (PGPR) and antagonistic activities. IAA production was noticed in four isolates: Providencia sp., Pseudomonas sp., Penicillium sp. and Trichoderma sp. Significant enhancement of seedling vigour in cowpea was observed with Pseudomonas sp. followed by Providencia sp. (seedling vigour index of 3058.66 and 2954.33 respectively). The fourteen isolates were further tested in
vitro for antagonistic activity against major soil borne pathogens (Rhizoctonia solani, Fusarium oxysporum, Sclerotium rolfsii, Pythium aphanidermatum and Ralstonia solanacearum). Most of the isolates inhibited R. solani whereas none was effective
against the bacterial pathogen R. solanacearum. Based on the efficiency of P-solubilization and plant growth promoting activities, three native isolates Providencia sp., Pseudomonas sp. and Trichoderma sp. were selected for evaluation in pot culture experiment, with cowpea (variety Bhagyalakshmi) as
the test crop. KAU commercial formulation of P-solubilizing microorganisms was used as reference culture. Three treatments of P were included as no P, rock phosphate and
factamphos.Pseudomonas sp. (PSB149) recorded maximum shoot weight, root weight, no. of nodules/ plant, pod length, no. of grains per pod and pod yield per plant, in combination
with rock phosphate. Chemical analysis of plants after harvest revealed that P content in roots and grains was highest in KAU reference bio-inoculant, whereas highest P in shoot
was observed in Pseudomonas sp. There was an increase in available P content of soil when Pseudomonas sp. (PSB149) was applied along with rock phosphate (17.67mg/kg).
Final population of P-solubilizing microorganisms in soil was highest in Pseudomonas sp. (33X104cfu/g). This is indicative of the rhizosphere colonization efficiency of the bioinoculant Pseudomonas sp.The in vitro and pot culture experiments revealed the efficiency of Pseudomonas sp. as a P-solubilizing microorganism. When tested in pot culture, this isolate in combination with rock phosphate gave the same pod yield as that of factamphos, indicating its suitability as a biofertilizer for Kerala soils. Evaluation under field conditions will be
required for validation of the results, before the bacterium can be recommended as a biofertilizer.