Utilization of plant growth promoting rhizobacteria as root dipping of seedlings for improving bulb yield and curtailing mineral fertilizer use in onion under field conditions

The present study was conducted on onion variety 'Punjab Naroya' in a factorial randomized block design during 2016 -17 and 2017 -18 in a loamy sand alluvial soil of semi-arid region. There were 14 treatments consisting of two factors. Factor I comprised two levels of mineral fertilizers (100% NPK and 75% NPK) and factor II consisted of six combinations of microbial inoculants along with a control. Microbial inoculants were applied through root dipping of seedlings for 30 min before transplanting. The study was aimed to identify microbial consortia that can reduce the use of mineral fertilizers by 25% without any loss in bulb yield and to identify microbial consortia that can act as potential supplement to recommended dose of mineral fertilizers (100% NPK) in improving bulb yield of onion. We have identified two treatments viz., 100% NPK + T-6 (Azotobacter sp. + Sphingobacterium sp. + Burkholderia sp.) and 100% NPK + T-1 (Azotobacter sp. + Bacillus sp.) that have registered 12.0% and 7.5% increase in bulb yield over uninoculated control (100% NPK). The study has also led to the identification of three treatments viz., 75% NPK + T-6, 75% NPK + T-1 and 75% NPK + T-5 (Azotobacter sp. + Bacillus sp. + Burkholderia sp.) manifesting 8.6%, 6.1% and 3.7% higher bulb yield over uninoculated control (100% NPK). Besides, various microbial inoculants significantly improved the plant growth parameters (plant height, number of leaves), yield attributes (bulb yield, 20-bulb weight and equatorial diameter of bulb), bulb quality attributes (ascorbic acid, total soluble solids, pyruvic acid and dry matter) and reduced the physiological loss in weight during 15-120 days of storage at room temperature. The triple inoculation treatment with Azotobacter sp. + Sphingobacterium sp. + Burkholderia sp. gave the best results for most of the traits followed by dual inoculation treatment with Azotobacter sp. + Bacillus sp. However, the soil chemical (pH, electrical conductivity, organic carbon, available nitrogen, phosphorus and potassium) and microbial properties (bacterial, actinomycetes and fungal count) exhibited non-significant improvement after harvest of onion crop. The interaction effects between mineral fertilizers and PGPRs were non-significant for all the traits. Therefore, it can be concluded that the integrated utilization of these microbial inoculants as root dip of seedlings along with 75% NPK offers the scope of reducing the application of mineral fertilizers to onion crop by 25% without any loss in bulb yield and to reduce environmental pollution.