Effect of nitrogen fixer, phosphate and potash solubilizing bacterial consortium on vegetative growth of rabi onion cv. n-2-4-1 (Allium cepa L.) under field conditions

Introduction

Onion (Allium cepa L.) is one of the most important commercial vegetable crops, widely cultivated for its culinary, nutritional and economic value. In India, successful onion cultivation largely depends on the development of a vigorous vegetative phase, as vegetative growth determines the plant's physiological efficiency and biomass accumulation. Onion is characterized by a shallow and sparsely branched root system, which limits nutrient exploration and makes the crop highly responsive to nutrient availability in the rhizosphere (Kumar, 2001; Brewster, 2008) [5,11] . Vegetative growth parameters such as plant height, number of leaves and leaf length are key indicators of onion plant vigor. A higher number of healthy leaves and greater leaf length enhance photosynthetic surface area, leading to improved metabolic activity and biomass production (Randle, 1997; Tiwari et al., 2012) [18,20] . Poor vegetative growth during early and mid-growth stages often results in reduced physiological performance, emphasizing the need for effective nutrient management strategies. Nitrogen, phosphorus and potassium play a crucial role in regulating vegetative growth of onion. Nitrogen supports leaf initiation and chlorophyll synthesis, phosphorus enhances root development and energy transfer, while potassium contributes to cell elongation, turgor maintenance and biomass formation (Marschner, 2012; Havlin et al., 2014) [10,13] . However, conventional chemical fertilizers often show low nutrient use efficiency in onion due to nutrient losses and disruption of soil microbial activity (Fixen et al., 2015; Bhattacharjee & Dey, 2014) [4,8] . Plant growth-promoting rhizobacteria (PGPR) offer a sustainable alternative by improving nutrient availability through biological nitrogen fixation, phosphate solubilization and potassium mobilization. The combined application of these microorganisms as a microbial consortium provides synergistic benefits, ensuring balanced nutrient supply and enhanced rhizospheric activity. Liquid biofertilizer formulations further improve microbial survival and effectiveness under field conditions (Bashan & de-Bashan, 2005; Meena et al., 2016; Muthusamy et al., 2023) [3, 14, 15] . Hence, the use of liquid microbial consortia represents a promising approach for improving vegetative growth of onion under sustainable production systems.

Objective of the Study

In view of the above considerations, the present study was undertaken to assess the effect of liquid nitrogen fixer, phosphate and potash solubilizing bacterial consortium on vegetative growth parameters of rabi onion (Allium cepa L.) cv. N-2-4-1 under field conditions.

Materials and Methods Experimental Location and Season

A field experiment was conducted during Rabi 2021-22 at the Post Graduate Institute, Mahatma Phule Krishi Vidyapeeth, Rahuri, to assess the effect of a nitrogen fixing, phosphate and potash solubilizing bacterial consortium on the growth and yield of Rabi onion (cv. N-2-4-1). The soil was deep, moderately fertile, black, calcareous, and well drained.

Land Preparation

The field was ploughed once and harrowed twice, with FYM applied uniformly at 20 t ha⁻¹ before tillage to achieve fine tilth. The experiment was conducted during Rabi 2021-22 in a Randomized Block Design with three replications. Gross and net plot sizes were 3.60 × 2.40 m and 3.00 × 2.00 m, respectively.

Raising of Nursery

Onion seeds (var. N-2-4-1) were treated with a consortium at 25 mL kg⁻¹ seed and sown in nursery beds at 10 cm row spacing and 1.5 cm depth.

Transplanting of Seedlings

Healthy, uniform seedlings were dipped in a consortium solution (1 mL L⁻¹) for 10 minutes and transplanted at a spacing of 15 × 10 cm.

Experimental

Application of Consortium

A microbial consortium comprising nitrogen fixing bacteria, phosphate solubilizing bacteria (PSB), and potash solubilizing bacteria (KSB) was applied to onion (var. N-2-4-1). Seeds were treated at 25 mL kg⁻¹, seedlings at transplanting with 10 mL L⁻¹ of water, and soil drenching was done 45 days after transplanting at 3 L ha⁻¹.

Growth Parameters Recorded

Vegetative growth was assessed at 30, 60 and 90 DAT using measurements from randomly selected plants in each plot, averaged for statistical analysis.

The following vegetative growth parameters were recorded:  Plant height (cm): Plant height was recorded from the pseudo-stem to leaf tip in five randomly selected plants per treatment at 30, 60, and 90 DAT.  Number of leaves per pseudo-stem: The number of leaves per pseudo-stem was counted in five randomly selected plants per treatment at 30, 60, and 90 DAT, and the mean was calculated.  Root length (cm): Root length was measured from the bulb base to root tip at 30, 60, 90 DAT, and at harvest, and the mean was calculated.  Fresh weight of shoot and root (g): Fresh shoot and root weights were recorded from five randomly selected plants at 30, 60, 90 DAT, and at harvest, and the mean was calculated.  Dry weight of shoot and root (g): Shoot and root samples were sun-dried and oven-dried at 60°C to constant weight, recorded at 30, 60, 90 DAT, and at harvest, and the mean was calculated.

Statistical Analysis

Data were statistically analysed using CRD for in vitro and RBD for field experiments. Critical differences (C.D.) were calculated at 1% for in vitro and 5% for in vivo studies, following Panse and Sukhatme (1985) [17] when significant differences occurred. Differences were significant at 5%, highlighting the synergistic effect of microbial consortia with NPK on shoot growth.

Results and Discussion

Fresh root weight increased with growth and nutrient treatments. The control (T1) had the lowest biomass (0.2 g at 30 DAT to 0.5 g at harvest), while T2 and T3 showed slight early improvements. T4 (50% NPK + consortium) reached 0.6 g by 90 DAT, and the highest weights were in T5 and T6

(75-100% NPK + consortium), peaking at 0.8 g. T7 (100% NPK + FYM) performed moderately (0.7 g). Differences

were significant at 5%, highlighting the effectiveness of higher NPK combined with microbial consortia in enhancing root biomass.

Shoot dry weight increased with microbial consortia and NPK treatments. The control (T1) ranging from 0.7 g at 30 DAT to 2.4 g at harvest, while consortia alone (T2) reached 2.8 g. Combined treatments showed higher gains: T3 (25% NPK + consortia) reached 3.1 g, and T5-T6 (75-100% NPK + consortia) peaked at 4.1 g. T7 (100% NPK + FYM) achieved 3.5 g. Differences were significant at 5%, highlighting that integrating chemical fertilizers with microbial consortia maximizes shoot biomass.

Root dry weight increased with NPK and consortium treatments. The control (T1) and consortia alone (T2) remained low (0.1 g at 30 DAT to 0.3 g at harvest).

Combined treatments showed progressive gains: T3 (25% NPK + consortia) reached 0.4 g, T4 (50% NPK + consortia) and T7 (100% NPK + FYM) 0.5 g, and T5-T6 (75-100% NPK + consortia) peaked at 0.6 g. Differences were significant at 5%, indicating that NPK plus microbial consortia effectively enhance root growth.

Discussion

The present study clearly demonstrates that application of a liquid bioconsortium consisting of nitrogen fixing, phosphate solubilizing and potash solubilizing bacteria significantly enhanced vegetative growth parameters of Rabi onion. Improvements in plant height, number of leaves, leaf length, fresh biomass and dry matter accumulation indicate superior physiological performance of consortium-treated plants over the control and RDF-alone treatments. Tinna et al. (2020) [19] reported that 100% NPK combined with microbial consortia increased yields by 7. [16] in tomato and by Dadhich and Somani (2007) [7] in sorghum, highlighting the role of rhizosphere microbes in nutrient mobilization and plant vigor. Gadge (2020) [9] reported that seed inoculation with a microbial consortium (Rhizobium, PSB, KMB) plus 75% RDF significantly improved growth and yield (19.48 q ha⁻¹), comparable to 100% RDF, allowing a 25% fertilizer reduction. Similar benefits of PGPR inoculation on onion growth and yield were reported by Colo et al. (2014) [6] and Mahala et al. (2019) [12] with Azotobacter, Bacillus, and Azospirillum combined with fertilizers. Enhanced nutrient uptake likely promoted cell division and elongation, increasing shoot growth and biomass. Similar effects were reported by Pandey et al. (2023) [16] in tomato, Afify et al. (2019) [1] in onion, and Ahmed et al. (2023), where PGPR consortia improved shoot and root growth, including fresh weight, even under stress. Afify et al. (2019) [1] reported that biofertilizers increased onion foliage height, leaf number, and dry weight, while [14] also reported positive effects of microbial inoculants on dry matter accumulation. Pandey et al. (2023) [16] reported that microbial inoculation in tomato increased plant height (86.8 cm), shoot dry weight (23.3 g plant⁻¹), and root dry weight (8.56 g plant⁻¹) in T7 (Azoto-B-24 + PSB-S-91), followed by T8 (Azoto-B-24 + PSB-S-68). These treatments also enhanced Azotobacter and PSB populations and showed the highest dehydrogenase activity (26.3 µg TPF g⁻¹ soil h⁻¹).

Conclusions

The application of a liquid Bioconsortium significantly enhanced the vegetative growth of Rabi onion (Allium cepa L.) cv. N-2-4-1 under field conditions. The consortium resulted in marked improvements in plant height, leaf number, leaf length, fresh biomass accumulation and dry matter production, compared to the control and recommended fertilizer dose alone. Increased vegetative vigor and improved photosynthetic surface area in consortium treated plants indicate enhanced physiological performance. The observed growth enhancement can be attributed to improved nutrient availability through nitrogen fixation, phosphate solubilization and potassium mobilization, along with synergistic microbial activity. This study establishes that the bioconsortium is an effective, ecofriendly solution for improving onion growth and can contribute to sustainable nutrient management by reducing reliance on 25% chemical fertilizers.

Acknowledgement

AcknowledgementAuthors express sincere thanks to the Head, Department of Plant Pathology and Microbiology, PGI, MPKV, Rahuri, Maharashtra, India, for providing necessary facilities and encouragement during investigation.

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