Crop Production
Microbial inoculants help crops grow better by making it easier for plants to get nutrients, grow stronger roots, and stay healthy. This helps plants grow more evenly and produce higher yields.
Microbial inoculants are used as biostimulants to improve plant growth and yield in crop production systems, as they can enhance crop tolerance to abiotic stresses such as drought and heat, which are becoming more frequent. Besides promoting plant growth, beneficial microorganisms can also protect plants against biotic stresses such as pathogens by producing antimicrobial compounds. Biostimulants can contribute to sustainable crop production as part of a mitigating strategy tailored to site-specific conditions.
Microbial inoculants mainly include nitrogen-fixing bacteria, arbuscular mycorrhizal fungi (AMF), and plant growth–promoting rhizobacteria (PGPR), and can contain single strains or mixtures of microorganisms showing additive or synergistic effects. These products are commonly available as liquid formulations, granules, powders, or pellets, and they carry living organisms, so climate conditions during transportation and storage, such as temperature and humidity, are extremely important. Some require a brief time, between treatment and planting, and proper application to ensure effectiveness under field conditions.
Application Methods & Challenges
Seed inoculation is the most common and practical technique compared to direct soil application, as it can deliver microorganisms directly to the plant rhizosphere. Table 1 highlights some of the benefits provided by microorganisms to the soil or plant. Although many studies in laboratory and greenhouse conditions show positive results from the use of microbial inoculants, consistent responses in the field are still limited. This inconsistency may be related to difficulties in microorganism survival and establishment on the seed or within the rhizosphere, due to unfavorable environmental conditions, biotic factors such as competition with the resident microbial community, or other reasons. Field studies across different soil and environmental conditions are necessary to improve confidence in these products. The objective of this study was to evaluate the field performance of different biological products applied as seed treatment across multiple locations and growing seasons in Alabama.
Table 1. Agronomic Benefits Provided by Seed-Applied Microorganisms
| Microorganism genus | Benefit |
|---|---|
| Azospirillum1 | Stimulates root growth and fixes nitrogen by releasing hormones at germination. This helps the plant quickly build a larger, deeper root system to absorb soil moisture and nutrients better. |
| Bacillus2 | Aids in disease control and phosphorus solubilization. These bacteria form tough spores on the seed; upon germination, they create a protective barrier against fungal pathogens and help free up unavailable soil phosphorus. |
| Bradyrhizobium3 | Essential for soybeans, these slow-growing bacteria specialize in biological nitrogen fixation (BNF). They nodulate roots to capture atmospheric nitrogen, converting it into plant-available fertilizer and eliminating the need for synthetic nitrogen. |
| Delftia4 | Improves the root environment by making phosphates available and producing compounds that suppress pathogens. This provides a clean, nutrient-rich zone for vigorous early growth. |
| Glomus5 | Arbuscular mycorrhizal fungi (AMF) act as an extension of the root system. They form a network of threads to explore more soil volume, significantly increasing water uptake during dry spells and capturing immobile nutrients such as phosphorus. |
| Kosakonia6 | Colonizes the plant to fix supplemental nitrogen and release volatile gases. These gases inhibit harmful fungi near the seed, protecting the germination process and reducing early season disease pressure. |
| Methylobacterium7 | Feeds on methanol released by the seed during germination. In exchange, it supplies hormones that speed up seedling emergence and improve tolerance to early season dry conditions. |
| Pseudomonas8 | Aggressively competes for free iron in the soil, which starves out disease-causing fungi. It also primes the plant’s defense mechanisms against environmental and biological stressors. |
| Rhizobium9 | Fast-growing bacteria focused on BNF and root development. They quickly infect young roots to form early nodules, supply nitrogen, and produce hormones that encourage strong early root growth and stand establishment. |
| Trichoderma10 | A beneficial fungus that aggressively attacks soilborne pathogens (mycoparasite). It targets the fungi responsible for damping-off and root rot, helping ensure a uniform, healthy soybean stand. |
Field Trial Setup & Conditions
During the 2022 and 2023 growing seasons, field trials were established at three different Alabama locations: E. V. Smith Research Center in Shorter (EVS), Tennessee Valley Research Center in Madison (TV), and Prattville Agricultural Research Unit in Prattville (PARU) in a no-tillage system, with rye as a winter cover crop. Fertilizers, herbicides, and pesticides were applied following the recommendations of the Alabama Cooperative Extension System. The same commercial soybean cultivar, treated with insecticide and fungicide, was used in all years and locations. Different biological treatments were applied to the seeds before sowing. Seeds were treated according to each label’s recommendation.
Data Collection & Economic Analysis
To evaluate the performance of the treatments, stands were counted to estimate plant population. At full maturity, the plots were harvested, and the yield was obtained. A sample of seeds from each plot was collected, and the seed protein and oil content were analyzed. To evaluate the economic return of each seed treatment, the return on investment (ROI) per hectare was calculated. The soybean price was based on the national average for each corresponding year, according to the United States Department of Agriculture. The cost of each seed treatment was determined based on the current commercial pricing of each specific treatment product.
Plant Stand Results
Microbial inoculation treatments did not affect the initial plant stand in most cases. At the same time, a specific variation occurred in Shorter, Alabama, during the 2023 season, the control treatment and most of the biologicals performed similarly. In practice, this demonstrates that seed inoculation has neither hindered nor provided an advantage for soybean seedling emergence.
Yield Performance Across Treatments
No statistical difference was observed among treatments for yield, so the plots originating from treated seeds produced a similar yield to the control (untreated seeds). However, when analyzing individual products, performance varied numerically by region and year. In 2022, for example, Treatment 7 achieved the best average at EVS (42.8 bu/ac), while Treatment 2 led at TV (25.3 bu/ac). In 2023, the yield peak occurred at EVS with Treatment 8 reaching 76.6 bu/ac, and the same treatment recorded the worst performance at TV that year. In PARU (2023), Treatment 1 achieved the highest numerical yield (43.8 bu/ac), while Treatments 7 and 8 recorded the lowest.
Environmental Influence on Results
Performance varied across locations and years, indicating strong environmental influence. In year 1 (2022), a severe drought in July and August directly affected soybean pod-filling stage and lowered final yield, especially at TV, compared to 2023. Soybean response to microorganisms varied according to soil properties, weather conditions, and the native microbial community already inhabiting the site. If conditions are not favorable, the microorganism applied to the seed does not survive long enough to benefit the plant’s growth. Grain quality results were consistent: none of the seed treatments outperformed the control across the five evaluated environments (no statistical difference observed). Soybean protein content ranged from 33.7 percent to 36.6 percent, and oil content remained between 18.5 percent and 20.1 percent. This confirms that seed inoculation did not affect the composition or nutritional value of the harvested grain.
Economic Return (ROI) Analysis
A critical point for producers is ROI, which varied substantially by treatment and location (tables 2 and 3). In 2022, most treatments resulted in financial losses (negative ROI) at EVS and TV. Only Treatment 7 covered the technology costs, delivering a profit in both areas: $25.10/ac and $5.00/ac at EVS and TV, respectively. Conversely, Treatment 2 (Urea) resulted in the worst overall financial impact, as the high cost of the input did not translate into harvest gains. In the 2023 season, the economic scenario was even more unstable. At EVS, Treatment 8 generated the highest profit ($43.70/ ac) due to its numerical yield gain. At TV, this same Treatment 8 resulted in a loss of $28.10/ac, due to a local yield drop. At PARU, the financial balance was predominantly negative for almost all tested products.
Therefore, the evaluation of microbial inoculant seed treatments in soybeans across five site years in Alabama showed no consistent improvement in plant stand, grain yield, seed composition, or ROI. The results varied widely across treatments, years, and locations, reinforcing that the efficacy of microbial inoculants is a highly context-dependent trait, influenced by environmental conditions, soil characteristics, and microbial interactions.
Although there is growing interest in microbial inoculants, many products have not shown consistent results under field conditions. For producers, this means there is currently no universally effective biological solution. Until more site-specific strains and targeted decision tools are developed, on-farm validation, such as testing new products side by side using a randomized strip design before broad commercial application, is essential. Only through this careful, context-specific evaluation can microbial inoculants be used effectively to support a more sustainable crop production without unnecessary financial risk.
Table 2. Soybean Yield, Yield Gain, and Return on Investment (ROI) for Microbial Inoculation Treatments in 2022 at Shorter, Alabama (EVS), and Madison, Alabama (TV)
2 Urea treatment (50 lb/ac), with no microorganisms.
3 not significant
| Treatment | EVS Yield (bu/ac) | TV Yield (bu/ac) | EVS Yield gain (bu/ac) | PARU Yield gain (bu/ac) | TV ROI1 ($/ac) | PARU Yield gain ($/ac) |
|---|---|---|---|---|---|---|
| Untreated control | 40.8 | 23.5 | N/A | N/A | N/A | N/A |
| Urea2 | 37.4 | 24.9 | -3.4 | 1.3 | -106.50 | -34.80 |
| Azospirillum brasilense, Bacillus licheniformis, B. amyloliquefaciens, B. subtilis, Pseudomonas fluorescens, Rhizobium | 38.7 | 23.7 | -2.1 | 0.1 | -36.30 | -3.00 |
| Trichoderma virens | 40.4 | 25.3 | -0.4 | 1.8 | -10.00 | 23.50 |
| Bradyrhizobium japonicum | 38.6 | 23.7 | -2.2 | 0.1 | -36.90 | -1.00 |
| Bacillus subtilis, Bacillus amyloliquefaciens, Bradyrhizobium japonicum | 40.1 | 24.9 | -0.7 | 1.3 | -15.20 | 16.20 |
| Bacillus velezensis | 39.0 | 23.7 | -1.8 | 0.1 | -30.80 | -1.60 |
| Bradyrhizobium elkanii; Delftia acidovorans; Bacillus velezensis | 42.8 | 24.1 | 1.9 | 0.6 | 25.10 | 5.00 |
| Bacillus velezensis | 38.4 | 23.7 | -2.4 | 0.1 | -39.30 | -1.30 |
| Glomus intraradices, G. mosseae, gG. aggregatum, G. etunicatum | 40.7 | 25.2 | -0.1 | 1.6 | -6.00 | 20.80 |
| P>F | ns3 | ns3 | N/A | N/A | N/A | N/A |
Table 3. Soybean Yield, Yield Gain, and Return on Investment (ROI) for Microbial Inoculation Treatments in 2023 at Shorter, Alabama (EVS), Madison, Alabama (TV), and Prattville, Alabama (PARU)
2 Urea treatment (50 lb/ac), with no microorganisms.
3 not significant
| Treatment | EVS Yield (bu/ac) | TV Yield (bu/ac) | PARU Yield (bu/ac) | EVS Yield gain (bu/ac) | TV Yield gain (bu/ac) | PARU Yield gain (bu/ac) | EVS ROI1 ($/ac) | TV ROI1 ($/ac) | PARU Yield gain ($/ac) |
|---|---|---|---|---|---|---|---|---|---|
| Untreated control | 72.9 | 63.8 | 41.7 | N/A | N/A | N/A | N/A | N/A | N/A |
| Azospirillum brasilense, Bacillus licheniformis, B. amyloliquefaciens, B. subtilis, Pseudomonas fluorescens, Rhizobium | 72.1 | 63.8 | 43.8 | -0.7 | 0.0 | 2.1 | -14.70 | -5.20 | 21.20 |
| Kosakonia cowanii strain SYM00028 | 75.0 | 65.4 | 40.5 | 2.1 | 1.6 | -1.2 | 20.50 | 14.80 | -21.10 |
| Rhizobium | 73.8 | 64.8 | 39.3 | 0.9 | 1.0 | -2.3 | 8.00 | 9.90 | -33.50 |
| Bacillus subtilis + Bradyrhizobium japonicum | 74.4 | 66.3 | 41.6 | 1.5 | 2.5 | -0.1 | 13.70 | 27.00 | -7.00 |
| Bacillus amyloliquefaciens strain PTA-4838 | 72.3 | 64.5 | 39.6 | -.06 | 0.7 | -2.1 | -12.10 | 4.90 | -31.00 |
| Methylobacterium hispanicum | 71.5 | 66.3 | 42.2 | -1.3 | 2.5 | 0.4 | -20.60 | 25.50 | 2.10 |
| Glomus intraradices, Glomus mosseae, Glomus aggregatum, Glomus etunicatum | 70.9 | 65.0 | 36.4 | -1.9 | 1.2 | -5.4 | -29.00 | 10.70 | -72.40 |
| P>F | ns3 | ns3 | ns3 | N/A | N/A | N/A | N/A | N/A | N/A |
| Bradyrhizobium elkanii, Delftia acidovorans + Bacillus velezensis | 74.7 | 62.9 | 36.1 | 1.8 | -0.9 | -5.7 | 18.70 | -15.30 | -75.80 |
| Bacillus velezensis | 76.6 | 61.8 | 35.3 | 3.7 | -1.9 | -6.4 | 43.70 | -28.10 | -84.80 |
References
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Hernández-Alcántara AM et al. 2023. Kosakonia cowanii Ch1 isolated from Mexican chili powder reveals growth inhibition of phytopathogenic fungi. Microorganisms. 11(7):1758.
Waliyang DI et al. 2023. The endophytic plant growth promoting Methylobacterium oryzae CBMB20 integrates and persists into the seed-borne endophytic bacterial community of rice.
Sandini IE et al. 2019. Seed inoculation with Pseudomonas fluorescens promotes growth, yield and reduces nitrogen application in maize. Int J Agric Biol.
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Mastouri F et al. 2010. Seed treatment with Trichoderma harzianum alleviates biotic, abiotic, and physiological stresses in germinating seeds and seedlings. Phytopathology. 100(11):1213-1221.
Maria Leticia Pacheco-Verdon, former Graduate Research Assistant, Crop, Soil, and Environmental Sciences, Auburn University; Diego Franca de Freitas, Associate Professor, Federal University of Vicosa; and Eros Francisco, Extension Grain Crops Agronomist, Assistant Professor, Crop, Soil, and Environmental Sciences, Auburn University
New May 2026, Evaluating Microbial Seed Treatments for Alabama Crops, ANR-3259
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