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Soybean with bacteria and humics

Table 3. Synergistic effects of humic substances and nitrogen-fixing bacteria on soybean growth under
various environmental conditions

Humic Bacterial partner (s) Environmental Physiological effects Agronomic References
substance condition outcomes
Humic acid Bradyrhizobium japonicum Acidic soil pH buffering; Nodulation 56,72,74
(HA) (pH < 5.5) Al ⁺chelation; (30–40%), yield
3
infection thread (20–25%)
formation; nodule
number ↑ by 25–45%
Fulvic acid Azospirillum brasilense Drought-stressed Auxin-like activity, Root mass (25%), 50,70,71
(FA) conditions antioxidant enzyme photosynthetic
expression and rate (15%) and
chlorophyll content grain yield
↑ by 15–30%
Humin (HU) Bradyrhizobium spp., Low-oxygen/ Acts as an Nitrogenase 28,72
Herbaspirillum spp. compacted soils extracellular electron activity (35%),
mediator, nod gene seed protein
expression, and root
length ↑ by 20–35%
HA+FA St griseoflavus, B. japonicum Saline soils Osmoregulation Germination rate 51
mixture (EC > 4 dS/m) support; membrane (20%), shoot
stability; shoot dry nitrogen content
weight ↑ by 18–40%
Artificial Bradyrhizobium+Pseudomonas Water-limited Water use efficiency; Grain yield 65,66
humic acids spp. semi-arid soils chlorophyll (30–40%), soil
biosynthesis; leaf residual N
area ↑ by 12–28%
HA-coated B. japonicum+Azospirillum Organic farming Controlled release; Long-term BNF, 35,76,77
inoculants consortium systems microbial survival crop resilience
and rhizosphere
colonization;
nitrogenase activity
↑ by 30–50%
Vermicompost Rhizobium, Azotobacter, mixed Nutrient-poor Microbial diversity; Total biomass 59,60,32
humates PGPR degraded lands enzyme activity; (20–30%),
root architecture; nutrient uptake
photosynthetic
efficiency ↑ by
10–25%
Abbreviations: BNF: Biological nitrogen fixation; EC: Electrical conductivity; FA: Fulvic acid; HA: Humic acid; HU: Humin;
PGPR: Ndfa: Nitrogen derived from the atmosphere; Plant growth-promoting rhizobacteria.

in nitrate leaching under field conditions in loamy In cropping systems where mineral fertilizers are
soils with co-application of Bradyrhizobium and often overapplied, the shift toward biological nitrogen
vermicompost-derived humates, compared to inoculants sources has been reported to reduce nitrate leaching and
alone. However, these effects were less pronounced in ammonia volatilization by up to 40%, as demonstrated
sandy soils, suggesting that mitigation efficiency is soil- in paired field trials. This has direct implications for
dependent. Compared to other mitigation strategies, both groundwater quality and atmospheric emissions,
such as biochar or controlled-release fertilizers, the advancing climate-smart agricultural practices. 78,79
humic-microbial combination offers a biologically Agronomically, consistent application of these
driven alternative with co-benefits for soil health. bio-based technologies leads to greater resilience in

Volume 22 Issue 6 (2025) 9 doi: 10.36922/AJWEP025230190