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Soybean with bacteria and humics

Table 2. Summary of humic products derived from agricultural waste: Sources, extraction methods,
composition, functions, and applications

Source Extraction method Humic Key agronomic functions Application References
fraction
Soybean Composting+alkaline Humic acid, Increases soil pH, reduces Acidic soils; 54-56
straw, cassava extraction fulvic acid aluminum toxicity, and enhances legume crops
waste nodulation
Palm bunch Thermochemical Humic acid Buffers soil acidity, improves Tropical soils, 31,56
compost humification microbial colonization, and BNF systems
enhances N fixation
Fish Enzymatic Highly Boosts microbial biomass, Organic 31,60
processing hydrolysis+compost bioactive enhances enzymatic activity, cropping
waste humates promotes root growth systems
Urban green Vermicomposting Fulvic Enhances micronutrient chelation Drought-stressed 59,60,61
waste acid-enriched (Fe, Zn), stimulates antioxidant or nutrient-poor
humus activity in soil soils
Digestate Hydrothermal Artificial Improves drought resistance, Arid regions; 60,64
(biogas carbonization humic supports microbial stability, and degraded soils
residue) substances buffers salinity
Orange peel, Microbial Fulvic acid, Improves auxin-like hormone Bioinoculant 54,50
fruit pomace composting phenolic-rich signaling, boosts rhizobial synergism with
colonization PGPR
Sewage Alkaline Mineral-bound Enhances P availability, detoxifies Urban-periphery 56,58
sludge+fly ash oxidation+filtration humic acids metals, and improves nodule soils; soybean
formation agroecosystems
Wheat Co-composting with Stable humin Acts as an electron mediator, Low-oxygen or 28,64
straw+biochar thermophiles fraction improving anaerobic N fixation compacted soils
efficiency
Abbreviations: BNF: Biological nitrogen fixation; PGPR: Plant growth-promoting rhizobacteria.

4. Synergistic effects of humic products and to more efficient nodulation and improved nutrient
nitrogen-fixing bacteria assimilation. 68,69
Recent transcriptomic studies confirm that humic
When used in combination, humic substances and substances enriched with micronutrients (e.g., iron)
nitrogen-fixing bacteria demonstrate synergistic effects modulate the expression of nodulation-related genes
that exceed the benefits of either input alone. These (e.g., nodA, nifH, and auxin transporters) in both
synergies are particularly important in sustainable Bradyrhizobium and soybean roots. For example, Peng
soybean cultivation, where optimizing BNF and et al. demonstrated that iron-enhanced fulvic acids
65
nutrient cycling is essential. Co-application improves significantly upregulated symbiotic signaling pathways
nodulation, nitrogenase activity, and plant biomass, and antioxidant defense genes, resulting in higher
especially under conditions of abiotic stress such as nitrogenase activity and nodule biomass under drought
drought, acidity, or low fertility. 51,65,66 stress. These omics insights provide a mechanistic
Humic products enhance the survival, colonization, explanation for the observed agronomic synergy and
and metabolic activity of symbiotic bacteria such as Bj support further bioformulation refinement based on
by acting as carbon sources and microbial stimulants. molecular markers.
This results in higher rhizobial population densities in The combined application of humic acids with free-
the rhizosphere, improved root adhesion, and enhanced living diazotrophs, such as Az and Herbaspirillum, also
infection thread formation in legume root hairs. In improves root morphology, water-use efficiency, and
67
addition, humic substances upregulate plant genes hormone signaling—traits that are critical under drought
involved in symbiosis and root development, leading or nutrient-limiting conditions. Studies indicate
50

Volume 22 Issue 6 (2025) 7 doi: 10.36922/AJWEP025230190