Humic and fulvic acids from vermicompost: the chemistry and mechanism of action

Humic acids and fulvic acids are the two main fractions of humic substances, the organic compounds that form when animal and plant organic matter decomposes and changes over time. It is customary to distinguish them by molecular weight and solubility: fulvic acids are the smaller-molecule fractions, which stay soluble even in acidic conditions, while humic acids are the heavier ones, which precipitate at low pH and to which an effect on soil structure and cation exchange capacity is mainly attributed.

How they originate from vermicomposting

It is commonly believed that in vermicomposting the transformation of organic matter into humic substances passes through the digestive tract of the earthworm, which fragments the material and speeds up its microbial degradation. A study that vermicomposted pig manure with isotopically labelled rice straw observed that the vermicomposting process increases the content of humic substances, humic acid and fulvic acid in the substrate residues, while leading to less transformation of humic acid into fulvic acid^[1]. In the same study, the humic acid obtained at the end of vermicomposting was richer in aromatic structures, with a higher degree of humification, and poorer in proteins and in substances similar to fulvic acids^[1]; however, the quotation given in the references does not state the point of comparison. It is widely held among agronomists that such a marked degree of humification depends on the two-stage process, mechanical (through fragmentation) and microbial (through the digestive activity of the earthworm), that vermicomposting adds to the purely microbial action of traditional composting.

The effect on the root system

The scientific literature associates humic substances with a direct action on the root system. A review published in Frontiers in Microbiology describes a stimulating action on the quantity and length of lateral roots and root hairs, a process that, according to its authors, appears to be mediated by signalling pathways involving several plant hormones^[2]. A 2025 study on rice, carried out under controlled conditions on young seedlings, measured the effect precisely: the application of humic acid increased root dry weight by 27% and lateral root length by 20% compared with the control^[3]. The same study attributes the result to the regulation of the genes involved in plant hormone signalling pathways^[3]. Similar results also emerge when vermicompost is applied not as an isolated extract but as a whole soil improver: on oilseed rape (Brassica napus), vermicompost, described as a growth-stimulating organic fertiliser rich in humic acid^[4], proved superior both to the humic acid extract and to indole-3-acetic acid, with an increase of about 8% in the dry weight of the root and above-ground part and a seed weight almost tripled^[4].

The mechanism behind nutrient uptake

The effect of humic substances does not stop at root morphology: their role in plant nutrition goes beyond the increase in root morphology, because they can form complexes with the cations present in the soil, improving the uptake of nutrients such as phosphorus, zinc and iron^[2]. At the membrane of the root cell the primary ion transport system comes into play, essential both for nutrient uptake and for the growth of the cell itself^[2].

Under water restriction the picture is more nuanced. A greenhouse study on soya, carried out with humic acids from vermicompost together with Bradyrhizobium, refers to the literature according to which the application of humic acid at low rates induces beneficial effects on the plant, improving its ability to take up nutrients and helping it overcome stress conditions^[5]. In plants not subjected to water restriction, humic acid gave greater growth and nodulation of soya than the control^[5]; after water stress was applied, however, the difference between the treatments disappeared^[5].

Why the process matters

It is commonly believed that the agronomic efficacy of a humic acid depends not only on the rate applied but on the raw material and the process that generated it: a humic acid from vermicompost is not chemically identical to one extracted from lignite or peat. For a fertiliser obtained from vermicomposting, the degree of humification and the richness of aromatic structures measured in the humic acid at the end of the process^[1] remain chemical data about the process: none of the sources collected demonstrates that it is precisely this degree of humification that produces the effects on roots and nutrient uptake measured in the other studies cited here. These are separate studies, on different matrices and crops, and the link between the chemistry of the process and the effect in the field remains a working hypothesis shared across the industry, not an experimental result.