The genie had barely settled back into the dark liquid when he heard the second wish:

“Don’t just feed my orchid. Make it grow stronger.”

This was a more serious request. Any well-formulated fertiliser can feed a plant. It supplies nitrogen, phosphorus, potassium and the other essential elements, and—as we established in Part I—a root does not run background checks on ions.

But make it stronger? Stimulate root growth, improve nutrient use, support microorganisms, increase tolerance to stress and, while we are at it, keep the leaves glossy, the flowers open for months and the owner from waking at three in the morning wondering whether the plant has been overwatered?

An ordinary fertiliser sounds rather pedestrian for a job like that. What we need is a biostimulant—a word that sounds as though it ought to switch on an extra reactor somewhere inside the orchid.

Enter vermicompost extracts, humic acids, fulvic acids and a whole family of dark-coloured products whose labels tend to show roots behaving as though they have just won an award for the world’s most enterprising underground system.

The question is: how much genie is there in the bottle, and how much very talented graphic designer?

A biostimulant: more than lunch

A biostimulant is not simply a fertiliser with an unusually helpful personality.

Under the definition adopted in European Union law, a plant biostimulant stimulates plant nutrition processes independently of the product’s own nutrient content. It may improve nutrient-use efficiency, increase tolerance to abiotic stress, enhance quality traits or make nutrients in the soil or root zone more available.

Put simply, fertiliser brings lunch. A biostimulant is supposed to help the plant make better use of it, cope more effectively with difficult conditions or gain better access to what is already present in its surroundings.

The distinction matters, although in practice the boundary can become blurred. Many products containing humic acids or vermicompost extracts also supply nitrogen, phosphorus, potassium and micronutrients. If a plant grows better after treatment, it may be difficult to tell whether the biostimulant acted, the nutrients acted, or both did. The plant may simply have been hungry and would have welcomed a less romantic bottle just as enthusiastically. The genie, naturally, would prefer to take all the credit.

Vermicompost: the worm was involved, but what did it actually do?

Let us begin with a name that has enjoyed a remarkable career on garden-centre shelves.

Vermicompost brings to mind dark, fertile earth, the natural recycling of organic matter and an industrious earthworm working three shifts a day. Strictly speaking, vermicompost is material produced when earthworms and microorganisms process organic matter. It may contain mineral nutrients, a wide range of organic compounds and a rich community of microorganisms.

A liquid product sold as “vermicompost” or “worm humus”, however, is not simply solid vermicompost poured into a bottle.

It may be a water extract, liquid collected during production, a filtered concentrate, an organic fertiliser enriched with mineral nutrients or a mixture containing humic substances. It may also be a product whose closest connection to an earthworm is the portrait on its label.

Its composition depends on the raw material, production method, aeration, filtration, storage and anything the manufacturer adds later. Two bottles standing side by side under the same “vermicompost” label may differ more than grandmother’s chicken soup and instant noodles, even though both technically qualify as liquids eaten with a spoon.

It is therefore worth turning the bottle around before using it. The story lives on the front. With luck, the numbers live on the back.

Where did humus come from?

To understand what humic products may do, we need to leave the shop shelf for a moment and visit a laboratory from roughly two centuries ago.

Researchers had noticed that good agricultural soils were usually dark. To separate organic matter from sand, silt and clay, they treated soil samples with strong alkaline solutions. The process yielded a dark mixture that eventually became known as humic substances.

The mixture was then divided according to its behaviour under different chemical conditions:

– humic acids dissolved under alkaline conditions but precipitated when acidified,
– fulvic acids remained soluble even under acidic conditions,
– humins did not dissolve during this extraction.

It was an elegant classification: three fractions, three names, and the orderliness of a drawer full of carefully labelled socks.

This became the basis of the classical theory of humification. Plant and animal remains were thought to be gradually decomposed by microorganisms until large, dark and exceptionally persistent humus molecules remained. In the textbook version, these molecules represented the final initiation rite of organic matter: the leaf passed away, bacteria feasted, and humus endured for decades or centuries, looking down indulgently on the brief life of a radish.

There was one weakness in the theory. Researchers could not conclusively find and describe such large, distinct molecules directly in undisturbed soil.

Is humus born in the soil—or in the test tube?

In 2015, Johannes Lehmann and Markus Kleber published The Contentious Nature of Soil Organic Matter in Nature.

They strongly challenged the classical model. Much of what we call humic acids, fulvic acids and humins became visible to us only after soil had been treated with highly aggressive reagents, including alkaline solutions with a pH of around 13.

Such a pH is virtually unheard of in ordinary soil. This is not a gentle rinse with rainwater; it is a chemical interrogation under a very bright lamp.

Under these extreme conditions, molecular properties change, some bonds break and new ones may form. We therefore cannot be certain whether the extracted material already existed in precisely that form in the soil, or whether extraction altered, rearranged or even partly created it.

Imagine trying to study the structure of a cake. Instead of examining it under a microscope, we soak it in strong alkali, boil it, separate the resulting mixture and then announce that we have discovered three natural layers of the original bake.

The chemist may be delighted, but the baker would have a few questions.

Lehmann and Kleber proposed a different picture. Soil organic matter need not be divided into ordinary residues and a final, fully formed, almost immortal humus. It may instead be a continuous spectrum of thousands of compounds at different stages of decomposition and transformation.

A vast recycling system, not a humus factory

Leaves, roots, dead organisms, excreta and secretions from plants and microorganisms all enter the soil. Bacteria and fungi break down larger molecules and use them as sources of energy and nutrients, but they are not merely a demolition crew.

Microorganisms build their own proteins, complex carbohydrates, cell membranes and genetic material. When they die, their bodies also become part of the organic matter. Other organisms use their remains, produce new compounds, break those down, rebuild them and pass them on.

This is no straight road from fallen leaf to a lump of everlasting humus. It is more like an enormous flea market where nothing disappears completely, everything changes owners, function and packaging, and an old chest of drawers may return next season as three shelves, an onion crate and a fiercely avant-garde plant stand.

Organic matter may persist in soil for many years without being chemically indestructible. It can adhere to clay minerals, bind to iron, aluminium or calcium, become enclosed in microscopic pores and soil aggregates, or end up beyond the easy reach of microorganisms and their enzymes.

An apple locked in a safe does not become immortal. It simply becomes harder to eat. In much the same way, some organic matter may persist because it has been bound, shielded or very effectively hidden.

So humus does not exist?

Easy now—we are not throwing out the compost bin. It all depends on what we mean by humus.

If we use the word as a convenient name for dark, highly decomposed organic matter in soil, it remains perfectly serviceable. A gardener who speaks of humus-rich soil is not committing a scientific offence and need not flee the soil-science police.

If, on the other hand, we imagine humus as a distinct class of enormous, uniform and nearly indestructible molecules produced at the end of a special humification process, we are stepping onto ground considerably muddier than a freshly watered flower bed.

Lehmann and Kleber did not prove that persistent organic associations do not exist in soil, nor that extraction creates everything from scratch. They challenged the assumption that laboratory-isolated fractions faithfully represent separate substances occurring naturally in intact soil.

Not every soil scientist has accepted this view in full. Some argue that humic substances do exist, although not as individual giant molecules. They may form variable supramolecular structures: assemblies of smaller compounds held together by relatively weak interactions and associated with the mineral components of soil.

The debate continues. It is one of those scientific disputes in which both sides use the word “humus” without necessarily putting exactly the same thing inside it.

If humus is under suspicion, why do humic acids work?

This is where the story becomes particularly interesting. Questioning the classical humus theory does not turn products sold as humic acids into brown-tinted water created for the greater glory of the marketing department.

They contain real mixtures of organic compounds, commonly extracted from leonardite, lignite, peat, compost or vermicompost. The fact that their names arise from a laboratory extraction method does not deprive them of chemical and biological properties.

Research suggests that some of these mixtures may:

– influence root architecture and branching,
– stimulate proton-pump activity in cell membranes,
– alter nutrient availability and transport,
– affect plant metabolism,
– influence microorganisms in the root zone,
– help plants respond to nutrient deficiency or stress,
– produce effects resembling some actions of plant hormones.

This does not mean that a single identified “growth factor” is floating in the bottle, ready to reach a root and press a green button marked GROW.

Humic products are complex mixtures. Their effects depend on the raw material, extraction method, composition, dose, plant species and developmental stage, growing medium, fertilisation regime and environmental conditions.

The genie exists, but he keeps irregular office hours.

A little may help; a lot may hinder

Studies of humic substances often reveal a characteristic dose response: the best result occurs at a particular, relatively low dose. Less may be ineffective, more may bring no additional benefit, and still more may inhibit growth.

The response curve may therefore look like a bell. The plant performs better and better until it reaches an optimum, after which it apparently decides that the show has gone on long enough.

This matters because home growing has its own ancient piece of folk wisdom: if one capful helps, two will help twice as much, and three will turn an orchid into a palm tree. They will not.

Too much product can increase salinity, change the solution’s pH, supply an excessive amount of a particular nutrient or disturb the balance within the pot. The margin for error is especially narrow for a moth orchid growing in bark. There is no large mass of soil to dilute, retain or buffer our enthusiasm.

Enter the moth orchid, dressed entirely in bark

Most research on humic acids and vermicompost has involved agricultural crops, vegetables, fruit crops or model plants. These were often grown in soil, peat-based media, nutrient solutions or laboratory conditions.

Phalaenopsis, meanwhile, is an epiphyte. In nature it does not sit up to its neck in rich black earth; it grows on tree trunks and branches. Its roots encounter bark, organic debris, rainwater, dust, microorganisms and whatever else collects in small crevices, but they do not function in ordinary garden soil.

Conditions in a household pot are more particular still.

Coarse bark:

– contains few nutrients,
– has a limited ability to retain ions,
– dries relatively quickly,
– is periodically flushed,
– gradually decomposes under the influence of moisture and microorganisms.

Many benefits attributed to humic substances involve improvements to soil structure, water retention, cation-exchange capacity and biological activity. In a pot filled with large pieces of bark, some of these mechanisms have far less room to operate.

A moth orchid is not a miniature wheat field. A pot does not become black earth merely because we pour a brown liquid into it.

What does orchid research tell us?

Here the genie lowers his voice. Some studies suggest that humic substances may support the growth of certain orchids, particularly while young plants are being acclimatised. We still do not have a large body of well-controlled research on mature Phalaenopsis growing in the kind of bark commonly used in homes.

Orchids differ from one another. A result obtained with a young Cymbidium, or with a plant emerging from in-vitro culture, cannot simply be walked through the sitting room and seated beside an adult Phalaenopsis.

Research on other species can show that a particular mechanism is possible. It cannot guarantee that the same product, at the same dose and under our conditions, will produce an equally visible effect. Science says cautiously: “Under these experimental conditions, at this dose and in this species, an effect was observed.” Advertising replies: “300% MORE ROOTS!”, while the small print on the back of the bottle quietly attempts to leave the film set.

Can vermicompost replace fertiliser?

It may supply nutrients, but that does not make every product a complete and sufficient fertiliser.

If a product contains very little nitrogen, phosphorus and potassium, and its label does not list all the necessary micronutrients, it is unlikely to meet the plant’s needs on its own. It may serve as a supplement or biostimulant without providing the full menu.

If, however, a “vermicompost” product declares a clear NPK analysis and has been enriched with mineral nutrients, part of its fertilising effect comes from those nutrients.

Before crediting a mysterious force of organic matter, we should therefore check whether the genie is hiding an ordinary bag of nitrate fertiliser behind his back.

A well-balanced mineral fertiliser remains a more predictable primary source of nutrients for a moth orchid. A humic product may be a useful supplement, but it should not conceal inadequate feeding or substitute for sound cultivation.

When might an effect be visible?

Biostimulants often show their abilities most clearly when a plant genuinely has a problem to solve.

In research, beneficial effects of humic substances may be particularly apparent under conditions of:

– nutrient deficiency,
– salinity,
– drought,
– unsuitable pH,
– reduced root activity,
– stress associated with repotting or acclimatisation.

If a plant already has healthy roots, adequate light, suitable temperatures, good water, an airy medium and proper nutrition, any additional effect may be small or difficult to see.

We do not give cough syrup to a healthy person in the hope of making them healthier still—and able to cough in reverse. Likewise, a biostimulant may produce no visible result when applied to a plant that already has everything it needs.

How should you choose a dark bottle?

Rather than judging by the depth of the brown colour or the number of earthworms pictured on the label, check:

– how the product is legally classified: fertiliser, soil improver or biostimulant,
– what raw material it was made from,
– whether it contains humic acids, fulvic acids or a vermicompost extract,
– its declared NPK analysis,
– whether micronutrient content is specified,
– the recommended dose for potted plants,
– whether the manufacturer provides clear directions and limitations,
– whether it is intended for plants grown in soilless media.

If the label promises everything—faster growth, more flowers, disease resistance, root regeneration, improved soil, a happier owner and probably a beneficial influence on the phases of the Moon—while providing almost no concrete information, the genie may work mainly in sales.

So is it worth trying?

A clearly described humic or vermicompost-based product can reasonably be tested as a supplement to the main fertilisation programme.

The safest approach is to:

– begin with a low concentration,
– avoid combining a full dose of the product immediately with a full dose of fertiliser,
– observe new roots and leaves over an extended period,
– flush the medium periodically with clean water,
– avoid changing five other aspects of cultivation at the same time,
– take particular care with weak or damaged roots.

If a plant produces more healthy root tips, resumes growth more readily or tolerates a particular stress better after treatment, there is a practical reason to keep using the product.

If no difference is visible after several months, we need not conclude that we have failed to believe in vermicompost with sufficient conviction. We can simply accept that, under our conditions, the product adds nothing the plant was not already receiving.

The absence of a miracle is not user error.

The second wish: granted, subject to conditions

Humic acids and vermicompost extracts contain genuine mixtures of compounds that may influence root development, nutrient availability and plant responses to stress. Products sold under the same “vermicompost” or “worm humus” label do not necessarily share the same composition or effects, and a healthy moth orchid does not require them in order to grow properly.

Two extra capfuls will not make a struggling orchid forget its rotten roots, poor light and exhausted bark. The genie would prefer not to be implicated in this matter.

Humic products may be valuable supplements, particularly under specific conditions and at an appropriate dose. They are not the essence of fertility, liquid healthy soil or a compulsory part of every orchid-care routine.

Healthy roots, adequate light, an airy medium, appropriate watering and balanced nutrition remain the essentials. Only once those foundations are in place does it make sense to ask whether the dark liquid might add anything more.

The genie has therefore granted the second wish, but added the conditions in very small print:

“I can help an orchid make better use of what it has. I cannot make it use what it does not have.”

One wish remains. The genie glanced nervously at the label, adjusted his turban and pretended that his warranty was about to expire.

Sources and Further Reading — expand +— collapse −

Lehmann, J., Kleber, M. (2015). The Contentious Nature of Soil Organic Matter.
https://doi.org/10.1038/nature16069

Gerke, J. (2018). Concepts and Misconceptions of Humic Substances as the Stable Part of Soil Organic Matter: A Review.
https://doi.org/10.3390/agronomy8050076

Olk, D.C. et al. (2019). Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters.
https://doi.org/10.2134/jeq2019.02.0041

Baveye, P.C., Wander, M. (2019). The (Bio)Chemistry of Soil Humus and Humic Substances: Why Is the “New View” Still Considered Novel After More Than 80 Years?
https://doi.org/10.3389/fenvs.2019.00027

Rose, M.T. et al. (2014). A Meta-Analysis and Review of Plant-Growth Response to Humic Substances: Practical Implications for Agriculture.
https://doi.org/10.1016/B978-0-12-800138-7.00002-4

Canellas, L.P., Olivares, F.L. (2014). Physiological Responses to Humic Substances as Plant Growth Promoter.
https://doi.org/10.1186/2196-5641-1-3

Calvo, P., Nelson, L., Kloepper, J.W. (2014). Agricultural Uses of Plant Biostimulants.
https://doi.org/10.1007/s11104-014-2131-8

© 2026 Marzenna Kielan. All rights reserved. Illustrations created by the author with the use of AI.