Humus shows up in nearly every soil on the planet, but it looks and behaves very differently depending on the climate, the organisms at work, and how the landscape is managed. A temperate forest floor layered with crumbly, dark organic matter, a waterlogged peatland, an ancient charcoal-enriched plot in the Amazon, and a commercial bag of humic acid granules are all examples of humus or humus-derived material. The variety is wider than most people expect, and understanding the differences between these forms reveals a lot about how soils actually function.
The Three Classic Humus Forms in Forests
Soil scientists have long grouped forest humus into three morphological types based on how organic matter sits on and mixes into the mineral soil beneath it. These forms are called mull, moder, and mor, and you can often tell them apart just by digging a shallow pit in a forest.
Mull humus is the type most gardeners would recognize. In a mull, the organic material has been thoroughly mixed with mineral soil by soil animals, especially earthworms. The boundary between the organic layer and the mineral layer is blurred or nearly invisible. Mull soils tend to have a near-neutral pH, good nutrient cycling, and a dark, crumbly texture. You find mull under broadleaf deciduous forests in Europe, under many temperate grasslands, and in well-managed agricultural soils. Research in European forests has shown that earthworm biomass and litter nutrient content reinforce each other in a positive feedback loop: richer litter supports more earthworms, which in turn create mull conditions and push soil pH upward.1Functional Ecology. Positive feedback loop between earthworms, humus form and soil pH reinforces earthworm abundance in European forests
Mor humus, sometimes called “raw humus,” is the opposite extreme. In a mor, decomposition is slow and driven mostly by fungi rather than soil animals. The organic layer sits on top of the mineral soil as a distinct mat, and you can usually see three sub-layers: fresh litter on top, partially decomposed but still recognizable fragments in the middle, and a dark, homogeneous humus layer at the bottom.2Forest Ecology and Management. Humus in northern forests: friend or foe? Mor forms under coniferous forests, heathlands, and acidic soils where earthworms are scarce. The boreal forests of Scandinavia and Canada are classic mor territory. Because fungal decomposition tends to be incomplete, nutrients get locked up in the organic layer rather than cycling back into the mineral soil.
Moder humus sits between these two. Some soil animals are present but not enough to fully blend organic and mineral layers. You get a recognizable organic horizon with visible fecal pellets from mites, springtails, and other small invertebrates, but without the thorough mixing that earthworms provide. A study comparing humus types on a forested slope in Japan found moder humus on the ridge and mull humus at the bottom, with earthworm biomass largely explaining the difference between the two sites.3Ecological Research. Soil macro‐invertebrates and litter disappearance in a Japanese mixed deciduous forest and comparison with European deciduous forests and tropical rainforests Research in the Western Italian Alps found that soils with mor humus were the most vulnerable to water erosion, while a fourth form called amphi, which combines thick organic layers with strong earthworm activity, was the most resistant.4Applied Soil Ecology. Humus forms affect soil susceptibility to water erosion in the Western Italian Alps
Wetland Humus and Peat
When soils are waterlogged for much of the year, decomposition slows dramatically because the organisms that break down plant material need oxygen. The result is a buildup of partially decomposed organic matter that can reach meters in depth over thousands of years. Peat is the most familiar example, and it hosts its own distinctive humus chemistry.
In peat, the traditional humic fractions, fulvic acid, humic acid, and humin, vary widely depending on the type of peat and how much it has been drained. Analysis of different peat types found that brown peat contained the highest amounts of humic acid and fulvic acid, while black peat and sapropel (lake-bottom sediment) had substantially less.5PubMed Central. Determination of Organic Compounds, Fulvic Acid, Humic Acid, and Humin in Peat and Sapropel Alkaline Extracts Draining peatlands for agriculture changes humus chemistry significantly: the humic acids in drained peat are less chemically mature than those in undisturbed bogs and fens, suggesting that aeration accelerates decomposition and reshuffles the organic matter.6PubMed Central. Influence of Drainage on Peat Organic Matter: Implications for Development, Stability, and Transformation
A less well-known wetland humus form is anmoor, which develops in soils that are wet and base-rich but not permanently submerged. Brook valleys, small floodplains, and areas fed by calcium-rich groundwater are typical settings. In anmoor soils, decomposition is handled partly by anaerobic bacteria and partly by large earthworms that can tolerate the wet conditions. The organic matter gets mixed into the mineral soil much as it does in mull, but under very different hydrological circumstances.7Applied Soil Ecology. Review Humusica 2, article 10: Histic humus systems and forms – Key of classification
Terra Preta and Ancient Human-Made Humus
One of the most striking humus examples on Earth was created by people, not just natural processes. Terra Preta de Índio, or “Amazonian Dark Earth,” refers to patches of unusually fertile black soil scattered across the central Amazon basin. These soils contain roughly three times more organic matter, nitrogen, and phosphorus than the surrounding infertile soils, along with about 70 times more charcoal.8PubMed Central. Prehistorically modified soils of central Amazonia: a model for sustainable agriculture in the twenty-first century They were created by pre-Columbian populations who added charred residues, organic waste, excrement, fish bones, and other refuse to the soil, either deliberately or as a byproduct of settlement life.
What makes Terra Preta remarkable from a humus perspective is how stable its organic matter has remained. The charcoal, or biochar, acted as a scaffold for humification: fungi played a larger role in stabilizing the organic matter than bacteria, which is the reverse of what typically happens in tropical soils.9Geochimica et Cosmochimica Acta. State of the scientific knowledge on properties and genesis of Anthropogenic Dark Earths in Central Amazonia (terra preta de Índio) Chemical analysis of the humic acids in these soils shows they are a two-component mixture: ordinary tropical humic acids containing carbohydrates, peptides, and lignin fragments, and a second component of oxidized charcoal with a highly aromatic, carboxyl-rich structure. The oxidized charcoal component persists for centuries or even millennia, even under active cultivation, and appears to be a major reason why these soils maintain their fertility so much longer than typical tropical soils.10Journal of the Brazilian Chemical Society. Lessons from the Terra Preta de Índios of the Amazon region for the utilisation of charcoal for soil amendment
Grassland Humus and Chernozems
Chernozem, from the Russian for “black earth,” is among the most humus-rich soils found under natural conditions. These soils develop under temperate grasslands where grasses die back each year, feeding a steady supply of root material into the soil. The organic matter is distributed deeply, sometimes a meter or more down, and the soils are prized for agriculture.
Even within chernozem landscapes, humus content and chemistry vary with local moisture. Research on chernozems in Western Siberia found that in drier areas, organic matter accumulated in terrain depressions, reaching concentrations above 5%, with fulvic acids dominating the mix and pushing soil acidity up. In the wetter, leached chernozems nearby, organic matter was lower, around 2%, but humic acids dominated instead.11Почвы и окружающая среда. Humus status of chernozem soils formed under deep hydrogen release and different humidification conditions The ratio between humic and fulvic acids is one of the standard ways soil scientists characterize humus quality in any soil, and chernozems illustrate how that ratio shifts even across short distances.
How Organisms Shape Humus
The biological players behind humus formation are just as varied as the humus types themselves. Earthworms are the headline act in many soils, but fungi, bacteria, mites, springtails, and beetle larvae all contribute, and the balance between them determines which humus form develops.
Earthworms matter so much because they physically move organic matter into the mineral soil and mix it there. Their casts, the soil they excrete after passing it through their gut, are enriched in microbially processed carbohydrates and coated with mucilage that binds mineral particles together. This creates a physically protected pool of carbon that stays stable until the casts themselves break down.12Applied Soil Ecology. Soil organic matter within earthworm casts of an anecic-endogeic tropical pasture community, Colombia Through this process, earthworms build the organo-mineral complexes that give mull humus its characteristic dark, crumbly structure.13ScienceDirect. Chapter 8 – Organisms and Humus in Soils
Fungi dominate where conditions are too acidic, too cold, or too nutrient-poor for earthworms. In coniferous forests, fungi are the main decomposers of lignin, the tough structural polymer in wood and bark. Selected fungi break lignin apart through fragmentation, oxygenation, and ring-opening reactions, producing enzymes like peroxidases and copper radical oxidases to do the work.14PubMed Central. Lignin Metabolism by Selected Fungi and Microbial Consortia for Plant Stimulation: Implications for Biologically Active Humus Genesis Because fungal decomposition is slower and less complete than what earthworms achieve, it produces the thick, layered organic horizons typical of mor humus.
The Scientific Debate About What Humus Actually Is
For most of the twentieth century, textbooks described humus as being made up of “humic substances,” large, chemically unique molecules formed through a process called humification. These substances were divided into humic acid, fulvic acid, and humin based on how they behaved when extracted with an alkaline solution. That framework is still widely used in applied soil science, but over the past two decades it has come under serious challenge.
A landmark review argued that the available evidence does not support the existence of large, persistent, chemically unique “humic substances” in soils. Instead, soil organic matter is better understood as a continuum of progressively decomposing organic compounds, from fresh plant fragments to small molecules, with no sharp chemical boundary marking a special class called “humus.”15PubMed. The contentious nature of soil organic matter The critique centers on the extraction method itself: when you soak soil in a strong alkali, the dark material that dissolves may simply be a grab-bag of whatever organic molecules happen to have ionizable chemical groups, rather than a distinct class of newly synthesized molecules. The alkaline extraction cannot tell the difference between material that went through a unique humification pathway and material that was just oxidized or broken down by ordinary decomposition.
This matters for how we interpret the “humus examples” discussed throughout this article. The morphological forms, mull, moder, mor, and their wetland equivalents, are real and observable features of soil that no one disputes. You can see and feel the difference between a mull and a mor. But the chemical fractions traditionally extracted from those soils, the humic acids and fulvic acids measured in peat or chernozem studies, may not represent distinct chemical entities so much as artifacts of the extraction procedure. Researchers who work with humic acids in farming, remediation, and water treatment are increasingly aware that the materials they use are operationally defined: “humic acid” means “the stuff that dissolves in alkali and precipitates in acid,” not a single molecule with a fixed structure.
Humic Acids in Farming
Despite the ongoing academic debate, humic acid products are a growing segment of the agricultural input market. Farmers and horticulturists apply them as soil amendments or foliar sprays, and the commercial products are derived from a range of humus-rich raw materials including leonardite (a soft, oxidized form of lignite coal), peat, compost, and vermicompost.
A broad review of laboratory and field experiments found that humic acid application can improve crop performance and soil quality, but the effects are highly variable. The source of the humic acid turned out to be the single most influential factor: leonardite-derived products, compost-derived products, and peat-derived products differ in their molecular structure, solubility, and functional-group content, and these differences translate into different effects on plants and soils.16Frontiers in Agronomy. Understanding the Role of Humic Acids on Crop Performance and Soil Health Application rate, soil type, and crop species also matter, which means a humic acid product that works well on sandy soil under tomatoes may do little for clay soil under maize.
A field trial testing a commercial humic acid on maize found that the amendment helped lower soil pH and increased micronutrient availability, but had no statistically significant effect on most growth and yield parameters. The one exception was total grain yield: a low application rate of humic acid came closest to matching the yield of a full recommended fertilizer treatment.17International Journal of Life Science and Agriculture Research. Characterization of a Commercial Grade Humic Acid (HA) and Its Impact on Soil Properties and the Yield of Maize Results like these suggest that humic acids are better thought of as soil conditioners than as fertilizer replacements, at least in the short term. They may improve soil structure, water retention, and nutrient availability gradually, but expecting a single-season yield boost is often unrealistic.
Humus in Pollution Cleanup
Humic acids have a well-documented ability to bind heavy metals, which has led to research on using humus-rich amendments for soil remediation. The general idea is that adding humic material to contaminated soil can lock metals into less mobile chemical forms, reducing the chance that they leach into groundwater or get taken up by crops.
In practice, the picture is more complicated than “humus grabs metals.” A study testing two different humic acids on soils contaminated with zinc, lead, copper, and iron found that both amendments successfully immobilized zinc and lead in an acidic soil, shifting those metals into forms that could only be extracted with the harshest reagents. But copper and iron were slightly mobilized by the same treatment, becoming more available rather than less.18PubMed. Fractionation of heavy metals and distribution of organic carbon in two contaminated soils amended with humic acids In a calcareous soil, the effects were smaller across the board. The takeaway is that humic acid amendments for remediation are not one-size-fits-all: the soil’s pH, the metals involved, and the type of humic acid all influence whether a given metal becomes more or less available.
Separate work on copper-contaminated soils compared humic acids from biosolid compost and leonardite and found that the two sources had very different binding behaviors. Leonardite-derived humic acid had a higher copper-binding capacity thanks to its greater aromaticity and abundance of acidic functional groups, but the overall pH of the soil-amendment system turned out to be the most important factor governing copper mobility. In other words, the humic acid’s direct metal-binding contributed, but the broader chemistry of the soil after amendment mattered more.19PubMed. In situ remediation of metal-contaminated soils with organic amendments: role of humic acids in copper bioavailability
Humus in Lakes and Steppe Ecosystems
Humus is not strictly a soil phenomenon. In many freshwater systems, dissolved organic matter originating from terrestrial humus washes into rivers and lakes, giving the water a characteristic tea-brown color. This dissolved humus plays a role in lake ecology that researchers are only beginning to quantify.
A study of steppe lakes found that the dissolved organic matter was composed of four distinct humus-type components, two of which were identified as terrestrial in origin, meaning they were carried into the lakes from surrounding soils rather than produced by aquatic organisms. The lakes with higher inputs of this exogenous humus actually produced lower greenhouse gas emissions. The humus components were negatively correlated with emission rates of carbon dioxide and methane, suggesting that terrestrial humus entering steppe lakes acts as a suppressant on gas production rather than fueling microbial activity the way fresher organic matter would.20PubMed. High exogenous humus inhibits greenhouse gas emissions from steppe lakes This finding complicates the common assumption that more organic matter in water always means more greenhouse gas output. The chemical maturity of the organic matter, how far along the decomposition continuum it has traveled, appears to determine whether it feeds microbial metabolism or resists it.
Aquatic humus also affects how metals and pollutants behave in water, how much light penetrates the water column, and how acidic or buffered a lake becomes. Brown-water lakes in boreal and subarctic regions owe their color almost entirely to dissolved humic substances leaching from surrounding peatlands and forests. For aquatic organisms in those systems, humus is not a background feature of the landscape; it defines the chemical environment they live in.