What is humus?
In the previous chapter, it was mentioned that one of the elements of soil analysis is determining the level of humus (organic matter). But what exactly is it?
Imagine a container with a volume of 1 liter — a “cube” measuring 10 × 10 × 10 cm. You fill it about halfway with finely crushed rocks (the largest particles are about 2 mm, the smallest are truly microscopic). Then you pour in 250 ml of water and tightly close the container. Did you know that you have just recreated soil in about 95%? Yes — theoretically, soil should consist of:
Minerals — about 46% (finely crushed rocks),
Water — about 25% (the glass of water poured into the container),
Air — about 25% (filling the remaining space once the container is closed).
A score of 95–96% in an exam would be almost perfect — so why isn’t it enough to create a medium where anything could grow? Because one key element is missing: the remaining 4–5% — humus. Even though humus is dead organic matter, it is at the same time an extremely living structure — an intelligent reservoir of water and nutrients for plants, which builds itself and regulates itself.
In short: humus (soil humus) is the natural, organic part of soil formed through the decomposition of plant and animal residues by small and large soil organisms. It is the most processed and stable form of organic matter in soil, rich in nutrients (nitrogen, phosphorus, potassium, and others) essential for plant growth.
Did you know? Although most soils contain only a few percent of humus, there are soils where this component exceeds 20%. These are so-called organic soils — peats, silts, mucks, or forest litter soils. On the one hand, they are extremely fertile; on the other, they are difficult to cultivate. They are soft, waterlogged, unstable, sensitive to drying out, and prone to acidification. That is why simplified, ultra-shallow, or no-till cultivation using light equipment is often the only rational solution. Here, every ploughing operation is a risk — not only of sinking the plough, but also of destroying what is most valuable in the soil. One poorly timed pass with heavy machinery can wipe out years of humus-building work in just a few seasons.
Rolmako offers a wide range of machines for no-till farming. More information can be found in the no-till farming section on rolmako.com. rolmako.com.
Why are there no yields without humus?
Humus is the foundation of soil fertility. Many farmers dream of reaching 4–5% organic matter content in their soils. Meanwhile, 63% of Polish soils contain only 1–2% humus, and about 4% contain less than 1%. How does this affect yields — and why?
Storage of nutrients
The finest humus particles (humus colloids) act as a storage system for nutrients and water, and as a framework for soil structure. These are “microscopic shelves” from which humus releases essential nutrients to plants when needed. In soils with low organic matter content, nothing holds valuable bioelements (present as cations such as K⁺, Ca²⁺, Mg²⁺), so they are easily leached away.
Buffering of soil reaction
Humus colloids also act like shock absorbers in the soil — thanks to them, soil reaction (pH) does not change abruptly, and nutrients remain in the root zone instead of being quickly washed into groundwater.
Water retention
Research shows that increasing humus content by just 1% allows soil on 1 hectare to retain an additional about 150 m³ of water that is readily available to plants. That’s a huge amount — roughly 150,000 liters, equivalent to 150 large 1,000-liter tanks. Humus-rich soil is therefore much more drought-resistant, as it stores rainfall very effectively. However, crop production does not depend on water alone, but on a balance between moisture and aeration. Excess water leads to root rot and reduced biological activity, which in turn lowers yields. Fortunately, humus handles this challenge exceptionally well.
Building a crumb soil structure
Well-moistened, humus-rich soil promotes the formation of a crumb (granular) structure. Thanks to humus and natural secretions of soil microorganisms, mineral particles are “glued” together into stable aggregates using natural binding agents. Such a stable structure protects soil against wind and water erosion and ensures better aeration — soil mesopores store plant-available water while still containing air-filled spaces.
In short: humus: stores nutrients and regulates their availability to plants, increases water retention, buffers (or “absorbs”) pH fluctuations, binds soil aggregates, creating a stable crumb structure. Each of these factors directly affects yield potential. - Soil without humus:
- retains water and nutrients poorly,
- is more prone to erosion, heats up and cools down faster, and is less resilient to climate variability,
- has poorer structure (often too compacted or, conversely, loose and easily compacted).
Plants grown on soils depleted of humus more often suffer from water and nutrient deficiencies, grow poorly, are more susceptible to diseases, and produce lower yields. This also increases the need for mineral fertilization and frequent field operations, raising production costs. In short — without organic matter, there is no fertile soil and no good harvests.
Ways to increase humus content in soil
Soil can be “fed” with organic matter in many ways — the key is to make it a routine practice on the farm. Any organic material left on or added to the soil can become a building block of humus, provided it is given enough time and the right conditions for microorganisms to decompose it.
Leaving straw and crop residues
Cereal straw, stubble, maize leaves, beet leaves, weeds from interrows — all of these are valuable sources of organic carbon for the soil. Soil microorganisms gradually decompose these residues, transforming them into humus. However, it is important to maintain a proper carbon-to-nitrogen balance. Cereal straw, which is rich in cellulose, should be finely chopped and ideally supplemented with nitrogen (e.g. by spraying with a urea solution or applying it together with organic fertilizer) to facilitate decomposition by soil bacteria.
Use of manure and slurry
Regular application of manure (e.g. every 2–4 years at a rate of about 30–40 t/ha) significantly increases organic matter levels in soil. Slurry and liquid manure can have a similar effect when used properly, provided that correct application techniques are followed (rates adjusted to crop needs and rapid incorporation into the soil to limit nitrogen losses).
- Manure is a solid material containing bedding (e.g. straw) and animal excreta. Thanks to its high content of organic matter at various stages of decomposition, it builds humus more slowly but more durably, with effects lasting even several years.
- Slurry and liquid manure are liquid fertilizers that decompose quickly, so they do not build humus in the long term. They are excellent as fast-acting nitrogen fertilizers, but they do not improve soil structure or increase organic matter content. They work very well, however, when applied as a spray on crop residues.
Compost and organic waste
Compost contains partially decomposed organic matter, which means it feeds soil humus relatively quickly. It can be applied at lower rates than fresh manure (e.g. 10–20 t/ha every year or every two years). Compost improves soil structure and water-holding capacity, and it is also safe to use — it contains stable humic compounds and very few weed seeds (provided the composting process is carried out correctly).
Cover crops and green manure
Cover crops supply large amounts of fresh organic matter, rich in easily decomposable compounds (sugars, proteins), which microorganisms quickly convert into humus. Particularly valuable are mixtures containing legumes (they supply nitrogen and have deep roots that loosen the soil) as well as multi-species cover crops — diversity of residues supports diversity of soil microflora. Research confirms that crop rotations including cover crops reduce organic matter losses and promote the development of beneficial soil microorganisms (more in e-book 2).
Important! Management of residues and organic fertilizers
Important! It is not enough to simply leave stubble, spread manure or compost, or sow cover crops. As explained in more detail in Chapter 8 of e-book 2, these materials should be thoroughly shredded and/or shallowly mixed into the soil, for example using knife rollers, harrows, or weeders.
Rolmako offers machines designed specifically for post-harvest cultivation, suitable for farms of all sizes. Recommended models include::
- SpringExpert mulch harrow,
- Cultivator with a mulch harrow ComboTill,
- SpeedCutter harrow for ultra-shallow cultivation,
- U693 compact disc harrow,
- U671 heavy stubble harrow,
- U497 universal cultivator,,
- U436 multifunctional tillage cultivator,,
- U624 chisel plough.
Detailed specifications of all these tools can be found in the post-harvest cultivation section at rolmako.com.
Preventing organic matter losses
Building humus through hard work makes little sense if, at the same time, accumulated organic matter is allowed to be lost rapidly. Unfortunately, some intensive tillage methods and agronomic neglect lead to accelerated humus loss from soil. The main causes of organic matter depletion include:
- Excessive mechanical tillage (frequent deep ploughing and other aggressive operations increase soil aeration and accelerate humus decomposition),
- Limited or no use of organic fertilizers (lack of fresh organic inputs),,
- Excessive mineral fertilization (high nitrogen rates may stimulate microorganisms to “consume” humus reserves),
- Leaving soil bare (soil heats up and dries out, intensifying humus mineralization; during heavy rainfall, erosion and loss of the fertile top layer occur).
Important! The greatest enemy of humus is excessive ploughing. Each inversion of soil to a depth of 20–30 cm strongly aerates it, breaks down crumb structures, and accelerates organic matter decomposition (disturbed and oxygenated microbes “consume” humus faster, releasing CO₂ into the atmosphere). Even reducing ploughing frequency (e.g. ploughing every second year instead of annually, alternating with reduced tillage) brings benefits. Completely abandoning the plough in favor of loosening operations (cultivating, grubber tillage, harrowing) and no-till seeding delivers even better results: soil retains more structure, loses carbon more slowly, and soil life remains less disturbed.
Did you know? : Burning residues in the field is still a common way of disposing of straw or maize stubble. However, this is a severe waste of humus and nutrients, which literally go up in smoke. Instead of fire, it is far better to use appropriate post-harvest cultivation machinery.
You can learn more about agronomic practices that prevent organic matter loss in e-book 2. e-booka 2.
