Photo Olivier Cor

by Olivier Cor

Directeur agronomie


How is humus used, how does the transformation of organic matter in the soil work, and what is its role in agriculture.

What is humus, and where does it come from? Following its elaboration through the transformation of organic matter in the soil allows a better understanding of its main role in soils, including agricultural ones.

1 - Where does humus come from ?

Humus is formed from stable residues resulting from the decomposition of fresh organic matter from plants, animals as well as microorganisms under the combined action of animals (macro and microscopic), plants, and soil microorganisms.

Humus is a complex mixture that is extremely varied in terms of composition, color, odor. In fact, it depends on organic sources, the fauna and flora of the soil as well as the environmental conditions. (pH, temperature, water conditions …).

As illustrated above, organic matter is an essential food source for the entire soil ecosystem, and humus is the stabilized residue resulting from this “digestion” process by the soil. Digestion allows the recycling of all elements except those found in humus.

2 - Organic matter in its humification phase is the food source for plants.

Olivier Cor, agronomy director at Lallemand Plant Care  explains that, for example, when a nitrogenous fertilizer is applied, the plant is directly fed by this contribution but only between 30% to 50% of its needs are met.
The major reasons behind this are:

  1. There is a competition between the plant and all other living beings in the soil.
  2. The contribution is punctual and the plant cannot absorb all the fertilizer at the time it is applied and it also has needs before and well after the fertilizer is applied. This is why fertilizers are only complementary to the basic nutrition provided by the soil. On their own, they are far from being sufficient to meet the needs of plants: “Organic matter, which is reorganized during the humification process, is a primary source of nitrogen for the plant, which enables it to feed itself regularly as soon as it needs it and not only when fertilizer is applied,” explains the agronomist.

Olivier Cor sees this concept as “essential” for agriculture, even more important than fertilization itself since it is what will (or not) allow us to achieve a good crop yield. “And if one were to compare the amount of nitrogen in humus and fertilizer, the nitrogen stock in humus would be the equivalent to several semi-trailers of ammonium nitrate” adds the agronomists with a laugh.

During humification, mineral elements are released. This mineralization of organic matter is mainly carried out by soil microorganisms which mobilize a large part of it before returning it to the plants.

We must keep in mind that there is no humus and organic matter on one side and mineral fertilizers on the other, everything is tightly connected, and “Indirectly, organic matter gives efficiency to mineral fertilizers, just as we balance forage rations between roughage and mineral supplements for animals”, completes the agronomist.

3 - Humus also plays a major role in soil structure and therefore in the circulation and storage of gases and water

Humus, the most stable part of organic matter, binds closely with the mineral matrix of the soil and in particular clays, forming structured assemblages commonly called “clay-humic complex” CHC.
These particles are globally negatively charged, and together they create an electric field in the soil solution.

It is the combination of these electric fields that causes the orientation of the particles and explains why soluble ions such as nitrate, which is also negatively charged, are pushed away from the solid particles, making it highly mobile in the soil allowing it to be so easily dragged along by the soil water.

Conversely, positive ions in solution such as calcium or potassium will stay close to the CHC particles where the electric field is the strongest.

These particles, through their ability to bind together, structure the soil creating micro and macroporosities, a veritable labyrinth that allows air and water to circulate in the soil.

This structure also provides the soil with stability against environmental damage such as: rain, soil compaction caused by the passage of agricultural machinery, etc…

“A soil that is well-balanced in organic matter through regular inputs will therefore restructure itself quicker, for example when agricultural machinery passes through” explains Olivier Cor.

Organic matter and the associated flora and fauna also give it a more natural resistance to various sources of stress and not just mechanical stress but also natural such as: heavy rainfall, drought, erosion.

4 - Humus and soil water

Humus and especially the humus/mineralization chain is essential to obtain a perennial soil structure that ensures efficient water air penetration and circulation as well as good root colonization.
It is these elements combined with the life in the soil that will ensure good exploitation of the land by healthy roots that are well supplied with water and minerals.

“It will be wrong to say that organic matter acts as a water pump,” says Olivier Cor. “On the contrary, it allows it to circulate and spread rapidly throughout the soil volume, limiting the risk of runoff and facilitating its storage even in the event of heavy rain!“.

The CHC, which retains on its surface the exchangeable cations (Ca2+, Mg2+, K+, Na+…) also protects the soil from the risks of leaching losses, losses that are detrimental to the plants, to the farmer but also to the environment and therefore to all of us!

Organic matter is used to feed plants and microorganisms in the soil: without organic matter, there would be no living beings to degrade it, therefore no functional carbon cycle, no more plants, and an unmanageable level of CO2 … a return to the very beginning of our earth’s history.

Earthworms, crustaceans, and insects also play a fundamental role in the production, structuring, maintenance, and productivity of agricultural soils. They also need fresh and decomposed organic matter for food and as a place to live.

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