The organic matter content of soil has reduced by 1 to 2% in 30 years. This decline has led to the decrease in organic soil fertility and agricultural productivity. To offset this decline in soil fertility, large quantities of inputs have been used. Agricultural productivity can be improved once again only by changing farming practices and restoring this stock of humus.
How is humus formed in the soil?
A number of stages and actors are involved in creating humus in the soil: Fresh organic matter is incorporated into the soil by biological activity, especially that of earthworms, and/or human activities (crop residues, cover crops, roots, compost, regeneration of the fauna and microflora of the soil, etc.). There are three processes by which humin (the basic component of humus) can be formed.
Process 1
Most of the organic matter (90%) is quickly decomposed into simple molecules (CO2, water, nutrient ions, etc.). These simple molecules have 5 outcomes:
- Released in the atmosphere
- Absorbed by the plants
- Fixed on the clay-humus complex
- Lost by leaching
- Recovered by microorganisms
These simple molecules recovered by filamentous fungi are used by the latter as an energy source and will yield a substance bonded to the mineral matter: humin (1).
Process 2
The plant constituents impregnated with lignin yield:
- Soluble humic compounds
- A weakly transformed lignin residue
Soluble humic compounds undergo a series of polymerisations, via filamentous fungi in the soil, yielding humic acid and then humin (2).
Process 3
The weakly transformed residues mix with the mineral matter by mixing in the digestive tract of the earthworms. They then yield humin (3).
Humin (1) + (2) + (3) forms the most insoluble, stable and long-lasting component of humus. This humus is then bonded to clay particles in the digestive tract of earthworms and forms the clay-humus complex of soil. This is the nutrient reservoir for plants.
What strategy must be adopted?
All practices aimed at respecting and developing earthworms and filamentous fungi will be ideal for increasing the humus stock of soil.
We can highlight:
- Elimination of tillage and reduction of inputs
- Cover cropping between all crops, to protect and enrich the land
- Maximum conservation of crop residues
- Use of leguminous plants in crop rotations
Humus and organic carbon
There are both agronomic and environmental benefits to improving the soil humus content. In fact, stocking humus is stocking organic carbon in the soil (https://www.4p1000.org/).
It means transforming the CO2 captured by plants for photosynthesis into an agronomic substrate essential for agricultural productivity and good plant nutrition while respecting the ecosystems.