The humus balance allows us to assess whether farming practices preserve, increase, or reduce the soil’s organic matter content. It weighs the carbon inputs from roots, crop residues, cover crops, and organic amendments against the losses due to mineralization. For farmers, this assessment provides a decision-making tool to adapt crop rotations, residue management, and organic inputs to the specific needs of each field.
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What is a humoral assessment?
The humus balance monitors the evolution of stable soil organic matter, often referred to as raw humus . It compares humus inputs from roots, crop residues, plant cover, and organic inputs with losses caused by mineralization.
The result indicates whether the cropping system increases the humus content, maintains it at a relatively stable level, or leads to its decrease. This approach makes it possible to assess the long-term effects of agricultural practices on the physical, mineral, and biological properties of the soil.
What is the purpose of the humus balance?
Humus balance can be used at several levels, from a one-off diagnosis of a plot to the comparison of cropping systems over several years:
- assess the organic state of the soil and anticipate the evolution of its carbon stock;
- check if the restitutions compensate for the losses through mineralization;
- compare the effects of crop rotations, cover crops, tillage and residue export;
- to measure the influence of organic inputs and compare different effluents;
- identify systems that consume more humus than they produce;
- guide the management of straw, crop residues and cover crops;
- to determine the input of organic matter according to the characteristics of the plot;
- set a target for maintaining or increasing organic stock over several campaigns.
The calculation does not only provide an annual value. It also helps to compare several scenarios before modifying a rotation, exporting more residues, or changing tillage practices.
The different compartments of organic matter
Soil organic matter comprises carbonaceous materials whose level of transformation, stability, and duration of presence in the soil vary greatly:
- Living organic matter includes roots, bacteria, fungi, earthworms, microflora, and other organisms present in the soil. It participates directly in biological transformations and the functioning of nutrient cycles.
- Fresh organic matter includes plant residues, animal debris, excrement, root exudates, and recently incorporated biomass. It provides an energy source for decomposing organisms.
- Transient organic matter corresponds to materials in the process of transformation. It represents an intermediate stage between fresh residues and more stable organic forms.
- Humified organic matter corresponds to raw humus. Composed of relatively stable compounds, it generally constitutes the largest part of the organic stock and contributes sustainably to the stability of the soil structure.
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Inputs and outputs taken into account
The entrances
Inputs refer to organic matter returned to or added to the soil and subsequently transformed into humus. This material comes from straw, aboveground residues, roots, cover crops, cover crops, and grasslands. Manure, compost, slurry, digestate, wood chips, and other organic products can also contribute to the soil’s organic matter supply. The actual amount returned depends on the crop species, yield, biomass produced, root development, and the proportion of residues removed from the field. Not all organic matter produces the same quantity of stable humus, as its carbon content, composition, and rate of decomposition vary.
The exits
The outputs primarily originate from the secondary mineralization of humus. Their level varies depending on the initial organic carbon stock, soil texture, clay content, presence of limestone, pH, depth of analysis, and climatic conditions. Temperature and humidity directly influence the biological activity responsible for the decomposition of organic matter. Cultivation practices also play a role: repeated soil aeration can stimulate microbial activity and accelerate mineralization. Some calculators therefore differentiate between direct seeding, simplified cultivation techniques, and systems involving more intensive tillage.
How do you calculate a humus balance?
The calculation is based on an estimate of the carbon inputs likely to form humus and the quantities of carbon lost through mineralization.
The historical method of Hénin-Dupuis
The Hénin-Dupuis method is based on a simplified representation of organic carbon inputs and outputs:
B = Σ (k₁ × m) − k₂ × C
In this equation, B represents the annual change in the organic carbon stock. The variable m corresponds to the amount of fresh carbon returned by plant residues or organic matter. The coefficient k₁ indicates the proportion of this incoming carbon that joins the stable organic stock. The variable C represents the carbon stock already present in the soil, while k₂ corresponds to the fraction of this stock mineralized during a year.
The carbon stock for the following year can therefore be expressed as follows:
Cₜ₊₁ = Cₜ + Σ (k₁ × m) − k₂ × Cₜ
The humification coefficient k₁ varies depending on the type of material added. Mature compost, straw, slurry, and young plant cover do not produce the same amount of stable organic matter. The mineralization coefficient k₂ depends on climate, soil texture, limestone content, moisture, temperature, tillage practices, and the initial organic matter content.
Coefficients should always be used in conjunction with their reference system. Some older methods apply the humification coefficient to dry matter, while others apply it directly to the carbon content of the product. Confusion between these units can significantly alter the final result.
The AMG model
The AMG model, whose name refers to Andriulo, Mary, and Guérif, describes the fate of organic carbon in greater detail. It distinguishes between fresh carbon added to the soil, an active carbon compartment subject to mineralization, and a stable carbon compartment whose renewal is much slower.
Its general principle can be summarized as follows:
Stock variation = Σ (k₁ × m) − k × active C
Some of the carbon from residues, roots, and organic matter enters the active compartment. This compartment changes according to new inputs and the rate of mineralization. The stable compartment represents a fraction of the carbon that remains in the soil for a significantly longer period.
Developed from French agronomic research and long-term trials, the AMG model integrates soil characteristics, climate data, farming practices, and biomass production. It allows for the simulation of carbon stock evolution over several years and the comparison of different rotation, soil replenishment, and tillage scenarios.
The quality of the result depends, however, on the data entered. Yields, quantities of residues, root biomass, carbon content of organic products and soil characteristics must be representative of the plot studied.
How should the results of the humus balance be interpreted?
The result must be analyzed over several campaigns, because the evolution of the organic stock remains slow and can be masked in the short term by the variability of the measurements.
A positive humoral assessment
A positive balance means that the inputs of humified carbon exceed the losses through mineralization. The theoretical humus stock increases, and the organic matter content can gradually improve. This situation can be linked to significant crop residue returns, the regular presence of cover crops, high root biomass, organic inputs, or reduced tillage intensity.
This evolution generally promotes structural stability, biological activity, water retention, and the soil’s ability to withstand climatic stresses. However, a very positive balance does not mean that the increase in organic matter content will be immediately measurable. Several years may be necessary before a clear change appears in soil analyses.
A negative humoral assessment
A negative balance indicates that mineralization exceeds the input of humified carbon. The humus stock gradually decreases, particularly when residue returns are low, cover crops are absent, biomass production is limited, or tillage strongly stimulates the decomposition of organic matter.
In the long term, this depletion can reduce aggregate stability, increase susceptibility to soil compaction and erosion, decrease water retention, and weaken soil resilience. A one-time negative result must be considered within the overall crop rotation: a crop that exports biomass can be offset by temporary grassland, a productive cover crop, or a crop that returns more biomass in subsequent growing seasons.
A balance sheet close to equilibrium
A result close to zero indicates a theoretical balance between humification and mineralization. The organic matter content then tends to be maintained, provided that the calculation assumptions correspond to the actual conditions of the plot. This balance may be a suitable objective for soils already rich in organic matter, while degraded or poorly organic soils may require a positive trajectory over several years.
The interpretation must take into account the initial carbon stock, soil type, depth considered, and the desired agronomic objective. The same annual variation does not have the same meaning in a shallow, carbon-poor soil as in a deep soil that already has a high carbon stock.
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