Assessing Soil Humus Content
Humus plays a central role in soil fertility and fulfills multiple functions: it provides nutrients, improves soil structure, increases the soil's water retention capacity, protects the soil from erosion, and promotes soil organisms.
To assess the humus content of the soil, there are various analysis methods to choose from. Laboratory soil analysis provides an indicative value of the current humus content level. Additionally, the spade test or calculating a humus balance can help estimate the humus content of one's own farm soil.
What is humus?
Humus refers to all the dead organic matter in the soil. All initial organic materials (of plant, animal, and microbial origin) are decomposed in the soil by microorganisms. Since the starting materials have different compositions, humus is a very heterogeneous mixture of various organic substances. Besides carbon (40 to 70% C) and nitrogen (5% N), humus mainly consists of oxygen, hydrogen, phosphorus, sulfur, and micronutrients.
Humus gives the soil a dark color. Therefore, soil color can provide approximate indications of humus content. In mineral soils, humus accumulates mainly in the upper soil layer, where there is the most organic matter and microbial activity is highest. That is why mineral soils generally become lighter towards the bottom.
How much humus is good in my soil?
Besides management and local climate, the humus content of soils primarily depends on their clay content, as clay is a component of stable clay-humus complexes. Thus, when evaluating humus content, one should not only consider the absolute humus content of a soil but also its ratio to the clay content.
Sandy soils with little clay can store only small amounts of humus. By nature, heavy soils generally have higher humus contents. But heavy soils also need more humus than sandy soils to achieve good soil structure. Sites are therefore different and should only be compared among themselves. The same humus content can be high for one site and low for another.
Swiss soils contain between 0.5 and 4.5 percent organic carbon in the top 20 centimeters. This corresponds to 10 to 100 tons of organic carbon, or 17 to 172 tons of humus per hectare.
| Humus content relative to clay content | Evaluation |
|---|---|
| < 12 % | Insufficient |
| 12–17 % | Moderate |
| 17–24 % | Good |
| > 24 % | Very good |
How to determine (analytically) the humus content?
To assess the humus content of the soil, there are various analysis methods to choose from. Soil analysis provides an indicative value of the current humus content level. These measured values can be useful for observing the effects of changes in agricultural practices. However, they must be interpreted with caution due to seasonal variations, high heterogeneity in the field, and other factors. To observe changes, follow-up measurements should ideally take place in the same season, at the same location, and at the same time in the crop rotation crops (e.g., after the maize harvest).
To determine humus content, the standard laboratory method is to measure the organic carbon content (Corg or OC) by burning a soil sample at over 900 °C in an elemental analyzer. Humus content can be calculated from this measurement (Corg × 1.725). Estimating color from a tactile test is too imprecise for proper determination of humus content. Therefore, a direct and exact determination of humus content in the field is not possible.
More detailed information on costs and specialized services for laboratory analyses is available in the technical sheet "Soil analyses for organic farms".
Complementary methods for evaluating humus content
The following tools can help evaluate the humus content of a soil:
A high humus content visibly influences soil structure. The spade test allows quick and easy evaluation of soil structure. Good soil structure with small rounded aggregates indicates a high humus content relative to clay. Large angular aggregates rather suggest a low humus content. Of course, a soil with high humus content can also be compacted, and spade test sample results do not always allow conclusions about humus content.
For an approximate estimate of a farm's humus balance, the free humus calculator from Agroscope at humusbilanz.ch is useful. The humus calculator indicates whether the cropping system tends to preserve or reduce humus content. The humus balance compares organic matter inputs and losses by mineralization, taking into account clay content, pH, and cultivated plants. The input of organic fertilizers is particularly considered. The humus balance allows estimation per parcel as well as for the entire farm.
For more information
- Technical sheet «Humus management» (FiBL Shop)
- Technical sheet «Soil analyses for organic farms» (FiBL Shop)
- Agroscope humus balance (Website humusbilanz.ch)
- SoilDok spade test (instructions, forms, videos, app): (Website testbeche.ch)
Original article
La version initiale de cet article a été rédigée par Tim Schmid, Raphaël Charles, Jeremias Niggli et Daniel Böhler.
