Humification – maintaining and improving soil fertility

Climate Organic Farming Organic Matter Fertility Management in Arable Crops Humus Climate Change

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 it from erosion, and promotes the activity of soil organisms. Soils rich in humus not only guarantee good yields but also improve the resilience of crops against long periods of drought or intense rainfall. Due to the effects of climate change, research is increasingly focusing on the role of humus in mitigating the effects of this disruption, far beyond the sole domain of organic farming. After a period of decline in the humus content of Swiss arable land, values have remained stable in recent decades. However, the potential for humus formation is far from fully exploited. Many measures could be easily integrated into current agricultural practices. Experience shows that a humus enrichment strategy adapted to the specific conditions of the farm allows making the most of these benefits. This technical sheet explains the basics of humus management, presents the main improvement measures, and provides advice for planning farm-specific strategies.

What is humus?

Humus is the total of dead organic matter in the soil. All starting organic materials of plant, animal, and microbial origin are transformed in the soil by microorganisms. Since the starting materials have different compositions, humus is a very heterogeneous mixture of different organic substances. Humus gives the soil a dark color. Therefore, soil color can provide initial indications of its humus content. In mineral soils, humus is mainly concentrated in the upper soil layer, where organic matter is abundant and microbial activity is most intense. This is why mineral soils are generally lighter in depth.

Why is humus important?

Structuring agent

Humus helps bind soil particles into stable aggregates. The soil structure becomes crumbly and porous. Good soil structure improves air circulation, accelerates spring warming, facilitates water infiltration, and reduces soil erosion by water and wind. It also promotes root growth and increases mechanical resistance to compaction. Humus can thus partially compensate for the negative properties of clay soils (poor aeration, slow water infiltration, susceptibility to compaction), silty soils (susceptibility to erosion and crusting), and sandy soils (poor water and nutrient retention capacity).

Habitat shaper

Illustration 1: Positive effects of humus. Humus has positive effects on both the chemical and physical and biological properties of the soil. Humus contributes to good plant growth, soil protection against erosion, and soil ecological diversity.

Humus is closely linked to living organisms in the soil. On one hand, it serves as a food source for most organisms. During biological transformation, the nutrients contained in dead organic matter are mobilized and made available to these organisms and plants. On the other hand, during transformation processes, part of the humus is tightly bound to the soil's mineral substance (clay-humus complexes) and is thus protected from further degradation. Humus bound to clay minerals significantly contributes to improving soil structure. All soil organisms, from unicellular organisms to earthworms, participate in these processes.

Nutrient reservoir and source

Besides carbon, humus mainly consists of oxygen, hydrogen, nitrogen, phosphorus, and sulfur. With an estimated annual mineralization rate of 1–3%, 10–300 kg of nitrogen are released per hectare, depending on the soil's humus content. About one-third is assimilable by plants. Humus is therefore an important source of nitrogen and phosphorus, but not of potassium. Due to microbial decomposition, the release of bound nutrients occurs continuously. Given its large exchange surface, humus has a very high nutrient retention capacity, which can exceed that of clay. Humus thus increases the soil's cation exchange capacity and prevents leaching of positively charged nutrients.

Carbon sequestration to slow climate change

Humus is composed on average of 50% carbon and constitutes the largest carbon pool in the soil. Increasing the soil's humus content helps reduce the amount of carbon dioxide, a powerful greenhouse gas, present in the atmosphere. Maintaining a high humus content in soils can significantly contribute to climate change mitigation. The "4 per 1000" initiative, launched at the United Nations Climate Conference in Paris in 2015, shows that an increase in humus content of 0.4% per year compared to the current value, in the top 30–40 centimeters of all soils worldwide, could stop the increase of CO2 in the atmosphere.

However, much of the continents consist of deserts or are covered by forests. On other surfaces, a decrease in humus content is expected due to global warming. Some agricultural areas have already reached their local potential, and humus can again be lost due to inappropriate management, leading to CO2 emissions.

Agriculture therefore has significant potential to slow climate change through better humus management, but it cannot accomplish this task alone.

Humus in a few figures

Humus is composed of 40–70% carbon (C), 5% nitrogen (N), and other elements such as oxygen, hydrogen, phosphorus, sulfur, calcium, magnesium, etc.
It is not possible to directly and precisely determine the humus content of a soil. The standard method is to measure the organic carbon content (Corg or OC) by burning a soil sample at over 900 °C in an elemental analyzer. This yields the Corg content. Humus content is calculated by multiplying Corg by the factor 1.725.
Swiss soils contain between 0.5 and 4.5% Corg in the top 20 centimeters. This corresponds to 10 to 100 t of Corg, or 17 to 172 t of humus per hectare. The spade test allows an easy and quick assessment of soil structure. Good soil structure with small rounded aggregates indicates a high humus content relative to clay.

What is the ideal humus content?

Besides cultivation methods, the humus content of soils primarily depends on their clay content, a component of stable clay-humus complexes. When evaluating humus content, one should therefore not only consider the absolute humus content but also the humus/clay ratio. Sandy soils with low clay content (<10%) can store only little humus. Heavy soils with clay content above 40% generally have higher humus contents. They need more humus than sandy soils to form a crumbly structure.

Table: *Evaluation of humus content according to clay content*
Humus content relative to clay content	Evaluation
< 12 %	Insufficient
12–17 %	Moderate
17–24 %	Good
> 24 %	Very good

Example 1: Clay content 15%, Corg 1.9%:

Humus content = Corg × 1.725 = 3.3%

Humus/Clay = 3.3% / 15% = 0.22 (22%; good)

Example 2: Clay content 35%, Corg 2.9%:

Humus content = Corg × 1.725 = 5.0%

Humus/Clay = 5.0% / 35% = 0.14 (14%; moderate)

Mastering influencing factors – exploiting potentials

To best utilize existing potentials and efficiently manage humus on the farm, it is important to understand the factors regulating soil humus content.

Site characteristics

Soil humus content depends on soil type, exposure, water regime, and climate. Most site characteristics cannot be influenced by cultivation techniques. But it is important to know a site well to assess the soil's humification potential.

Main site-related rules

Soils depleted in humus have a higher enrichment potential than fertile soils.
Sandy soils have a low humus accumulation potential.
Under mild and humid climates, organic matter humification and humus mineralization are faster than under cold and dry climates.
In soils rich in organic matter and well-drained, humus mineralization is particularly high.
On sloping terrain, erosion can also cause humus losses.
Humus contents can vary greatly within the same plot.

Important to know

Clay content must be considered when evaluating humus content.
Each site is unique and difficult to compare to others. The same content can be evaluated as good for one site and insufficient for another.
Climate and weather also influence humus content. It is therefore advisable to always take soil samples at the same time of year to allow comparison.

Crop rotation

Permanent grasslands have the highest humus contents. On arable land with a large proportion of row crops or open-field vegetables, humus loss is greater than on farms with a large share of temporary grasslands, cereals, grain maize, and seed legumes. Indeed, the mandatory return of crop residues (roots, stalks) or optional (straw) throughout the rotation compensates for the natural mineralization of humus. On the other hand, intensive soil tillage and absence of plant cover accelerate humus loss. Crop rotation determines the frequency and intensity of soil tillage, how soil biological activity will be stimulated, as well as the duration and density of the plant cover.

Besides economic and agronomic criteria, the effects of crop rotation on humus content must also be considered when planning crops. Excessive humus decomposition due to highly nutrient-demanding crops can also have negative financial consequences due to decreased soil fertility.

Detailed article : Principles of soil fertility

Important to know

To reduce humus losses due to erosion and promote humus formation through a plant cover, ensure as continuous soil cover as possible by undersowing and catch crops.
Humus-consuming crops should, if possible, be followed by humus-producing crops.

Plant covers

Many plant covers have been developed in recent years. Pure sowings are increasingly replaced by mixtures that ensure high biomass production and whose roots explore different soil horizons. The combination of different species and plant families also promotes the development of highly diverse microorganism populations.

Covering the soil

Undersowing of grasslands, catch crops, or plant covers, as well as sowing of companion plants, can contribute to continuous soil cover. These different techniques also help regulate pests (especially in oilseed rape), hinder weed development, prevent erosion (especially in row crops), provide protection and food for beneficial insects, and regulate soil temperature. Covers rich in legumes supply nitrogen to the following crop. Companion plants fulfill their function at the beginning of the crop and then freeze, as in oilseed rape cultivation, while undersowing plays its role after the main crop harvest, as in cereal cultivation.

To cope with failures of plant covers sown after harvest during heat waves, the undersowing technique is gaining importance. It involves anticipating their establishment before the heat to ensure high biomass production as soon as water is again available.

However, undersowing is more demanding in crop management because if unsuitable, they may compete with the main crop for water, nutrients, and light. Undersowing with too vigorous development can negatively affect the yield and quality of the main crop and make harvesting more difficult. Therefore, the goal of undersowing is to anticipate the establishment of plant cover to produce biomass immediately after harvest and during the intercrop period. The more intensive the main crop management, the less the undersowing is likely to develop, possibly leading to sparse, low-productive vegetation.

Undersowing in cereals can be used after harvest as forage catch crops or multiannual temporary grasslands, but their harvest forfeits their primary objective of humus regeneration. Mixtures specially designed for undersowing produce less biomass than standard forage mixtures established after harvest.

Traditionally, undersowing is established in spring, with or after the last pass of a harrow or tine harrow. In regions where spring is dry, undersowing can also be done in autumn simultaneously with the main crop sowing. In this case, it is recommended not to use vigorous forage mixtures.

Considering the C/N ratio

The maturity degree of plant cover biomass at incorporation determines what happens in the soil. A high N content (C/N ratio <15, e.g., green manure with a high proportion of legumes or very young biomass) favors rapid decomposition, and excess nitrogen will be available for the following crop. However, rapid decomposition also carries a risk of nitrate losses if the following crop cannot use the available amounts. A C/N ratio of 15–20 is optimal for good biomass decomposition. Balanced mixtures based on legumes, as well as covers at the beginning of flowering, generally have such composition. A C/N ratio above 20 (e.g., a lignified cover after winter) has a high C content. If the nitrogen deficit is not compensated by slurry incorporation or commercial fertilizer application, the nitrogen needed for decomposition is taken from the soil solution. This can result in competition for nitrogen between soil organisms and the crop, called nitrogen hunger, and a slowdown of mineralization. A high (wide) C/N ratio rather promotes humus formation, a low (narrow) C/N ratio increases the amount of available nitrogen.

Important to know

It is essential that the soil is protected by a plant cover during winter. Species remaining during winter must be at the right physiological stage and tolerate low temperatures.
Do not use species from the same family as the main crop. If peas are integrated into the crop rotation, do not sow green manures based on peas or vetch in the following crop. Other legumes, such as clover, pose fewer problems.
Green manure mixtures rich in diverse species are preferable to pure sowings because they have more positive effects on soil fertility, weed control, emergence reliability, and biodiversity.

Transfer mulch – targeted biomass transfer

Transfer mulch is a way to close nutrient cycles on farms without livestock. The mulch layer helps improve thermal and water balance in intensive and water-demanding crops such as potatoes or open-field vegetables. It can also help control weeds. The supply of fresh matter and better water and thermal balance stimulate soil biological activity.

The C/N ratio of the mulch determines its effect on humus and nitrogen supply. To prevent weed development, a "wide" C/N ratio is preferable so that the mulch does not decompose too quickly. A "narrow" C/N ratio is important for a rapid nutritional effect.

Different substrates are suitable for transfer mulch:

Green biomass from an external area
Silage in the form of fermented mulch
Hay/reeds as dry mulch

Types of transfer mulch vary according to farms, and their availability is subject to more judicious use. Depending on the vegetation type of the donor area and the desired mulch layer thickness, the ratio between donor and recipient areas varies from 1:1 to 1:4. The required donor area must not be underestimated, and spreading must be well planned. Besides appropriate machinery, adaptations on recipient areas are sometimes necessary (traffic lanes). The pros and cons must be weighed.

Soil tillage

Soil tillage is an important factor for improving soil fertility. Studies have shown that superficial soil tillage without turning it over enriches the top layer with organic matter. This results in better soil structure and better bearing capacity of the arable layer. It also helps combat erosion and preserve the biological activity of the soil. Fungi and earthworms, in particular, are less disturbed and can better exert their soil-stabilizing effect.

Each soil tillage operation weakens the humus, and the more intensive the tillage, the more deleterious the effects. It is about finding the ideal balance between agronomic usefulness and the damage caused by humus mineralization. The principle for soil tillage is always: "as much as necessary, as little as possible."

Minimize plowing

In many organic farms, plowing is used to control perennial weeds. Moreover, it facilitates breaking up (turning over meadows). However, the continuous development of superficial soil tillage tools now allows for complete or substantial abandonment of deep plowing. A superficial plowing (ideally with a plow stubble plow) helps reduce humus decomposition.

No-till soil work (without turning) presents some challenges, however:

The warming of the more compact soil is slowed in spring and may delay juvenile crop development.
The delay in juvenile development can lead to stronger weed pressure and a greater risk of infestation by harmful organisms.
Residues from the previous crop can clog harrows and hoes. Chopping the previous crop residues (chopper) helps reduce these risks.

Deep loosening to reduce compaction

Machine traffic, grazing, and natural compaction processes make the soil denser below the arable horizon. This reduces water and air permeability, which can lead to reduced root growth and increased drought stress. To avoid this, it may be advisable, depending on soil type, to loosen it more deeply.

Deep loosening can be mechanical or biological by establishing deep-rooted crops. Similarly, mechanical deep loosening must be stabilized by a root system before traffic can resume on the soil, which might otherwise collapse again.

Important to know

It is recommended to change soil tillage techniques gradually to better assess the effects.
Mechanical deep loosening should be done just below the old plow pan (20–30 cm depth). Before deep loosening, determine with a probe, spade test, or soil profile whether loosening is necessary and at what depth, and if the soil is sufficiently dry.
Ideally, deep loosening is done on a plot with green manure or temporary grassland, or just before sowing a deep-rooted crop (alfalfa, Chinese cabbage, etc.).
Practical experience has shown that soil is particularly sensitive to compaction if loosening has not been stabilized by plant roots for several weeks.

Crop residues

Surface and soil crop residues also contribute to the formation of soil organic matter. What applies to cover crops in terms of C/N ratio also applies to crop residues: the higher the lignin content (and thus the C/N ratio), the slower they decompose and thus contribute to humus formation. Straw can therefore immobilize nitrogen in the soil. That is why the ideal in farms with livestock is to return it to the soil as composted and stabilized manure. For farms without livestock, a straw-manure exchange is a solution. Otherwise, the role of legumes is predominant: straw management must be considered in rotation planning, choice, and management of cover crops.

Important to know

If crop residues with a high C/N ratio remain on a plot, this can cause nitrogen immobilization at the beginning of the next crop because available nitrogen is needed by microorganisms for residue decomposition.
Applying slurry on cereal stubble reduces the C/N ratio of the stubble and thus promotes its decomposition. However, slurry spreading can cause nitrogen losses through ammonia volatilization, and nutrients may be partially leached due to lack of plant uptake.

Farmyard manure

In most organic farms, farmyard manure forms the basis of humus management. But generally, there is still great potential for improvement in its use.

Spreading

Unlike mineral fertilization, organic fertilization does not deplete humus reserves and can even contribute to humus formation in the soil. However, improper use can lead to loss of a large part of the nutrients. These are then no longer available to crops and can pollute the environment.

Important to know

Apply nutrients at the right time, i.e., when they can be absorbed by plants.
Avoid applications on bare soil, water-saturated soil, during severe drought, or during plant dormancy.
To promote humification, it is better to avoid rapidly assimilable nitrogen fertilizers such as slurry or fermented slurry.
Large quantities of slurry harm earthworms. Therefore, no more than 25 m³ of slurry per hectare per application should be spread.
Diluting slurry can reduce its toxic effect on earthworms.
Sampling slurry at key times of the year provides useful information on nutrient concentration. These data can also be used in subsequent years.
To reduce ammonia losses (and odor emissions), spread slurry in cool temperatures (preferably in the evening) and close to the soil (trailing shoe).
In arable crops, shallow incorporation is recommended immediately after spreading.

Preparation

The treatment of farmyard manure aims to improve its value for soil and plants and reduce nutrient losses. Manure can be composted; slurry can be separated and diluted.

Humification effects are better if manure is already decomposed or composted.

Composting manure

Composting manure reduces the C/N ratio and, in the case of straw-rich manure, improves nutrient availability and avoids nitrogen immobilization. The carbon contained in composted manure is transformed into humus and thus remains stored in the soil longer.

The workload to produce composted manure is relatively high and requires some experience. Decomposed manure is a compromise between composted and fresh manure. It can be produced by mixing layers and loosely storing them on the manure pad. Decomposed manure is better tolerated by plants than fresh manure and, if the latter is low in straw, the nitrogen effect will be faster than with composted manure. However, composted manure has a more positive effect on humus formation than decomposed manure. Fresh manure has the least effect on humification.

Table 2: Effects of different types of manure
Stacked manure
Little effect on humification Possible nitrogen immobilization Possible formation of phytotoxic substances
Decomposed manure
Moderate effect on humification Welcome food for earthworms Reasonable workload Good nitrogen effect
Composted manure
Maximum effect on humification Significant extra work Good nitrogen effect No nitrogen immobilization even if manure is rich in straw

Separating slurry phases

Separating the liquid and solid phases of slurry allows targeted use of solid and liquid parts and helps reduce nutrient losses during spreading. Moreover, separating solids slightly increases slurry storage volume.

Most farms dilute slurry with water in outdoor runs. The more dilution, the better slurry is tolerated by plants and soil. However, limited storage space, water availability, and spreading efficiency may limit slurry dilution.

Use of additives

Various additives are marketed, such as rock powders, organic extracts, bacterial or homeopathic preparations, to reduce emissions from farmyard manure and increase nutrient efficiency.

So far, scientific studies have rarely confirmed positive effects of additives for farmyard manure, and results were not always reproducible. From a scientific standpoint, no additives can be unreservedly recommended.

Biochar is a very stable organic matter. In common humus measurement methods, it is considered humus. Thus, biochar increases the measured humic value but not the actual value. Its use is of great practical interest, but very large quantities are needed to increase the soil's water and nutrient retention capacity. It mainly acts in sandy soils.

There are highly variable qualities, and only certified biochar is recommended. Regarding its use in slurry, stables, animal feed, or composting, scientific data are not yet sufficient to make recommendations.

Observations show that additives for farmyard manure are quite effective when already incorporated into feed or litter.

Important to know

During composting, starting materials, optimal moisture, regular turning of windrows, and other factors are decisive for good results.
Depending on specific farm conditions, such as exposure or water protection zones, field-edge composting is not always possible or only limited. Producing decomposed manure on the manure pad is an alternative.
When using additives, apply treatments appropriately and evaluate product effectiveness.

Application of other organic fertilizers

Besides farmyard manure, organic fertilizers from other farms can be used within the fertilization balance. Well-decomposed compost or composted manure are particularly suitable for humus management. Organic farms can also cover up to 50% of their nutrient needs by sourcing from biogas plants. There is a distinction between digestates from agricultural biogas plants and those from industrial biogas plants. Digestate from industrial biogas plants contains more than 20% cosubstrates, more nitrogen, and often more foreign substances. Liquid digestates must be listed in the input list and, from a humus management perspective, should be used sparingly and diluted due to their high concentration of readily soluble nitrogen. With non-agricultural biogas plants and composting plants, the load of foreign substances must be checked when purchasing digestate or green waste compost. When contaminated inputs are used for several consecutive years, foreign substances, e.g., plastic, can accumulate in soils. Moreover, Bio Suisse standards also include provisions on allowed nutrient quantities and maximum transport distances.

Applying organic fertilizers can increase humus content on one’s land but does not store excess atmospheric carbon in the soil.

Important to know

To avoid importing pathogens and problematic weeds, organic fertilizers must come only from reliable sources.
Using green waste compost and products from non-agricultural methanization plants carries a risk of foreign substances. Regular use of organic fertilizers containing plastic, even in small amounts, can heavily contaminate fields.

How to measure soil humus content?

Humus contents must be interpreted with caution due to seasonal variations, high field heterogeneity, and other factors. However, measured values can be useful to observe the effects of changes in cultivation techniques. To obtain reliable results on humus content, some principles must be respected.

The technical sheet Soil analyses for organic farms (see p. 20) describes the procedure to follow and provides guidance on choosing the appropriate analysis method. Estimation based on color from a tactile test is too imprecise to be used properly. If the method is not specified, inquire at the laboratory. Detailed monitoring, as required for example in humus regeneration projects as a climate protection measure (climate), costs more than a standard tactile analysis.

For an approximate estimate of the humus balance of a farm or plots during their rotation, the Agroscope humus calculator is available free of charge at humusbilanz.ch.

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 considering clay content, pH, and cultivated plants. Organic fertilizer inputs are notably taken into account. The humus balance allows estimation per plot as well as for the entire farm.

Humus content analysis

Organic carbon materials are determined by combustion at over 900 °C in an elemental analyzer. The amount of CO2 emitted during combustion allows calculation of organic carbon content. Humus content corresponds to this value multiplied by 1.72 (e.g., 1.8% organic carbon × 1.72 = 3.1% humus). Loss-on-ignition or wet chemical methods (potassium dichromate) according to Walkley-Black are no longer used.

Source

Paul Mäder et al. (FiBL), 2022, Humification – maintaining and improving soil fertility. Available at: https://www.fibl.org/fileadmin/documents/shop/1315-gestion-humus.pdf