Introduction to integrated methods in the vegetable garden
Humus; formation and evolution
Chapter : Crop soil
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⇒ Humus; formation and evolution
Introduction to integrated methods in the vegetable garden
Chapter : Crop soil
Previous or next articles ; click on a title to go to the page
⇒ Humus; formation and evolution
Humus is derived from the decomposition of dead organic matter commonly known as Fresh Organic Matter (FOM) from forestry or agricultural activities. These organic materials are used by a variety of soil organisms as a source of energy and materials for building their cell structures and reproduction. During this process, various organic compounds such as starches, sugars, proteins, cellulose, lignins and resins are transformed by a process known as mineralisation into simpler bodies that can be assimilated by plants or enter into other processes leading to the formation of humus.
Studies carried out over the last fifty years on humus have revealed physico-chemical properties that are particularly interesting for agriculture: increased water retention capacity, favourable effect on soil structure, stimulation of microbial activity, improved root exploration, easier absorption of fertilising elements, release of nitric nitrogen, etc. However, not all humus are suitable for agriculture. Some are too acidic or too rich in carbon. Only mull-type humus, suitably prepared with composts or by cultivation techniques with soil preservation, is considered indispensable for maintaining or rebuilding fertility.
As the name implies, it includes all forms of living organic matter of animal or plant origin, including plant roots, microflora, earthworms, fungal mycelium, etc. LOM represents only 1-3% of total soil organic matter.
refers to all dead organic matter of plant and animal origin including rapidly decomposing materials. FOM includes all more or less fractionated elements that can be easily identified. Because of their short life span, FOM represents only a small part of the total organic matter of the soil (about 20%).
is derived from the above substances and is commonly referred to as humus. Humus is made up of long and complex molecules that are often closely linked to mineral matter. Humus is not strictly speaking the final residue of decomposition. Humification is characterised by the formation of larger and larger molecules from the simpler residues of decomposing SOM. The soil microflora is not only responsible for this humification. Chemical and physical processes (polycondensation, polymerisation, inheritance humification characterised by reduced lignin decomposition) are also involved. Humus has different fine structures depending on the initial composition of the plants, without recognising their origin with the naked eye. Humus is a colloidal substance that is insoluble in water and has the consistency of a soft, airy, dark-coloured material with a characteristic odour. As their life span can be several decades, or even several hundred years under specific conditions, humus accounts for about 80% of the organic matter in soils.
In a forest, humus losses are compensated by the addition of new organic elements from plants (leaves, dead branches) and faecal matter produced by animals. These inputs form a first layer of organic matter at different stages of decomposition, which is called surface litter (horizon O in the pedological reference system). It constitutes an important source of energy for all the useful auxiliaries; insects, earthworms, fungi and micro-organisms that participate in its decomposition. The thickness of this litter is more or less important depending on the nature and density of the forest and the local climate.
In soil science, horizons refer to layers of different structures found in a soil section called solum. The profile is the sequence of information from top to bottom of a horizon designated by a letter. The reference horizons are listed by the Association française pour l’étude du sol (AFES). These horizons are classified into groups and subgroups in relation to the evolution of pedogenesis (soil history) and the proportion of certain constituents. The horizons that are mainly relevant to agriculture are the following:
Horizon-O: In relation to the surface, this horizon describes the first layer corresponding to different degrees of decomposition of organic matter. The horizon-O coincides with the surface litter found in forests, which is essentially made up of more or less organised organic matter (branches, dead leaves, animal excrement, etc.).
Horizon-A: also called "topsoil", this horizon is located below the horizon-O. It is made up of a mixture of mineral matter and well-decomposed organic matter, much of which is transformed into humus.
Horizon-L: This corresponds to the ploughed area in which allochthonous elements have been added, modifying its initial characteristics (fertilisers, amendments, etc.).
Horizon-S: Pale in colour compared to the A horizon, it contains smaller quantities of organic and mineral elements from the A or L horizons, including mineral salts that can be assimilated by plants. This horizon was formed by the alteration of primary minerals, releasing clays in particular. It is from this more or less thick underlying layer that the deep roots of certain plants bring up nutrients from the alteration of the parent rock (designated by other letters and which may be hard or loose or composed of scree or other).
In a regularly ploughed soil, the horizon-L has a homogeneous colouration that contrasts with the underlying layer. The upper part of this underlying layer is called the ploughing surface.
In a forest, humus losses are compensated by the addition of new organic elements from plants (leaves, dead branches) and faecal matter produced by animals. These inputs form a first layer of organic matter at different stages of decomposition, which is called surface litter (horizon O in the pedological reference system). It constitutes an important source of energy for all the useful auxiliaries; insects, earthworms, fungi and micro-organisms that participate in its decomposition. The thickness of this litter is more or less important depending on the nature and density of the forest and the local climate.
The constant renewal of fine roots produces underground litter which is responsible for 92% of the subsoil biomass. The thickness of this litter is often very high in the vicinity of trees. This can be easily seen in public gardens by observing the soil surface below the trees in the wet season. The soil surface is often covered with small mounds of soil called "turricules" in French. These are the result of the activity of earthworms that feed on this natural organic litter.
In a field or vegetable garden, the surface litter is often small or even absent. The farmer must then add crushed or composted crop residues or manure to try to maintain the soil's humus reserve. These surface residues often represent 50% and sometimes more of the organic matter of the cultivated land. In the PACA (Provence-Alpes-Côte d’Azur) region, where durum wheat is grown extensively, the carbon-rich straw is often crushed before being buried to replenish humus reserves.
Underground litter from crop roots also helps to maintain the humus pool. This underground litter is often underestimated because it cannot be seen. However, cultivated plants fix an enormous amount of carbon, especially when their crop cycle is brought to an optimum by fertilisation and adequate irrigation. Some of the carbon is exported through the crops. The other part is found in the surface residues (such as straw) and in the roots.
Public garden of Oraison; Turricules of plough worms
grass with part of its root
Plant roots are usually covered with root hairs (radicles) that are invisible to the naked eye and form a large surface area for the assimilation of water and mineral nutrients from the soil. These root hairs are fragile and disappear quickly. A single example of winter rye can produce 620 kilometres of roots in only 0.5 cubic metres of soil (4). These root hairs are constantly renewed to compensate for losses. It is not easy to estimate the biomass of plant roots because the volume of rootlets is difficult to sample. Nevertheless, as a general rule, plants allocate more biomass to the roots if the limiting factor for growth is below ground (e.g. water), whereas they allocate more biomass to the aerial parts if the limiting factor is above ground (e.g. reduction of light caused by obstacles such as taller plants).
In temperate zones, root and rootlet decomposition contributes 30-50% of the organic matter in a forest, with the remainder coming mainly from leaf fall (4). For field crops, here are some examples in kg/are of stable humus from roots:
* For the definition of K1 see below
Ces chiffres montrent que 1/5 à 1/8 de la matière sèche des racines est transformé en humus.
Pour les cultures maraîchères, voici quelques exemples de productions d’humus exprimées en kg/are provenant des litières de surface et souterraine.
Quantity of stable humus provided by the harvest residues of some vegetables, taking as average K1 13,5* (from data extracted from A ; Anstett " niveau humique des sols " 1962) ; table published in " la fertilisation des cultures légumières " of the CTIFL - indices brought back for a surface of 100 m² (1 are).
** This figure is the result of multiplying the average amount of dry matter in kg/are by K1, i.e. 13.5 in this case; example: carrot 32 x 13.5 = 4.32
The indices in this table show that the residues of vegetable crops produce little stable humus, which must therefore be supplemented by other inputs when trying to correct annual humus losses (for the calculation of annual losses for an area of one acre, see here).
Mull is a low-acid humus found in forests containing many deciduous trees. Many earthworms participate in the biological activity. The surface litter is decomposed after one year. This humus is characterised by its high nitrogen content (C/N = 10 to 15). The soil structure is rich in clay-humus complexes and has a granular structure. A variant, carbonate mull, which originates on calcareous soils, has very interesting biological characteristics that can be reproduced in agriculture on identical soils by adding organic amendments or by using soil conservation techniques such as semi-direct cultivation. Humus is strongly bound to clay by calcium bridges producing highly flocculated structures. Rendzines are humus rich in limestone, magnesium and iron, black in forest areas, reddish if iron is very present. In mountainous areas with a rich deciduous vegetation, the humid and cold climate favours the accumulation of humus to form humo-chalky soils.
Moder is a humus that originates on poorer soils characterised by thick litter and a less diverse fauna and flora. The surface litter often consists of a mixture of leaves and needles where fungal filaments predominate. The bacterial population is less numerous. The colour of the moder is very dark. Its pH is acidic (< 5) and the C/N ratio varies from 15 to 25. Rankers are moder humus with low nitrogen content that can evolve over time by fixing calcium, potassium, iron hydroxide and aluminium ions from the bedrock to become less and less acidic.
Mor is a humus found on siliceous soils where many conifers or heather grow in difficult environments (high mountains, rocky areas along certain maritime coasts, sandy soils, boreal areas, etc.). The soils are very acidic with low biological activity. The surface litter decomposes very slowly. The pH can go down to 3.5 and the C/N ratio is above 20. Due to their high acidity, this type of humus lasts for a very long time and can accumulate and evolve in some areas to form infertile humus layers.
Humus in grassland and in cultivated areas is related to human activity. Compared to cultivated areas, grasslands are often richer in humus. The quality of agricultural humus depends on several factors, not all of which are anthropogenic, e.g. climate. The nature of the crops, tillage, organic inputs are factors that modify to a greater or lesser extent the content and qualities of this humus. Poor soil maintenance leads to a reduction in humus, whereas good agricultural practices have the opposite effect. There are very specific and ancient humus linked to human activities such as the Brazilian black earth (terra preta) which is exceptionally fertile and consists of a high concentration of charcoal, organic matter and nutrients mixed with pottery shards.
Humus contains complex organic acids including fulvic acids (low molecular weight, molecules soluble in acidic or basic reactions), humic acids (higher molecular weight, molecules insoluble in acids and alcohols), and humin (an extremely stable high molecular weight substance consisting of various products linked together, molecules insoluble in all solvents). Humin makes up 50-70% of humus.
Fulvic acids are known to dissociate the parent rock, thereby increasing the mineral resources available to plants. These organic acids also have the advantage of increasing the availability of phosphorus.
As for humic acids, because of their flat structures linked together, they improve water retention in the soil (they absorb about 16 times their own weight). Like fulvic acids, humic acids can form compounds with metal ions, especially iron and copper, or with other ions to form food reserves for plants. In addition, humic acids play an important role in the degradation of pesticides and their metabolites present in the soil.
The mineralisation of humus is mainly dependent on the microbial biomass of the soil. Soil aeration, temperature and moisture are important factors in humus mineralisation. Compared to organic fertilisers, humus mineralisation is slower and contributes little to the enrichment of the crop soil in mineral salts. This contribution is usually nil for long-lasting humus.
Agricultural humus, like all other humus, is not eternal. It is gradually mineralised by micro-organisms (with production of mineral salts and gases such as CO₂). Humus losses vary from 1 to 5% per year (1). Soil pH, ploughing depth, precipitation, climate are factors that intervene more or less in humus loss. Warm, moist and aerated soils have high humus loss rates. Very dry and hot soils, on the other hand, lose less humus. Mountain areas are also known to have very low humus loss, probably due to the much longer cold periods than in plains and valleys. Generally speaking, a ploughed clay soil can lose 2% of its organic matter per year while a sandy loam soil can lose 4% (2). Without a constant supply of fresh organic matter or composts, humus will eventually disappear.
When organic matter contains plant debris rich in cellulose and lignin, their very slow decomposition does not contribute much to the increase of the biological activity of the soil. The larger the granulometry of the organic matter, the longer it will take to integrate carbon into the humus pool. Tree bark and shredded tree branches will take more than a year to integrate into the humus pool. However, the yield of stable humus is higher than that of rapidly decomposing organic matter.
Preserving the humus capital of cultivated soils is essential, especially as it can take a very long time to be reconstituted. In a cereal field soil containing 2% organic matter, this quantity of humus present in a 20 to 25 cm deep topsoil layer represents the product of the transformation of straws from a hundred years of cereal cultivation (3).
In order to predict the volume and nature of organic inputs, the estimation of humus losses from a cultivated soil is detailed by clicking here.
The fertilizing properties of humus are defined by the carbon to nitrogen (C/N) ratio. A C/N of 20 means that humus contains 20 times more carbon than nitrogen. Nitrogen-rich plant debris produces humus with a C/N of around 15. Plant debris containing a lot of cellulose produces humus with a C/N often higher than 20 and is in a way poor humus. However, they are valued for their soil restructuring properties because they produce very stable agglomerates with clay. The lower the C/N, the faster the humus degrades.
In a natural environment (forest) or artificial environment (fields, vegetable gardens, etc.), if there is too much carbon in the fresh organic matter, micro-organisms will consume a large part of the nitrogen reserves present in the soil, resulting in a nitrogen deficiency for the plants. This deficiency is also known as "nitrogen starvation", which occurs when the farmer spreads organic matter that is too rich in carbon (e.g. straw that has not been transformed into manure) in the field. Nitrogen starvation is characterised by stunted plants and pale green foliage. But this nitrogen is temporarily not available to the plants. Some of this nitrogen will be returned with the death of micro-organisms following the reduction of excess carbon. Soil that is too rich in carbon can be corrected by adding mineral nitrogen (e.g. pearl urea, ammonium nitrate), the dose of which is determined by a nitrate analysis using laboratory test strips (see here).
It is interesting to know the amount of humus from the different types of fresh organic matter. The isohumic coefficient (or K1 coefficient) defines the amount of stable humus remaining after 3 years of burial of a given amount of fresh organic matter. For each type of fresh material, the K1 coefficient applied to the dry matter specifies what will remain as stable humus. For example: for a K1 of 15, which corresponds to a green manure with a dry matter production rate of 20%, one tonne of FOM will produce 30 kg of stable humus. It is immediately obvious that green manure produces very little stable humus, whereas a tonne of well decomposed manure with a K1 of 50 and a dry matter of 20% will produce 100 kg.
The K2 coefficient expresses in % the rate of humus destroyed each year by mineralisation. As a general rule, a loss of humus of 2% in the north of the Loire and 3% in the south of the country is accepted for vegetable crops.(1)
1) Magdoff et Weil, 2004
2) À McGuire - Can Manure Sustain Soils? – 7 février 2018
3) Frayssinet – fertilisation organique des sols
4) Global Soil Biodiversity Atlas 2017
5) La fertilisation des cultures légumières – Ctifl ; H Zuang – Edition 1982
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