The humus is part of the total organic matter in the soil . It is primarily subject to the activity of the soil organisms ( edaphon ), which through their metabolism continuously contribute to the build-up, remodeling or degradation of the humus. In the true sense of the word, only the decomposed organic part in the soil is considered humus, while the undecomposed part is called detritus . Since the respective conversion stages are fluid, an exact delimitation is not possible. Neither humus nor detritus are dead matter, but are heavily infused with soil bacteria .
Humus consists of a multitude of complex compounds which, after dying, release organic matter and are chemically converted both spontaneously and enzymatically by soil organisms. The compounds differ considerably in their degradability by microorganisms. Low-molecular carbohydrates and proteins are broken down quickly, complex compounds such as cellulose or lignin are broken down more slowly. Certain humus components therefore remain in the soil for only a few weeks or months ( nutrient humus ), while others remain for centuries or millennia ( permanent humus ).
First, find biochemical reactions ( hydrolysis - and oxidation processes by tissue enzymes ) organisms substances native place in the plant, for. B. the breakdown of chlorophyll leads to autumnal discoloration of the leaves. In the preliminary phase of decomposition, the cell structure remains intact.
During this phase, hydrolysis and oxidation of high polymer compounds occur. In addition, water-soluble components (e.g. sugar , amino acids , organic acids) are washed out. This leads to a sharp increase in the number of microorganisms that live in particular from the substances released.
The macrofauna sometimes bites into litter, sometimes eats it and excretes it in modified form. These compounds are then worked into the soil by little bristles (e.g. earthworms , Enchytraeidae ) and arthropods (e.g. insects , millipedes , arachnids ) and thus generally promote accessibility for the mesofauna (collembola, mites, nematodes).
Dismantling and conversion phase
The organic fragments are now split enzymatically and simple inorganic components such as CO 2 , H 2 O , NH 4 + , NO 2 - , NO 3 - , PO 4 3− (= mineralization ) are released. There is a relative accumulation of poorly degradable substances (e.g. lignin , lipids ). Lignin is broken down and converted (ligninolysis) by specialists (e.g. white rot fungi ).
The phases described above initially create nutrient humus (labile organic substance), in the further stage permanent humus (stable organic substance) is created.
Products of humus formation
Nutrient humus are the organic substances that are rapidly broken down in the soil. In addition, there is the body substance of all dead soil organisms. The nutritional humus of plant origin has the following composition:
- Carbohydrates ( cellulose , sugar , starch ; usually over 50%)
- Lignin ( 10–40% depending on the degree of lignification of the plants )
- nitrogenous compounds (usually less than 10%)
Nutrient humus serves as a source of food for most soil organisms and is therefore the prerequisite for the biological activity of the soil. Incorporated flat or finely distributed as a root mass (after the roots have broken down, a finely branched system of tubes remains), it promotes ventilation and thus the metabolism. With the cell breakdown, the plant nutrients bound in the organic substance are returned to the material cycle . This makes them available for feeding new plants. The nutrient humus provides the building blocks for building the humic substances in permanent humus.
In contrast to nutrient humus, permanent humus is only broken down very slowly. It arises from further degradation of nutrient humus or in the final stage of composting.
It can bind both water and nutrients and release them back to the plants. The water and nutrient binding capacity is many times that of clay. Permanent humus is an essential building and stabilizing element of the soil structure through the formation of clay-humus complexes and stable soil aggregates. The permanent humus makes up most of the soil's organic matter (generally over 90%) and contains most of the soil's nitrogen. It causes the dark color of the humus topsoil and thus promotes the warming of the soil surface. Due to its properties, permanent humus has a decisive influence on soil fertility .
The humus quality can be measured by the nitrogen content, namely by the carbon / nitrogen ratio (C / N). In the freshly dead substance, the C / N ratio is high, but with strong differences depending on the type of plant , plant part and age of the plant (C / N ratio of winter wheat: 71; sugar beet: 20). The C / N ratio is reduced by the degradation in the soil. An optimal C / N ratio is 10 to 15.
For the rough calculation of the nitrogen content of the humus topsoil, the humus content and the thickness of the A horizon play a role. A nitrogen content of 1/17 of the humus content and a weight of the crumb soil of 1500 t / 10 cm / ha can be assumed. Under humid and temperate climatic conditions, it is expected that around 1–2% of the organically bound nitrogen in the crumb is converted every year and thus becomes available to plants (= nitrogen replenishment from the soil).
Due to the high C content of the humus (approx. 60%), carbon can be bound in the soil at the same time as the high humus content.
Due to their pore volume, soils rich in humus can store more water than others.
Carbon and other elements are food for microorganisms that settle and continuously multiply in soils that are particularly rich in humus.
L denotes the scatter horizon ( English litter - "scatter"). It contains dead plant remains that are not or only slightly decomposed. These are still recognizable as such without restriction and can be classified according to plant species. The proportion of fine substance is less than 10% by volume.
O (from organic) denotes a horizon of organic matter with more than 10% by volume of fine matter. Plant remains are already clearly decomposed. The horizon contains less than 70 mass-% mineral matter. The O-horizon can be subdivided into Of- and Oh-horizon.
Of (rotten, from Swedish: Förmultningsskiktet) is an O-horizon in which the proportion of fine organic matter (10 to 70 vol.%) Clearly emerges. The plant remains have already largely decomposed through fermentation and disintegration. Structures of plant tissue can still be recognized, but these are already mixed with humus particles.
Oh (from humos ) denotes the permanent humus horizon with dark-colored humic substances. No plant structures can be identified from the material it contains. The decomposition of the plant material has reached a very advanced stage. The proportion of fine organic matter (over 70% by volume) predominates.
The A h horizon is not part of the humus layer . This is the mineral topsoil and mostly contains humus brought in by animal activity (e.g. earthworms and moles) or human activity (e.g. plowing). The humus content here is a maximum of 30 percent.
The extent to which the humus layer is pronounced and which of the horizons described it has depends in particular on the extent to which the existing environmental conditions (temperature, humidity, soil acidification, nutrient supply, exposure, location, latitude, climate and others) promote humus accumulation. In general, the more unfavorable the environment is for the activity of the microorganisms, the greater the humus accumulation.
Easily decomposable organic material (leaf litter) and optimal decomposition conditions in the mineral topsoil are characteristic of the humus form Mull. As a result, only a thin layer of humus forms (L horizon). Scatter decomposition takes place mainly in the Ah horizon . Mull is significantly involved in the formation of a void-rich, stable crumb structure of base-rich soils. Ground-burrowing representatives of the Edaphon such as earthworms, woodlice, millipedes and fly larvae predominate. Earthworms ensure that the topsoil is continuously supplied with newly formed gauze through their steady excrement deposits on the soil surface and in their tunnels. The humus substances formed in the gauze are highly polymeric and therefore hardly mobile.
The raw humus is the most unfavorable form of humus. Raw humus consists of vegetation residues ( litter ) that are difficult to decompose . A too cool or humid climate also leads to inadequate conversion of the plant waste.
For example, the litter of conifers is more difficult to decompose than that of many deciduous trees. In general, constituents such as waxes , resins , tannins and lignin are difficult to convert, consequently dead plant parts with high proportions of these substances also survive much longer and often produce only inferior humus. The undecomposed litter and the small amount of fine organic matter are clearly separated from each other.
The humic substances can be washed out of the topsoil into the subsoil (B-horizon). In addition, poorly decomposed litter from raw humus soil can form organic acids. This also makes the iron in the soil more capable of migration and can also be washed out into the B horizon. Raw humus thus promotes podsolization .
Moder is a hybrid between gauze and raw humus.
Hydromorphic humus forms
Very high water contents inhibit the oxygen supply to a soil and thus the decomposition of organic matter, which consequently accumulates. Depending on the water balance, the resulting humus forms are referred to as wet humus, wet humus and swamp humus. Extremely wet locations lead to peat formation. At the bottom of water there is the sea humus (see Mulm ).
Rock humus soil
Column humus or rock humus soil describes the formation of humus in rocks and mountain walls; the humification takes place from the inside out through the biocenosis of various lithobionts . It consists of clay-yellow to chestnut brown calcareous clays and has high moisture and high detritus content. It arises on rocks in so-called cart fields and provides the basis for alpine evergreen plants .
Humus content of the soil
The humus content of the soil can fluctuate within wide limits. It can be calculated from measured values for the organic carbon content of the soil by multiplying these values by a factor of 1.72 (factor 2 for peat and humus). Depending on the humus content of the soil horizon , the vegetation , the climate , the soil moisture and land use . There are also great differences with regard to the distribution of humus in the soil : In forest soils, the main mass of humus lies as a more or less thick layer (see humus forms) above the mineral soil (humus, raw humus). In agricultural mineral soils, the humus is intimately mixed with the mineral content. The salary decreases rapidly from top to bottom. The mean humus content of plowed mineral soils is 1.8–2.5% in the crumb, with grassland soils in the mean of the upper 10 cm around 5–8%. Higher humus contents are typical for clayey soils, moist to wet soils and soils in a climate rich in precipitation. Heavily aerated, sandy soils have lower humus contents (1–2%).
In the bottom of a continuous mining and construction will take place from humus. In a stable ecosystem (e.g. forest , old grassland ) both processes are balanced, i. H. the humus content hardly changes. The tillage increases the humus decomposition. Therefore, there must be a sufficient supply of organic matter (humus supply). The influence of arable farming on the humus content of the soil can be clearly demonstrated by plowing up grassland: The higher humus content under grassland sinks rapidly in the first years of arable use and gradually adjusts to a low value that varies from location to location. When new grassland is sown, they gradually increase again. If the residual humus content, as it occurs in our arable soils, is also relatively stable, it is not invulnerable. It can be run down, for example, by growing humus consumers such as sugar beets , potatoes , silage maize or vegetables. As part of cross compliance , great importance is attached to the preservation of organic matter in the soil. In certain cases, farms are obliged to have the humus content determined by means of a soil analysis.
The discovery of the Edaphon and the functions of humus made it possible to look for alternatives in the form of organic fertilization , after the mineral fertilizer was increasingly criticized in the last quarter of the 20th century (see history of fertilizer ).
The supply of organic matter is conventionally carried out by
- the crop residues ( roots , stubble , straw , sprouts) left on the field after harvesting ,
- the targeted cultivation of catch crops for green manure
- Manure ( dung , manure , compost , sewage sludge fertilizer )
- Bark humus
The amount of organic matter added to the soil is determined by the type of plant being grown and the method of harvest. It is less important that nutrients are supplied to the soil in large quantities, but that they can be "processed" by the soil. Crop residues, catch crops, manure and liquid manure are to be assessed differently in terms of their effect on the humus content. Harvest residues are decomposable substances that break down faster the softer (less lignified) and higher in protein they are.
- Othmar Nestroy : Understand the soil: structure, types, fertility . Stocker, Graz 2015. ISBN 978-3-7020-1193-2 .
- W. Amelung, H.-P. Blume , H. Fleige, R. Horn, E. Kandeler , I. Kögel-Knabner , R. Kretschmar, K. Stahr , B.-M. Wilke: Scheffer / Schachtschabel textbook of soil science. 17th edition. Heidelberg 2018. ISBN 978-3-662-55870-6 .
- Scheffer / Schachtschabel Textbook of Soil Science , 15th edition, 2002, ISBN 3-8274-1324-9 .
- Kuntze / Roeschmann / Schwerdtfeger Bodenkunde , 5th edition, 1994, ISBN 3-8252-8076-4 .
- Diedrich Schroeder: Soil Science in Key Words , page 36, Bern 1983, ISBN 3-266-00192-3 .
- Soil Science Mapping Instructions , 5th edition, Hanover 2005, ISBN 3-510-95920-5 .
- Annie Francé-Harrar: Soil life and fertility . Bayerischer Landwirtschaftsverlag 1957.
- Humification , paragraph humus forms and humus types , menu systematics: Soil information / soil development / humification, Hypersoil project - learning and working environment on the subject of "soil" in the classroom, Westfälische Wilhelms-Universität Münster u. a., 2002.