Humus form
The humus layer is formed from dead plant remains that are decomposed. When the activity of burrowing animals is low, a distinct O horizon forms above the mineral soil, as in mor 1, mor 2, and moder. When activity is high, the material is mixed into the soil, as in mull‑like moder and mull.
Organic matter in the soil – properties
In addition to various minerals, forest soils contain organic matter. This consists of dead plant and animal remains that have been decomposed to varying degrees by different organisms that utilize the energy and nutrients contained in the organic material.
Over time, a humus form characteristic of the particular forest ecosystem develops in the soil. The classification into humus forms is based on:
- the amount of incompletely decomposed organic material (the fermentation layer, or Of layer) in relation to the amount of well‑decomposed material (the humic substance layer, or Oh layer),
- the degree of mixing with mineral soil as a result of the activity of burrowing soil animals, primarily earthworms,
- whether the accumulation of organic material at the soil surface is due to high water content and thus reduced oxygen availability required for decomposition (H horizon).
The fermentation layer (Of) consists solely of organic material at various stages of decomposition. However, more than 50% by volume consists of dead plant parts that have partly retained their original structure. The remaining material mixed with these plant remains consists of structureless humus (strongly decomposed material). The visible plant remains are interwoven with fungal hyphae and fine rootlets. In addition, small pellets in yellow and greyish‑white shades are often present. The fermentation layer is not mixed with mineral soil. Locally, however, small accumulations of mineral grains may be encountered as evidence mainly of the activity of so‑called “soil” ants.
The humic substance layer (Oh) consists of at least 75% organic material, of which at least 50% by volume is a dark paste (crumbly when dry). Plant parts with a more or less preserved structure may make up the remaining portion of the organic material. A maximum of 25% mineral soil admixture may therefore occur.
Taking into account the presence of fermentation and humic substance layers, mineral soil admixture, and aggregate formation, the humus layer is divided into different humus forms. These are intended to provide a qualitative expression of the properties of the humus layer, primarily from a fertility perspective (availability of plant nutrients).
On the most nutrient‑poor sites, decomposition of organic material proceeds slowly while the input of new litter is very low. This results in an O horizon consisting of an Of layer that is thicker than the Oh layer, and the humus form is termed mor. The mor humus form is further divided into two classes, mor type 1 and mor type 2, where the former has an Of layer constituting more than 75% of the entire organic horizon (see below). If the Oh layer dominates and mineral soil mixing is still limited, the humus form is moder. Where the humus layer is clearly separated from the mineral soil and consists only of a fermentation layer and/or a humic substance layer without the presence of aggregates (aggregates are clumps of organic material and/or mineral soil particles, ranging in size from a grain to a pea; they consist of earthworm and insect excreta and are more stable the higher their clay content), the humus form is thus mor or moder.
Towards more nutrient‑rich forest soils, the humus form changes to mull‑like moder and then to mull. The difference in character between humus forms of the mor type and the mull humus form is created by increasing earthworm activity and finer‑textured, more nutrient‑rich soils.
The humus forms peat‑like mor and peat develop on moist sites where the groundwater table lies close to the soil surface and decomposition is inhibited by oxygen deficiency. The humus layer of these humus forms is usually designated H.
The main principles underlying the differences between the humus forms mentioned above are illustrated in the figure below. Where aggregates occur, the humus form may be mull‑like moder or mull. Crack formations that “apparently” resemble aggregates occur on sites with special environmental conditions where the humus form is termed gyttja soil.
The figure below schematically shows the relationships between the Of, Oh and A horizons (denoted “A” in the figure) for humus forms of the mor and mull types. (The figure is not relevant for H horizons, i.e. humus forms of the peat type.)
Humus, which most often constitutes the main component of the organic material in the humus layer, consists of decomposed (or decomposing) animal and plant remains. The degree of decomposition can vary from the stage where the process has just begun—when the structure of plant remains and similar materials is still visible—to the stage where the material has been completely transformed into a more or less homogeneous mass that, when moist, forms a dark brown paste. This variation means that, in some cases, the humus layer can be subdivided into different sublayers (Of and Oh, respectively; see below). The thickness of these sublayers may vary over time depending on differing environmental conditions which, in one case, may result in decomposition of organic material exceeding the input from litter production, and in another case the opposite situation.
Certain substances that contain readily available energy or valuable nutrients are decomposed rapidly. Other substances are broken down much more slowly or are transformed into chemical molecules that are more or less stable. Lignin, a substance found in wood, is very difficult to decompose. The most stable compounds in the soil, the humic substances, are considered to consist largely of somewhat transformed lignin compounds. During the decomposition of organic material, nutrients that are important for the growth of trees and other plants are released. Nitrogen (N) and phosphorus (P) are important nutrients that are mineralised—converted from being bound in organic material into inorganic, plant‑available compounds. Organic matter also functions as an effective ion exchanger and can bind positively charged ions such as Ca²⁺, Mg²⁺, and K⁺. The humus layer therefore constitutes an important reservoir of plant nutrients. It supplies nutrients that are part of the biological cycle and binds nutrients that are released from the inorganic parent material.
Contact
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PersonJohan Stendahl, head of department and researcherBiogeochemistry of Forest Soils