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17/3/2006

The humus layer

The humus layer is formed from decomposing litter, which is dead leaves, roots, branches etc. In areas where the number of excavating soil organisms is small, their activity is limited, and the humus layer form a well-defined organic horizon (O-horizon) on top of the mineral soil. This applies to mor1, mor2 and moder. If there is a large number of excavating soil organisms, like for example earthworms, the organic matter is worked into the mineral soil resulting in a mineral soil with humus content (A-horizon) like in the humus forms moder and mull.


Dominerande humusformsklass Saknas Mår Mull
Torv

Maps were produced by merging SK's classes for humus form according to the following:

Class

Consists of

Missing

(0) No humus layer

Mor

(1) Mor type 1 + (2) Mor type 2 + (7) Peatlike mor

Mull

(3) Moder + (4) Mull-like moder + (5) Mull

Peat

(6) Gyttja soil + (8) Peat with mineral grain admixture + (9) Peat without mineral grain

 

Mean thickness of the humus layer

Mean thickness of the humus layer

Characteristics of the humus layer

In course of time a humus layer characteristic of the particular forest ecosystem is developed. The classification of humus forms is based on:

  • amount of incompletely decomposed organic matter (litter layer or Of-layer) in relation to the amount of well decomposed organic matter (humus layer or Oh-layer)

  • degree of admixture with the mineral soil

  • if the accumulation of organic matter at the soil surface is caused by a high water content and thereby a reduced access to oxygen needed for decomposition (H-horizon)

At the most nutrient poor sites, decomposition of organic matter is very slow and litter supply small. The result is an O-horizon consisting of an Of-layer thicker than the Oh-layer and the humus form is denoted mor. The humus form mor is, in turn, divided into two classes, mor1 and mor2, where mor1 has an Of-layer constituting more than 75% of the total organic horizon. If the Oh-layer dominates and if there is only a slight admixture with mineral soil, the humus form is moder. Towards more nutrient rich forest soils the humus form gradually turns into, first, mull-like moder, and thereafter to mull. The differences between the two humus forms of mor type and mull is caused by increasing earthworm activity and more fine-grained, and nutrient rich soils. The humus forms peat-like mor and peat are formed at wet soils where the groundwater table is close to the soil surface and decomposition is restrained by lack of oxygen.

Soil organic matter

Apart from different minerals, forest soils also contain organic matter consisting of remnants from plants and animals. Soil living organisms decompose the organic matter and thus feed on energy and nutrients released. Some of the organic matter contains energy and nutrients easily available for the decomposing organisms and the turnover therefore is fast. But some of the organic matter decompose much slower or is transformed to more or less stable chemical molecules. Lignin, a substance found in wood, is very difficult to decompose. The main part of the most stable compounds in forest soils, humic substances, are considered to consist of somewhat transformed lignin substances. Through the decomposition of organic matter, nutrients important for growth of trees and other plants are released. Nitrogen (N) and phosphorus (P) are examples of two important nutrients that are mineralised. They are bound in the organic matter. Through the decomposition process they become part of inorganic compounds easily available for plants. The organic matter is also an effective ion exchanger and can bind positively charged ions like Ca2+, Mg2+ and K+. The humus layer therefore is an important nutrient reservoir. It delivers nutrients being part of the biological cycle and binds nutrients released from the inorganic mother material.

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