Definition of soil organic matter

Agriculturist since ancient times have recognized significant benefits of soil organic matter
(SOM) to crop productivity. These benefits have been the subject of controversy for centuries and some are still debated today.
Many of the benefits of SOM have been well documented scientifically, but some effects
are so intimately associated with other soil factors that it is difficult to ascribe them uniquely to the organic matter. In fact, soil is a complex, multicomponent system of interacting materials, and the properties of soil result from the net effect of all these interactions.

One of the major problems in communicating in the field of humic substances is the lack
of precise definitions for unambiguosly specifying the various fractions. Unfortunately, the terminology is not used in a consistent manner. The term humus is used by some soil scientists synonymously with soil organic matter, that is to denote all organic material in the soil, including humic substances. Contemporary, the term humus is frequently used to represent only the humic substances.

The term SOM is generally used to represent the organic constituents in the soil,
including undecayed plant and animal tissues, their partial decomposition products, and the soil biomass. Thus, this term includes:

  1. identifiable, high-molecular-weight organic materials such as polysaccharides and proteins,

  2. simpler substances such as sugars, amino acids, and other small molecules,

  3. humic substances.

It is likely that SOM contains most if not all of the organic compounds synthesized by
living organisms.

SOM is frequently said to consist of humic substances and nonhumic substances.
Nonhumic substances are all those materials that can be placed in one of the categories of discrete compounds such as sugars, amino acids, fats and so on. Humic substances are the other, unidentifiable components. Even this apparently simple distinction, however, is not as clear cut as it might appear.

Distribution SOM is shown on the picture:

Organic compounds of soil - live organisms and their undecomposed, partly decomposed and completely decomposed remains as well as products of their transformation.

Living organisms alive - edaphon.

Soil organic matter - non-living components which are a heterogeneous mixture composed largely of products resulting from microbal and chemical transformations of organic debris. Soil organic matter can exist in different morphological patterns, which are the bases of the classification of so called forms and types of humus.

Unaltered materials - fresh and non-transformed components of older debris.

Transformed products - (humus) - bearing no morphological resemblance to the structures from which they were derived.These transformed components are reffered to as the humification processproducts.

Humic substances- a series of relativelyhigh-molecular-weight, brown to black colored substances formed by secondary synthesis reactions. The term is used as a generic name to describe tocolored material or its fractions obtained on the basis of solubilitycharacteristics:

Nonhumic substances- compounds belongingto known classes of bichemistry, such as :
The chemical and colloidal properties of SOM can be studied only in the freestate, that
is, when freed of inorganic soil components. Thus the first taskof the researcher is to separate organic matter from the inorganic matrixof sand, silt, and clay.
Methods for the extraction of soil organic matter have evolved from the research and
thinking of many scientists.

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