Quoted from: Chertov, O. G., and A. S. Komarov. "SOMM-a model of soil organic matter and nitrogen dynamics in terrestrial ecosystems." In Evaluation of Soil Organic Matter Models, pp. 231-236. Springer, Berlin, Heidelberg, 1996. https://link.springer.com/chapter/10.1007/978-3-642-61094-3_16 

      SOMM - a simulation model of soil organic matter mineralisation, humification and nitrogen release has been further developed. The model takes into account the rate of processes that are dependent upon the nitrogen and ash content of litter fall, temperature and moisture. The activity of the main groups of soil decomposers is reflected in the model. The model represents a system of linear differential equations with variable coefficients.

      SOMM represents a quantification of the humus types concept (Mor, Moder and Mull) existing in forest pedology since last century (Muller, 1887; Wilde, 1958; Duchafour, 1961). It has been created as a tool of theoretical analysis and natural ecosystems' simulation firstly. 

      The experimental basis for the SOMM model is a set of classical laboratory works on the rate of organic litter decomposition in controlled conditions in dependence of temperature, moisture and chemical composition of the material (Kostychev, 1889; Waxman & Tenney, 1927; Waxman & Gerretsen, 1931; Kononova, 1951; Mikola, 1954; Alexandrova, 1970). The experiments allowed quantification of the activity of the complex of micro-organisms (Fungi, Actinomycetes and Bacteria) with micro fauna. The procedure of obtaining the kinetic parameters from the data set has been described in detail previously (Chertov, 1985; 1990). Data from the laboratory experiments of Chernova (1978) for soil arthropods and Perel & Sokolov (1964) and Striganova et al. (1987) for earthworm activity have been used to quantify the role of decomposers in organic debris humification. 

      The idea of a complex of humic substances with undecomposed plant debris (CHS) was proposed for use in the model. This complex is responsible for the slow decomposition phase of plant debris, which still contains many easily mineralisable substances. This occurs because CHS, being impregnated with humic acids, is transforming at the rate of "true humus". It allows a new explanation for the "soil protective effect" (Van Veen & Paul, 1981), which has previously been attributed to the increasing lignin content in decomposing matter. CHS is believed to be the main inhibiting or modifiying factor of decomposition and is the "forgotten" compartment in the studies of SOM biochemistry. The CHS consists of sub-horizons F and H in forest floor with a superficial input of litter fall. It corresponds to a fraction of "labile (active) humus" in soils with root litter input.