General Microbiology - Schlegel H. 1987

Decomposition of natural substances
Humus formation

Most PLANT AND ANIMAL residues decompose within the soil (Fig. 14.5). Easily degradable Materials undergo rapid and fairly complete oxidation, whereas substances that are resistant to microbial breakdown persist in the soil for extended periods as its organic components. Soil organic matter consists partly of partially decomposed plant remains and partly of humus. Humus is defined as an amorphous, typically dark-colored material of biological origin found in the soil. It comprises compounds that are difficult for microorganisms to degrade—primarily Lignin, as well as fats, Waxes, CARBOHYDRATES, and protein components. These are transformed into polymeric substances that defy precise chemical characterization. In addition to Bacteria and Fungi, Protozoa and various types of worms also participate in humus formation.

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Fig. 14.5. Transformation of plant matter in the soil and humus formation. (Modified from Flaig W., Landw. Forschung, 21 [1968], 103.)

Concurrently with the humification of plant material, it becomes enriched with nitrogen. While the carbon-to-nitrogen ratio in plant residues is approximately 40:1, in humus it is about 10:1. A significant portion of the nitrogen is converted into Organic compounds—that is, into a form unavailable for plant uptake. Lignin binds nitrogen particularly tightly, serving as a source of lignoproteins and heterocyclic compounds. Humus exists in a state of dynamic equilibrium: on the one hand, its supply is continually replenished by the influx of organic residues; on the other, it diminishes as a fraction of the humus undergoes complete oxidation. The humus content of a soil is higher the more favorable soil conditions are for its formation and the less favorable they are for its decomposition. The low humus content of tropical soils is attributed to the rapid microbial breakdown of all organic matter, a process fostered by the tropical climate. Steppe chernozem develops in regions with prolonged cold winters and dry summers. However, The amount of accumulating humus depends not only on climatic and soil conditions, but also on The Nature of the plant residues. Cereal straw and steppe plant residues yield readily degradable humus, whereas forest tree leaves, and especially pine needles, produce a coarse humus that is highly resistant to decomposition.

During The process of humus formation, numerous carboxyl groups are released or generated within the organic compounds. Consequently, the presence or absence of bases is of critical importance for the quality of humus and the rate at which it is processed by microorganisms. In soils deficient in mineral components, particularly alkaline cations (such as podzols, heath soils, and coniferous forest soils), fulvic acids accumulate (acid humus). In the presence of adequate alkaline minerals, base-neutralized humus colloids are formed, which, combined with clay colloids, constitute the soil's sorption complex. The organic fraction of this complex can be regarded as a high-molecular-weight natural ion exchanger that maintains a specific ionic balance for soil inhabitants, including plants and microorganisms. The formation of mild humus stimulates biological activity in the soil; fungal hyphae and mucus bind soil particles together, thereby conferring a favorable crumb Structure.

Whereas purely mineral soil is poor in microorganisms, humus-rich soil harbors a vast diversity of species. This community, which is present even in unfertilized soil, is termed autochthonous, in contrast to the zymogenic community that dominates when organic matter is added to the soil. Thus, the stabilizing effect of humus on soil dynamics is also linked to its role in maintaining a rich soil microflora.