Humus (from the Latin humus, “soil, earth, terrain”) represents the complex of organic substances, generally in the colloidal state, constituents of the soil and derived from the complete chemical and biological decomposition of the remains of plant and animal organisms (humification), by the intervention of microorganisms and following the action of physical and chemical agents of the environment.

Humus represents the most chemically and physically active part of the soil organic matter and interacts with the mineral fraction and the circulating solution influencing the chemical and physical properties of the soil. The discovery of the importance of humus for plant nutrition was made by the German agronomist Albrecht Thaer.

The color of humus is normally dark or blackish, as it occurs in the classic case of “black soil” of steppe, also characterized by a high content of calcium carbonate; in lateritic soils of tropical regions, however, it can take light colors, more difficult to perceive.

Chemically speaking, humus is made up of numerous organic complexes (lignin and its derivatives, proteins, etc.) to which are associated, in smaller proportions, carbohydrates, fats, waxes, organic acids and alcohols. Among the organic acids are significant humic, ulmic, cremic, fulvic, etc.. which, reacting with the bases contained in the soil, give the corresponding salts, soluble and therefore washable. Because of these characteristics, humus assumes the role of a reservoir of nutrients for plants; even if it is not absorbed by plants, except for a portion of the soluble fraction, humus favors their mineral nutrition. Moreover, it confers important physical characteristics to the soil.

Humus favors, in fact, the formation and preservation of a glomerular structure that facilitates the circulation of air and water; it increases the absorptive power of the soil, that is the ability of colloidal substances to retain mineral ions that can then be absorbed by plants; it maintains the pH of the soil within the neutrality values to which the cultivated flora and microbial flora are better adapted; it raises the thermal and water capacity of the soil; it increases the biological activity of the soil; it accelerates the root uptake of mineral ions present in the solutions circulating in the soil.

The protective function of humus against soil erosion is therefore twofold, in the sense that, in addition to promoting the development of plant cover, it actively contributes to maintaining soil particles in place, preserving them from runoff. The loss of humus that takes place in the presence of intensive cultivation not accompanied by adequate returns of organic matter, both plant (stubble, straw and other residues) and animal (liquid and solid manure), reduces fertility, which can not be recovered with only inputs of mineral fertilizers. Such conditions are frequent in intensive plant monocultures.

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