Life produces organic material

The organic part of soil comes from organisms that live over and under its surface. The variety of these organisms and the exorbitant number of substances that they synthesize, that range from simple amino-acids to large natural polymers, such as lignin, explain the great diversity of organic material present in the earth. In the soil not all the remains of vegetable and animal organisms are biodegradable in the same way. A good part of these residues remains practically unchanged for even rather long periods, accumulating in time. In addition to this, in many environments such as peat-bogs and moors of high latitudes or altitudes, the climate is dominated for most of the year by low atmospheric temperatures that slow down decomposition phenomena.
When soil’s organic material remains in time, a superficial layer is formed made up of a brown-coloured matrix that is capable of staying in this phase for indefinite periods of time even though it has lost the macroscopic characteristics of the original materials from which it derives. The substance obtained is called humus.
Humus has been studied for a long time, especially from the point of view of its chemistry; nevertheless, to date, it has not been possible to describe its composition in detail. For example, it is known that part of the organic nitrogen that it contains derives from the presence of amino-acids, amino-sugars and nucleic acids, however, the nature of the remaining part is not totally clear. What can be stated, however, is that humus is fundamental in guaranteeing a reservoir of organic material that is useful for soil organisms.

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