What, Exactly, Is Humus Made Of?

By Herwig Pommeresche

The term “soil humus content” refers to the totality of all the organic substances present in the soil. It is often expressed in terms of carbon content percentage, as carbon is the basic building block of organic material. But this definition is insufficient as it only reveals the sum of all the carbon atoms contained in the soil. How much of that is valuable compost, living soil biota, liquid manure, or other organic substances is not clarified.

What is humus made of

A relevant quote comes from M. M. Kononova’s treatise, “The Soil’s Humic Substances — Results and Problems in Humus Research” (1958):

“The history of humus research is rich in incorrect approaches to clarifying important questions, which has led to contradictions and confused ideas about the nature of humic substances, their origins, and the role they play in forming the soil and determining its fertility.”

But if we primarily understand “humus” as referring to the abundance of organic substances present in the soil, we overlook its mineral content.

The proportion of minerals has increased in cultivated soils during our era in comparison with past eras in which consistently humid heat promoted the formation of organic soil material over huge swaths of forest for thousands of years. The ratio between the organic and mineral portions of the material has shifted, to the detriment of the soil.

Incidentally, this is a particularly strong example of the importance of using the right terms with the right meanings: the word “mineral” is here used correctly to refer to everything from rocks, gravel, and sand to the very finest mechanically ground particles — it has absolutely nothing to do with NPK fertilizers or other salt ions.

One small calculation is sufficient to get an idea of the significance of plant roots in the soil:

“The formation of root hairs greatly increases the root’s surface area. Rye (Secale cereale) has about 13,000,000 roots with a surface area of 235 square meters, and 14,000,000,000 root hairs with a surface area of 400 square meters in 1/22 of a cubic meter of soil. [. . .] The surface area of the underground portions is thus 130 times as large as that of the above-ground portions” (Jurzitza 1987, 28).

One single rye plant has the equivalent surface area of an entire garden in direct contact with the soil in which it grows. What this soil is made up of has to be crucially important.

Annie Francé-Harrar (1957) wrote the following about how healthy soil should look for plant roots to be able to optimally carry out their work:

“Ideal soil should have the following composition: 65 percent organic material, 20 percent edaphic organisms, 15 percent mineral substances. [. . .] But this kind of abundance of organic material exists hardly anywhere on the planet any more, the highest concentrations being in untrodden corners of tropical jungles, but never in our growing soil. But it is possible to restore the organic-inorganic balance in growing soil within a practical timespan through systematically employed humus management.”

These recommended ratios also provide a target to work toward in systematic humus management. But she was already well aware of how difficult it is to put this into practice: “But this [. . .] means a radical agricultural revolution, much larger than the one triggered by Liebig in his time” (20).

How does the humusphere form?

Topsoil formation is very much a classic case study in the movement of living material from the waste material of living things into plants, of the descent of living material into Mother Earth. It’s also a study in the soil, of its many functions, of its conversions and storage until its reappearance in the world of above-ground organisms. The bulk of the soil material first becomes clearly visible as nutrient-forming chlorophyll, but that chlorophyll would never exist without the work of the countless organisms in the soil.

The conceptual model of mineralization — the complete breakdown of all organic material into inorganic base materials — is first of all (and I cannot emphasize this enough) a technically incorrect use of the terminology. Second, it is logically improbable that it takes place, because that would leave only one possible explanation for the new life that forms, that being the concept of spontaneous generation, which has been rejected by the same scientific establishment.

The same species of bacterial symbionts appear in almost all animal and plant organisms, the lactic acid bacteria. In fact, soil probes from all over the world, even if the soil in question is only slightly fertile, always contain large quantities of lactic acid bacteria. The soil contains more of them, and of better varieties, the more fertile it is. This is further evidence that the cycle of living material takes place in the topsoil through the mediation of bacteria. The remnants of biological processes on the surface, processed by countless species of small creatures, are first processed into precursors by budding fungi species, predominantly yeasts and molds, and then passed along to the bacterial symbionts in the soil. According to the most recent research, these symbionts — lactic acid bacteria in this case — can be directly consumed and digested as food via endocytosis by plant root hairs (Rateaver and Rateaver 1993), and they leave all kinds of organic material behind after they die, especially in the fall.

These particles, as well as the bacteria themselves (i.e., the living material in the soil bacteria), are a prerequisite for the formation of high-quality soil: topsoil that is aerated, loose, water-retaining, capable of biological tillage (Sekera 2012), safe from erosion, and fertile, the result of the functions of the edaphon, as outlined by Henning (2011). The adhesiveness of the microorganism residues cements the inorganic mineral substances of rock erosion into soil crumbs.

In contrast to the views of agrochemists, it is this alone that deserves the name “humus” in the biological sense: a conglomeration of organic and inorganic material. And this means that it is completely impossible to describe humus as a dead, chemical substance!

Humus formation is a sort of “organic predigestion” for plants; and at the same time, humic soil serves as a pantry of living nutrients during the growing season, when plants can grow only if supplied with sufficient warmth, water, and sunlight.

Otherwise, however, the parallels between animal and plant digestion are unmistakable. In both cases, microorganisms serve as an intermediate station, as “nutrient facilitators,” and in both cases organic or inorganic material can be extracted as needed from the nutrient substrate and used to build cells and tissues.

In purely spatial terms, the humusphere is the sphere between the atmosphere (the gas sphere) and the lithosphere (the rock sphere) and constitutes the biosphere together with the hydrosphere (the water sphere). In the humusphere, the entire metabolism of all dead and living material is carried out in a continuous cycle. It is driven by the oldest life-forms that we know of: microorganisms.

According to the cycle of living material model, we can attest that humus is created by life, out of life, for life.

Front cover Humusphere bookSource: Humusphere: Humus, a Substance or a Living System?

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