Humus

Nature (humus)

A great part of the organic material that reaches the soil is broken down by the action of microorganisms, resulting in mineral components that can be taken by the roots of plants. In this way the nitrogen (nitrogen cycle) and the other nutrients (nutrient cycle) are recycled. This process is called mineralization. Depending on the conditions in which the break down is carried out, a fraction of the organic matter does not continue into mineralization, but instead goes in the contrary direction, forming new organic chains (polymers). These organic polymers are stable, that is resistant to the action of microorganisms, and constitute humus. This stability implies that once formed humus integrates the permanent structure of soil, contributing to its improvement.

It is difficult to define humus precisely; it is a highly complex substance, which is still not fully understood. Humus should be differentiated from decomposing organic matter. The latter is rough-looking material and remains of the original plant are still visible. Fully humified organic matter, on the other hand, has a uniform dark, spongy, jelly-like appearance, and is amorphous. It may remain like this for millennia or more. It has no determinate shape, structure or character. However, humified organic matter, when examined under the microscope may reveal tiny plant, animal or microbial remains that have been mechanically, but not chemically, degraded. This suggests a fuzzy boundary between humus and organic matter. In most literature, humus is considered an integral part of soil organic matter.

Transformation of organic matter into humus

The process of "humification" can occur naturally in soil, or in the production of compost. Organic matter is degraded into humus by a combination of mycorrhizal fungi, bacteria, microbes and animals such as earthworms, nematodes, protozoa and various arthropods. The importance of chemically stable humus is thought by some to be the fertility it provides to soils in both a physical and chemical sense, though some agricultural experts put a greater focus on other features of it, such as its ability to suppress disease. It helps the soil retain moisture by increasing microporosity, and encourages the formation of good soil structure. The incorporation of oxygen into large organic molecular assemblages generates many active, negatively charged sites that bind to positively charged ions (cations) of plant nutrients, making them more available to the plant by way of ion exchange. Humus allows soil organisms to feed and reproduce, and is often described as the "life-force" of the soil.

Plant remains (including those that passed through an animal gut and were excreted as faeces) contain organic compounds: sugars, starches, proteins, carbohydrates, lignins, waxes, resins, and organic acids. The process of organic matter decay in the soil begins with the decomposition of sugars and starches from carbohydrates, which break down easily as detritivores initially invade the dead plant organs, while the remaining cellulose and lignin break down more slowly. Simple proteins, organic acids, starches and sugars break down rapidly, while crude proteins, fats, waxes and resins remain relatively unchanged for longer periods of time. Lignin, which is quickly transformed by white-rot fungi, is one of the main precursors of humus, together with by-products of microbial and animal activity. The end-product of this process, the humus, is thus a mixture of compounds and complex life chemicals of plant, animal, or microbial origin that has many functions and benefits in the soil. Earthworm humus (vermicompost) is considered by some to be the best organic manure there is.

Stability

Much of the humus in most soils has persisted for more than a hundred years (rather than having been decomposed to CO₂), and can be regarded as stable; this is organic matter that has been protected from decomposition by microbial or enzyme action because it is hidden (occluded) inside small aggregates of soil particles or tightly attached (sorbed or complexed) to clays. Most humus that is not protected in this way is decomposed within ten years and can be regarded as less stable or more labile. Thus stable humus contributes little to the pool of plant-available nutrients in the soil, but it does play a part in maintaining its physical structure. A very stable form of humus is formed from the slow oxidation of blue carbon, after the incorporation of finely powdered charcoal into the topsoil. This process is thought to have been important in the formation of the fertile Amazonian dark earths or Terra preta do Indio.

Horizons

Humus has a characteristic black or dark brown color and is organic due to an accumulation of organic carbon. Soil scientists use the capital letters O, A, B, C, and E to identify the master horizons, and lowercase letters for distinctions of these horizons. Most soils have three major horizons—the surface horizon (A), the subsoil (B), and the substratum (C). Some soils have an organic horizon (O) on the surface, but this horizon can also be buried. The master horizon, E, is used for subsurface horizons that have a significant loss of minerals (eluviation). Hard bedrock, which is not soil, uses the letter R.

Criticism of humification theory

The theory that a 'humification' process created 'humus' predates a sophisticated understanding of soils. Products of a humification process have not been observed in soil. Although 'humification' theory is unsupported by evidence, "the underlying theory persists in the contemporary literature, including current textbooks."

Benefits of soil organic matter and humus