I have written before on composting and the recycling of one’s leaves, garden debris and food waste to make rich compost that can be used in next year’s garden. When I was co-teaching the gardening for fun and profit class with Elsa Sanchez, we always would invite Rick Stehouwer from the department of ecosystems science and management to come and make a presentation on “Composting for the Home Gardener.” He would impart the principles and science of composting to the students. As I drove across the mountain the other day I observed the leaves swirling around eventually to settle down on the forest floor and begin the process of creating humus, or nature’s life-giving compost.
In soil science, humus refers to the fraction of soil organic matter that is amorphous and without the “cellular cake structure characteristic of plants, micro-organisms or animals.” Humus significantly influences the bulk density of soil and contributes to moisture and nutrient retention. Soil formation begins with the weathering of humus. In agriculture, humus is sometimes also used to describe mature, or natural compost extracted from a forest or other spontaneous source for use to amend soil. It is also used to describe a topsoil horizon that contains organic matter (humus type, humus form and humus profile). Humus is the dark organic matter that forms in the soil when plant and animal matter decays. Humus contains many useful nutrients for a healthy soil — nitrogen being the most important of all.
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 up by the roots of plants. In this way the nitrogen and other nutrients are recycled. This process is called mineralization. Depending on the conditions in which the breakdown 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. These organic polymers are stable, that is resistant to the action of microorganisms, and constitute humus. The stability implies that once formed humus integrates in 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 a rough-looking material and remains of original plant material 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 of the literature, humus is considered an integral part of soil organic matter.
The transformation of organic matter into humus occurs through the process of humification and occurs 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, through some agricultural experts put a greater focus on other features of it, such as the 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 find 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.
The end product of this process, the humus, is thus a mixture of compounds and complex 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.
Remember when you see the leaves swirling around in the air above you that they are going to settle on the forest floor and begin the process of generating more humus, the “life force” of the soil.
Bill Lamont is professor emeritus of vegetable crops in the department of plant science at Penn State and can be reached by e-mail at firstname.lastname@example.org.