Building Soils for Better Crops, Third Edition

Organic Matter: What It Is and Why It’s So Important


Follow the appropriateness of the season, consider well the nature and conditions of the soil, then and only then least labor will bring best success. Rely on one’s own idea and not on the orders of nature, then every effort will be futile.


As we will discuss at the end of this chapter, organic matter has an overwhelming effect on almost all soil properties, although it is generally present in relatively small amounts. A typical agricultural soil has 1% to 6% organic matter. It consists of three distinctly different parts—living organisms, fresh residues, and well-decomposed residues. These three parts of soil organic matter have been described as the living, the dead, and the very dead. This three-way classification may seem simple and unscientific, but it is very useful.

A nematode feeds on a fungus, part of a living system of checks and balances
Figure 2.1. A nematode feeds on a fungus, part of a living system of checks and balances. Photo by Harold Jensen.

The living part of soil organic matter includes a wide variety of microorganisms, such as bacteria, viruses, fungi, protozoa, and algae. It even includes plant roots and the insects, earthworms, and larger animals, such as moles, woodchucks, and rabbits that spend some of their time in the soil. The living portion represents about 15% of the total soil organic matter. Microorganisms, earthworms, and insects feed on plant residues and manures for energy and nutrition, and in the process they mix organic matter into the mineral soil. In addition, they recycle plant nutrients. Sticky substances on the skin of earthworms and other substances produced by fungi help bind particles together. This helps to stabilize the soil aggregates, clumps of particles that make up good soil structure. Organisms such as earthworms and some fungi also help to stabilize the soil’s structure (for example, by producing channels that allow water to infiltrate) and, thereby, improve soil water status and aeration. Plant roots also interact in significant ways with the various microorganisms and animals living in the soil. Another important aspect of soil organisms is that they are in a constant struggle with each other (figure 2.1). Further discussion of the interactions between soil organisms and roots, and among the various soil organisms, is provided in chapter 4.

A multitude of microorganisms, earthworms, and insects get their energy and nutrients by breaking down organic residues in soils. At the same time, much of the energy stored in residues is used by organisms to make new chemicals as well as new cells. How does energy get stored inside organic residues in the first place? Green plants use the energy of sunlight to link carbon atoms together into larger molecules. This process, known as photosynthesis, is used by plants to store energy for respiration and growth.

Partially decomposed fresh residues removed from soil.
Figure 2.2. Partially decomposed fresh residues removed from soil. Fragments of stems, roots, and fungal hyphae are all readily used by soil organisms.

The fresh residues, or “dead” organic matter, consist of recently deceased microorganisms, insects, earthworms, old plant roots, crop residues, and recently added manures. In some cases, just looking at them is enough to identify the origin of the fresh residues (figure 2.2). This part of soil organic matter is the active, or easily decomposed, fraction. This active fraction of soil organic matter is the main supply of food for various organisms—microorganisms, insects, and earthworms— living in the soil. As organic materials are decomposed by the “living,” they release many of the nutrients needed by plants. Organic chemical compounds produced during the decomposition of fresh residues also help to bind soil particles together and give the soil good structure.

Organic molecules directly released from cells of fresh residues, such as proteins, amino acids, sugars, and starches, are also considered part of this fresh organic matter. These molecules generally do not last long in the soil because so many microorganisms use them as food.

The well-decomposed organic material in soil, the “very dead,” is called humus. Some use the term humus to describe all soil organic matter; some use it to describe just the part you can’t see without a microscope. We’ll use the term to refer only to the well-decomposed part of soil organic matter. Because it is so stable and complex, the average age of humus in soils is usually more than 1,000 years. The already well-decomposed humus is not a food for organisms, but its very small size and chemical properties make it an important part of the soil. Humus holds on to some essential nutrients, storing them for slow release to plants. Humus also can surround certain potentially harmful chemicals and prevent them from causing damage to plants. Good amounts of soil humus can both lessen drainage and compaction problems that occur in clay soils and improve water retention in sandy soils by enhancing aggregation, which reduces soil density, and by holding on to and releasing water.

Another type of organic matter, one that has gained a lot of attention lately, is usually referred to as black carbon. Almost all soils contain some small pieces of charcoal, the result of past fires, of natural or human origin. Some, such as the black soils of Saskatchewan, Canada, may have relatively high amounts of char. However, the interest in charcoal in soils has come about mainly through the study of the soils called dark earths (terra preta de indio) that are on sites of long-occupied villages in the Amazon region of South America that were depopulated during the colonial era. These dark earths contain 10–20% black carbon in the surface foot of soil, giving them a much darker color than the surrounding soils. The soil charcoal was the result of centuries of cooking fires and in-field burning of crop residues and other organic materials. The manner in which the burning occurred—slow burns, perhaps because of the wet conditions common in the Amazon— produces a lot of char material and not as much ash as occurs with more complete burning at higher temperatures. These soils were intensively used in the past but have been abandoned for centuries. Still, they are much more fertile than the surrounding soils—partially due to the high inputs of nutrients in animal and plant residue—and yield better crops than surrounding soils typical of the tropical forest. Part of this higher fertility— the ability to supply plants with nutrients with very low amounts of leaching loss—has been attributed to the large amount of black carbon and the high amount of biological activity in the soils. Charcoal is a very stable form of carbon and apparently helps maintain relatively high cation exchange capacity as well as biological activity. People are beginning to experiment with adding large amounts of charcoal to soils—but we’d suggest waiting for results of the experiments before making large investments in this practice. The quantity needed to make a major difference to a soil is apparently huge— many tons per acre—and may limit the usefulness of this practice to small plots of land.

Normal organic matter decomposition that takes place in soil is a process that is similar to the burning of wood in a stove. When burning wood reaches a certain temperature, the carbon in the wood combines with oxygen from the air and forms carbon dioxide. As this occurs, the energy stored in the carbon-containing chemicals in the wood is released as heat in a process called oxidation. The biological world, including humans, animals, and microorganisms, also makes use of the energy inside carbon-containing molecules. This process of converting sugars, starches, and other compounds into a directly usable form of energy is also a type of oxidation. We usually call it respiration. Oxygen is used, and carbon dioxide and heat are given off in the process.

Soil carbon is sometimes used as a synonym for organic matter. Because carbon is the main building block of all organic molecules, the amount in a soil is strongly related to the total amount of all the organic matter—the living organisms plus fresh residues plus well-decomposed residues. When people talk about soil carbon instead of organic matter, they are usually referring to organic carbon. The amount of organic matter in soils is about twice the organic carbon level. However, in many soils in glaciated areas and semiarid regions it is common to have another form of carbon in soils—limestone, either as round concretions or dispersed evenly throughout the soil. Lime is calcium carbonate, which contains calcium, carbon, and oxygen. This is an inorganic carbon form. Even in humid climates, when limestone is found very close to the surface, some may be present in the soil.

It is believed that the unusually productive “dark earth” soils of the Brazilian Amazon region were produced and stabilized by incorporation of vast amounts of charcoal over the years of occupation and use. Black carbon, produced by wildfires as well as human activity and found in many soils around the world, is a result of burning biomass at around 700 to 900°F under low oxygen conditions. This incomplete combustion results in about half or more of the carbon in the original material being retained as char. The char, also containing ash, tends to have high amounts of negative charge (cation exchange capacity), has a liming effect on soil, retains some nutrients from the wood or other residue that was burned, stimulates microorganism populations, and is very stable in soils. Although many times increases in yield have been reported following biochar application— probably a result of increased nutrient availability or increased pH—sometimes yields suffer. Legumes do particularly well with biochar additions, while grasses are frequently nitrogen deficient, indicating that nitrogen may be deficient for a period following application. Note: The effects of biochar on raising soil pH and immediately increasing calcium, potassium, magnesium, etc., are probably a result of the ash rather than the black carbon itself. These effects can also be obtained by using more completely burned material, which contains more ash and little black carbon.

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