2.1: Introduction

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.

—Jia Sixie, 6th century, China

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 1–6% organic matter by weight. It consists of three distinctly different parts: living organisms, fresh residues and molecules derived from 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 in understanding soil organic matter.

The living. This 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. The range of organisms in soil is so great that it is estimated that they represent about 25% of the world’s total biodiversity. 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 materials produced by fungi help bind particles together. This helps to stabilize the soil aggregates, which are clumps of particles that make up good soil structure. Sticky substances on plant roots as well as the proliferation of fine roots and their associated mycorrhizae help promote development of stable soil aggregates. 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, and much of this energy ends up as residues in the soil after the plant dies.

The dead. 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.

Some 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. Their structure makes them easy to decompose because so many microorganisms use them as food. Some cellular molecules such as lignin are decomposed, but it takes longer for organisms to do so. This can make up a large fraction of the soil organic matter in poorly drained soils, like peats and mucks, as well as wetlands that have been taken into agricultural production. These hold large amounts of organic matter that was not decomposed due to waterlogging, but they don’t provide the same benefits as the fresh residues.

The very dead. This includes other organic substances in soils that are difficult for organisms to decompose. Some use the term humus to describe all soil organic matter. We’ll use the term to refer only to that relatively stable portion of soil organic matter that resists decomposition. Humus is protected from decomposition mainly because its chemical structure makes it hard for soil organisms to utilize.

Identifiable fragments of undecomposed or partially decomposed residue, including remains of microorganisms, can be held inside aggregates in spaces too small for organisms to access. In a sense they behave as if they were “very dead” because of being inaccessible to organisms. As long as organic residue is physically protected from attack by microorganisms it will behave as part of the “very dead.” When these aggregates are broken up by freezing and thawing, drying and rewetting, or by tillage, entrapped organic fragments and simple organic substances adsorbed on clays can be made accessible to microorganisms and are readily decomposed. Because much of soil organic matter is so well protected from decomposition, physically and chemically, its age in soils can be as high as hundreds of years.

But even though humus is protected from decomposition, its chemical and physical properties make it an important part of the soil. Humus holds on to some essential nutrients and stores them for slow release to plants. Some medium-size molecules also can surround certain potentially harmful chemicals, like heavy metals and pesticides, and prevent them from causing damage to plants and the environment. The same types of molecules can also make certain essential nutrients more available to plants. Good amounts of soil humus and fragments of crop residues can lessen drainage and compaction problems that occur in clay soils. They also improve water retention in sandy soils by enhancing aggregation, which reduces soil density, and by holding on to and releasing water.

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

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

Char. Another type of organic matter, one that has gained a lot of attention lately, is usually referred to as black carbon or char. Many 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, presumably from naturally occurring prairie fires. However, an increased interest in charcoal in soils has come about mainly through the study of the soils called dark earths, the 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, which gives 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—produced 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 remain much more fertile than the surrounding soils, partially due to the high inputs of nutrients in animal and plant residue that were initially derived from the nearby forest, and they 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 (even centuries after abandonment). Charcoal is a very stable form of carbon that helps maintain relatively high cation exchange capacity and supports biological activity by providing suitable habitat. However, char does not provide soil organisms with readily available food sources as do fresh residues and compost. People are experimenting with adding biochar to soils, but this is likely not economical at large scales. 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, gardens and container plants, or as a targeted additive coating seeds. Also, benefits from adding biochar should be considered in comparison to what might be gained when using the same source materials like wood chips, crop residues or food waste added directly to the soil, after composting or even after complete combustion as ash.

Biochar as a Soil Amendment

It is believed that the unusually productive “dark earth” soils of the Brazilian Amazon region and other places in the world were produced and stabilized by long-term incorporation of charcoal. Black carbon, produced by wildfires as well as by human activity and found in many soils around the world, is a result of burning biomass at around 600–900 degrees Fahrenheit 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 partially a result of increased nutrient availability or increased pH—sometimes yields suffer. Legumes do particularly well with biochar additions, while grasses frequently become nitrogen deficient, indicating that nitrogen may be deficient for a period following application.

Biochar is a variable material because a variety of organic materials and burn methods can be used to produce it, perhaps contributing to its inconsistent effects on soil and plants. The economic and environmental effects of making and using biochar depend on the source of organic material being converted to biochar, whether heat and gases produced in the process are utilized or just allowed to dissipate, the amount of available oxygen during biochar production, and the distance from where it is produced to the field where it is applied. On the other hand, when used as a seed coating, much less biochar is needed per acre, and it may still stimulate seedling growth and development.

Note: The effects of biochar on raising soil pH and immediately increasing calcium, potassium, magnesium, etc., are probably mostly 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.

Carbon and organic matter. Soil carbon is sometimes used as a synonym for organic matter, although the latter also includes nutrients and other chemical elements. 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, or the amount of carbon in organic molecules in the soil. 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 (mineral) form of carbon. Even in humid climates, when limestone is found very close to the surface, some may be present in the soil. In those cases the total amount of soil carbon includes both inorganic and organic carbon, and the organic matter content could not be estimated simply by doubling the total carbon percent. 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.