Organic Matter: What it is and why it’s so important

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 a 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. 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 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 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. The fresh residues, or “dead” organic matter, consist of recently deceased microorganisms, insects, earth-worms, 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 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 a good structure. Organic molecules directly released from cells of fresh residues, such as proteins, amino acids, sugar