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Soils - Part 3: Soil Organic Matter

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Where Does Soil Organic Matter Come From?

As indicated on the previous page, most soil organic matter, at one time, was alive. Plants and animals that live in the soil also die in the soil and contribute their remains to the “pool” of materials. If it all stopped there, though, we would be hip-deep or worse in plant and animal parts. When the plant or animal dies, smaller organisms begin their work. Insects, including earthworms, are primary decomposers because they begin the work of breaking down the remains. Fungi, such as those you find on the forgotten leftovers in your refrigerator, soon follow. Most of the work, however, is done by the smallest soil organisms, the bacteria. Bacteria have two self-serving functions in working on organic remains. First, they use some of the carbon for energy, much as we do when we eat a sandwich. Second, they assimilate some of the carbon (and other elements in the remains) for their bodies.

When bacteria use carbon for energy, they usually produce carbon dioxide gas. This gas seeps out of the soil and is lost as a possible source of soil organic matter. The carbon that bacteria use for their bodies, however, is solid. Through many generations of reproduction and death of bacteria, this carbon begins to look entirely different from the original plant or animal remains. It has become humus. Even though humus is organic, it is more complex than the plant or animal from which it came. It is not accurate to say that humus is simply decomposed organic matter. One result of the complexity of humus is that it becomes more difficult for other microorganisms to break down. For this reason, humus can have a lifetime of tens of years in a soil.

The previous explanation is what occurs when the process works as it should. If bacteria are absent or inactive for any reason, the process slows or stops. By far the most common reason for bacteria to be inactive is if the soil is cold. There is little organic matter decomposition and humus formation in the winter or in permanently cold climates. Vivid examples of this are the well-preserved bodies of prehistoric-age peoples recently found in cold mountain regions. Other reasons for slowed decomposition are an absence of oxygen and water. These two points are really the opposite of each other because the most common cause for a lack of oxygen in soil is if the soil is flooded. Swamps and peat bogs have large amounts of poorly decomposed plant and animal remains because the water there excludes the oxygen which most bacteria need. The opposite extreme, desert climates, has so little water that bacteria also function poorly. This, possibly as much as the Egyptians’ skills as embalmers, has helped preserve the mummies for thousands of years.

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