Of all the components of soil, organic matter is probably the most important and most misunderstood. Organic matter serves as a reservoir of nutrients and water in the soil, aids in reducing compaction and surface crusting, and increases water infiltration into the soil. Yet, it's often ignored and neglected. Let's examine the contributions of soil organic matter and talk about how to maintain or increase it.
Many times we think of organic matter as the plant and animal residues we incorporate into the soil. We see a pile of leaves, manure or plant parts and think, "Wow! I'm adding a lot of organic matter to the soil." This stuff is actually organic material, not organic matter.
What's the difference between organic material and organic matter? Organic material is anything that was alive and is now in or on the soil. For it to become organic matter, it must be decomposed into humus. Humus is organic material that has been converted by microorganisms to a resistant state of decomposition. Organic material is unstable in the soil, changing form and mass readily as it decomposes. As much as 90 percent disappears quickly because of decomposition.
Organic matter is stable in the soil. It has been decomposed until it is resistant to further decomposition. Usually, only about 5 percent of it mineralizes yearly. That rate increases if temperature, oxygen and moisture conditions become favorable for decomposition. This often occurs with excessive tillage. Stable organic matter is analyzed in a soil test.
An acre of soil measured to a depth of 6.67 inches weighs approximately 2 million pounds, which means that 1 percent organic matter in soil weighs about 20,000 pounds. Remember, it takes at least 10 pounds of organic material to decompose to 1 pound of organic matter, so it takes at least 200,000 pounds (100 tons) of organic material applied or returned to the soil to add 1 percent stable organic matter under favorable conditions.
In soils formed under prairie vegetation, organic matter levels are generally comparatively high because organic material was supplied from both the top growth and roots. We don't usually think of roots as supplying organic material, but a study in the Upper Great Plains showed that a mixed prairie had an above-ground (shoot) yield of 1.4 tons of organic material per acre, while the root yield was about 4 tons per acre. The plants were producing roots that were more than twice the weight of the shoots.
Soils that have developed under forest vegetation usually have comparably low organic matter levels. There are at least two reasons for these levels:
Soils formed under prairie vegetation usually have native organic matter levels at least twice as high as those formed under forest vegetation.
Building soil organic matter is a long-term process but can be beneficial. Here are a few ways to do it:
A good supply of soil organic matter is beneficial in crop or forage production. Consider the benefits of this valuable resource and how you can manage your operation to build or at least maintain the organic matter in your soil.
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