The Role Of Organic Matter In Soil

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 is often ignored and neglected.

- Eddie Funderburg

Let’s examine the contributions of soil organic matter and talk about how to maintain or increase it.

What is organic matter?

People often think of organic matter as the plant and animal residues incorporated into the soil. They see a pile of leaves, manure, or plant parts and think, “Wow! I’m adding a lot of organic matter to the soil.” This is actually organic material, not organic matter.

What is 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 micro-organisms 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% of it 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 around 5% of it mineralises yearly. That rate increases if temperature, oxygen, and moisture conditions become favourable for decomposition, which often occurs with excessive tillage. It is a stable organic matter that is analysed in the soil test.

Amount of organic matter in soil

An acre (0,4ha) of soil measured to a depth of six inches (15cm) weighs approximately 2 000 000 pounds (907 tons), which means that 1% organic matter in the soil would weigh approximately 20 000 pounds (nine tons) per acre. Remember that it takes at least ten pounds (4,53kg) of organic material to decompose to one pound (0,45kg) of organic matter, so it takes at least 200 000 pounds (90 tons) of organic material applied or returned to the soil, to add 1% stable organic matter under favourable conditions.