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Soil organic matter in cropping systems

Soil organic matter matters!

Soil organic matter

  • maintains a healthy, productive soil by providing food and a home for microbes.
  • protects our soils - a non-renewable resource -  from erosion losses.

Practices that build and maintain soil organic matter result in a number of benefits. A healthy soil

  • captures and filters water.
  • retains nutrients and makes them available for plant update.
  • provides the housing for the vast beneficial and diverse soil organisms.

What is soil organic matter?

We frequently hear that organic matter is one of the most important components of soil. But what is it, exactly? One textbook definition is

The organic fraction of the soil that includes plant, animal and microbial residues in various stages of decomposition, biomass of soil microorganisms and substances produced by plant roots and other soil organisms.

Simply, it is the soil material that's derived from living organisms - whether it's a carcass, waste product or other substance released from living organisms. Even though microbial cells are alive, they experience rapid population turnover - much like dead residues - and are often included in the definition of soil organic matter.

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Why are the Upper Midwest soils so rich in organic matter?

There are several reasons why soils in the Upper Midwest have higher levels of soil organic matter relative to the rest of the world. 

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Classifications of organic matter

Many different materials in soil fall under the definition of organic matter; however, not all organic matter is created equal. For example, a mouse carcass and a rotten log are both considered organic matter, but they are very different in their chemical nature and in how fast they decompose.

While there are different organic matter descriptions (or fractions) based on their chemical properties, we’ll use a more simplified way to think about organic matter: active and stable organic matter. 

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Benefits of organic matter

Even though soil organic matter may make up a small portion of the overall soil mass, it has a disproportionately large influence on soil function. Here’s why:

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Building organic matter

It is clear that soil organic matter is vitally important for promoting soil health. Soil microbes play a big role in that, as well. In order for the microbes to grow and do their many jobs, they need (1) food, (2) a strong house, and (3) freedom from drastic physical and chemical disturbances. These same characteristics provide conditions for increasing a soil’s organic matter levels. When you want to build organic matter, build the belowground habitat.

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Consider soil organic matter inputs

In addition to providing the resources to support microbial activity, it is important to consider both the quality and quantity of organic matter inputs into a soil. As mentioned earlier, a healthy soil has a steady supply of both active and stable forms of organic matter. This means that building organic matter in soil requires (1) the time and space that are occupied by plant roots and residues is maximized (quantity), and (2) plant residues with a variety of C:N ratios (quality).

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Destroying organic matter

moldboard plowing in field
Figure 18. Moldboard plowing. Photo: Jodi DeJong-Hughes

Soil organic matter has a natural tendency to build when the soil is occupied by vegetation and not disturbed.  For example, soil organic matter levels slowly increase with time in native prairie or forest soils. 

Soils managed for crop production are a different story. As we work the soil, remove crops and residues, and intensively manage these soils, the natural tendency to build organic matter is overcome and these soils experience losses instead of gains (Figure 18). This is a challenge that can be minimized with management, but in order to understand how to minimize these losses, we need to understand why the losses occur.

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Points to ponder

While it can be a challenge to change a farming system, a soil managed with organic matter in mind is a soil that will be strong, healthy, and resilient long into the future—and that matters! 

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Soil organic matter (print-friendly version)

Caley Gasch, soil scientist, North Dakota State University and Jodi DeJong-Hughes, Extension educator

Reviewed in 2019

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