Cold winter air is one reason that we have highly productive soil in Illinois and the Midwest. Temperature plays a major role in how much organic matter is present in soils. Soil organic matter is carbon-based material, such as organic substances given off by plant roots, that has been broken down into a more slowly decomposing form of organic matter. Most of the work for this change is from bacteria. 

When I talk to groups about this, I first have everyone go through a thought experiment. Imagine you are in a lush tropical rainforest. It's warm all year long, and you are surrounded by many different plants. With this image, would you think there would be a little or a lot of organic matter in the soil?

If you answered a lot, that is what would seem to be a logical answer. But those locations have very little soil organic matter present. Because of the year-round warm temperatures, soil bacteria are constantly breaking organic substances down, and nutrients released by that process are very quickly taken back up by existing growth. For that reason, tropical and even sub-tropical soils typically have very little soil organic matter present. They do well as long as the ecosystem isn’t disturbed, but if, for example, the forest is cleared, it quickly becomes very poor soil for growing anything else.

You might also think that placec like northern Canada would not have much soil organic matter due to the very short growing season. The opposite is true. Because of the very long and cold winter, soil bacteria are active for a short period of time each year to break down organic substances, being dormant during cold weather. This leads to a buildup of organic matter in the soil.

What does this mean for Illinois?

Illinois and the Midwest are in between these extremes. We have a nice growing season, but our winters are cold enough for soil bacteria to go dormant, allowing soil organic matter levels to be high enough to make our soils very productive for native plants, gardens, and field crops. 

There are several other reasons that Midwest soils are highly productive. These soils are geologically young, had prairies growing on them for thousands of years which had massive roots that oozed organic substances, and were also poorly drained, which slowed bacterial action. 

How will warming winters affect soil organic matter?

The winter season is warming faster than other seasons in the Midwest. Studies in the middle latitudes suggest that with less cold weather dormancy of soil bacteria, soil organic levels may decrease.

More Resources

Nature Communications, Effects of winter soil warming on crop biomass carbon loss from organic matter degradation 

JGR Biogeosciences, Decreased Soil Organic Matter in a Long-Term Soil Warming Experiment Lowers Soil Water Holding Capacity and Affects Soil Thermal and Hydrological Buffering