Soil is essential to life. One reason is that soil protects plant roots, animals, and microbes from freezing in the winter. As air temperatures drop below 320F (00C), water within the top layers of the soil will eventually freeze. This is commonly known as the frost layer. So, while you think that once the ground is frozen, life stops in the soil, that’s very untrue. What’s going on under your feet is exciting stuff!
The frost layer can be several feet deep, though many factors influence how far down it goes. If a lot of snow falls on the ground early in the winter, it can serve as a blanket for the soil underneath. Organic matter plays a role in insulating soil, holding in heat stored below ground during the warmer months. The organic matter can be mulch or compost that gardeners add around the plants, or leaves that fall naturally. Dried leaves from plants, if left for spring removal, also provide soil and root insulation. (To learn more about mulches and soils, visit this blog post: https://soilsmatter.wordpress.com/2014/10/06/can-mulches-help-water-quality/).
Perennial plants that grow in colder climates, such as many grasses, trees, and shrubs, are able to withstand freezing. They develop root systems below the frost layer. The root systems of these plants perform a number of tasks that protect them from the cold. Roots can release a lot of water from their cells into the surrounding soil. This allows roots to endure colder temperatures without the risk of internal water expanding and damaging root cells. Water within root cells also contains higher concentrations of sugars and salts. They both assist in lowering the freezing point of water inside and between the cells (much like antifreeze!)
Many soil-dwelling animals burrow below the frost layer to survive the winter months. These include insects, frogs, snakes, turtles, worms, and gophers. Some will hibernate. Others simply live on the food that they have collected for their long “vacation” deep underground.
What is even more fascinating? A great number of soil animals have evolved to withstand temperatures below freezing. At least five frog species in North America make their own natural antifreeze. This allows them to become completely frozen for long times without suffering any serious damage to the structures of their cells.
Even soil microbes – bacteria and fungi that live in the soil year round – can be active in winter months. Studies in Antarctica show microbial life in permanently frozen ground (permafrost). In North America, once spring comes, the microbes become even more active. This ensures the biodiversity that is so important to keep plant and animal life healthy. (To read more about soil microbes, visit this blog post: https://soilsmatter.wordpress.com/2015/07/14/is-it-true-bacteria-live-in-the-soil-isnt-that-bad/
Next time you are out braving the cold on a wintery day, try to imagine the root systems and living creatures below ground. We can thank soil for protecting and insulating its inhabitants. Whether they are hibernating or snacking on stored food, they are alive and well.
Answered by Mary Tiedeman, Soil Science Society of America member