What Happens To Soil In Winter?

Excerpted from: Soil In Winter

Man wearing sneakers standing on the ground

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 32° F, 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 soil under your feet is still teeming with life, even in the frozen temperatures of winter.

Snow Protects Soils

Snow can be appreciated for more than its good looks. It also plays a crucial role in maintaining healthy soils throughout the winter months. One of snow’s major jobs is to serve as a natural blanket, or insulator. Its effectiveness depends on the amount that collects before the weather gets too cold. Early winter snowfalls insulate the ground by preventing heat from escaping into the atmosphere and by blocking cold air from moving into the soil.

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.

A blanket of snow traps heat energy, snow restricts the depth of the frost layer, or area of soil containing ice. In other words, soils with deep snow cover often have thinner frost layers than those without snow. The area below the frost layer serves as a refuge for animal and plant life that call it home. In turn, thinner frost layers provide more room for organisms to live during the winter months.

Soils without snow cover freeze to greater depths, and undergo “freeze-thaw” cycles. If you’ve ever put water in the freezer, you’ve likely noticed that it expands when it turns to ice. This also happens to soil water in the frost layer. As water in the ground becomes solid, it expands and squeezes soil materials together. Ice also pushes soils upward, causing the ground to heave. Freeze-thaw cycles can severely damage plants by pushing them out of the ground and by ripping and pinching their roots apart.

Soils that are well insulated tend to thaw more quickly in the spring time than their bare counterparts. Thawed soils are far better at taking in and storing meltwater than frozen soils. If snow melts before the soil has thawed, the soil cannot absorb the water. Instead, ponds of water will form, or it will run off the surface of the land, leading to water erosion.

What's Happening Under Your Feet

Organic matter layer

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

Perennial Plants

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!)

Spring Bulbs

While winter soil may freeze to depths beyond which the bulbs are planted, soil temperature will rarely fall below 29-30° F. At these temperatures, water in the cells of the bulb may freeze, but the cells will not be harmed. The cold temperature triggers starches in the bulbs to break down into glocuse and other molecules which acts to lower the temperature at which water freezes and protects the cells of the bulbs.

Soil-dwelling Animals

Field mouse in winter

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.

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.

Soil Microbes

Soil Microbes — bacteria and fungi that live in the soil year round — can be active in winter months. In Wisconsin, once spring comes, the microbes become even more active. This ensures the biodiversity that is so important to keep plant and animal life healthy.

Further Reading:

 Beavers — Nature's Hydrologist, Part 2
 Garter Snakes — The Gardener's Friend
 Wisconsin Native Salamanders
 Goundhog or Woochuck: All The Facts
 Voles, Both The Good and The Bad