What Happens To Soil In Winter?
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.
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.
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 by developing 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 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.
Snow is known as poor man’s fertilizer. As snow falls through the atmosphere, nitrogen
attaches to the snowflakes, providing a gentle natural fertilizer boost to plants.
Snow can provide the natural moist cooling period that many of our native seeds require in order to
germinate. This propagation condition is known as stratification. In the fall, try sowing native seeds
with a stratification requirement, such as goldenrods and blazing stars.
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.
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. Gardeners add organic matter around
the plants, or allow leaves that fall naturally to remain around the plants. Dried leaves from plants, if left
for spring removal, also provide soil and root insulation
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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!
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 glucose
and other molecules which acts to lower the temperature at which water freezes and protects the cells
of the bulbs.
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 — 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.
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