What Happens to Soil in Winter? Does Everything Die?


The soil under your feet is still teeming with life, even in the frozen temperatures of winter. All photos: Soil Science Society of America

What follows is an article by Mary Tiedeman from the excellent site, Soils Matter, Get the Scoop! of the Soil Science Society of America, a go-to source for valuable and honest information on the soils we garden in.

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 (0 °C), 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 (1 meter or so) 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 also provide soil and root insulation. (To learn more about mulches and soils, visit the blog What Do Mulches Do.)

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Organic matter layer of dead and decomposing leaves, etc. (30 cm = 12 inches) above Alaskan “permafrost” provides insulation throughout the colder months, as well as nutrients for plant life during growing season. Photo: Mary Tiedeman

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.

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Soil microbes and creatures living in soil work year round on agricultural fields, even when snow covered.

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 the blog Is It True Bacteria Live in the Soil. Isn’t 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.


When Does Rock Become Soil?


A guest blog by Barbara-Ann G. Lewis of the

Soil Science Society of America

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The sandstone, top layer, is weathered by climate, organisms and time. Lichens—a mixture of algae and fungi—help break the rock down. Eventually plants can grow in the space, adding organic matter as they grow and die. Photo: SVFisk

Rock starts becoming soil the moment it is exposed to the environment. But it’s a long transformation process from exposed rock to a mature soil. Depending on the nature of the rock and other factors in its surroundings, that time period can range between tens to tens of thousands of years!

The sandstone, top layer, is weathered by climate, organisms and time. Lichen—a mixture of algae and bacteria—help break the rock down. Eventually plants can grow in the space, adding organic matter as they grow and die. Photo: SVFisk

Soil is not simply weathered rock. Soil is a dynamic natural resource. It is comprised of minerals, water, gases, organic material, and living creatures including soil microbes and tiny animals. Calling a soil “mature” doesn’t mean that soil formation has stopped. It means the changes in the soil have become practically imperceptible as the soil comes into dynamic equilibrium with its environment. That means the rate at which soil is forming is about equal to the rate at which the soil is breaking down or naturally eroding away.

Soil scientists look at five major factors in the soil-forming process. These factors are: climate (Cl), organisms (O), relief (R) or topography and drainage of the land, the rock or other parent (P) material that will become the soil, and how much time (T) has gone by.  These can be combined into a “recipe” for soil known as ClORPT. Soil scientists can study each factor separately, or in combinations, to find out how a particular soil became what it is. They can even predict what kind of soil will eventually mature in that place.

Let’s look at what it takes to turn rock into a soil. Under the action of heat, cold, rain, wind, and other atmospheric factors, the rock breaks down physically into small fragments that become the parent material of the soil. The rock also chemically changes as the compounds in the rock dissolve in rain or react with air. Rock is also broken down biologically by living organisms in contact with the rock and its fragments. These processes are collectively called weathering.

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Roots of plants help break up parent materials, and provide organic matter to maturing soils. Photo: D. Weindorf

As the soil-forming process continues over time, plants become established. Wind, birds, and other animals deposit seeds. When water comes in contact with the seeds, they may sprout. Their roots penetrate the disintegrated rock fragments, and their shoots are supported by the material. Roots take up chemicals released by the rock as it breaks down, and use some of the chemicals as nutrients. In turn, plants add organic material in the form of roots and leaves to the disintegrating rock environment. This encourages the growth of microorganisms and tiny animal populations. These living things degrade the fallen leaves and dead roots into organic fragments, which provide more nutrients. It also improves the capacity of the material to store moisture necessary for continual growth of plants in the forming soil.

Weathering continues until everything that can be weathered is weathered, and the rock is no longer recognizable as rock. Disintegrated rock, however, is not yet a soil. This point must be emphasized. It is not until the dynamic equilibrium mentioned above is achieved that we truly have soil.

The rates of all these processes change with time. Rain percolating through young soil carries smaller particles and soluble compounds downward. Evaporation carries soluble compounds upward toward the surface. Sudden floods or avalanches can tear through the area, carrying weathered material away and bringing new material in its place. Human activities can change the nature of the surface.

The processes of weathering and soil formation go on, not only on the surface, but also below the surface within the soil itself. When mature, a soil can extend downward to bedrock from a few inches below the surface to up to eight feet or more, depending on environmental conditions and the original nature of the rock.

Soil is an amazing substance; human survival depends on it. The type of rock that is its parent will influence many characteristics of the soil. But climate, organisms, the shape of the land and time also influence soil formation. For more information on soils, visit www.soils.org/discover-soils.20170826A SVFisk sfisk@sciencesocieties.org