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Soil frost situation
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What is soil frost?
Soil frost occurs when water present in between soil particles freezes. Ice binds these particles together and makes the ground go hard. The effects of soil frost depend on the soil type.
How is soil frost formed?
Frost starts forming when soil temperature drops below freezing. It begins close by the ground surface and penetrates deeper and deeper. Frost depth is affected by the severity of the winter and thickness of snow cover and, among other things, the soil type. A period of at least a week or two of very cold temperatures is needed in order for the soil to freeze to a depth of 10 to 20 centimetres. Snow acts as an efficient insulation: it only takes a snow layer of 10 to 15 centimetres to stop the soil from freezing.
Where does soil frost occur?
The frost depth and duration vary by region: in Eastern Finland, where the snow cover is usually thick, soil frost only reaches to a depth of 10 to 20 centimetres, whereas in Western Finland the frost depth is usually greater. In Northern Lapland the frost depth can reach one metre and sometimes even more. The frozen soil layer typically is at its thickest in late February or early March, or slightly later in Lapland, however with major yearly variations. Climate change is reducing soil frost.
Many types of soil frost
The characteristics of soil frost are influenced by the soil type. In soil with a crumby structure water freezes on cavity walls, creating cavity frost with rod-like crystals. Massive frost develops in soil with a more compact structure, into which capillary action continuously pulls more water. The increased water volume and freezing expand the volume of the soil, resulting in frost heave. When soil frost melts, puddling may occur. The third type is layered soil frost.
Fun facts about soil frost?
Frost improves soil structure
Soil frost is helpful for farmers. Especially fields with clay soils are prone to compaction, but soil frost breaks up the soil in winter and makes it soft and easy to cultivate. The crumby structure produced by soil frost provides good conditions for plants to take root, with enough space around the roots for water and air. This helpful impact of soil frost will be reduced by climate change.
Soil frost facilitates timber harvesting
Harvesting trees on soft and soggy peatlands can be tricky. In particular, strip roads used to transport timber out of the forest are exposed to a high level of stress. Deep ruts may be created, and tree roots can be damaged. This is why timber harvesting on peatlands has traditionally taken place in winter once soil frost has hardened the ground.
Soil frost affects construction
The effects of soil frost must be accounted for in construction to avoid frost damage to building foundations and wall structures. Damage can be prevented by means of appropriate frost insulation and by excavating the foundations below the frost limit. The soil on the site can also be replaced with a type less prone to frost heaving. The soil should be examined for its frost properties before the construction starts.
Soil frost may damage roads
When spring comes, many unpaved roads are in a very bad condition. Soil frost prevents meltwaters from being infiltrated into the soil, making the road surface sludgy. Water may erode the road, causing large potholes to appear, and ice lenses may create bumps on it. Frost heave may damage the road badly by shifting and breaking culverts. On paved roads, soil frost may break the paving.
Earthworms survive soil frost
The earthworm species living in Finland have adjusted to soil frost. The common earthworm digs its vertical tunnel to a depth which soil frost does not reach, keeping the worm safe from freezing. Small earthworm species living close to the ground surface have a different survival mechanism. They prepare for winter by producing anti-freeze substances in their tissues.
Invertebrates overwinter in soil
The temperature never drops very low underground even in winter, which is why many invertebrates overwinter in soil. Soil frost is no obstacle to them; ice-free cavities can be found in plant root systems and beside stones. Some make burrows for themselves or use holes dug by other organisms. Overwintering as an egg or larva, or hibernating, are common strategies. Eggs and larvae may endure extremely low temperatures.
Soil frost and snow – a good combination
The combination of soil frost and snow protects arable fields from erosion. This protection is compromised by warmer winters because heavy rains on snow-free and frost-free fields can carry away sediment and nutrients and transport them into ditches, rivers and lakes, increasing pollution. Soil frost also helps control plant diseases: if the soil remains unfrozen under a snow cover, certain fungal diseases may get the upper hand.
Palsa mounds preserve soil frost
The oldest frozen soil in Finland is found in the palsa mounds of mires in Northern Lapland. The mounds start rising in places where soil frost penetrates deeper than usual. While the peat on the surface melts in spring, the ice deep in the core of the mound persists. The palsa mound grows bigger each winter and can achieve the age of 2,000 years and height of 4 to 7 metres. Finally the peat on the top of the mound tears and wears away, the ice core melts and the mound collapses.