by KEITH REID

An outcome of this difficulty is the common assumption that very little happens in the soil during the winter. While it is true that things move slowly during the cold months, it is unlikely that any processes in the soil can be said to stop completely during the winter.

Some work has been done on the physical aspects of cold soils, primarily because of their importance to engineers who design roads or buildings. There is less understanding of the chemical and biological processes that happen in the soil, but recent work with greenhouse gas emissions suggests that what happens below the surface of the soil in winter should not be ignored.

The extent of physical change in the soil depends not only on the air temperature above the soil, but on the amount of snow cover. Snow is an excellent insulator, because of the air trapped within the snow crystals. Small mammals like mice and voles know this well, living comfortably beneath the snow even though the air temperature plunges to minus 30 or more. It is not uncommon, in areas of high snow accumulation, for the soil to freeze only rarely, and then to thaw within a few days because of heat conduction from deeper in the soil.

It is a much different situation in areas where there is little snow, or where the snow is removed regularly. Without the insulating value of the snow, sub-zero temperatures penetrate into the soil and the soil water begins to freeze. As heat is lost from the surface, the depth of the frozen layer increases.

At the bottom of the frozen layer, strange things begin to happen. As water is removed from the liquid phase by freezing, more water diffuses towards the freezing front to replace what has gone. This is exactly the same process that happens during the summer, when water is converted into vapour through evaporation, but the outcome is quite different. The frozen water is still present in the soil, and the water that has diffused to the frost front also freezes, so a layer of ice eventually accumulates within the soil. These "frost lenses" can range from just thick enough to be visible up to several inches in thickness, and there can even be multiple frost lenses as freezing conditions penetrate deeper into the soil.

There are several consequences to these frost lenses. First, these are layers of solid ice, unlike the normal frozen soil that is made up of ice films on the soil particles, cementing them together but not completely filling the pores. An ice lens will effectively block any vertical movement of water or solutes through the soil.

These layers also take up space within the soil, so they will lift any structure that is above the frost. Building design for areas with cold winters includes either the ability to "float" up or down without damage, or foundations that penetrate deep enough that the frost never penetrates below their bases.

The final consequence is the impact of large volumes of water released when the frost lens melts. In the field, this appears as a sudden increase in the wetness of the soil as the "bubble" of water is forced to the surface and the soil above it collapses into the area that had been occupied by the ice.

The same process can occur on roadways if the water cannot drain quickly from the melting ice, causing "frost boils." This is the reason for half-load season on most secondary roads, as the strength of the soil below the road is reduced even if the water never comes to the surface.

Not all of the effects of soil freezing are negative. On a gross scale, the formation of ice lenses helps to break up soil compaction, without any intervention by tillage. On a smaller scale, freezing and thawing play an important role in producing a friable soil structure. Ice crystals that form in the soil break apart large clods and squeeze the smaller granules closer together to generate a more stable structure.

The idea of fall plowing to "let the frost get into the ground" was not totally wrong, although it is not clear how much the tillage actually increased frost action. It is clear that spring plowing that did not have this freezing and thawing resulted in a much cloddier seedbed.

Freezing actions at the soil surface are also responsible for incorporating small seeds into a zone where they can germinate. This is well known with "frost seeding" of red clover. The success of this practice has less to do with the carrying capacity of the soil on a frosty morning than with the movement of the seeds down into the soil by alternate freezing and thawing.

In the next issue, I will discuss the impact of winter conditions on some of the chemical and biological processes that occur in the soil. BF