Does Freeze-Thaw Action Alleviate Soil Compaction?

Farmers and service providers often ask if freeze-thaw action takes out soil compaction. Research supports that this indeed happens, but it is limited to the surface soil. The reason freeze-thaw cycles increase porosity is that the density of frozen water is about 9% less than that of liquid water – this explains why ice floats to the top of liquid water. So when water in soil pores freezes, it expands, and this leads to pore expansion. When the temperature increases above freezing, water melts and fills these pores, and the soil freezes up again, then the pores expand a bit more, and so forth.

The key, therefore, is that the soil goes through freeze-thaw cycles. The surface soil goes through more freeze-thaw cycles than the subsoil, and therefore, this process is most effective to restore porosity in compacted surface soil, but not in the subsoil.

In a trial in Montana, researchers measured that the surface 12 inches of the soil went through about 15-20 freeze-thaw cycles. They measured penetration resistance in soil that was compacted once in the fall of 2009, and in the spring of the following year (See Figure 1).

They used heated blankets to maintain soil unfrozen in some plots as comparison. They determined that in the frozen plots, penetration resistance decreased the most (60-70%). In the unfrozen plots, penetration resistance also decreased, but a little less - 50-60%. The authors assumed that in the unfrozen plots, compaction alleviation happened because of drying and wetting and microbial activity. Their soil contained 37-38% smectite clay - this is a clay mineral that swells/ shrinks depending on the moisture content.

In our soils, this type of clay mineral is not common, so this process is not likely to help in our Pennsylvania soils. Freeze-thaw action, however, also helped in alleviating soil compaction in this case to a depth of 12 inches. If we check central Pennsylvania soil temperature (See Figure 2), we conclude that our soil does not go through as many freeze-thaw cycles.

In fact, the soil at 12 inches depth never froze at this location. At 8" depth, the soil froze repeatedly between the end of January and mid-February. At 4" depth the soil froze more frequently. At higher altitude or further north, soil might freeze and thaw more frequently, to deeper depth. It seems that freeze-thaw effects therefore will be limited to 4 to 8 inches depth in PA, with most effect at the shallow depth. The effects of freeze-thaw action is most evident in a honeycomb structure at the surface of the soil.