Significance of Ground Freezing on Soil Bulk Density Under Zero Tillage

Soil bulk densities remain persistently higher in Ontario on soils on which zero tillage is practiced, than on soils which are conventionally tilled. This occurs in spite of the fact that the formation of ice lenses over winter on soils which are frost susceptible may introduce porosity, which is sufficiently large to cause displacements of the ground surface of several centimeters. Studies were carried out over two winter seasons on soils of two different textures and on which two different tillage treatments (conventional and zero tillage) were practiced in order to determine the reason for this anomalous behavior. Soils under zero tillage accumulated more snow over winter and did not freeze as deeply as soils that were plowed in the fall. However the amounts of water which accumulated in the frozen zone were similar per unit depth of frost penetration. The greater depth of frost penetration resulted in significantly greater surface displacement on the fall plowed soil. However the soils under both tillage treatments quickly reconsolidated upon thawing and returned to near prefreezing bulk densities prior to spring planting. The formation of ice lenses appears to create pores which are inherently unstable and collapse as the ice melts and the soil drains. The loss of pores created by ice lenses occurs in a manner analogous to the removal of a slice of meat from between two slices of bread. The unstable nature of pores created by ice lenses appears to be in contrast to the characteristics of pores created by tillage, which on account of their formation by the vertical and horizontal displacement of peds, are more stable.