The over-winter period, that is, from the end of one growing season to the start of the next,is often quoted as an important control in soil structural management, and in fact, is a primary reason behind fall ploughing in conventional tillage. In spite of a considerable body of literature, however, there remain conflicting opinions over whether over-winter conditions have a positive or negative effect on structural form and stability

Some view the over-winter period as one of structural degradation, while others see it as a period of potential structural rehabilitation. Similarly, many researchers have noted increases in the stability of soil aggregates over the winter months, while traditional thought suggests that aggregates experience a loss in stability over the winter months.

Some of the discrepancy regarding the effects of over-winter changes in soil structure may be due to the poorly defined time frame of previous studies. Most studies have concentrated either on the short term effects of single processes (i.e. freezing and thawing), or the net long term effects from the fall to the spring, without considering the transient processes operating within this time frame. The period in question actually involves the time from the end of one growing season to the start of the next. In this time, wetting-drying cycles are occurring, as is age hardening. Colder nights initiate diurnal freeze-thaw cycling at the soil surface, followed by deeper seasonal freezing in areas of colder climates. Whenever the soil surface is exposed while frozen, the possibility of moisture loss by sublimation exists. As temperatures begin to rise in the spring and melting begins, the soil may reach saturation and begin experiencing wetting-drying cycles, and age hardening again, as well as thaw consolidation. This complex series of events occurring over this time period may easily confound the results of previous studies.

The primary objectives of this study were to establish how structural form and stability change from the end of one growing season to the start of the next and what the mechanisms are governing the changes in these soil physical properties. Freezing-thawing, wetting-drying and age hardening processes, acting together over winter, were observed for their beneficial and detrimental effects on soil structural form and stability. It was found that there was not a simple single improvement or degradation occurring, but rather alternating periods of improvement and degradation, resulting in an either positive or negative net change.

Dr. Pieter H. Groenevelt
Dept. of Land Resource Science
University of Guelph
Guelph, Ontario  N1G 2W1
CANADA
Phone: (519) 824-4120 x3583
Fax: (519) 824-5730
E-mail: pgroenev@lrs.uoguelph.ca