FALLOW MANAGEMENT AND OVERWINTER EFFECTS ON WIND ERODIBILITY IN SOUTHERN ALBERTA

Aggregate size distribution (ASD) is influenced by tillage and climate and greatly affects wind erodibility of soils. This study determined the effects of fallow management and winter effects on ASD in the chin ook belt of Alberta. Geometric mean diameter (GMD), geometric standard deviation (GSD), and erodible fraction (EF) <0.84 mm were determined in fall and spring on five fallow systems on a Lethbridge clay loam so il (fine-loamy, mixed, Typic Haploboroll). The fallow systems were (i) tillage only, (ii) minimum tillage (herbicides and tillage), and (iii ) zero tillage (herbicides only). In fall, one-way disk and heavy-duty cultivator fallow treatments had GMD values about 1 mm higher than bl ade cultivator or zero tillage treatments. The 1989-1990 winter with 9 0 cm of snowfall, 67 d of snow cover, and 101 d with a mean 2-cm soil temperature between +2 and -2 degrees C resulted in greater aggregate breakdown than the 1988-1989 winter, which had 129 cm of snowfall, 104 d of snow cover, and 68 d with 2-cm soil temperatures between +2 and -2 degrees C. Estimates of initiating wind velocities for erosion, bas ed on residue cover and EF, were 13.3 m s(-1) for the tillage treatmen ts, 13.7 m s(-1) for the minimum tillage treatment, and 15.4 m s(-1) f or the zero tillage treatment. However, the minimum and zero tillage s ystems had EF values >60% at more sampling times than the tilled treat ments, indicating that wind erosion problems may occur if the residue cover was jeopardized.