SOIL AGGREGATION AS INFLUENCED BY CULTURAL-PRACTICES IN SASKATCHEWAN .2. BROWN AND DARK BROWN CHERNOZEMIC SOILS

The size distribution and stability of aggregates are important characteristics influencing tilth, erosion, water infiltration and nutrient dynamics in soil. We need to understand how soil management influences agregation so that we may make suitable modifications to farming practices to enhance soil stability. Two long-term crop-rotation experiments in southern Saskatchewan were used to examine the effects of management on aggregation. In 1991, soil was collected three times during the summer from the top 5 cm of soil as part of a 25-yr experiment at Swift Current (Orthic Brown Chernozemic soil) and twice as part of a 27-yr experiment at Scott (Orthic Dark Brown Chernozemic soil). The two medium-textured soils were dry-sieved by rotary sieve and then wet-sieved after (i) slow wetting and (ii) fast wetting of the 1-2-mm size fractions. The wind-erodible fraction (<0.84 mm) of soil and geometric mean diameter (GMD) of aggregates varied with antecedent precipitation. Thus, wind-erodible aggregates increased at both sites, even in cropped rotation phases, when conditions were dry during the last 6 wk of the 1991 summer. However, the presence of crops and management practices that increased crop residues (e.g., feitilizers) tended to reduce the wind-erodible fraction and increase GMD. A very close relationship existed between the wind-erodible fraction (y) and GMD at the two sites: y = 16.2 + 105/GMD, (r2 = 0.95***) for the pooled data for the two sites. In contrast to results for the wind-erodible fraction, dry growing-season weather conditions tended to increase the water stability of aggregates. Fast wetting was more effective than slow wetting in delineating management effects on aggregate stability. Aggregate stability was increased by frequent cropping and by adequate fertilization, presumably as a result of increased production of crop residues. At Swift Current, chemical fallow increased aggregate stability, likely as a result of reduced soil disturbance.