SOIL AGGREGATION AS INFLUENCED BY CULTURAL-PRACTICES IN SASKATCHEWAN .1. BLACK CHERNOZEMIC SOILS

The impact of cultural practices on soil aggregate characteristics which determine the susceptibility of the soil to wind and water erosion was studied at two long-term (> 30-yr) crop rotation sites on Black Chernozemic soils at Indian Head and Melfort, Saskatchewan. Surface soil (top 5 cm) taken in spring and fall, 1991, was air-dried and sieved by rotary sieve to measure aggregate size distribution. The water-stability of soil aggregates (1-2 mm) was determined after: (i) slow wetting, and (ii) fast wetting. Both rotation studies employed conventional tillage management until 1990 when the Indian Head experiment was converted to zero-tillage. Summerfallowing increased the wind-erodible (< 0.84-mm) fraction of soil and decreased the geometric mean diameter (GMD) of aggregates. One year of cropping was sufficient to significantly reduce the proportion of wind-erodibile aggregates. Fertilization and legume green manure and hay crops reduced the wind-erodible fraction at Indian Head, but had no effect on the higher organic matter soil at Melfort. In monoculture wheat systems at Indian Head there was an inverse relationship between the wind-erodible fraction and cropping frequency; this was credited to the positive influence of cropping frequency on crop residue production. The wind-erodible fraction (Y) was related to GMD at Indian Head: Y = 11.8 + 117/GMD (r2 = 0.80***), and at Melfort, Y = 11.9 + 91/GMD (r2 = 0.82***). When subjected to rapid wetting, both the difference between cropped and native grassland soils, and the influence of cultural practices on water stable aggregates were pronounced. Aggregate stability was more closely related to the long-term management than to recent (< 1 yr) cultural treatments. Frequent cropping, fertilization, and use of legumes increased water stable aggregates, particularly at the Indian Head site with its lower organic matter content.