Effect of soil moisture and soil organic matter on soil bulk density during a growing season
We measured surface (0-7.5 cm) soil bulk densities at nine sampling times for two and four long-term crop rotations at Swift Current and Indian Head respectively, to determine the effect of crop management and time. In a non-swelling medium textured soil at Swift Current, surface bulk density varied about 0.2 Mg m3 over the growing season. The variation could not be readily related to any causes. Compared with continuous wheat and the cropped phase of the fallow-wheat rotation, the fallow phase of fallow-wheat had an consistently higher bulk density. This was attributed to the compacting effects of tillage traffic and of rains failing on the moist surface soil. Surface bulk density measured at any single sampling may not be representative of the cropping system, multiple samplings would yield a more representative bulk density. For a heavy clay soil at Indian Head, surface bulk density changes were dominated by soil shrinking and swelling in response drying and wetting. Although the bulk density appeared lower for the cropping systems with more soil organic carbon when the soil was driest, this effect was consistent with differences in aggregation that would change the location of soil shrinkage voids. In particular, the better aggregated soils having more organic C probably had much of the shrinkage occur as individual soil aggregates. Many of the inter-aggregate shrinkage voids would be included in the bulk density measured with a small diameter soil core. However, for the more poorly aggregated soil with less organic carbon, the soil probably shrank more as large structural unit between vertical shrinkage cracks. The cracks were not included in the small diameter soil core, so the apparent bulk density of the soils with less organic C was higher. However, theoretically, the actual bulk density measured over a large area would be the same for all the systems on the heavy clay. Generally, when using small cores to compare bulk densities across cropping management systems on a clay soil with a large shrink-swell behaviour, the most useful bulk densities will be obtained when the soil is as wet (i.e. as swollen) as practical. Considering only the two samplings when the soil was wettest, there was no effect of crop management on the surface bulk density at Indian Head.
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