Micromorphological and selected physical properties of a moldboard-plowed soil with and without residue cover

Changes in micromorphology and the following physical properties, air permeability, bulk density, volumetric moisture content and air-filled porosity, of bare (NR) and residue-covered (RM) soil surfaces (75 mm) that had been tilled with a moldboard plow were examined. Sampling was conducted during the growing season (May–September) and following application of artificial rain, equivalent to a 50-year storm. The soil was a Udalf formed in loess (76% silt; 21.5% clay in surface) overlying bedrock, and had a low surface organic-carbon content (1.42–1.76%).

Results showed that residue cover provided an effective barrier against early-season surface sealing. Porosity (>40-μm equivalent diameter) following rainfall simulation was much higher in the upper 15 mm of the RM surface (22.8%) than in the NR surface (7.7%). Large planar pores (> 1000 μm) in the RM subtreatment accounted for much of the difference and this was associated with a much higher air permeability in the RM subtreatment (186.6 μ²) than in the NR subtreatment (45.1 μ²). However, with a decline in residue cover during the growing season the protective effect of residue diminished. Internal slaking and associated consolidation was prominent in both subtreatments at all sampling times and was characterized by fabrics indicating marked disaggregation and particle sorting. The most extreme example of this type of particle reorganization occurred in a RM replicate from the rainfall-simulation experiment. The replicate was underlain by a zone compacted by wheel traffic. A major drough period occurred during July, and samples taken from both subtreatments at the end of the month immediately after a 38-mm rain were sealed and internally consolidated. The RM subtreatment showed some evidence of reaggregation by the beginning of September. This may have been associated with a net increase in its subsurface residue content and an increase in soil moisture content during August. The observed differences in surface-sealing characteristics among subtreatments were manifested in significant differences in air permeability. The other physical measurements were less sensitive to these short distance changes in pore geometry.