Air permeability varies with soil structure and is influenced in part by soil water content and porosity. This study determined the extent to which freezing and thawing affects tillage-induced soil structure by measuring changes in soil air permeability. Tillage plots, located near Lamberton, Minnesota, were established in 1985 and cropped in alternate years to corn and soybeans. Tillage treatments were arranged in a randomized block design and included moldboard plow, ridging, and no tillage. Soil cores (0.3 m diameter) were obtained after corn harvest and moldboard plow in the fall of 1993 by pushing plastic pipe 0.3 m into the soil. Cores were excavated from the plots and transported to the laboratory. Rubberized asphalt was poured between the soil column and inside edge of the pipe to minimize side-wall flow. The cores were placed in de-aerated water to achieve soil saturation, after which the soil was allowed to drain for 24 hr and then covered to minimize evaporation. An air permeameter facilitated measuring air flow through the soil columns prior to soil freezing and after 1, 5 and 10 freeze-thaw cycles. Each cycle consisted of a one-dimensional freeze of the soil profile followed by a thaw. Prior to the first freeze-thaw cycle, air permeability was greatest for moldboard plow (0.04 mm2) compared to ridging and no tillage (0.02 mm2). Structural changes were evident in all tillage treatments after one freeze- thaw cycle as indicated by an increase in permeability. Moldboard plow resulted in the largest increase in permeability, more than doubling (0.10 mm2) after one cycle. Changes in air permeability with freezing and thawing were transient for ridge and no tillage; permeability was the same prior to one freeze-thaw and after 10 freeze-thaw cycles. Structural changes in moldboard plow caused by freezing and thawing appeared to be more enduring as permeability was greater after each freeze-thaw cycle than prior to one cycle. Freezing and thawing can create fracture planes and pathways in soils for more rapid air or water movement.
Brenton Sharratt USDA-ARS 803 Iowa Avenue Morris, MN 56267 (320) 589-3411
email at: bsharratt@mail.mrsars.usda.gov