Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 10, 1997
Publication Date: N/A
Interpretive Summary: Large machinery with heavy axle loads on American farms has caused compaction both in the plow layer and in the adjacent subsoil. Measured soil density for 10 years after a heavy axle load shows no change in the adjacent subsoil of a typical Corn Belt soil due to natural forces of freezing and thawing. Soil quality (as noted in drainage and crop yield) has diminished some but not to the extent expected from the soil density change. Soil shrinkage responses to water losses from this machinery compacted soil confirm that freezing and thawing and wetting and drying should not be expected to reduce the soil density and improve the somewhat reduced soil quality. Small reductions in tile drainage efficiency were shown to be as large a factor as heavy axle loads for compacted subsoils. This information should be helpful worldwide to assess which soils and practices have the greatest compaction potential for degrading soil quality.
Technical Abstract: Freezing-thawing and wetting-drying in the northern Cornbelt have not ameliorated an adversely high bulk density of a compacted subsoil. A soil shrinkage curve may explain amelioration failures, but has not been measured as related to soil compaction. Shrinkage and bulk density of undisturbed clods (about 200 cm**3), as well as water retention of undisturbed cores, were measured to evaluate long-term compaction effects in two sets of two axle-load treatments relative to their control. One set of compaction comparisons had better tile drainage than the other set. Wet clods were sampled from the Ap and subsoil horizons of a Normania clay loam (fine-loamy, mixed, mesic Udic Haplustoll) in the spring of 1991 without further fragmentation after the soil had a full winter to swell following moldboard plowing in the fall. Clods were further saturated, coated with a film, then allowed to air dry. Weight and volume were determined periodically for eight months to measure shrinkage. Maximum reduction of clod volume in the Ap layer (0.232) was significantly greater than in the subsoil layer (0.152), but compaction effects were not significant in either layer. The dry bulk density of subsoil clods (1.77 g cm**-3) was significantly greater than in the Ap layer (1.68 g cm**-3), but no statistical differences were observed among compaction treatments. Maximum shrinkage was always less than 1 and averaged 0.61 in the subsoil compared to 0.80 in the Ap layer, which indicates nearly all structural and residual shrinkage after immediate air entry during gravity drainage. Such a shrinkage curve may explain why no bulk density changes have occurred since 1981. The water retention characteristic of the compacted and control treatments were still significantly different in a better drained subsoil.