SHRINKAGE AND WATER-RETENTION CHARACTERISTIC IN A FINE-TEXTURED MOLLISOL COMPACTED UNDER DIFFERENT AXLE LOADS

In areas where heavy vehicles are used, the subsoils often become very compacted. Freezing-thawing and wetting-drying have not been effectiv e at reducing compaction. In this study, the type of soil shrinkage re lated to compaction was investigated to explain these amelioration fai lures. In conjunction with a shrinkage curve, the water retention char acteristic was also measured because both can be related to compaction . Shrinkage and bulk density of undisturbed clods (about 200 cm(3)), a s well as water retention of undisturbed cores, were measured to evalu ate long-term compaction effects in two sets (better and poor tile dra inage) of two axle-load treatments (9- and 18-Mg axle loads) relative to their control. Wet clods were sampled from the Ap (0 to 25 cm) and subsoil (30 to 45 cm) horizons of a Normania clay loam (fine-loamy, mi xed, mesic Udic Haplustoll) in the spring of 1991 without fragmentatio n after the soil had a full winter to swell following moldboard plowin g in the fall. Clods were further saturated, coated with a film, then allowed to air-dry. Mass and volume were determined periodically for e ight months to measure shrinkage. Maximum volume reduction (m(3) m(-3) ) of clods in the Ap layer (0.232) during shrinkage was significantly greater than in the subsoil layer (0.152), but compaction effects were not significant in either layer. Dry bulk density of subsoil clods (1 .77 Mg m(-3)) was significantly greater than in the Ap layer (1.68 Mg m(-3)), but no statistical differences were observed among compaction treatments. Maximum shrinkage was always <1 and averaged 0.61 in the s ubsoil compared to averaged 0.80 in the Ap layer, which indicates near ly all structural and residual shrinkage after immediate air entry dur ing gravity drainage. The water retention characteristic of the origin al compacted and control treatments were still significantly different in the better drained subsoil but not in the more poorly drained subs oil, which showed that the 9-Mg axle load overall since 1987 has compa cted the subsoil nearly as much under wet soil conditions as the 18-Mg axle load initially. These soil structural measurements explain the f ailure of natural forces to reduce bulk density of the compacted subso il. (C) 1997 Elsevier Science B.V.