PERSISTENCE OF SUBSOIL COMPACTION FROM HEAVY AXLE LOADS

As the persistence of subsoil compaction has a major impact on agricultural sustainability, responses of soil pore characteristics and plant roots to subsoil compaction were examined as related to heavy axle loads. A Ves clay loam was originally compacted with three axle loads of less than 4.5 (control), 9, and 18 Mg in the fall of 1981 (soil dry (D) ) and in the fall of 1982 (soil wet (W) ). The 9 Mg treatment was recompacted on both trials in the spring of 1988 with an 18 Mg axle load, and maize and soybean were grown in rotation. Field measurements in 1988 and 1989 included bulk density, biopore area, root number and location, ponded and negative water pressure infiltration, saturated hydraulic conductivity (K(sat)), and ped size distribution. Bulk densities measured under the row in 1989 for the W plots were increased in the 10-28 cm layer by the new 18 Mg load, whereas the original 18 Mg treatment had higher bulk densities in the 28-68 cm depth. The control had lower bulk densities than both compacted treatments for the 18-58 cm increment in 1989. The number and areas of biopores in the D control plots were at least ten times greater than in W control plots. Compaction at 35 cm reduced K(sat) and ponded infiltration compared with the control. Mean ped diameter ranged from 1.3 to almost four times greater in the compacted treatments than in the control. In spite of increased soil density, a substantial reduction in root growth was not observed. Roots grew around dense clods in the compact treatments allowing continual vertical extension. There was no evidence of poor drainage hindering crop growth in the heavy axle load treatments.