Persistence of soil compaction due to high axle load traffic .2. Long-term effects on the properties of fine-textured and organic soils

The long-term effects of high axle load traffic on soil structure were investigated in three field experiments. Two of the experiments were located on fine-textured mineral soils (Vertic Cambisol). The clay soil had 48 g clay (particle size less than 2 mu m) per 100 g in the topsoil and 65 g per 100 g in the subsoil, and the loam soil had clay contents of 30 g and 42 g per 100 g in the topsoil and subsoil, respectively. One experiment was located on an organic soil (Mollic Gleysol) consisting of well-decomposed sedge peat mixed with clay from 0.2 to 0.4-0.5 m depth, and underlain by gythia (organic soil with high clay content). In the autumn of 1981, one pass and four repeated passes with a heavy tractor-trailer combination compacted the soils to 0.4-0.5 m depth. The trailer tandem axle load was 19 Mg on the clay and 16 Mg on the other soils. For 9 years after the experimental traffic, the main crops grown were spring cereals. During this time, the maximum axle load applied during field operations was 5 Mg and the maximum tyre inflation pressure was 150 kPa. The clay and loam froze to 0.5 m depth for 6 and 2 years, respectively. During several growing seasons all three soils dried and cracked. In the ninth year after the loading, soil penetrometer resistance, saturated hydraulic conductivity (K-sat), macroporosity and number and area of cylindrical biopores were measured and the visual structure of the soils examined. Compaction in the plough layer was alleviated by ploughing and natural processes, whereas in the subsoil the effects of the compaction were still measurable, in all experiments, in the ninth year after the high axle load traffic. In the clay soil in the 0.3-0.5 m layer and in the organic soil in the 0.28-0.4 m layer, the penetrometer resistance was 22-26% greater and the soil structure more massive in the plots compacted with four passes than in the control plots. In the 0.4-0.55 m layer in all soils, the loading with four passes decreased K-sat by 60-98% and macroporosity (diameter greater than 300 mu m) by 37-70%. In the fine-textured mineral subsoils, cylindrical biopores were found in all treatments. The trend of the results was, however, for biopores to be fewer in compacted than in control plots.