Visualizing soil compaction based on flow pattern analysis

Soil compaction modifies the pore system, often in the sense of degrading or destroying the soil structure. As a consequence, not only the soil mechanical parameters like the pre-consolidation load or bulk density are being changed, but also the transport properties of the pore system. Compaction induced changes of water infiltrability and availability of water and air to plants and microorganisms may hamper the functioning of the soil environment. We studied the effects of the mechanical impact applied by a sugar beet harvester on soil porosity, bulk density and on the water infiltration regime under field conditions on a sandy loam in Switzerland. Three treatments were compared: multiple vehicle passage, single passage and control (no traffic). Bulk density, total porosity and macroporosity were determined in the laboratory. In the field, a dye tracer solution was homogeneously applied onto the plots of the three treatments. Vertical profiles were prepared and color slide pictures taken with a normal photographic camera. The images were processed by digital image analysis in order to analyze the spatial distribution of the stained areas. An obvious effect of the mechanical impact was an increase of preferential flow. The water was ponding on the soil surface of the trafficked plots and funneled into preferential flow ports, mainly worm burrows. Wetting of the main root zone decreased because a significant fraction of the infiltrating solution bypassed the matrix. The effect was more pronounced in the multiple passage plot than in the single passage plot. These results agree well with the laboratory measurements. In the single passage plot, a significant effect was observed down to a depth of 15 cm. The plot with the multiple passage showed a stronger effect down to greater depth. The laboratory measurements indicate subsoil compaction, which cannot be concluded from the results of the tracer experiments. The flow patterns, on the other hand, visualize the compaction effects and yield qualitative information about compaction induced changes of the infiltration regime of the soil. (C) 2002 Elsevier Science B.V. All rights reserved.