Soil compaction in agriculture: from soil stress to plant stress

This thesis is based partly on material from an extensive programme of field experiments carried out in Sweden, 1960-1990. The data have been analyzed and complemented with new field and laboratory experiments. Crop yield data in the field experiments were used to create an empirical model for estimating yield losses due to compaction. This model can be used in an advisory capacity since it calculates the yield losses at the farm level. In a series of 96 field experiments, direct effects of wheel traffic on crop yield and soil physical properties were only to a minor extent affected by soil texture. Organic matter reduced bulk density, and increased the need of recompaction to maximize yield. There were systematic differences in bulk density and compression index depending on soil texture and organic matter content when field data were compared with results from a uniaxial compression test using the same soils. In a separate field experiment, plant nutrient uptake was reduced in loose and very compact compared to moderately recompacted soil, but this result could not be reproduced in the laboratory. Effects of field traffic persisted after mouldboard ploughing, resulting in lowered yields on soils with high clay content. Yield reduction was related to an increase in soil strength rather than bulk density. Chisel ploughing to 20 cm loosened the soil almost as effectively as mouldboard ploughing in a longterm field experiment. It also increased yield and reduced soil strength compared to shallow tillage. Soil stress, rut depth and penetration resistance were highly correlated to inflation pressure in a comparison between four tractor tyres. At a given inflation pressure, soil stress was greater under bias-ply than radial tyres. A new method was developed to determine soil displacement under wheel traffic and could prove important to make guidelines for avoiding subsoil compaction. In the past there has been too little interaction between basic and applied research on soil compaction. A future challenge is to link effects of wheel traffic into models predicting impact on soil properties and processes, longterm soil quality, crop yield and the environment.