The effects of tree crops on the soil physical properties of former agricultural land were compared with those of ley in a rotation with cereals on adjacent sites. Five sites in southern Sweden were investigated focussing on soil water retention characteristics, dry bulk density, macroporosity and saturated hydraulic conductivity. Three of the sites were on light textured soils and two on clay soils.The tree crops were 30 to 35-year-old hybrid aspen, Populus deltoides, and silver birch, Betulapendula, and the ley crops were one to five years old.The light-textured soils under tree crops showed bimodal pore size distributions in the macropore region, whereas under ley crops they showed unimodal distributions. Dry bulk densities were generally smaller and the macroporosities larger under tree crops compared with leyicereal crops. Saturated hydraulic conductivities tended to be larger under tree crops. Slopes of the linear regression lines between saturated hydraulic conductivity and each of the parameters dry bulk density, porosity and macroporosity were steeper in the soil under agricultural crops than under tree crops.Observed differences in physical properties were considered to be an effect of land use, which had brought about changes in aggregate stability, pore size distribution and pore continuity.

The effects on a number of soil physical and aeration parameters of compaction during spring pre-sowing operations were measured on a clay soil (49% clay). A soil-tyre contact stress of 200 kPa was applied by tractor tyres.Yield of an oat crop was reduced by 30% as a result of compaction. Total porosity of the soil was reduced by 6% vjv owing to loss of pores > 60 pm, and water retention was increased. The resultant decrease in air-filled porosity greatly reduced gas diffusion and air permeability coefficients of the soil, and, for a time, 0, content of the soil atmosphere was significantly lowered in the compacted treatment. Penetrometer resistance after sowing was 3.5 MPa in the control and 4.5 MPa in the compacted treatment; in the latter, root growth was inhibited until the soil dried and cracked. By the end of June, canopy temperature measurements indicated water stress in the oat crop on compacted soil but not in that on the control. The results obtained indicated that air permeability, measured in the field, of 1 mm s s ' provides a satisfactory single value below which crop growth is likely to be reduced.

SUMMARY The effects on soil aeration of varying both the intensity and quantity of water applied per irrigation were investigated in a field experiment on a clay soil. Soil physical and soil aeration parameters were measured before, and for up to 7 days after irrigation. Irrigation increased the volumetric water content of the surface layer by approximately 8% v/v. However, periods of waterlogging were short on this soil. Significant decreases in soil oxygen content in the days after irrigation were caused by increased soil and root respiration. In terms of subsequent crop growth, the practice of applying 30 mm irrigation water at an intensity of 5 mm h−1 was shown to be superior to irrigations providing half this quantity (15 mm) or double this intensity (10 mm h −1). The lower intensity irrigations decreased soil permeability by homogeneous wetting and swelling, and thus more of the water applied was recovered in the soil. Approximately one third of the water applied at the higher intensity was lost through cracks, and wetting of the soil was less uniform.