application of slurry (50 kg NH4-N/ha) to winter wheat and spring barley. The treatments were only carried out once in each plot, and therefore the number of treated plots per site increased from 4 in 2000 to 16 in 2003.
Penetration resistance 0-60 cm depth (Olsen, 1987) was recorded yearly in the treated plots in the years 2001 to 2004. Detailed measurements of soil physical properties (bulk density, and water content and air permeability at –100 hPa matric potential) were carried out at Foulum in 2003. Aboveground plant growth was followed during the 2002/03 growing season using spectral reflectance measurements according to Andersen et al. (2005). At Foulum, root growth was followed in winter wheat during the 2002/03 growing season using the minirhizotron technique described by Thorup-Kristensen (2001). Crop yield was determined every year in all treated plots.