Subsoil loosening eliminated plough pan but had variable effect on crop yield

Summary

The effect of subsoil loosening to c. 35 cm depth was studied in an organic crop rotation experiment at Flakkebjerg (sandy loam) and Foulum (loamy sand). In each of the years 2000-2003 half of 4 plots per site was loosened in the autumn in a young grass-clover. Penetration resistance was recorded yearly. Detailed soil physical properties were determined for the Foulum site in 2003. Aboveground winter wheat growth was followed during the 2002/03 growing season using spectral reflectance measurements. Root growth was followed during the 2002/03 growing season at Foulum using the minirhizotron technique. Crop yield was determined every year in all treated plots. The Paraplow treatment effectively loosened the plough pan. Loosening in the grass-clover crop resulted in reduced growth. Apparently, this unintended effect of loosening had marked effects on the growth of the succeeding winter wheat crop. In wet years, yields were lowest in loosened and unmanured soil. In a dry year, loosening resulted in increased yield at Flakkebjerg. This appears to be an effect of the grass-clover crop that interferes with the results of loosening. Generally, our results suggest that under Danish conditions subsoil loosening can only be recommended in case of very severe subsoil compaction.