SUBSOIL COMPACTION IN DUTCH MARINE SANDY LOAMS - LOOSENING PRACTICES AND EFFECTS

Subsoil compaction occurs on a large scale in arable land in the marine sandy loam soils of The Netherlands, which comprises 12% of the arable land. The changes in subsoil structure, resulting from three loosening procedures, i.e. deep rota-digging, blade-type subsoiling and plough-mounted subsoiling, were followed over 5 years using micromorphological and physical methods. The results were used to simulate soil-water regimes for a 30 year period to obtain quantitative information on the influence of soil structure, as an expression of different soil-management practices, on land qualities such as moisture deficit, aeration status and workability. As reference, the structure of permanent pasture land was added to show the contrast with a well-developed structure. If no changes in management practices were made after deep tillage these sandy loam soils were recompacted within 3 years with the same or worse physical properties and land qualities. Between secondary and primary ploughpans differences in moisture content and aeration status were significant, as a result of the presence of isolated packing voids instead of root channels. When the primary ploughpan needs to be opened up the best method is using a small plough-mounted subsoiler every few years under favourable soil conditions. Then the zones broken up are too narrow to be recompacted in a few years by machine wheels and during that time enough vertical root channels have been formed to maintain an improvement of soil physical properties. Because of the high faunal activity the most favourable soil physical properties and related land qualities occurred in permanent pasture land. Under Dutch climatological conditions maximum benefits of a pasture land subsoil structure were a decrease of about 10% in the number of years in which a moisture deficit occurs; in dry years the deficits decrease by about 50%. The number of workable days and days with adequate soil aeration will increase by 20 and 10%, respectively. Consequently, changes in soil management, namely reduction of tillage practices, increase of organic and green manures and a reduction of pesticides and inorganic manures, leading to a more biologically determined soil structure, will result in improved soil physical properties and land qualities and a positive crop response.