EFFECTS OF ECOLOGICAL SOIL-MANAGEMENT ON WORKABILITY AND TRAFFICABILITY OF A LOAMY SOIL IN THE NETHERLANDS

Effects of ecological and conventional farming in an identical loamy s oil were expressed in terms of the land qualities workability and traf ficability using conditions in an old meadow as a reference, Threshold values for workability, determined by the lower plastic limit occurre d al matric potentials of -120, -45 and -35 cm for the ecological, con ventional and old meadow system, respectively. The corresponding traff icability threshold values, obtained by penetrometer measurements, wer e -160, -15 and -120 cm matric potential. An additional field-traffic experiment showed that deleterious effects of driving over a wet field were relatively small for the conventional system, and more pronounce d for the old meadow and the ecological system. A dynamic simulation m odel for water flow was applied, using measured soil hydraulic paramet ers, to calculate soil water content and workable and trafficable peri ods during the year by use of the threshold values. Thirty years' clim atic data were used in order to obtain probability graphs. The probabi lity of a field to be workable and trafficable was largest for the con ventional system, and least for the ecological system, while the old m eadow had a high probability of being workable and a low probability o f being trafficable. The occurrence of five consecutive days with an a ppropriate topsoil moisture content for workability and trafficability was considered to represent the potential start and end of any growin g season. The probability of being able to sow or plant at what is con sidered the optimum dare by agronomists was high for the conventional field (77% for cereals; 93% for potatoes and sugar beet) and low for t he old meadow (10% and 33% respectively) and very low for the ecologic al field (0% and 17% respectively). The moisture supply capacity of th e soil, defined as the ratio between actual and potential transpiratio n, was most favourable for the old meadow, least favourable for the co nventional field and with the ecological field in between. Potential p roductivity of the ecological system was thus higher than the conventi onal system, but the risk of compaction was higher as well. putting re latively high demands on the management abilities of ecological farmer s.