Modelling structural changes in tilled topsoil over time as a function of cropping systems

The optimization of tillage practices requires the evaluation of the long-t erm effects of cropping on changes in soil structure. A model is presented that simulates changes in soil structure in the tilled horizons of cultivat ed fields. The indicator of soil structure that we have modelled is based o n a morphological description of the structure and the assessment of the pe rcentage of severely compacted zones and clods showing no visible porosity in the tilled layer of a soil profile. The model takes into account the spa tial variation of the structure induced by traffic and tillage. Severely co mpacted zones are created under wheel tracks, they are cut and transferred during ploughing, and they are fragmented into fine soil during secondary t illage. The model was evaluated over 7 years in a field trial on loamy soil in three experimental plots, where the crop sequences and field operations induced contrasted changes in the percentage of severely compacted zones a nd clods. The model satisfactorily reproduced the changes with time of the indicator, and the correspondence between simulated and observed values was fairly good (r=0.82, model efficiency =0.61). However, some variations fro m one year to the next were poorly predicted. The sensitivity analysis did not suggest modification to the parameter values but showed that the perfor mance of the model could be improved if the fragmentation (due to weather a nd tillage) were more precisely described in the model. Nevertheless the pr esent version of the model and the corresponding simulation program (SISOL) can be used to compare the effects of different technical choices on the e volution of soil structure.