Modelling vertical and lateral seed bank movements during mouldboard ploughing

The vertical distribution of weed seeds in the soil is of fundamental importance because seedling emergence depends on seed depth. The lateral displacement of the earth during mouldboard ploughing contributes to the dispersal of the weeds inside the tilled field. In order to model vertical and lateral seed displacements during ploughing, an existing model describing soil particle movements for different ploughing characteristics (depth and width) and soil structures was tested on a multilocal field trial. The trials were carried out in 1996 and 1997 and comprised two soil textures and three soil structures; tillage was performed with a mouldboard plough at varying ploughing widths and depths. Seeds were simulated by beads that were introduced immediately before ploughing with an auger at different depths and lateral positions (relative to the future passage of the coulter) within and just below the ploughed horizon. Lateral displacement and the final vertical position of the heads were measured and compared to the simulations obtained with the model. The model correctly simulated the final vertical seed co-ordinate and lateral seed displacement as a function of soil structure, ploughing width and depth and initial seed position, if ploughing depth is lower than ploughing width. If, however, the former exceeds the latter and/or if the furrows are not properly rotated, the model does not simulate the seed movements correctly. The model was then used to calculate seed transfer matrices describing vertical seed movements between seed bank layers for different conditions and plough modes and to determine the optimal ploughing mode for a given seed bank distribution. For instance, if most seeds are located in the top layer ploughing should be as deep as possible, with a low depth to width ratio to maximise soil inversion and seed burial. If, however, the seeds are concentrated in the bottom layer, the model can be used to decide how shallowly to plough in order to avoid disturbing the deeper seeds and what ploughing width to associate to this depth in order to minimise soil inversion and leave as many seeds as possible undisturbed. Ways of improving the model are suggested, particularly the necessity to simulate the effect of a skim coulter. (C) 2000 Elsevier Science B.V. All rights reserved.