Seedlings often use cracks to emerge in crusted soils. This study describes crack patterns for different types of sowings with the final aim to predict crack formation on soil surfaces in a crop emergence model. Crack patterns and their changes through time, from sowing to the end of emergence, were observed on the seedbed surfaces of several crops. The experiments were carried out in a silt loam soil (0.2 kg kg-1 clay content). The seedbed structures were very different, depending on soil tillage and sowing operations and soil water content during the operations. We obtained two states of seedbed: fine, composed of less than 0.05 kg kg-1 aggregates over 3 cm, or coarse with more than 0.20 kg kg-1 of these aggregates. By the end of the experiment, the stage of crust development was from structural to beginning of sedimentary.
An image analysis tool was produced to detect automatically the visible cracks on soil surface images using morpho-mathematic methods. The crack network was described by several measurements that include the total length of the cracks and certain characteristics of the plates delimited by them, such as plate area, number of intersections by plate and individual segment sizes.
The total length of the cracks varied from 15 to 35 cm.100cm-2 ; and was higher for the most crusted plots. The plate areas also increased with the crust stage. In contrast to the changes in areas, the plate shape did not change between plots and tended to be hexagonal whatever the initial structure of the soil and crust development. Plates are delimited by about decimetric segments.
These data will be used to evaluate and adapt an empirical 2D stochastic model of crack growth (Horgan and Young, 2000). We must also analyse more situations, in particular different soil textures to generalise our predictions.
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