Summary
The recent increase in conservation tillage offers the possible benefit of decreasing soil compaction risk by wheeling. Excessive compaction has damaging consequences for agriculture and the environment. Direct drilling is likely to change soil porosity and soil carbon content in the long term. This paper analyses the effect of both of these factors on soil mechanical strength through measurements of the compression index Cc, the swelling index Cs, and the pre‐compression stress pc*. Oedometer tests were performed on remoulded soils taken from an experiment with a wheat‐maize rotation, where three soil tillage systems were compared over 31 years: annual mouldboard ploughing, superficial tillage (10 cm depth), and no tillage. The results show that initial structural porosity (i.e. macro‐porosity) increased Cc slightly and decreased pc* exponentially. Consequently, the risk of compaction in the direct drilling systems should decrease when a decrease in soil porosity is found, compared with conventional systems. However, this change in soil porosity in direct drilling is not always observed. The other variables that can be influenced by tillage system, i.e. carbon content or soil moisture, have also been examined to assess the possible benefit of direct drilling on soil compaction risk. For soils with similar initial structural porosity, our results show that the increase in carbon content of the superficial soil layers in direct drilling systems tends to increase the soil’s susceptibility to compaction by increasing Cc in wet conditions and by decreasing pc* in dry conditions. Moisture conditions at wheeling determine the degree of soil compaction as a function of tillage system.
Sensibilité des sols au tassement par les engins agricoles: analyse de l’effet du travail du sol pour un sol limoneux
Résumé
L’adoption croissante du semis direct pourrait permettre de diminuer le risque de tassement des sols lors du passage des engins agricoles. Les tassements sévères du sol ont des conséquences importantes sur l’environnement et l’agriculture. A long‐terme, la technique du semis direct modifie la porosité et la teneur en carbone du sol. Cet article propose de quantifier l’effet de ces deux facteurs sur la résistance mécaniques du sol par des mesures de l’indice de compression Cc, de l’indice recompression Cs et de la pression de preconsolidation pc*. Des essais oedométriques ont été réalisés sur des échantillons de sols remaniés (sol limoneux) prélevés sur un essai comparant trois systèmes de travail du sol depuis 31 ans pour une rotation blé/maïs: labour, travail superficiel (10 cm), semis direct. Les résultats montrent que la macroporosité appelée porosité structurale du sol induit une faible augmentation de Cc et une décroissance exponentielle de pc*. Ceci doit diminuer le risque de tassement dans les systèmes où le semis direct s’accompagne d’une réduction de la porosité. Cette évolution de la porosité en semis direct n’est cependant pas systématique. L’effet des autres facteurs doit être considéré, à savoir ...
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