Loads and soil deformations from agricultural vehicles and their influence on soil water retention properties

Agricultural sites are often influenced by soil deformation and compaction through vehicular traffic. Both can lead to changes in pore-size distribution and consequently in changes at the water retention properties. Compaction usually has the disadvantage that crop yields are reduced and, conversely, avoiding soil compaction has the potential to increase yields and is also more sustainable for the soil. By compaction mainly the macro pores are affected and the lost in pore volume comes from a transformation of macro pores and coarse pores to middle and fine pores.

In this study presented here agricultural sites in Lower Saxony (Germany) were soil sampled before and after vehicle wheelings with different machines. The samples were lab investigated to become soil moisture to pore pressure pairs and based on these pairs samples retention curves were yield by fittings with the traditional constrained van Genuchten model using the HYPROP-Fit software. The goal of the study is mainly to compare the van Genuchten parameters θs (saturated water content) and α (van Genuchten shape parameter) between unwheeled and wheeled situations and this in relation to the used vehicles, depth and position in the field.

The results show that soil compaction mainly occurs as a result of vehicular traffic, which is reflected in predominantly decreasing θs values. In most cases, the van Genuchten parameter α is significantly reduced after traffic loading, indicating that the mean and dominant pore size range is shifting towards finer pores. Based on these results, it may be possible in the future to identify vehicles and types of traffic where the slightest possible negative soil changes are to be expected.