Transit Wet Soils Does Not Always Cause Harmful Effects On Topsoil Porosity.

This research was driven by previous results obtained in cattle breeding ranches in the Flooding Pampa of Argentina (Typic Natraquolls, loam texture)  and more recently in integrated crop-livestock production farms (Typic Argiudolls, silty loam texture) in the Rolling Pampa. In both cases no soil structural damage was found because of livestock and/or agricultural transit on wet soil conditions.  In order to elucidate why the wet soils were not affected by transit, the variations of topsoil bulk density (BD) and gravimetric water content (W) with time were compared in nearby non transited (fenced enclosures) and normally transited situations.  Topsoil BD always decreased significantly in both non transited and transited conditions while ponded or saturated. Soil volume (V = 1/BD) and calculated air volumes (Va = V - W) were plotted against W using laboratory (shrinkage curves) and field data. In the laboratory soil V varied little with W and Va increased on drying, showing a low soil swelling behavior. In the field, both soils increased their V and Va  when wet, showing the occurrence of air trapping, regardless the soil was transited or not.   Soil swelling was not caused by swelling clay mineralogy but by a process of air inflation. The occurrence of trapped air was a main factor to withstand external loads applied to wet soils, thus preventing the occurrence of harmful effects on topsoil porosity.