Compared with forces occurring in soil mechanics problems in civil
engineering, the forces that are applied to soil in farming operations
generally have a short duration, less than a few seconds, a small loaded
area, no more than a few square decimeters, and small intensities, 10
bar being a high value. On the other hand, soil properties vary widely
between those of a weak mud and a stone-like dry soil. Tillage and
related applications of force to soil are practiced worldwide in
farming. Tillage operations are performed on one hectare of land for
every three human beings. This means that for the food production for
each individual daily, something like one cubic meter of soil is
stirred, or about 20 times his body weight. Theoretical knowledge of
this most common human activity, which largely determines the surface
shape of the fertile part of the earth, is still very limited. In this
book the authors have tried to give an outline of the present state of
the art. One of the starting points was a course in soil dynamics taught
by the authors at the Agricultural University at Wageningen, The
Netherlands. We hope to reach interested readers who have no more
theoretical knowledge than high school level, as well as readers who
want to go beyond the level of a third year university student. For the
chapter on wheels and tires we received substantial support from F. G.
J. Tijink of the Tillage Laboratory at Wageningen.