Geoscience Reference
In-Depth Information
of untouched soil between the neighboring rows, the soil was frequently cross loos-
ened by mutually perpendicular paths. The ox-improved tractive force, later
improved with a horse and sometimes vice versa, was also used with the vertical
stick being replaced by a moldboard. At the end of this period, wheels replaced the
runner and the compressive role of the moldboard plow weight was reduced. Earlier,
the wheelless plow had to be dragged across the fi eld. The shape of the moldboard
plow aided horizontal cutting as well as turning the loosened topsoil layer upside
down so that its original surface lay on the bottom of the plowed trench and its base
became the new soil surface. Finally, the tractor replaced the horse and the depth of
tillage was increased. Consequently, plant nutrients and humus in lower portions of
the A horizon reach the soil surface, while the earlier impoverished surface soil is
shifted deeper into the profi le. Because the positive effects of repeated tillage to the
same depth steadily decreased, it was necessary to plow the soil to greater depths to
renew those positive features of churning the A horizon. However, greater depths
brought higher friction from an increased soil resistance that was overcome by
using a stronger tractive force. Horses were replaced by tractors. And, the stronger
the tractor, the deeper the soil could be plowed. On the other hand, the increased
weight of stronger tractors progressively compressed the soil causing a loss of the
structural stability of the original loosely packed aggregates of friable soil. During
compression, individual soil particles are pushed together reducing pore space and
increasing the soil bulk density. With the coarse pores being wiped out, the porosity
is greatly reduced. We must also remember that this physically created condition is
more intensive when there has been a loss of soil humus, especially of humins and
glomalin, that accelerates the disintegration of soil aggregates.
Soil compression is most apparent within the A horizon. But the top part of the
horizon is loosened to a maximal porosity during plowing to prepare the soil surface
for the next crop. After planting, during the vegetative season and after harvest, the
porosity continually diminishes until the soil is once again plowed and prepared for
another crop. However, the layer below the plowing depth is permanently com-
pressed, and after a long time, a distinct permanently compressed horizon (plow
sublayer) is formed. The regularly “loosened” top part of the A horizon also retains
marks of compression for a long time, but they are less impressive than those
observed below the depth of plowing in the plow sublayer. Plowing is also a very
effective tool for burying weeds. Its effectivity is at least as powerful as the contem-
porary use of herbicides. Plowing has the great advantage that it does not endanger
groundwaters and surface waters in lakes by herbicides or their derivatives after
being chemically transformed in soil and in surface waters.
Soil compression also has unfavorable consequences. The rooting of plants is
worsened, and when the roots of agricultural plants reach the depth of the perma-
nently compressed plow sublayer, they are not apt to penetrate through it.
Consequently, the plants start to suffer from a lack of nutrients. The reduced poros-
ity brings reduced soil permeability, and the lower is the permeability (or hydraulic
conductivity), the smaller is the portion of rainwater penetrating the profi le. Most of
the rainwater attacks the sloppy soil surface causing erosion and eventually the
potential collapse of the soil vegetation owing to incoherent environmental
