-3-


Penetrometer Resistance
 -Bars-


 0



 10


 20
E
U

30


 50
 50
 so0


Effect of soil moisture con-
tent on penetrometer re-
sistance in a Norfolk soil.
Average per cent moisture
by volume is shown for 0
to 30 cm and 30 to 60 cm
for two separate observa-
tions.


Figure 2.


Penetrometer Resistance
 -Bars-
 10 20 30


Effect of soil moisture content
on penetrometer resistance in
a Troup soil. Average per
cent moisture by volume is
shown for 0 to 30 cm and 30
to 60 cm for two separate ob-
servations.


 From an energy viewpoint the most desirable moisture content for subsoiling
is at field capacity or when the soil first becomes dry enough for tillage following
rainfall. It may be desirable to subsoil when the soil is dry in order to shatter
the tillage pan as much as possible but the increased yield response may not off-
set the added cost of energy. A decrease in moisture content in the Norfolk soil
of 3% below field capacity would about double the power requirement for subsoiling.
A decrease of only 1% moisture below field capacity would double the power re-
quirement for subsoiling in the Troup soil. Furthermore, substantial yield in-
creases have been observed in corn and soybeans as a result of subsoiling when
soil moisture content was near field capacity (7, 8).

 Soil compaction has been attributed mainly to the use of a disc-harrow, by
many people. However, four trips over a subsoil crevice with an offset disc-
harrow recompacted the soil to a PR value of less than 5 bars (Fig. 3). The
graph shows the depth of subsoiling at about 14 inches (35 cm) and the depth of
the harrow at about 6 inches (15 cm). One trip over a subsoiled crevice with a
tractor tire caused greater recompaction of the soil than 4 trips with a harrow
(Fig. 4). Four trips over the crevice with a tractor tire recompacted the soil to
resistance levels of over 15 bars as measured with the recording penetrometer.
There is a high probability that tractor tires will pass over the subsoil crevice


Figure 1.