- 2-,


as follows: 1) no treatment, 2) subsoiled only, 3) subsoiled followed by one trip
with the offset harrow, 4) subsoiled followed by two trips with the harrow, 5) sub-
soiled followed by four trips with the harrow, 6) subsoiled followed by one trip
with the tractor tire directly over the subsoiled furrow, 7) subsoiled followed by
two trips with the tractor tire as in no. 6, and 8) subsoiled followed by four trips
with the tractor tire as in no. 6.

 Resistance to penetration was measured with a recording penetrometer to a
depth of two feet (60 cm). Four measurements were taken each time per treat-
ment and averaged. Soil-moisture content was measured with a neutron moisture
probe when penetrometer measurements were made.

 Penetrometer measurements were taken to correspond to different levels of
soil-moisture content.

 Power requirements were estimated from the following equation:
 5280 1
 HP = PR x 14.5 x Ax 3 mph x 6 x 500-
 where HP = horsepower
 PR = penetrometer resistance in bars
 A = area of chisel point in square inches
 mph = miles per hour

These estimates may be slightly high since the angle of the chisel point with re-
spect to direction of travel was not considered.


 RESULTS AND DISCUSSION

 Soil moisture content has a significant effect on resistance to penetration of
the soil profile. The traffic pan is located in the top foot (30 cm) in most coastal
plain soils with a long history of cultivation. Therefore, the moisture content in
the upper part of the soil profile will have a pronounced effect on penetrometer
resistance. Penetrometer resistance (PR) was reduced from 36 to 18 bars in the
top 30 cm of a Norfolk soil when the moisture content increased from 17.4% to
20.6% (Fig. 1). This corresponds to a power requirement change of 25 HP per
chisel or 100 HP for a four row subsoiler (Table 1). The change in moisture con-
tent corresponds to 0.18% per bar of change in PR. Similar results were observed
in the Troup soil except the moisture change was much less, corresponding to .09%
per bar change in PR (Fig. 2).


Table 1. Power required to pull a single subsoil chisel through the soil with
 various levels of resistance to penetration at a speed of 3 miles per
 hour. Chisel point dimensions 2 inches by 6 inches.

 Penetrometer Horsepower
 Resistance (bars) per chisel

 5 7
 10 14
 20 28
 30 42
 40 56