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INFLUENCE OF SOIL$MOI
AND COMPACt )W AO4
ENERGY FOR SUBSOILING
WITH MINIMUM TILLAGE
BY F.M. RHOADS AND D.L.WRIGHT
Florida Agricultural Experiment Stations
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
AREC, Quincy Research Report 81-2
INFLUENCE OF SOIL MOISTURE AND COMPACTION ON ENERGY
FOR SUBSOILING WITH MINIMUM TILLAGE
F. M. RHOADS AND D. L. WRIGHT1
Tillage pans were identified and characterized in four Coastal Plain soil series
occurring throughout the Southeastern United States (5). Depth to the pan was
11 to 15 cm, pan thickness was 13 to 14 cm, and root growth within the pan was
Deep tillage and deep placement of lime, fertilizer, and nematicides have been
tested on various crops at several locations with inconsistent results (1, 2, 3,
6, 9, 10, 11, 12). Subsoiling under the row increased seed cotton yields 41% but
bedding, deep placement of lime, and addition of a nematicide had no influence on
yield (1). Subsoiling increased soybean yields in 7 of 16 experiments, whereas, a
nematicide increased yields in 10 of 16 tests (6). However, the combined treat-
ment of subsoiling, plus a nematicide, increased yields significantly in 13 of 16
experiments (6). Subsoiling, in New Jersey, with and without deep placement of
lime and fertilizer on a Collington sandy loam soil, did not produce significant
yield increases of several vegetables (2). However, residual effects of subsoiling
significantly increased water movement into this soil for 3 years after the last
deep tillage operation.
In-row subsoiling before planting produced highest soybean yields in North
Florida (7). Depth of rooting of corn was increased with subsoiling (8). Re-
sponse to subsoiling on sandy soils appears to be related more to increased
nutrient availability than to availability of water. Yield response to subsoiling
has been most consistent where under-the-row subsoiling was practiced.
Energy requirements for subsoiling are quite high and considerable savings
could be achieved if the subsoiling operation was not necessary every growing
season. However, under normal tillage operations the soil is recompacted each
year and subsoiling is required on an annual basis for maximum crop yields.
There is a possibility that recompaction of the soil following subsoiling could be
minimized under minimum tillage production of crops. Avoiding travel over crop
rows from the previous season with tillage implements and tractor wheels.should
reduce soil compaction. This can be accomplished with minimum tillage operations
where succeeding crops are planted directly in stubble rows of the previous crop.
This report contains test results from experiments designed to measure the
effect of soil-moisture content on resistance to soil penetration and the effects of
a disc-harrow and a tractor wheel on soil compaction. Power requirements for
subsoiling at different levels of soil penetrometer resistance were also estimated.
Eight tillage and compaction treatments were applied to three soil types during
the winter of 1979-80. The soils were Orangeburg loamy fine sand, Norfolk loamy
fine sand, and Troup sand. All treatments were harrowed with an offset disc-
harrow before tillage and compaction treatments were applied. Treatments were
1F. M. Rhoads, Professor of Soil Science, D. L. Wright, Extension Agronomist,
AREC, University of Florida, Rt. 3, Box 638, Quincy, Florida 32351.
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
Power requirements were estimated from the following equation:
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.
Resistance (bars) per chisel
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-
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-
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
three or four times during a single year where conventional tillage is used. This
is why most growers have planters attached directly behind the subsoiler chisel in
order to avoid recompaction of the soil between the subsoiling and planting operation.
Minimum tillage provides a way to avoid recompaction of soil in the subsoil slit be-
tween crops since the location of the rows from the previous crop are visible during
the planting operation. Therefore, the tractor operator can run the tractor wheels
between rows and plant directly over the subsoiler slit made for the previous crop.
Perhaps as a result of. this practice the subsoiling operation would only be necessary
every other year. Thus, a significant savings of energy would be accomplished.
PENETROMETER RESISTANCE BARS
0 10 20 30 40
PENETROMETER RESISTANCE BARS
10 20 30
before subsoiling and in
the subsoiler crevice be-
fore and after four trips
with a disc harrow.
Effect of a tractor tire on
recompaction of soil in the
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cotton yield in "stunt" areas of Georgia. Agri. Exp. Sta. Univ. of Ga. Res.
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