Effect of narrow opener geometry on lateral surface soil movement and
implications for no-till seeding
Solhjou AA, J. Fielke J, Desbiolles J
Barbara Hardy Institute – Agricultural Machinery Research and Design, University of
South Australia, Mawson Lakes, SA 5095; [email protected]
Keywords: No till, Soil movement, Rake angle, Narrow opener, Herbicide incorporation,
Bent leg opener
Introduction
Australian no-till farming often uses narrow point openers to open the soil and place seed and
fertilizer in the soil. They are generally used in conjunction with spraying pre-emergence
herbicides for weed control and are followed by press wheels to pack soil over the seeds.
These openers can create excessive soil disturbance and soil throw, with the effect of
increasing the depth of soil cover on adjacent furrows (Desbiolles and Saunders, 2006),
increasing stimulation of weed seed germination (Chauhan et al., 2006) and enhancing
seedbed soil moisture loss (Chaudhuri, 2001). In Australian farming systems, pre-emergence
herbicides are often mechanically incorporated by the sowing operation, and excessive lateral
soil throw at seeding can result in herbicide contaminated soil reaching adjacent seed rows
(Derpsch, 2007, Desbiolles and Saunders, 2006). The factors previously identified in the
literature affecting soil movement include: soil condition such as texture, moisture and
structure (Sharifat, 1999), tool settings such as speed and depth (Sharifat, 1999; Godwin,
2007) and opener geometry (Sharifat, 1999; Godwin, 2007; Chaudhuri, 2001). This paper
reports on the soil movement created by a range of narrow points with various rake angles
(angle contained between the active face of the opener and the direction of travel) and the use
of a commercial bent leg style opener. The work used small aggregate size cubes in a grid
pattern over the depth profile and across the path of the opener, acting as tracers to indicate
3D soil movement. This report outlines the results of surface tracer displacements only, as an
estimate of the likely movement of pre-emergence herbicide sprayed on the soil surface by
the mechanical incorporation process. This movement can affect both the efficacy of weed
control and the potential risk of crop damage.
Materials and Methods
Experiments were undertaken in remolded soil bin conditions using the seed placement test
rig at the University of South Australia (UniSA). Four flat narrow point openers of 16 mm
width with rake angles of 35°, 53°, 72° and 90°, respectively and a bent leg opener with a 10
mm wide foot were tested using a forward speed of 8.2 km/h and work depth of 120 mm. A
randomized complete block design with four replications was used to evaluate the effect of
rake angle on the soil movement. A subsequent comparative test was undertaken with 2
replications for the bent leg opener. The 3 m long soil bin comprised a reconstituted sandyloam soil with a 10.6% soil moisture content (w/w, dry basis) and 1.37 g/cm3 soil bulk
density (dry basis). Soil movement was measured by placing PVC cubic tracers (1 cm x 1cm
x 1cm) in a reference grid within the soil profile prior to tillage and measuring their final
position after tillage using a 3 dimensional digitizing frame. The 13 surface tracers were
positioned with one on the center of opener travel path and the others symmetrically set at 10,
20, 30, 60, 90 and 120 mm from the centre. These surface tracers were placed at half their
height (5 mm) into the soil. The furrow loosening result was assessed over a 0.5 m length of
furrow using a scanning laser soil profile meter and included the surface profile of the
loosened soil and the furrow soil failure boundaries measured after excavation of the
loosened soil.
Results and Discussion
Figures 1 to 4 show ‘whole-of-trial’ tracer data for the 4 rake angle openers, including 4
replications with left and right pseudo-replications obtained by symmetry, while the furrow
profiles shown are the average furrow cross-sections (note: the variability in the lateral soil
throw over the 0.5 m long sample is not displayed by the average profile but is reflected in
the position of individual tracers). The results showed that each of the various rake angle
tools cleared all of the surface tracers from above the centre of the furrow below which the
seeds would commonly be expected to be placed (seed zone). The tool with the low rake
angle of 35° had the narrowest band of cleared tracers and this would indicate that preemergent spray placed on the surface would be moved a smaller distance from the furrow
centre (±45 mm) giving the narrowest zone through which weeds may be able to emerge.
The tools set at the other rake angles achieved a wider band of cleared tracers and this would
likely result in more weed competition near the seed row. The tool with the 53° rake angle
gave the widest band of soil with no tracers (±90 mm). Typical row spacing (Lc) in Southern
Australian no-till farming systems ranges from 225 to 300 mm while small proportions of
farmers may operate at other row spacing, as narrow as 180 mm and as wide as 380 mm. All
tools threw a considerable volume of soil beyond the furrow boundaries. Their furrow
backfill factor, defined as the proportion of furrow size filled with loose soil, was 93%,
92.1%, 97.4% and 98.1% at the 35°, 53°, 72° and 90° rake angles, respectively. A significant
quantity of loose soil reached over the width of 180 mm, but by 250 mm the soil volume
reduced considerably, as shown by both the height (depth) of the loosened soil and the large
majority of tracers contained within. This indicates that, at narrower no-till row spacing, these
tools will move significant topsoil contaminated with pre-emergent herbicide onto the
adjacent furrow, with the potential for causing crop damage depending on herbicide solubility
and absorption pathways. The soil-bin tests were conducted at 8.2 km/h and at 120mm depth,
and the extent of lateral soil throw and interaction with adjacent furrows would increase at
higher speeds but reduce at shallower depth (Desbiolles and Saunders, 2006). In contrast, as
shown in Fig 5 the bent leg style opener shows that the majority of loosened soil can be
retained within its respective furrow, this is explained by the offset and bevel edge shank at
the soil surface not contributing to lateral soil throw. As a result, the furrow backfill factor
was comparatively assessed at 100%. In operation, the bent leg opener would not throw soil
into adjacent furrows and affect the depth of soil cover over their seed zone nor change their
profile of incorporated pre-emergent herbicide. The bent leg profile used in the experiment
showed a 65 mm wide band of soil near the shank which did not include any tracers, in line
with the action of the bevelled edge shank, and which could be suitable for a seed row
location. Further work is studying the effect of bent leg opener geometry on various aspects
of soil movement. The work is aiming to shed light on the opportunities for opener design
and operational setting to assist with fine tuning the effectiveness of an increasingly
mainstream weed management practice in Australia.
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References
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Derpsch for WANTFA, GRDC and DAFF. 31/03/2005
Desbiolles, J and Saunders, C 2006 Soil throw characteristics of no-till furrow openers: a
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