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Experimental subsoiling by in-line shallow and deep tines.

Introduction

Soil compaction and, consequently, bulk density increase with increasing soil depth due to the equilibrium between the weight of the soil overlying a soil profile and soil compaction within that soil profile (Hartge 1988). This means that soil compaction could occur naturally in virgin soils or subsoil layers; however, in some virgin bush soils in Western Australia, no sign of compaction has been recorded (Lemon 2009). Intensive cropping and grazing and short crop rotations accompanied by inappropriate soil management have led to severe soil compaction in different regions in the world (Hamza and Anderson 2005).

When soil is compacted, the large pores, which are effective in storing and moving water and gases, are reduced in numbers, thus leading to a decrease in the ability of the soil to transport nutrients and water and restricting gaseous exchange (DeJong-Hughes et al. 2001; Hamza and Anderson 2003). However, all soils, and especially sandy soils, can sometimes improve water-holding capacity by subsoil compaction, which can be revealed in dry growing seasons with moderate amounts of compaction (Soane and Van Ouwerkerk1994). Also, root growth and potential soil nutrient uptake is retarded in compacted soil due to increasing soil strength (Venezia et al. 1995; Laker 2001). Compacted soils cannot be treated by chemical means to remove compaction but must either be physically treated, for example by being deep-tipped to the compacted depth (Hamza and Anderson 2005), or have mechanisms such as freeze-thaw, swell-shrink, or biological activity present to reduce the soil strength.

Soil tippers differ in their capacity to loosen the soils, their draft, and their power requirements, depending on the tine shape, tine angle of contact with the soil, and tine arrangement (Slattery and Desbiolles 2003; Slattery et al. 2004). Tine spacing and ripping depth also play an important role in these operations (Riethmuller and Jarvis 1986; Palmer and Mead 1998; Hamza and Penny 2002). Often the draft force of the tillage equipment increases at a much higher force than the proportionate increase in tilling depth, making deeper tillage an expensive process because it involves high energy input (Harrison 1990). This is one of the reasons that most farmers are reluctant to rip their compacted soils. In addition, in rain fed areas such as Western Australia, if the soil is not tipped at the optimum soil moisture, which occurs in the field over a relatively short period, large-sized clods are formed on some soils (Hamza and Penny 2002), and this restricts subsequent farm operations and results in poor crop establishment. After ripping the soil, a controlled-traffic farming system (Blackwell et al. 2003) is best used to avoid recompaction.

Spoor and Godwin (1978) pioneered the technique of a shallow leading tine (SLT) ripper for an offset configuration to reduce the overburden pressure on the following deeper tines. In the SLT technique, they used two sets of tines, each doing part of the job, compared with a single deep tine used in conventional rippers. The shallow tine works ahead of, and between, the deeper tine to break the soil along a short failure surface, rather than along the whole surface as with a conventional ripper. This improves the work efficiency (volume of soil loosened per unit drawbar pull) of deep ripping (Lacey et al. 2001) because it increases the critical depth of the ripping implements (Spoor and Godwin 1978) and decreases the draft force and produces better soil tilth (Kirby and Palmer 1992).

In shallow ripping, tines move only a small volume of soil from a small, disturbed area where the soil moves in the direction of least resistance, usually upward and outward away from the tine. This process can relieve the overburden pressure on a following deeper tine and may reduce the specific draft by >50% (Spoor and Godwin 1978). Specific draft has been shown to be reduced by up to 95% with SLT, but results can be variable, with a tendency for offset leading tines to reduce specific draft with greater depths of ~500 mm compared with in-line leading tines (Palmer and Kirby 1992). The economic advantages of the SLT do not come from the lower draft required by SLT alone but may also come from the elimination of the cost of subsequent operations to break down very large soil clods. Farmer experience shows this is less of a problem when single deep tines are used on deep sands, as surface clods rarely occur. By comparison, deep-tipping tines move a lot of soil and cause failure along a very large surface, disturbing a large area. Because the resistance at the tip of the tine is much higher than that at the soil surface, the soil moves sideward instead of upward or outward, causing soil compaction close to the tine tip, which in turn increases the draft.

The depth at which such compaction occurs is called the critical depth (Spoor and Godwin 1978). Critical depth can be defined as the depth below which soil is not lifted towards the surface but rather is compressed to the sides of the tool and moved in a horizontal plane to form a slot at the lowest depths and a Y-shaped, rather than a V-shaped cross-section of disturbance (McKyes 1985). The inability of a narrow cutting tool to lift soil up over its entire depth (thus defining the critical depth for that particular tool) has long been recognised (Zelenin 1950). The critical depth depends on the width, inclination, and lift height of the tine foot and upon soil moisture and density. The attachment of wings to the tine foot along with using shallow tines to loosen the surface layers ahead of the deep tine increases soil disturbance, particularly at depth, reduces the specific resistance, increases the critical depth, and allows more effective soil rearrangement. Complete soil loosening at depth and smooth soil surfaces can also be assisted by selecting appropriate tine spacing. This spacing can be increased using wings and shallow leading tines (Spoor and Godwin 1978).

Hamza et al. (2011) demonstrated in a similar soil type (i.e. to that used in the present study) that the SLT ripper reduced the draft force by 3.1 kN or 10.5%. However, the economic efficiency of SLT ripper, i.e. specific draft and expected fuel savings, over that of conventional rippers was not determined. The objective of the present study was to investigate the ability of the SLT ripper to reduce the specific draft required for ripping the soil, thus significantly reducing the cost to mitigate or eradicate compaction, which improves the economic efficiency of the SLT compared with the conventional ripper.

Materials and methods

Site and soil

This experiment was carried out at the Merredin Research Station (31[degrees]29'S, 118[degrees]14'E) on a site that was in a pasture--cereal rotation. The site was composed of compacted loamy sand soil (11% clay, 10% silt, and 79% sand). Most of the soil compaction was likely to be caused by farm machinery and sheep grazing. Gravimetric soil moisture was 6.97%, 8.34%, and 8.54 for soil depths 0-10, 10-20, and 20-40 cm, respectively, at the time the treatments were applied. This soil moisture corresponds to ~60% of field capacity.

Experimental apparatus

The ripper and its attachments used in this experiment were similar to those used in Hamza et al. (2011). A ripper (Agrowplow[R], Wellington, NSW) (Fig. 1) with four tine sets arranged side-by-side at a spacing of 44.5 cm was used to rip the compacted soils once and in one direction at a speed of 4.3 km [h.sup.-1] The ripper was pulled by a hydraulic, front-wheel-assist tractor. A drawbar dynamometer (made by Gatton College, University of Queensland) was used to measure the draft force, and it consisted of a floppy link with four bonded strain gauges on a centre element that was protected by a shear pin. The output from the temperature-compensated Wheatstone bridge (with zeroing adjustment) was displayed on a voltmeter calibrated in kilonewtons (kN) and recorded with a video camera. The floppy link angle was <3[degrees] to the horizontal, and so the vertical component of the force was ignored. Tine depth was controlled with stroke control units on the lifting hydraulic cylinders attached to the arm holding the 14.9-28 (SW 378 mm, OD 1367mm; 8-ply R1) tyres.

Soil strength was measured using a CP20 cone penetrometer (Rimik Pty Ltd, Toowoomba, Qld) at standardised soil moisture (soil irrigated then left overnight to reach field capacity, when it almost stops draining). Penetrometer parameters were: depth interval 20 mm, insertion speed 1.83 m [min.sup.-1], cone tip 30[degrees], and cone diameter 12.83 mm (ASAE 2009).

The disturbed soil profile area was measured by digging out by hand the disturbed soil and measuring vertically down from a surface reference across a width of 1.95 m in 3-cm steps across the soil profile.

Treatments

Individual tine arrangements were regarded as treatments (Table 1) and replicated four times. Plot dimensions were 20 m long by 1.78 m wide. The treatments were as follows:

(i) Conventional ripper, one tine at 33 cm depth;

(ii) SLT A, one 8-cm-deep SLT in front of a 33-cm-deep tine;

(iii) SLT B, one 13-cm-deep SLT in front of a 33-cm-deep tine;

(iv) SLT C, one 18-cm-deep SLT in front of a 33-cm-deep tine;

(v) SLT D, two SLTs (13 and 23 cm deep) in front of a 33-cm-deep tine (Figs 2 and 3);

(vi) SLT offset, two 18-cm-deep SLT, 22 cm offset from a 33cm-deep tine (Fig. 4).

All treatments were ripped at ~60% of field capacity (6.97-8.54% soil moisture). At this soil moisture, the soil is relatively easy to plough and soil would not stick to the tines.

Statistical analyses

Results were analysed using GENSTAT (VSN International, Hemel Hempstead, UK) analysis of variance (ANOVA) at P=0.05 for assessing least significant differences (1.s.d.). To analyse the draft force and specific draft force data, ANOVA was also used.

Results and discussion

Soil strength

Soil strength values measured at field capacity and expressed as cone index (CI) in MPa are shown in Fig. 5. All soil strength values are the average of three Cl values measured at the same position and a few centimetres apart. Although the top 2 cm of the topsoil was not compacted, soil compaction increased dramatically until it became highly compacted at ~10 cm deep, exceeding 3.0 MPa, which is beyond the ability of plant roots to grow (Mason et al. 1988; Taylor 1971). However, soil compaction <1.5MPa began to appear after 2cm depth (Fig. 5), which, though still relatively high for optimal plant growth, imposes less restriction on plant root growth.

Draft force

Treatments SLT A (one 8-cm SLT in front of a 33-cm main tine) and SLT B (one 13-cm SLT in front of a 33-cm main tine) showed a significant decrease in draft force of 7.0 kN or 25.3% and 5.9 kN or 21.2%, respectively, relative to the conventional treatment (Table 2). Treatment SLT D (two SLT tines 13 and 23 cm deep in front of a 33-cm main tine) also tended to show a decrease in draft force relative to the conventional treatment, but the difference was not statistically significant and was less than that of treatments SLT A and SLT B. Treatments SLT C (one 18-cm SLT in front of a 33-cm main fine) and SLT offset showed numeric increases in draft force relative to the conventional treatment, but the increase in the SLT C treatment was very small and not statistically significant.

These data clearly demonstrate that one shallow leading tine attached in line and ahead of the main tine could cause a significant reduction in draft force when ripping compacted soil. According to the data reported here, the best depth of the SLT is 2540% less than the depth of the main tine. Hamza et al. (2011) reported almost half the amount of draft force reduction reported here when using SLT, and found a similar pattern of increasing draft force for the SLT offset treatment as found here. This could be attributed to two factors: (1) the difference in soil texture, with the soil in that experiment having more sand and less clay than the soil in the present experiment; and (2) the soil moisture was higher in the present experiment. Soil moisture is very important in determining the draft force in soil because, at all compaction levels, the penetration resistance increases with decreasing soil water potential (Lipiec et al. 2002). That is, increasing soil moisture content causes a reduction in the load support capacity of the soil (Kondo and Dias Junior 1999), thus decreasing the permissible ground pressure (Medvedev and Cybulko 1995). However, draft force shows only how much energy is consumed through ripping the soil; it does not show how much of the soil volume is disturbed. The economy of ripping the soil depends on the amount of force that is applied to rip a given cross-section of soil (which determines the specific draft force).

Specific draft force

Specific draft force (SDF) can be defined as the draft force (DF) used to rip (disturb) a given cross-section of soil (A), thus: SDF= DF/A. Table 3 shows the specific draft force and the disturbed soil areas along with values for changes in the specific draft force of SLT treatments relative to the conventional ripper treatment. The greatest significant decrease in specific draft force of SLT treatments relative to the conventional treatment was in SLT B, at 14.4kN [m.sup-2] or 15.2%, followed by SLT A, at 10.3kN [m.sup-2] or 10.9%. The only other SLT treatment that showed a numeric decrease in specific draft force relative to the conventional treatment was SLT offset, at 4.9 kN [m.sup-2] or 5.2%. However, this decrease was not significant at P= 0.05. The treatments SLT C and SLT D showed an increase (nonsignificant) in specific draft force of 5.4 kN [m.sup-2] or 5.6% and 7.4 kN [m.sup-2] or 7.8%, respectively.

The largest profile area of the soil that was disturbed by SLT treatments was that of SLT offset, but because the draft force used to produce such disturbance was higher than in the other treatments, at 30.7 kN, the specific draft reduction was less than in SLT A and SLT B. Even though SLT B disturbed 0.027 [m.sup.2] more soil than SLT A, it required more draft force than SLT A, by 1.1 kN; however, SLT B was more efficient in its specific draft force. From the current data, the volume of disturbed soil is crucial when the economy of ripping the soil is considered. The reduced draft force required and the larger soil profile area disturbed in a reduced draft force makes SLT B the most economical option to rip the soil.

Comparing the disturbed soil profile data in Table 3 with data from a similar experiment reported by Mielke et al. (1994) is difficult, because they measured only the lifted soil area rather than the disturbed soil area. Also, the two sets of data are hard to compare because those authors attached 300-mm wings to the foot of the shallow shank, and the shapes and configurations of the our SLTs were different from the shapes and configurations of the shanks used in that experiment. In addition, the effective depths were, in general, higher in that experiment.

Economy of using SLT ripper

Fuel consumption (L [ha.sup.-1]) can be shown to be directly related to the specific draft force using the ASAE (2011) standard by assuming the following: a four-wheel-drive tractor is working at full power; the tractor is working on firm soil with a tractive efficiency of 0.76; the disturbed soil area is 0.29[m.sup.2]; the implement has a width of 1.78 m and a speed of 4.3 km [h.sup.-1]. The conventional ripper would then have a fuel consumption of 13.6 L [ha.sup.-1], whereas the SLT B treatment used only 11.5 L [ha.sup.-1], which is a saving of 2.1 L [ha.sup.-1]. Assuming a fuel price of AUD $1.50[L.sup.-1], the SLT B treatment would save $3.15[ha.sup.-1] in fuel. Furthermore, there would be less wear on the tractor due to the lower draft, resulting in a 15.2% increase in work rate with the same tractor power if a wider implement were used.

Soil tilth

Ripping clay soils with a conventional ripper usually produces large soil clods, which interfere with plant germination during subsequent farm operations. The soil clod size is very important for crop establishment; smaller clods are usually associated with a higher crop establishment percentage and a smoother soil surface, leading to a more even distribution of moisture after rain. Large soil clods restrict seed emergence and can lead to a yield decrease. Figure 6 shows the relative size of soil clods resulting from ripping a compacted sandy clay loam soil with a conventional v. SLT ripper. The conventional ripper produced large soil clods ranging, in general, from 20.4 to 85.1 [cm.sup.2] in surface area, whereas the SLT ripper produced much smaller soil clods ranging from 2.47 to 24.5 [cm.sup.2] in surface area. After using the SLT ripper, most of the soil clods were smaller after the first rain due to the action of water, and the soil surface appeared smoother, ensuring a better seedbed for germination and establishment. In the plots treated with the conventional ripper, the clods were also reduced in size by the first rainfall but were still large enough to prevent homogenous germination and even distribution of rain. Hamza et al. (2011) reported a similar pattern of reduction of soil clod size between the conventional and the SLT ripper. Those data showed a slightly larger clod size, which may have been due to the lower soil water content.

Since soil ripping produces large pore spaces and results in lower soil bulk density, water storage of the soil increases, leading to a greater length of time for sowing before the soil dries out.

Conclusion

Farmers are reluctant to reduce soil compaction by ripping because of the high cost involved, high fuel consumption, and time with adequate soil moisture. The SLT ripper uses significantly less force and therefore renders ripping of compacted soil less costly by reducing the fuel consumption by up to 2.1 L [ha.sup.-1] as well as reducing implement and tractor depreciation. Thus, the ripping work rate (ha [h.sup.-1]) can be higher for the same tractor power with an SLT ripper, and the soil moisture 'window' for ripping is wider. The most effective tine configuration of those studied is two tines in line. The first tine is the shallow leading tine, around 25-40% shallower than the main tine. The second tine or main tine rips the soil to the required depth. The reduction in specific draft force ranged between 11 and 15% compared with the conventional ripper. Furthermore, the SLT ripper produced much smaller soil clods, resulting in improved plant establishment and water distribution after rain.

http://dx.doi.org/10.1071/SR13129

Received 24 April 2013, accepted 10 August 2013, published online 6 November 2013

Acknowledgements

This study was supported by the Grains Research and Development Corporation and the Department of Agriculture and Food, Western Australia. The authors thank Paul Blackwell for reviewing the manuscript and for his constructive suggestions and comments, and Kevin Boyd, Laurie Maiolo, Laurie Mackay, Gavin Elms, and the manager and staff of the Merredin Research Station for their technical assistance.

References

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M. A. Hamza (A,D), G. P. Riethmuller (B), and W. K. Anderson (C)

(A) Department of Agriculture and Food, Albany, WA 6330, Australia; Current address: 31 Ardeana Crescent, Yakamia, Albany, WA 6330, Australia.

(B) Department of Agriculture and Food, Dryland Research Institute, Merredin, WA 6330, Australia.

(C) School of Plant Biology, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.

(D) Corresponding author. Email: mhamza1@outlook.com

Table 1. Tine configurations (tine depth, cm) for different
treatments for the conventional and shallow leading tine
(SLT) ripper

For SLT offset, the SLT was 22 cm offset from the main tine

                  Main                 SLT
Treatment      First tine   Second tine   Third tine

Conventional       33
SLT A              33            8
SLT B              33           13
SLT C              33           18
SLT D              33           23            13
SLT offset         33           18

Table 2. Comparison of draft forces for the shallow leading
tine (SLT) and conventional rippers

Differences in draft forces between the SLT treatments and the
conventional treatment are shown in kN and as percentage
changes (%). Bold values refer to significant decreases or
increases in draft forces of the SLT treatments relative
to that of conventional treatments

Treatment         Draft force   Change in draft   Change in draft
                     (kN)         force (kN)         force (%)

Conventional         27.8
SLT A                20.8            -7.0*            -25.3*
SLT B                21.9            -5.9*            -21.2*
SLT C                28.0             0.2               0.7
SLT D                26.1             1.7              -6.1
SLT offset           30.7             2.9              10.5

I.s.d. (P=0.05)       3.4

Note: Values refer to significant decreases or increases
in draft forces of the SLT treatments relative to that
of conventional treatments indicated with *.

Table 3. Disturbed soil profile area and specific draft force (SDF)
for all shallow leading tine (SLT) treatments and the conventional
treatment

Decreases in SDF between SLT treatments and the conventional
treatment are shown as kN [m.sup.-2] and as percentage changes.
Bold values refer to significant decreases or increases in the SDF
of the SLT treatments relative to that of conventional treatments

Treatment         Disturbed surface         SDF         SDF difference
                  area ([m.sup.2])    (kN [m.sup.-2])        (%)

Conventional            0.293              94.8
SLT A                   0.246              84.5             -10.9*
SLT B                   0.273              80.4             -15.2*
SLT C                   0.280             100.2               5.6
SLT D                   0.255             102.2               7.8
SLT offset              0.342              89.9              -5.2
I.s.d. (P=0.05)                             7.8

Note: Values refer to significant decreases or increases in the SDF
of the SLT treatments relative to that of conventional treatments
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Author:Hamza, M.A.; Riethmuller, G.P.; Anderson, W.K.
Publication:Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Sep 1, 2013
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