Changes in the properties of a Vertisol and responses
of wheat after compaction with harvester traf®c
B.J. Radford
a, B.J. Bridge
b, R.J. Davis
c, D. McGarry
d,*,
U.P. Pillai
e, J.F. Rickman
f, P.A. Walsh
g, D.F. Yule
haQueensland Department of Natural Resources, LMB No. 1, Biloela, Qld 4715, Australia
bHonorary Research Fellow, CSIRO Division of Land and Water, PO Box 2282, Toowoomba, Qld 4350, Australia cBureau of Sugar Experiment Stations, PO Box 651, Bundaberg, Qld 4670, Australia
dQueensland Department of Natural Resources, Meiers Road, Indooroopilly, Qld 4068, Australia eAgriculture Department, The University of Queensland, Brisbane, Qld 4072, Australia fQueensland Department of Primary Industries, PO Box 597, Dalby, Qld 4405, Australia
gKondinin Group, 26 The Esplanade, Wagga Wagga, NSW 2650, Australia
hQueensland Department of Natural Resources, PO Box 736, Rockhampton, Qld 4702, Australia
Received 12 July 1999; received in revised form 18 November 1999; accepted 6 December 1999
Abstract
Soil compaction has been recognised as the greatest problem in terms of damage to Australia's soil resource. Compaction by tractor and harvester tyres, related to traf®cking of wet soil, is one source of the problem. In this paper an array of soil properties was measured before and immediately after the application of a known compaction force to a wet Vertisol. A local grain harvester was used on soil that was just traf®cable; a common scenario at harvest. The primary aim was to determine the changes in various soil properties in order to provide a ``benchmark'' against which the effectiveness of future remedial treatments could be evaluated. A secondary aim was a comparison of the measurements' ef®ciency to assess a soil's structural degradation status. Also assessed was the subsequent effect of the applied compaction on wheat growth and yield in the following cropping season. Nine of the soil properties measured gave statistically signi®cant differences as a result of the soil compaction. Differences were mostly restricted to the top 0.2 m of the soil. The greatest measured depth of effect was decreased soil porosity to 0.4 m measured from intact soil clods. There was 72% emergence of the wheat crop planted into the compact soil and 93% in the uncompact soil. Wheat yield, however, was not affected by the compaction. This may demonstrate that wheat, growing on a full pro®le of stored soil water as did the current crop, may be little affected by compaction. Also, wheat may have potential to facilitate rapid repair of the damage in a Vertisol such as the current soil by drying the topsoil between rainfall events so increasing shrinking and swelling cycles. If this is true, then sowing a suitable crop species in a Vertisol may be a better option than tillage for repairing compaction damage by agricultural traf®c.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Compaction; Bulk density; Hydraulic conductivity; Penetration resistance; Soil shear strength; Soil deformation
*Corresponding author. Tel.:61-7-38969566; fax:61-7-38969591.
E-mail address: [email protected] (D. McGarry)
1. Introduction
Worldwide, soil compaction is said to affect 68 million hectares of land principally from vehicular traf®c (Flowers and Lal, 1998). In the Australian context, soil compaction has been ranked as the great-est problem in terms of damage to Australia's soil resource (Williams, 1998) and is ubiquitous to all soils and farming systems across several regions of at least one State (McGarry, 1993). Alakukku (1996a,b) and Wu et al. (1997) link the increase in the weight of farm machinery in recent decades with progressive subsoil damage and review work on persistent subsoil physi-cal degradation. Tullberg (1990) estimated that over 30% of ground area is traf®cked by the tyres of heavy machinery even in genuine zero tillage systems (one pass at sowing). Under the more realistic circum-stances of minimum tillage the ®gure is likely to exceed 60% and in conventional tillage practice it almost certainly exceeds 100% during one cropping cycle (Soane et al., 1982; Erbach, 1986). The ®rst pass of a wheel is known to cause a major portion of the total soil compaction (Burger et al., 1983; Koger et al., 1983; Pollock et al., 1984). The depth of the effect of compaction varies widely. Flowers and Lal (1998) review works where increased bulk density has been measured in the 0.1±0.35, 0.05±0.1, to 0.5 and to 0.6 m layers.
There is often a tenuous link between measuring and/or observing soil compaction and measuring asso-ciated reductions in crop yield due to complex inter-actions between crop growth, soil properties and seasonal weather conditions (Earl, 1997; Connolly, 1998). Flowers and Lal (1998) give examples where high axle loads have both in¯uenced and not in¯u-enced yields of subsequent crops. In a review of crop susceptibility to compaction in the tropics, Kayombo and Lal (1994) do not present wheat (the crop grown in the current experiment) as susceptible. Pillai and McGarry (1999), assessing the relative ability of four tropical crops to biologically alleviate soil compaction of a Vertisol, found that wheat was relatively slow in ameliorating a compacted upper subsoil.
Damage from high axle loads increases when the soil is wet because wet soil has reduced strength (Kirby and Kirchhoff, 1990). Wheel slip also increases wheel damage (Soane et al., 1981) due to shear processes in the soil, which occur particularly when
the soil is wet (Kirby and Kirchhoff, 1990). High tyre in¯ation pressures also increase wheel damage (Soane et al., 1981; Rickman and Chanasyk, 1988). In a study that aimed to determine the soil moisture contents at which a sandy loam and clay loam are most compac-tible, Mapfumo and Chanasyk (1998) concluded that the ®ner textured clay loam was compactible over a wide range of moisture contents Ð from ®eld capacity to below the plastic limit.
In this paper several soil properties were measured before and immediately after the application of a known compaction force to a wet clay soil. The primary aim was to determine the change in these soil properties to relate changes with wheat yield measured in the following cropping season. A sec-ondary aim was a comparison of the measurements' ef®ciency to assess a soil's structural degradation status. Future work will measure the nature and rate of repair of the structure degradation at the experi-mental site under various, practicable farm manage-ment strategies.
2. Methods and materials
2.1. The site, soil and cropping system
The site is located in a ®eld adjoining the Queens-land Department of Primary Industries Research Sta-tion, Biloela, Qld, Australia (latitude 248220
S, longitude 1508310
Some physical and chemical properties of the soil pro®le (0±1.5 m)
Depth increment (m) Soil property
PSAa R1b pHc ECd
(mS cmÿ1)
Cle (mg kgÿ1)
CECf (meq%)
ESPg (%)
Organic Ch(%)
Total Ni(%)
Pj (mg kgÿ1)
CS (%) FS (%) S (%) C (%)
0±0.05 3 19 31 45 0.39 7.3 0.157 7 30 0.64 1.7 0.12 96
0.05±0.1 3 19 28 48 0.40 7.3 0.152 6 34 0.67 1.6 0.11 94
0.1±0.2 2 19 27 53 0.59 7.8 0.126 18 36 1.30 1.2 0.07 70
0.2±0.3 2 18 27 51 0.58 7.9 0.128 33 33 1.50 0.9 0.06 57
0.3±0.4 2 17 22 55 0.63 8.0 0.145 50 35 1.70 1.0 0.06 52
0.4±0.5 1 21 24 53 0.63 8.2 0.167 65 ± ± ± ± ±
0.5±0.6 3 22 24 51 0.66 8.3 0.189 85 ± ± ± ± ±
0.6±0.9 3 25 26 47 0.65 8.5 0.270 146 ± ± ± ± ±
0.9±1.2 3 30 21 43 0.68 8.4 0.351 269 ± ± ± ± ±
1.2±1.5 7 37 20 35 0.73 8.3 0.386 373 ± ± ± ± ±
aParticle size analysis: CS, coarse sand (>200mm); FS, ®ne sand (20±200mm); S, silt (2±20mm); C, clay (<2mm). bDispersion ratio±%(siltclay) dispersed in water% total (siltclay) (Bruce and Rayment, 1982).
c1:5 pH (water).
d1:5 electrical conductivity. e1:5 extractable chloride.
fCation exchange capacity (pH 8.5). gExchangeable sodium percentage (pH 8.5). hOrganic carbon (Walkley and Black, 1934). iTotal nitrogen (Kjeldahl digest).
jBicarbonate extractable P (Colwell, 1963).
B.J
.
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/Soil
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illage
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155±170
and chemical data from the site are given in Table 1. In the 0±0.2 m layer, the liquid limit (drop cone), plastic limit (rolling bead) and ÿ1.5 MPa (wilting point of <2 mm sieved soil by pressure plate) values are 56, 22 and 17% gravimetric water content, respectively. The mean annual rainfall at the site is 685 mm with 73% falling in the summer months (October±March). The mean annual evaporation (from a class ``A'' pan) is 1868 mm.
2.2. Experiment design
The compaction treatment applied to the soil con-sisted of traf®cking a wet soil with a harvester, running on lugged rubber tyres. This procedure is locally considered a prime cause of soil compaction. Harvest-ing under such wet soil conditions is a short-term economic necessity, when grain is ready for harvest and threatening summer storms are liable to cause substantial yield losses.
Compaction was applied with a Ford New Holland 8060 harvester, loaded with a full bin of sorghum grain. Average mass on each front wheel of the harvester was 4925 kg and on each rear wheel was 990 kg, giving a 10 Mg and 2 Mg load on the front and rear axles, respectively. Five days before compacting, in mid-April 1993, the soil was wetted with 50 mm of spray irrigation. In the ®ve days after wetting, there was zero rainfall and 32 mm of evaporation from a class ``A'' pan. The entire area of each compacted plot received a single pass of the front wheel of the harvester by spacing successive passes one front wheel spacing apart. In this way, uniform compaction was applied to the entire plot. The experimental treatment is not considered unrealistic because ran-dom wheel traf®c during a fallow (conducted to cultivate-out weeds) typically covers most of the ground area. The rear tyre tracks were superimposed on part of the front tyre marks but the rear tyres made little impression on the soil surface following the pass of the front tyres. In¯ation pressures were deliberately kept high (but not unrealistically high) to maximise compaction: 235 kPa (front) and 205 kPa (rear). Tyre widths were 587 mm (front) and 284 mm (rear), and tyre diameters 1605 mm (front) and 1103 mm (rear). The radius of the loaded front tyres was 710 mm. Maximum lug height on the tyres was 65 mm (front) and 25 mm (rear). The lugs comprised approximately
16% of the surface area of the front tyres. Operating speed was 7 km hÿ1
and wheel slip was negligible. The energy input of the front tyres was calculated to be 12 kJ mÿ2
.
2.3. Soil measurements
All soil properties were measured ®rst in the uncompact soil and then in the compact soil imme-diately after applying the compaction treatment. The following soil properties were measured.
Surface elevation with a dumpy level and staff at 24 locations before and after compaction. Elevations after compaction were measured both in the bottom of the tyre lug marks and in the interlug area.
Soil bulk density at (a) 48 locations with a 0.04 m diameter coring tube, at 0.05 m depth intervals from 0 to 0.2 m and 0.1 m intervals from 0.2 to 0.6 m; (b) 11 locations by twin probe gamma ray density meter (Campbell Paci®c Nuclear Model MC-24S Strati-gauge) in 0.05 m intervals to 0.5 m.
Soil shear strength by torsional shear box (Lloyd and Collis-George, 1982) at three locations and seven depths (0, 0.02, 0.05, 0.1, 0.15, 0.2 and 0.3 m), the soil increment sampled being 3 mm below each nominated depth. Either a 50 mm or 75 mm diameter shear cylinder (the size chosen as conditions dictated) was used to take the reading. Three measurements were taken at each depth in the uncompact soil and four measurements at each depth in the compact soil. Shear strengths were analysed by univariate analysis of variance.
Penetration resistance by Geotester pocket penet-rometer (with tip diameters of 2, 6.45, 10 and 15 mm chosen as soil conditions dictated) at 10 locations at the same depths used for the shear strength measure-ments.
Soil water content (gravimetric) at the same depth intervals (and on the same samples) used for the bulk density determinations.
weeks of equilibration, the water potential of the soil was obtained by weighing (wet and oven-dry) the middle ®lter paper. Water potentials were analysed by univariate analysis of variance.
Cone index by recording cone penetrometer in 0.03 m increments to 0.3 m at 40 locations (three sets of readings per location). Post-compaction measure-ments were taken in the inter-lug areas only. The penetrometer was ®tted with a standard 308circular stainless steel cone of 12.83 mm diameter with a 9.83 mm shaft (ASAE Standards, 1992).
Hydraulic conductivity by disc permeameter (Per-roux and White, 1988) at depths of 0 (soil surface) and 0.1 m. The four supply tensions correspond to the ®lling of soil pores with effective diameters smaller than 3, 1.5, 1 and 0.74 mm, respectively (Coughlan et al., 1991). Eight locations were tested before and after compaction. For each measurement, a level site was selected, a 200 mm diameter3 mm high ring placed on the undisturbed surface, and moist contact sand put in the ring and smoothed ¯at. The ring was removed, a disc permeameter placed on the sand pad, and the ¯ow of water into the soil measured with time. Supply tensions ofÿ40,ÿ30,ÿ20 andÿ10 mm H2O
were tested in succession on the same sand pad. At the completion of the test, the sand pad was scraped off the soil surface, and the water content of a sample of the wet soil was determined gravimetrically. An adja-cent sample of dry soil was also taken for determina-tion of its water content. The cumulative in¯ow of water into the soil was plotted against the square root of time, and the sorptivity calculated from the slope of the line at small values of time (Smettem and Clothier, 1989). This analysis gave conductivity values for mean supply tensions of ÿ35, ÿ25 and ÿ15 mm H2O together with an estimate of saturated
conduc-tivity (zero supply tension). The cumulative in¯ow was also plotted against time, and the hydraulic con-ductivity calculated from the slope of the line at large values of time (Smettem and Clothier, 1989). Esti-mates of pore numbers per unit area were calculated from the same ®eld data. By equating Darcy's Law and Poiseulle's Law, the change in hydraulic conductivity with each change in permeameter supply tension can be translated into an equivalent number of straight cylindrical pores of a certain size class (Coughlan et al., 1991). The size classes were 3±1.5 mm dia-meter, 1.5±1 mm diameter and 1±0.74 mm diameter.
Clod shrinkage was determined on soil clods (0.05± 0.1 m diameter), sampled within 0.05 m intervals from 0 to 0.2 m and 0.1 m intervals from 0.2 to 0.4 m. Three clods were collected from each depth increment at three locations in each of the lug and non-lug areas of the compact soil, and in the uncompact soil. Clod bulk density and clod shrinkage were determined using the Saran1 resin technique (McGarry and DanielIs, 1987). All clods were Saran1-coated at the sampled water content. The following ®ve parameters were derived from lines ®tted to the shrinkage data (plots of the reciprocal density of the soil clod vs. water content) after Cough-lan et al. (1991):
r the slope of the line in residual shrinkage
n the slope of the line in normal shrinkage yA the soil water content at the onset of
residual shrinkage (Mg Mgÿ1
)
Pa the specific volume of air-filled pores at zero water content (m3Mgÿ1
)
PA the specific volume of air-filled pores at the onset of residual shrinkage (m3Mgÿ1
).
sampling sleeves was determined by oven-drying (1058C) a 0.05 kg soil sample collected from ®ve depths (0, 0.075, 0.15, 0.2 and 0.3 m) down the pro®le of the small soil pits that remained after excavation of the soil sampling sleeves. The data for each of the four soil structure attributes were analysed by univariate analysis of variance. To facilitate the analysis of variance, data were averaged across 10 mm depth increments (McGarry et al., 2000) and plotted at 5, 15, 25 mm, etc. to 215 mm.
Soil deformation was determined at 0.05 m inter-vals upwards from a depth of 0.35 m using a pin displacement method (Van den Akker and Stuiver, 1989). Positioning of the point grid in the soil involved the excavation of a pit 0.55 m deep0.5 m wide 1.1 m long. Matchsticks were inserted in the two long sides of the pit (which were perpendicular to the direction of travel) in a 0.05 m square grid pattern using a perspex sheet with 3 mm diameter holes. The perspex sheet was levelled on two datum pegs inserted at the bottom corners of the pit. These two pegs are placed at a depth where it is assumed that there will be no soil deformation. In the current experiment, the assumption was veri®ed by checking the depth of the pegs before and after the passage of the harvester using a datum external to the pit. A simple jig was used to insert the matchsticks into the faces of the walls to ensure horizontal insertion beyond the sheet. The per-spex sheet was then removed, the pit re®lled, and the soil compacted to as close as possible to the density of the original undisturbed soil. After compaction, the pit was re-excavated, and the ®nal positions of the match-sticks recorded on a sheet of plastic ®lm. The relative vertical and horizontal movements of each matchstick were calculated, and the ®nal positions were super-imposed on the original grid. Results are reported for the second pit face traversed by the harvester wheels as this face is less likely to be damaged by the pass of the harvester, by being pushed out into the pit.
2.4. Cropping detail and crop measurements
Wheat (cv. Hartog) was sown in compacted and uncompacted plots at 70 kg haÿ1
in 275 mm rows with a zero till planter on 7 June 1993 (48 days after compaction). Rainfall/irrigation between compaction and sowing totalled 63 mm. The experimental design comprised two replications of a randomised block of
two main plots (rainfed and irrigated) split for two control (uncompact) and 12 compact subplots. Four of the compact subplots were rotary hoed to a depth of 0.1 m, 27 days after compaction. Subplots measured 309 m2. The rainfed plots received 209 mm of rain between sowing and harvest, and the irrigated plots an additional 75 mm of spray irrigation at anthesis.
Crop emergence percentage was counted as the number of wheat seedlings in 25 m lengths of row 14 days after sowing in the eight uncompact and 40 compact subplots planted to wheat. Emergence per-centage was based on the seed output from the planter. Above-ground dry matter at anthesis was recorded in a 1 m2area, sampled in each subplot sown to wheat 78 days after sowing, and the samples were dried (408C) to constant weight. Grain yield was obtained by using a small plot harvester to harvest 56 m2 per subplot with grain yields standardised to 12% moisture con-tent. Soil water storage at sowing was determined in 0.1 m increments to 1.5 m by neutron moisture meter in one access tube per subplot.
3. Results
3.1. Surface depression
The harvester tyres depressed the soil surface an average of 30 mm in the areas between the lug marks and 86 mm at the deepest part of the lug marks. The difference (56 mm) is comparable to the maximum lug height of the tyres (65 mm). Lug marks comprised 16% of the depressed surface area. The depth of soil surface depression was also measured by means of a benchmark at the bottom of the soil displacement pit. Mean vertical displacement of the surface as deter-mined by this method was 33 mm.
3.2. Soil water content and soil water potential
sig-ni®cantly between the compact and uncompact soil (Fig. 1). Hence changes in strength below 0.02 m can be compared directly.
3.3. Soil bulk density
The applied compaction gave a statistically signi®-cant increase in bulk density to a depth of 0.11 (soil cores) and 0.16 m (gamma probe) below the original soil surface (Fig. 2).
3.4. Soil shear strength
The applied compaction treatment signi®cantly increased torsional shear strength to a depth of 0.15 m below the tyre lugs and to 0.2 m below the non-lug areas (Fig. 3). At 0.3 m, the shear strength of the soil was not affected by the compaction.
3.5. Penetration resistance
Compaction signi®cantly increased pocket penet-rometer resistance to a depth of only 0.1 m in both the lug and non-lug areas of compaction (Fig. 4). Below 0.15 m, the penetration resistance was unchanged by the compaction.
3.6. Cone index
Compaction signi®cantly increased cone index to a depth of 0.18 m in two zones: 0±0.115 m and 0.13± 0.18 m below the original soil surface (Fig. 5).
3.7. Soil deformation
Compaction caused vertical displacement of match-stick markers to a depth of 0.35 m (Fig. 6). This result suggests that the marker displacement technique is a sensitive measure of the effects of compaction on soil. However, small vertical displacements of markers do not necessarily imply soil structural degradation to an extent suf®cient to inhibit the growth of plant roots.
3.8. Hydraulic conductivity and pore numbers
Hydraulic conductivity of the uncompact soil was greater both at the soil surface (0 m) and the 0.1 m depth for the 15 and 0 mm suctions (Fig. 7). The decrease in hydraulic conductivity from the compac-tion was greater at 0.1 m than on the soil surface; approximately a one-third reduction at each of the 35, 25 and 15 mm average suctions. Compaction decreased the number of pores per unit area in each of the three size ranges at both the 0 and 0.1 m depths (Table 2). The decrease was again greatest at 0.1 m with compaction giving up to a eight-fold reduction in pore numbers.
3.9. Clod shrinkage
Results of the analysis of variance are summarised in Table 3. All ®ve parameters measured gave a signi®cant treatmentdepth interaction with three being strongly signi®cant (p<0.001). Pa gave the greatest number of signi®cant main effects and inter-actions (equal with PA) and represents the overall
trend for the clod results. The means of the signi®cant Fig. 1. Soil matric potential at time of sampling. The notation A:B,
treatmentdepth interaction forPaare given in Fig. 8 where data points are plotted at the mid-point of each sample layer. The compact soil had signi®cantly less air from 0 to 0.35 m (i.e. in the 0 to 0.4 m sampled layer) than the uncompact soil. To 0.175 m, the com-pact soil was strongly signi®cantly (p<0.001) less porous than the compact soil. From the soil surface to 0.2 m, the tyre lugs signi®cantly reduced porosity compared with the non-lug areas.
not be plotted (Fig. 10). For each attribute, three layers were evident with an obvious reversal in trend in the data between the zones. In the top 30 mm, the applied
compaction did not signi®cantly change porosity; from 30 to 100 mm compaction signi®cantly (p<0.01) reduced porosity; and below 95 mm there was again no change in porosity from the compaction. The very low porosity below 100 mm was principally Fig. 3. Torsional shear strength to 0.3 m for the uncompact and
compact soil (both lug and non-lug). The notation A:B, etc. is used to depict comparisons of treatment A with treatment B, etc.
Fig. 4. Penetration resistance to 0.3 m using a pocket penetrometer for the uncompact and compact soil (both lug and non-lug). The notation A:B, etc. is used to depict comparisons of treatment A with treatment B, etc.
Table 2
The number of soil pores per unit area of three size classes as calculated from the disc permeameter data
Depth (m) No. of pores per square meter
Uncompacted Compacted
(a) (0.74±1.0 mm diameter)
0 92.9 49.2
0.1 55.7 20.9
(b) (1.0±1.5 mm diameter)
0 33.5 17.3
0.1 25.4 6.25
(c) (1.5±3.0 mm diameter)
0 5.42 1.29
0.1 5.38 0.678
Table 3
Results of the analysis of variance for the ®ve clod parameters with main effects and interactions expressed as probability levels of the F(variance ratio) statistic
Parameter Treatment Depth Treatmentdepth
r ** ** *
n *** *** **
yA ** *** ***
Pa *** *** ***
PA *** *** ***
caused by the low water potentials (Fig. 1) that would have caused cracks and pores to close. There may, however, have been some residual compaction in the soil from past cultivations. The compaction treatment signi®cantly (p<0.01) reduced values of surface area in the 20±110 mm layer, due to the loss of the small pores, and signi®cantly increased solid star length in
the 20±110 mm layer, demonstrating the greater pro-portion of solid in that layer.
3.10. Pro®le water content at sowing
There were no treatment effects on the ®nal quantity of water stored in the soil (0±1.5 m) at sowing, which Fig. 5. Penetration resistance to 0.3 m using a RIMIK recording penetrometer for the uncompact and compact soil. The depths with signi®cant differences (p<0.05) between the two treatments are notated with a *.
averaged 545 mm. The demonstrated changes in soil hydraulic conductivity after compaction either did not affect storage of the 63 mm of rainfall between com-paction and sowing or had effects which were too small to detect. However, this result facilitated mea-surement of the effects of the compaction treatment on crop growth and yield without the complicating effect of different amounts of stored soil water in the soil pro®le of each treatment.
3.11. Wheat emergence
The compaction treatment signi®cantly reduced seedling emergence from 93 to 72%. It should be noted that a state-of-the-art zero till planter was used and sowing was done at the optimum time. Under less optimum sowing conditions, the difference between the compacted and uncompacted treatments would have been greater. The rotary hoeing operation in the compacted treatment did not alter emergence percentage.
As a result of the differences in emergence, the established wheat plant populations averaged 1.8 mil-lion plants per hectare in the control and 1.4 milmil-lion per hectare in the compacted treatment. This differ-ence in plant population is unlikely to affect grain yield in central Queensland (Stevens, 1965).
Fig. 7. Hydraulic conductivity from disc permeameters, averaged between the four ®eld suctions and predicted for saturation (0 mm suction) for the compact and uncompact soil at each of two depths.
Fig. 8. The means of the clod shrinkage parameter Pa for the
3.12. Plant dry matter at anthesis
The compaction treatment had no signi®cant effect on the mass of above-ground dry matter at anthesis, which averaged 5746 kg haÿ1
. Rotary hoeing of the compacted treatment had no signi®cant effect on plant dry weight.
3.13. Grain yield
The compaction treatment had no signi®cant effect on grain yield, and rotary hoeing after compaction had no signi®cant effect on yield. Mean grain yield was 5242 kg haÿ1
.
4. Discussion
Our compaction treatment by harvester traf®c, con-sidered locally to be representative, affected the soil to a maximum depth of 0.4 m. Reviewing the results, statistically signi®cant changes in measured properties of this Vertisol occurred to a depth of 0.1 m (porosity from image analysis), 0.1 m (pocket penetrometer resistance), 0.11 m (surface area and solid star length from image analysis), 0.11 m (empirical bulk density), 0.16 m (gamma probe bulk density), 0.18 m (cone pene-trometer resistance), 0.2 m (torsional shear strength) and 0.4 m (the soil clod parameter,Pa). Displacement of matchstick markers occurred to a depth of 0.35 m but displacements may well have been an artefact of the experimental technique, which involved prior loosening of the soil to a depth of 0.55 m. All vertical displacement at the depth of 0.35 m was upwards, indicating that the loosened soil in the pit was churn-ing rather than solely movchurn-ing downward. Walsh (1994) has developed a modi®ed technique that provides the accuracy of direct measurement of soil displacements while minimising disturbance prior to compaction.
Pa determined from the clod shrinkage measure-ments provided a most sensitive measure of change in this soil resulting from applied compaction. Discri-mination between compact and uncompact soil was obtained from the soil surface to the 0.3±0.4 m layer, and between lug and non-lug areas to the 0.1±0.15 m layer. It remains speculative why this one parameter proved more sensitive than the other techniques. Pre-vious studies have also found Pa to be a sensitive measure of soil structure degradation (Coughlan et al., 1991) and soil structure repair (Pillai and McGarry, 1999) of Vertisols.
which resulted in poor seed±soil contact and poor coverage of the seed with soil. Local farmers and agronomists have commented that much larger reduc-tions in emergence would have occurred in the com-pact treatment if traditional wide sowing points on spring-loaded tines had been used.
Nevertheless, the results presented here show no adverse effects of compaction on crop growth or yield at a level of grain yield which was more than twice the district average (Harris and Walters, 1993). It is speculated that under certain seasonal growing con-ditions wheat can tolerate compaction by heavy wheel traf®c. In the current experiment the experimental site had not been cropped in the 18 months prior to the experiment and fallow weed control had been strictly controlled. The current wheat crop, therefore, was planted on a full pro®le of soil water. Lal and Tanaka (1992) recognised that the magnitude of the compac-tion effect on crop yield depends on the soil texture and the during season moisture regime. Soane and van Ouwerkerk (1994) noted that the negative effects of soil compaction on crop production can often be compensated by an increased supply of soil water (and nutrients) as well as by during season weather cycles. McGarry (1990, 1995) reported a strong nega-tive response on the yield of a cotton crop growing in a compacted soil where irrigation water was withheld. In the current experiment, slow early growth of the wheat was observed, though not measured. Growth retardation in the compact surface soil may have conserved soil water in the early stages of crop growth, leaving more available at critical stages such as anthesis later in the season. The wheat plants may also have facilitated some level of repair of the compaction by drying the topsoil between rainfall events and increasing the number of wetting and drying cycles and hence shrinking and swelling cycles in this Vertisol. If this is true, then sowing a suitable crop species in Vertisols, which by nature do have varying levels of inherent repair potential (McGarry, 1996), may be a better option than tillage for repairing compaction damage by agricultural wheel traf®c. Kayombo and Lal (1994) discussed the use of plant root systems to alleviate soil compaction and Pillai and McGarry (1999) demonstrated the effect with four crops on intact cores from a Vertisol in the laboratory. Biological repair by growing a crop is also a more productive option than mechanical repair by tillage
implements as the crop is potentially harvestable and sellable. To improve soil structure resulting from harvester damage, Sommer and Zach (1992) advo-cated a combination of soil loosening (susbsoiling) that is then stabilised with a cover crop.
These results provide no support for the costly mechanical repair practice of deep ripping, in which soil is loosened to depths as great as 0.5 m to remove a so-called ``hardpan''. Additionally, no data presented here support the presence of a ``hardpan'' in this soil Ð i.e. a layer above and below which soil structure condition is better. Rather, compaction as produced by the harvester tyres was most severe at the surface and differences in physical condition between the compact and uncompact diminished with depth. Hence, based on the soil examined here and the measurements undertaken, unless a prescription for deeper and worse agricultural compaction damage can be presented, it is recommended that depth of tillage should not exceed 0.2 m below the soil surface.
5. Conclusions
The traf®cking of a moist±wet Vertisol by a laden harvester gave a signi®cantly poorer structural state to a maximum depth of 0.4 m. There was a large varia-tion in depth of signi®cant effect between measures of structural state. Soil porosity derived from soil clod shrinkage data provided the greatest depth of effect. Wheat yield of the subsequent crop was not reduced by the applied compaction perhaps as soil water was not an additional limiting factor.
Acknowledgements
We thank the Land and Water Resources Research and Development Corporation (LWRRDC) and the Grains Research and Development Corporation (GRDC) for funding; Mr. Alan Key for assistance with the ®eld work.
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