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Australian Journal
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Aust. J. Soil Res., 2001, 39, 239–247
? CSIRO 20010004-9573/01/02023910.1071/SR00017Traffic and residue cover effects on infiltration
Yuxia Li A , J. N.Tullberg A , D. M.Freebairn B
A School of Agriculture and Horticulture, The University of Queensland, Gatton, Qld 4343, Australia;email: S002476@https://www.doczj.com/doc/ad3505367.html,.au
B Queensland Department of Natural Resources, PO Box 318, Toowoomba, Qld 4350, Australia.Abstract
Wheel traffic can lead to compaction and degradation of soil physical properties. This study, as part of a study of controlled traffic farming, assessed the impact of compaction from wheel traffic on soil that had not been trafficked for 5 years. A tractor of 40 kN rear axle weight was used to apply traffic at varying wheelslip on a clay soil with varying residue cover to simulate effects of traffic typical of grain production operations in the northern Australian grain belt. A rainfall simulator was used to determine infiltration characteristics.
Wheel traffic significantly reduced time to ponding, steady infiltration rate, and total infiltration compared with non-wheeled soil, with or without residue cover. Non-wheeled soil had 4–5 times greater steady infiltration rate than wheeled soil, irrespective of residue cover. Wheelslip greater than 10% further reduced steady infiltration rate and total infiltration compared with that measured for self-propulsion wheeling (3% wheelslip) under residue-protected conditions. Where there was no compaction from wheel traffic, residue cover had a greater effect on infiltration capacity, with steady infiltration rate increasing proportionally with residue cover (R 2 = 0.98). Residue cover, however, had much less effect on infiltration when wheeling was imposed.
These results demonstrated that the infiltration rate for the non-wheeled soil under a controlled traffic zero-till system was similar to that of virgin soil. However, when the soil was wheeled by a medium tractor wheel, infiltration rate was reduced to that of long-term cropped soil. These results suggest that wheel traffic, rather than tillage and cropping, might be the major factor governing infiltration. The exclusion of wheel traffic under a controlled traffic farming system, combined with conservation tillage, provides a way to enhance the sustainability of cropping this soil for improved infiltration, increased plant-available water,and reduced runoff-driven soil erosion.
Additional keywords: conservation tillage, controlled traffic, soil compaction, wheeling, wheelslip, rainfall simulator.
Introduction
In dryland crop production with highly variable, summer-dominant rainfall, crop yield depends on soil water stored during fallow (Freebairn et al . 1986a ), so reduced infiltration can limit yield. Runoff associated with low infiltration can also cause soil erosion (Freebairn et al . 1986b ), and this underlines the interest in soil management strategies to improve infiltration and soil water storage for crop use, and to reduce damage to the environment.
Traditional farming practices in Queensland involve frequent tillage to control weeds and prepare seedbeds, but these practices increase surface sealing and reduce steady infiltration rate (Connolly et al . 1997). Conservation tillage, in which crop residues are left on the soil surface as protection from raindrop impact, minimises surface sealing, increases infiltration, and reduces runoff (Blevins and Frye 1993). Despite a general understanding that zero-till is the optimal system in terms of soil and water conservation, few farmers have been able to avoid tillage completely (Daniells 1999). Soil compaction induced by wheel
240Y. Li et al. traffic may reduce infiltration capacity, limit the benefits of residue cover, and generate major practical problems in zero-till systems.
Most soil compaction occurs when soil is wheeled (compaction plus slippage) by tractors and other agricultural vehicles, because weed control and planting operations usually occur shortly after rainfall, when the soil is moist and susceptible to structural deformation (McGarry 1990, 1991). The process of soil compaction reduces total porosity and increases bulk density, resulting in changes in soil physical properties (Horton et al. 1994). This compaction can persist for many years (Hakansson et al. 1988). Shear deformation caused by wheelslip (i.e. a relative movement between the tyre and the surface) can further damage the soil (Soane et al. 1980/1981).
Wheel traffic induced compaction can be minimised in controlled traffic farming (Taylor 1983), where traffic lanes and crop areas are permanently separated to provide optimal conditions for both crop growth (not trafficked), and traction (consolidated). Studies comparing controlled traffic with soil compaction have demonstrated a significant improvement in soil structure (Campbell et al. 1986) and crop yield (Williford 1980; Chamen et al. 1992). Controlled traffic farming systems are being rapidly adopted in Queensland (Yule 1998), but qualification of its effect on infiltration would be valuable to farmers and their advisers (Freebairn 1999).
Soil compaction is a simple physical description, but field demonstration of its impact is often confounded by the difficulty of establishing a non-wheeled soil condition. This research was carried out within a long-term controlled traffic experiment, where non-wheeled soil had been achieved by a controlled traffic zero-till layout for 5 years. Compaction was created by a single wheel pass.
The objective of this work was to assess the effects of wheeling and different wheelslip levels with varying residue cover on infiltration under simulated rainfall, and to interpret these results in terms of practical impact.
Materials and methods
Site description
The experimental site was at the University of Queensland, Gatton (27°30′S, 152°27′E), with mean slope of 6–8%, on a self-mulching black earth (Table 1). The soil plastic limit gravimetric water content was 27% (G. Sharp, pers. comm.). Average annual rainfall is 785 mm with 70% of the total falling in the period October–March.
Table 1.Soil characteristics at Gatton experimental site
Adapted from Powel (1982), Agricultural Chemistry Branch, QDPI Depth pH Cation exchange Particle size distribution Moisture holding (cm)capacity(% of total)capacity (% g/g)
(cmol c/kg)Coarse
Fine sand Silt Clay1/3 bar15 bar
sand
0–108.34731923594426
20–308.24931520635430
50–608.44621418665634
80–908.751131816574627 110–1208.853*******
140–1508.74633629364323
Traffic and residue cover effects on infiltration 241
A controlled traffic experiment, providing a 2.5-m-wide by 30-m-long cropped ‘seedbed ’ and 0.5-m permanent traffic ‘lanes ’, was set up in 1994 to investigate the effect of controlled traffic on runoff and crop production. The experiment was designed as a ‘split plot ’: wheeled and non-wheeled (controlled traffic),comprising 3 tillage treatments (stubble mulch, minimum, and zero till). Details of the field experiment are given by Tullberg et al . (2001).
Experimental layout
T wo plots of controlled traffic zero-till seedbeds were randomly selected for wheeling and rainfall simulation. Each plot was split lengthways into 2 main parts, non-wheeled (controlled traffic) and wheeled soils, the latter being subject to 3 levels of wheelslip: 3% (self-propulsion), 10%, and 19% (Fig. 1).
A single wheeling was imposed lengthways taking 3 passes side by side to cover the whole wheeled area,using a 2WD tractor (John Deere 4040 with 40 kN of rear axle weight) fitted with a commercial Doppler radar slip unit. This was calibrated for zero slip at self-propulsion on a bitumen surface. Wheeling with 10%and 19% wheelslip was induced by towing a 30 kN tractor (which travelled outside the plot area) to apply drawbar pull. The tyre pressure of the tractor rear wheels was about 150 kPa and working speed was approximately 3–4 km/h throughout the study. The site received 80 mm rainfall 1 week prior to application of the treatments, and the average gravimetric soil moisture contents at wheeling were 18% and 28% at 5and 10 cm depth, respectively.
The surface soil of the wheeltrack area was lightly hand raked after wheeling to remove the influence of surface roughness on infiltration. Any surface crusts of non-wheeled areas were also lightly disturbed by raking to ensure that any previous surface seal did not affect infiltration. Soil surface cover for each rainfall simulation was created by adding wheat straw after wheeling. Levels of residue cover were estimated using method of photostandards for winter cereals (Molloy and Moran 1991).
T wo levels of surface protection (0 and 80% residue cover) were used for the wheeled soil and 4 levels (0, 40%, 80%, and 100%) for the non-wheeled soil. The layout, illustrated in Fig. 1, provided 3 replicates of all treatments except for bare wheeled soil. A single replicate of this treatment was dictated by the limited availability of 5 years non-wheeled soil.
Rainfall simulation
A portable field simulator, based on the design of Bubenzer and Meyer (1965), with 3 oscillating Veejet
80100 nozzles was used to apply simulated rainfall. The water supply pressure was maintained at 60 kPa to Fig. https://www.doczj.com/doc/ad3505367.html,yout of wheeling
and residue cover treatments on
the experimental site showing
approximate location of rainfall
simulation plots. Wheeling was
imposed lengthways. There
were 2 levels of residue cover
(0 and 80%) for all wheeled
soil and 4 levels (0, 40%, 80%,
and 100%) for non-wheeled
soil. There were 3 replicates of
all treatments except for bare
wheeled soil. The values in
each subplot indicate the
percentage of residue cover.
242Y . Li et al.produce drop size and energy similar to natural rain (Loch 1996). Rainfall was applied at a constant rate of 122 mm/h for about 60 min to simulate the high intensity storms that produce much of the long-term runoff and soil loss (Wockner and Freebairn 1991). This rainfall intensity is about that of once in 10-year, 20-min design rainfall event in Gatton (Pierrehumbert 1977). Time to ponding was recorded, together with rainfall and runoff using tipping bucket methodology (Ciesiolka et al . 1995). Infiltration rate and total infiltration were calculated by subtracting runoff rate and total runoff from rainfall rate and total rainfall at any time,neglecting surface retention (Silburn and Connolly 1995). Each run of the rainfall simulation covered 2subplots (0.75 m by 2 m each), so that a total of 24 rainfall simulation subplots were conducted (Fig. 1). Data were analysed by fitting an unbalanced analysis of variance model using the general linear model (GLM) procedure of SAS version 6.12. A common variance for all treatments was assumed. Treatment variation was partitioned using contrasts to estimate the effect of increasing residue cover on a non-wheeled soil, and the effect of different levels of wheelslip at 80% residue cover.
Results
A single wheeling significantly reduced time to ponding, steady infiltration rate, and cumulative infiltration compared with non-wheeled soil, irrespective of wheelslip or residue cover (Fig. 2). Residue cover had a significant effect on infiltration under non-wheeled conditions, but the response to residue cover was much less (no significance) on wheeled soils, which had lower infiltration rates overall.
Time to ponding was unaffected by different levels of wheelslip and residue cover under wheeled conditions (Fig. 2a ). All the wheeled soils began ponding after 5–7 min of rainfall Non-wheeled 3%10%19%Wheelslip
Wheelslip Wheelslip 020
40
60
80
100
120
20
40
60
80
100
120
10
203040
T i m e t o p o n d i n g (m i n )S t e a d y i n f i l t r a t i o n r a t e (m i n /h )C u m u l a t i v e i n f i l t r a t i o n (m m )(c )
(b )(a )Residue-protected (80%)Non-protected Fig. 2.Wheel traffic and wheelslip effects on (a ) time to ponding, (b ) steady infiltration rate, and (c ) cumulative infiltration with residue-protected soil and non-protected soil under 60 min of simulated rainfall (122 mm). The vertical capped lines indicate the standard error of mean (n = 3 for all treatments except a single replicate for non-protected wheeled soil, P =0.05).
Traffic and residue cover effects on infiltration 243and reached a steady infiltration rate after approximately 15 min. Time to ponding for non-wheeled plots, however, was significantly different for the residue-protected soil (28 min)and non-protected soil (11 min).
Wheelslip (10% and 19%) further reduced steady infiltration rate (Fig. 2b ) and cumulative infiltration (Fig. 2c ) compared with self-propelled wheeling (3% wheelslip) for the residue-protected soils. There was no significant difference found between 10% and 19% wheelslip. Mean steady infiltration rate of non-wheeled soil was 4–5 times that of wheeled soil. Mean total infiltration into non-wheeled soil was 77% of total rainfall (122mm) compared with 25% for the wheeled soil.
Detail of infiltration response to residue cover for non-wheeled soil is shown in Figs 3and 4. When residue cover increased from zero to 100%, time to ponding and steady infiltration rate significantly increased by about 3 and 1.3 times on average over 60 min,respectively (Fig. 3a ). Residue cover significantly increased total infiltration until it reached up to 80%, and after that value, future residue cover had no effect (Fig. 3b ). There was a strong linear relationship (R 2 = 0.98) between residue cover and infiltration for non-wheeled soil, with steady infiltration rate increasing by 0.66 mm/h with each 1%increase in residue cover (Fig. 4).
Discussion
This study has clearly demonstrated that wheel traffic is complimentary to, but of greater importance than, residue cover in terms of its effect on infiltration. Connolly et al . (1997)reported steady infiltration rates of 80 mm/h for bare virgin black vertosols in Queensland,whereas infiltration rates of only 20 mm/h and 4–12 mm/h were found, respectively, by Silburn and Connolly (1995) and by Freebairn et al . (1984) for these soils when subjected to long-term cultivation. In the present experiment a single wheel treatment reduced infiltration values of 102 and 48 mm/h to mean values (3 levels of wheelslip) of 16 and 0%40%80%100%(b )(a )020406080100120140Cumulative rainfall (mm)
Rainfall time (min)
I n f i l t r a t i o n r a t e (m m /h )C u m u l a t i v e i n f i l t r a t i o n (m m )20
406080100120140020
406080100120140
10203040506070
Fig. 3.Effects of residue cover on (a ) time to ponding and infiltration rate and (b ) cumulative infiltration during 60 min of simulated rainfall (122 mm) under non-wheeled conditions. The arrows in (a ) indicate the time to ponding for 4 levels of residue cover. The line type for different percentage covers for both figures is shown in (b ). The horizontal bar indicates the l.s.d value for time to ponding; vertical bars show the l.s.d. values for (a ) steady infiltration rate and (b ) total infiltration, respectively (n = 3, P = 0.05).
244Y . Li et al.11mm/h for residue-protected and bare soil, respectively (Fig. 2b ). Infiltration under a controlled traffic, zero tillage regime appears to be similar to that of virgin soils, but is reduced to that of long-term cultivated soil when wheeling is imposed. The implication of these results is that wheeling is responsibility for a pervasive sub-surface layer of degraded soil, and this might be the major factor affecting infiltration. This is supported by Tullberg et al. (2001) who demonstrated that tillage did not undo this wheeling effect in the same area.
The wheeling treatment here was intended to simulate the tillage/planting operations of broadacre grain production, so the tractor used was a common 2WD unit with a rear axle weight of 40 kN, and this treatment was applied 1 week after rainfall. It might be argued,however, that the pressure used in the rear tyres was at the upper end of the ranges (80–160kPa) and that the application of wheeling to the complete plot area does not simulate farm practice.
Tyre pressure is generally acknowledged to have a smaller effect on the subsoil (Hadas 1994). Since infiltration behaviour appears to be a function of the subsurface layer in compacted areas, where ameliorative change occurs relatively slowly, the complete wheeling treatment is likely to be similar in its outcome to that of the random wheelings that occur over time during cropping. Kuipers and van-de Zande (1994) have also demonstrated that tractor wheels traffic an area of 0.5–5 times the crop area in each crop production cycle, suggesting that a wheeling of the complete plot area in this study is not excessive compared with common practices.
The results show that infiltration rates of non-wheeled soil were directly proportional to the degree of soil surface protection provided by crop residue (Fig. 4), but this protection had no effect on infiltration when the soil was wheeled (Fig. 2), indicating that infiltration was governed by a degraded subsurface layer. In general, infiltration will be limited by the layer that has the lowest infiltration capacity. Evidence presented here suggests that infiltration will be improved and runoff will be reduced when a controlled traffic system protects the subsurface soil and residue cover protects the soil surface. This is largely achieved in the controlled traffic conservation tillage farming systems recently adopted over approximately 500000 ha in Australia.
Wheelslip had a further negative effect on infiltration rate when the subsurface was wet at wheeling, presumably due to the increased disruption of pore continuity with additional shear deformation. McHugh et al . (1996) reported that 10% wheelslip had no further effect on infiltration on the same soil, but their wheeling and wheelslip treatments were applied Residue cover (%)
020*********
20
406080100120
S t e a d y i n f i l t r a t i o n r a t e (m m /h )Fig. 4.Relationship between residue cover and steady infiltration rate for non-wheeled soil. Steady infiltration rate (mm/h) = 0.66(±0.03) × (Residue cover %) + 47.32 (±2.19); R = 0.98, P = 0.0001.
Traffic and residue cover effects on infiltration245 when soil was substantially drier than the plastic limit for both top and subsurface layers. Davies et al. (1973) demonstrated a similar effect to that found in this study, and Akram and Kemper (1979) have shown that infiltration rate decreased with soil water content at wheeling for a given compacting load.
The contrast between total 60-min runoff from non-wheeled soil with 80% residue-protection (7 mm), and soil wheeled with 10% moderate wheelslip (90 mm) after 122-mm intense (120 mm/h) simulated rainfall is very clear (Fig. 2c). This difference between the soil conditions achieved in current conventional cropping practice and that of controlled-traffic zero-till farming might be of considerable importance in terms of soil erosion, much of which occurs in the high intensity storm rainfall events that are a characteristic of the northern Australian cropping zone.
Tullberg (2000) recently showed that more than 50% of the energy output of an agricultural tractor could be dissipated in the process of creating and disrupting wheeltrack soil compaction. This study has demonstrated the large improvement in infiltration characteristics relative to that with wheeled traffic with a heavy clay soil, and suggests that this unnecessary dissipation of energy in traffic-induced soil deformation is a major factor in the degradation of heavy clay soils.
Conclusions
The effect of wheel traffic on infiltration on a 5-year controlled traffic and zero-tilled clay soil was investigated under simulated rainfall in northern Australia.
1. Wheel traffic had a large and significant effect on the infiltration of a heavy clay soil,
compared with a controlled traffic system where the high infiltration rates of non-wheeled soil were maintained when wheel traffic was excluded. Steady infiltration rate for non-wheeled soil was 4–5 times that of wheeled soil.
2. Residue cover had no effect on infiltration when the soil was wheeled, but had a sub-
stantial effect when the soil was non-wheeled in a controlled traffic system.
3. The data suggest that infiltration be controlled by surface sealing when wheel traffic
compaction was excluded in the controlled traffic system, but was governed by a degraded subsurface layer when the soil was wheeled.
4. Wheelslip further reduced infiltration with no change for wheelslips greater than 10%.
5. The energy dissipated in the soil and associated with wheel trafficking can alter the
hydrological performance of heavy clay soils to a significant extent. Acknowledgments
We thank Dr Hongwen Li for his generous support during his visit in Australia, Mr Allen Lisle for the statistical advice, and Dr Usha McGarry, Mr Robin Connolly, and Dr Jack Huge for the critical comments. We also thank Mr Brett Jahnke, Geoff Growth, Levi Victor-Gordon, and James Flemming for their help in the field work. This work was supported by the Australian Centre for International Agricultural Research, under project number 9209 and 96143.
References
Akram M, Kemper WD (1979) Infiltration of soils as affected by the pressure and water content at the time of compaction. Soil Science Society of America Journal43, 1080–1086.
Blevins RL, Frye WW (1993) Conservation tillage: an ecological approach to soil management. Advances in Agronomy51, 33–78.
Bubenzer GD, Meyer LD (1965) Simulation of rainfall and soils for laboratory research. Transactions of the ASAE8, 73–75.
246Y. Li et al. Campbell DJ, Dickson JW, Ball BC, Hunter R (1986) Controlled seedbed traffic after ploughing or direct drilling under winter barley in Scotland, 1980–1984. Soil and Tillage Research8, 3–28.
Chamen WCT, Vermeulen GD, Campbell DJ, Sommer C (1992) Reduction of traffic-induced soil compaction: a synthesis. Soil and Tillage Research24, 303–318.
Ciesiolka CA, Coughlan KJ, Rose CW, Escalante MC, Hashim GM, Paningbatan EP Jr, Sombatpanit S (1995) Methodology for a multi-country study of soil erosion management. Soil Technology8, 179–192. Connolly RD, Freebairn DM, Bridge BJ (1997) Change in infiltration characteristics associated with cultivation history of soils in south-eastern Queensland. Australian Journal of Soil Research35, 1341–1358.
Daniells I (1999) Improving soil structure with conservation tillage. In ‘Confarm 21’. Proceedings of The Second National Conservation Farming and Minimum Tillage Conference. Toowoomba, 1999. pp.
92–94. (Conservation Farmers Inc. Qld, Moree Conservation Farmers Association NSW: Toowoomba, Qld)
Davies DB, Finney JB, Richardson SJ (1973) Relative effects of tractor weight and wheelslip in causing soil compaction. Journal of Soil Science24, 399–409.
Freebairn DM (1999) Conservation cropping in southern Queensland—experiences and developments. In ‘Confarm 21’. Proceedings of The Second National Conservation Farming and Minimum Tillage Conference. Toowoomba, 1999. pp. 195–203. (Conservation Farmers Inc. Qld, Moree Conservation Farmers Association NSW: Toowoomba, Qld)
Freebairn DM, Loch RJ, Glanville S, Boughton WC (1984) Use of simulated rain and rainfall-runoff data to determine ‘final’ infiltration rates for a heavy clay. In ‘The properties and utilization of cracking clay soils’. (Eds JW McGarity, EH Hoult, HB So) pp. 348–351. (University of New England: Armidale, NSW)
Freebairn DM, Ward LD, Clarke AL, Smith GD (1986a) Research and development of reduced tillage systems for vertisols in Queensland, Australia. Soil and Tillage Research8, 211–229.
Freebairn DM, Wockner GH, Silburn DM (1986b) Effects of catchment management on runoff, water quality and yield potential from V ertisols. Agricultural Water Management12, 1–19.
Hadas A (1994) Soil compaction caused by high axle loads—review of concepts and experimental data.
Soil and Tillage Research29, 253–276.
Hakansson I, V oorhees WB, Riley H (1988) Vehicle and wheel factors influencing soil compaction and crop response in different traffic regimes. Soil and Tillage Research11, 239–282.
Horton R, Ankeny MD, Allmaras RR (1994) Effects of compaction on soil hydraulic properties. In ‘Soil compaction in crop production’. (Eds BD Soane, CV Onwerkerk) pp. 141–165. (Elsevier Science B.V.: Amsterdam)
Kuipers H, van-de Zande JC (1994) Quantification of traffic systems in crop production. In ‘Soil compaction in crop production’ (Eds BD Soane, CV Ouwerkerk) pp. 417–445. (Elsevier Science B.V.: Amsterdam)
Li Y, Ziebarth PJ, Tullberg JN (1998) Controlled traffic and surface runoff experiments at Gatton College, south-east Queensland. In ‘Proceedings of the Second National Controlled Traffic Conference’. pp.
126–130. (The University of Queensland: Gatton, Qld)
Loch RJ (1996) Rainfall simulator—components, methods and management. In ‘Rainfall simulator training school’. (QDPI: Toowoomba, Queensland)
McGarry D (1990) Soil compaction and cotton growth on a V ertisol. Australian Journal of Soil Research 28, 869–78.
McGarry D (1991) The effect of wet cultivation on the structure and fabric of a Vertisol. Journal of Soil Science40, 199–207.
McHugh AD, Tullberg JN, Ziebarth PD (1996) The effect of wheelslip on infiltration and runoff. In ‘Proceedings of Conference on Engineering in Agriculture and Food Processing’. pp. 96–100. (The University of Queensland: Gatton, Qld)
Molloy JM, Moran CJ (1991) Compiling a field manual from overhead photographs for estimating crop residue cover. British Soil Use and Management Journal7, 177–183.
Pierrehumbert CL (1977) Rainfall intensity–frequency–duration estimation. In ‘Australian rainfall and runoff—flood analysis and design’. (Eds A Pattison, JKG Ward, TA McMahon, B Watson) pp. 2–58.
(The Institution of Engineers, Australia: Sydney)
Silburn DM, Connolly RD (1995) Distributed parameter hydrology model (ANSWERS) applied to a range of catchment scales using rainfall simulator data. I: Infiltration modelling and parameter measurement.
Journal of Hydrology 172, 87–104.
Traffic and residue cover effects on infiltration247 Soane BD, Blackwell PS, Dickson JW, Painter DJ (1980/1981) Compaction by agricultural vehicles: a review II. Compaction under tyres and other running gear. Soil and Tillage Research1, 373–400. Taylor JH (1983) Benefits of permanent traffic lanes in a controlled traffic crop production system. Soil and Tillage Research3, 385–395.
Tullberg JN (2000) Wheel traffic effects on tillage draft. Journal of Agricultural Engineering Research75, 375–382.
Tullberg JN, Ziebarth PJ, Li Y (2001) Tillage and traffic effects on runoff. Australian Journal of Soil Research 39, 249–257.
Williford JR (1980) A controlled-traffic system for cotton production. Transactions of the ASAE23, 65–70. Wockner GH, Freebairn DM (1991) Water balance and erosion study on the eastern Darling Downs—an update. Australian Journal of Soil and Water Conservation4, 41–47.
Y ule DF (1998) Controlled traffic farming—the future. In ‘Proceedings of Second National Controlled Traffic Conference’. (Eds JN Tullberg, DF Y ule) pp. 6–12. (University of Queensland: Gatton, Qld) Manuscript received 7 April 2000, accepted 10 July 2000
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