Changes in soil cone resistance due to cotton picker traffic during harvest on Australian cotton soils

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    Soil & Tillage Research 140 (2014) 2939


    Contents lists available at ScienceDirect

    Soil & Tillage

    jou r nal h o mep age: w ww.e1. Introduction

    It is well known that crop yield is reduced by soil compaction(Soane and van Ouwerkerk, 1994) and subsoil compaction (>0.4 mdepth) is of concern due to the increase in size and weight ofagricultural equipment (Horn et al., 2000). Cone penetrometers areoften used to assess soil strength in relation to soil compaction androot growth (Bengough et al., 2001). Resistance to penetration isaffected by soil type; soil texture, organic matter content and claymineralogy (Stitt et al., 1982), while within a soil type it is affectedby soil water content, bulk density and structure. Soil resistancegreater than 2 MPa is considered to limit root growth (Hamza andAnderson, 2005). However, soils with a resistance less than 2 MPahave been shown to reduce cotton yield (Carter and Tavernetti,

    1968) while root growth in repacked soil columns ceased at2.5 MPa (Rosolem et al., 2008). Recently Kulkarni et al. (2010)indicated that although cotton growth was affected by soilresistance as low as 1.6 MPa (measured range 1.62.9) on a loamsoil in Arkansas, there was no yield penalty.

    The plastic limit is an arbitrary measure of the soil watercontent where the soil changes from brittle and fracturing tobecoming plastic and ductile. The soil plastic limit (PL) has beenused to dene the point where the soil is susceptible todegradation from tillage operations or harvesting trafc and thatdamage will be limited when the soil is drier than the PL (Kirby,1990, 1991). Soil degradation due to picker trafc will beinuenced by picker and soil parameters; whether the pickerhas tyres or tracks, the total load of the picker, the contact area ofthe tyres or tracks and the speed of travel while soil factors includethe soil strength which is a function of water content, texture andstructure (Kirby, 1988). As the clay fraction and soil organic matterincrease, so will the PL as these parameters affect soil watercontent. Soil beneath a tyre or track is subject to both compression

    Received 14 August 2013

    Received in revised form 30 January 2014

    Accepted 4 February 2014


    Soil compaction





    Cone penetrometer

    tyres. These machines weigh twice that of previous basket pickers, usually on single tyres, being

    replaced. This raises some concern about implications for subsoil compaction (>0.4 m depth) from

    harvest trafc. The objective of this study was to quantify changes in soil strength due to picker trafc

    during harvest. Measurements of soil strength were undertaken before and after trafc by new round

    module baler (32 t) and current basket (16 t) pickers during one cotton picking season. Soil cone

    resistance, water content and plastic limit (PL) were measured in the upper 0.6 m depth at eight sites

    during normal picking operations. Results showed that soil strength increased after trafc of either

    picker compared with before trafc and increases were detected to a depth of 0.6 m. Despite differences

    in soils and prole water content, the change in strength was similar under the round module baler and

    the basket pickers. A zone of greater soil strength (3 MPa) occurred closer to the soil surface under the

    round module baler (0.3 m) compared with the basket picker (0.4 m). Zones of increased soil strength

    were also detected at 0.6 m depth under both pickers indicating possible subsoil compaction. The OZCOT

    cotton simulation model was used to determine the frequency at which the soil prole was wetter than

    the PL for both irrigated and dryland systems. Simulations showed that the soil prole could be expected

    to be wetter than the PL 75% and 14% of the time under irrigated and dryland systems, respectively, at

    harvest over the period from 1960 to 2012. This indicates that cotton picking in irrigated systems has a

    high probability of occurring when the soil is susceptible to compaction, with the risk of subsoil

    compaction greater with the round module baler.

    2014 Elsevier B.V. All rights reserved.

    * Corresponding author. Tel.: +61 2 6799 1500; fax: +61 2 6793 1186.

    E-mail addresses: (M.V. Braunack), (D.B. Johnston).

    0167-1987/ 2014 Elsevier B.V. All rights reserved.Changes in soil cone resistance due to harvest on Australian cotton soils

    M.V. Braunack *, D.B. Johnston

    CSIRO, Plant Industry and the Cotton Catchment Communities CRC, Locked Bag 59, Na

    A R T I C L E I N F O

    Article history:

    A B S T R A C T

    Australian cotton growerstton picker trafc during

    ri, NSW 2390, Australia

    e rapidly adopted new picking technology of round module balers on dual


    l s evier . co m/lo c ate /s t i l l

  • and shear force and soil degradation is due to the soil response tothese stresses.

    A difculty in assessing the risk of soil degradation isidentifying when the soil is susceptible to compaction. By usingcrop simulation models and simulating the cropping system over along period it is possible to gain an idea of the frequency that a soilprole may be wetter or drier than the PL at harvest. Augment thiswith the amount of rainfall in the ve days prior to harvest and thedata provides information on the risk for compaction on anoperational basis (Littleboy et al., 1998).

    To reduce the effect of machinery trafc on soil degradation,manufacturers have used dual tyres and tracks to reduce groundpressure. The cotton industry uses both rubber tyres and tracks ontractors for planting and in-crop operations and rubber tyres onpickers. Soil stress beneath two and four wheel drive cotton pickerswas similar and much greater than beneath a rubber trackedtractor (Kirby et al., 1991). With respect to dual versus singlewheels Kirby and Blunden (1993) have demonstrated thatcompaction near the soil surface is dependent on ground contactpressure while that at depth is dependent on total axle load. Themeasurement of stress transmission/distribution through eld soilproles also reects this (Lamande and Schjnning, 2011a,b). Inreality as the weight of equipment increases, the tyre size shouldincrease to maintain ground contact pressure and minimisesurface compaction and the number of axles should increase to

    Growers are adopting new harvesting technology of roundmodule balers. These pickers build a round module on the go in anintegrated baling mechanism and drop the wrapped module whilebuilding another, compared to a basket picker which collectscotton seed in a basket on the go and then transfers this to amodule builder on the headland. The new pickers offer severaladvantages over the basket picker: a reduction of in-eld labour,greater picking efciency (Willcutt et al., 2009) and less equipmentto clean down and move between locations all reducing the costof production. These pickers are considerably larger and weighmore than current basket pickers and pose a risk in generatingsubsoil compaction, especially if wet soil conditions occur atharvest or the soil has not dried sufciently at depth after the lastirrigation or signicant rainfall. Growers need to be proactive indeveloping strategies to minimise the risk of subsoil compactionwhich is difcult to ameliorate and can limit crop performance.

    The objective of this work is to quantify changes in soil strengthdue to cotton picker trafc on Australian cotton soils and identifythe potential risk of subsoil compaction using long-term cropsimulation modelling.

    2. Materials and methods

    2.1. Field measurements











    M.V. Braunack, D.B. Johnston / Soil & Tillage Research 140 (2014) 293930share the increased weight to minimise subsoil compaction(Hakansson and Reeder, 1994).

    Research was undertaken during the period from 1981 to 2002in response to planting and harvesting on wet soils and theamelioration of soil degradation (Daniells, 1989; McGarry andChan, 1984; Stewart et al., 2002). Sullivan and Montgomery (1998)concluded that subsoil compaction in cotton elds was due to in-eld trafc and not clay translocation. Although it is claimed thatVertosol soils (Isbell, 1996) are self repairing due to the shrinkswell behaviour, it may take in the order of 11 wet/dry cycles torepair structural degradation as blocks of compressed soil remainbetween the large cracks in the prole, and especially in subsoildue to overburden (Sarmah et al., 1996). This raises an issue as towhether cotton growers still suffer from subsoil degradation frompast years when operations were undertaken on wet soils.

    Table 1Details of sites, soil type and equipment measured.

    Site Soil Equipment

    Auscott (1) (Narrabri) Vertosola (grey cracking clay)

    Light clay (35% clay, 00.1 m)

    to heavy clay (>50% clay, 0.11.2 m)

    Round module (

    Auscott (2) (Narrabri) Vertosol (grey cracking clay)

    As above

    Round module (

    Hillston (1) Chromosol (red brown clay)

    Silty-clay (35% clay, >25% silt,

    00.1 m)

    to Clay (45% clay, 0.10.9 m)

    Round module (

    Hillston (2) Chromosol (red brown clay)

    (As above)

    Basket (single ty

    Boggabilla Vertosol (black cracking clay)

    Medium clay (45% clay, 00.9 m)

    Basket (single ty

    Bourke Kandosol (red earths)

    Loamy-clay (30% clay, 01.0 m)

    Basket (single ty

    Myall Vale Vertosol (grey cracking clay)

    Medium clay (40% clay, 00.1 m)

    to heavy clay (>50% clay, 0.11.0 m)

    Basket (dual tyre

    St George Sodosol (solodic soils)

    Silty-Loam (25%, >25% silt, 00.1)

    to Clay-loam (30%, clay, 0.11.2 m)

    Basket (single ty

    PL, plastic limit measured (%); WP, wilting point (%, 15 bar); DUL, drained upper lima Australian Soil classication (Isbell, 1996). Field texture & approximate clay or sEight typical cotton elds were selected during the 2011 cottonharvest covering a range of soil types and soil moisture conditionsat harvest (Table 1). Soil cone resistance was measured, to depth ofup to 0.6 m at intervals of 0.02 m, with a recording penetrometerinserted at a constant rate (ASAE, 1986) (12.3 mm dia. cone, 308included angle) across twelve or eight furrows (round modulebaler and basket picker, respectively) and crop rows before andafter the passage of a fully laden cotton picker operating in the eldat the time. It was not possible to insert the cone penetrometer to0.6 m at all sites due to dry soil or trafc pre-history of the eld. Thepenetrometer was mounted in a metal frame and inserted at aconstant rate by a battery driven ram; this eliminated operatorfatigue and ensured consistent strength data. Measurement alwaysstarted and nished in a non-trafc furrow across twelve or eightrows with strength being recorded at 20 mm depth intervals; the

    Weight (t) Prole (00.6 m) soil water (%)

    Empty Full Front


    Pre- &



    al tyres) + trailer 38 47 21 32 25 22 40

    al tyres) + trailer 38 47 21 24 22 22 40

    al tyres) 32 34 21 19 17 13 36

    s) 17 20 14 23 22 13 36

    s) 16 18 12 22 19 18 44

    s) 15 17 13 20 21 19 35

    19 20 16 23 21 19 40

    s) 15 18 13 17 19 15 26

    (%) (WP & DUL from APSoil database).

    content and prole depth are given in parenthesis.

  • M.V. Braunack, D.B. Johnston / Soil & Tillage Research 140 (2014) 2939 31distance between readings being 0.25 m. The mean for eachposition of measurement (before and after trafc) was used forgenerating prole contour maps, with the mean of all positionsbeing used for before and after trafc analyses. Soil samples werecollected from a furrow, a crop row and equidistant between thetwo, at the same time from 00.1, 0.10.2, 0.20.3, 0.30.4, 0.40.5to 0.50.6 m depths for gravimetric water content and assessmentof plastic limit (PL, Australian Standards Association, 1995). Twotransects ve metres apart were measured 20 m in from the taildrain end of the eld perpendicular to the direction of picker travel.All penetrometer readings, for all sites, were corrected for surfacegeometry using the height difference between the top of the croprow and furrow base and to a common soil water content (usingthe average prole soil water content at each site after Busscheret al., 1997). Soil resistance data were contoured using SigmaPlot12.0 (Systat Software, 2011). Picker parameters collected includedempty and loaded weights, tyre size and ination pressure, andtrack width.

    In order to determine signicant changes in soil cone index dueto picker trafc the soil strength data was analysed as a multiplesplit plot (site/eld position/depth) with soil water content as acovariate using Genstat v14 (VSN International, 2011). Due to thenature of the eld measurements there was a wide range in pre-history and prole moistures at each site, so a multiple regressionwas tted to soil strength after picking to account for pre-historysoil strength, soil moisture, picker weights and prole depth. Theequation was of the form:

    SqrtAH 0:662 0:01462 D 0:456 BH 0:000453 Pw 0:00787 W 0:00673 BH Pw 0:00409 D BH (1)

    where AH is the after harvest soil strength (MPa), D is depth (m),BH is the before harvest soil strength (MPa), W is the weight(tonnes) of the picker and Pw is the prole moisture content. Thesite by depth interaction was not included in the model as it wasnot a signicant factor in regression. The regression accounted for64% of the variability in the data. Eq. (1) integrates many of thefactors associated with measured changes in soil strength afterpicking in these measurements; it indicates that the soil strengthbefore picking (reecting pre-history of the eld) had the greatesteffect on soil strength after picking.

    It is not possible to make direct comparisons between roundmodule baler and basket pickers as both were not operating in theeld at the same time. The sites, soils, pickers, soil water content atthe time of trafc and the corresponding soil plastic limit (PL),wilting point (WP) and drained upper limit (DUL) are given inTable 1. The round module balers were twice the weight of thebasket pickers depending on conguration (Table 1).

    2.2. Simulation

    The soil compaction model SoilFlex (Keller et al., 2007) wasused to simulate vertical stress distribution in the soil under thewheels of the round module builder and basket picker assuming anelliptical contact area; this model has the advantage of using tyresize (520/85R42 & 20.8-38 for round module & basket picker), andination pressure, (270 kPa for both) as measured inputs.

    The cotton crop simulation model OZCOT (Hearn, 1994) wasused to determine the frequency that the soil prole water content(00.6 m depth) at harvest, for the seven sites at which soilstrength was measured, was higher or lower than the measured PLto determine the risk of soil compaction at harvest (which canoccur up to three weeks after defoliation at 60% open bolls).Rainfall may occur in this intervening period which will re-wet thesurface soil. OZCOT uses the Ritchie water balance model (Ritchie,1972) and has been widely calibrated and validated across cottonsoils and growing regions in Australia (Cull et al., 1981; Hearn andConstable, 1981, 1984; Hearn, 1994).

    Simulations were conducted for both irriga...


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