J. agric. Engng Res. (1966) 11 (3) 170-187

Traffic Compaction of Soil and Tillage Requirements

W. ARNDT*; C. W. ROSEt

I. The Distribution of Traffic in Tillage Operations

W. ARNDT*

The traffic patterns for classical draught animal systems and new mechanized systems are describedand contrasted. In the classical systems the units of soil compaction are small discs more or lessrandomly distributed so that they can only be relieved by treating the whole field uniformly each year.This probably accounts for the agricultural tradition of uniform tillage treatments. The overalleffort was generally minimized by deferring ploughing for 3 a. In the mechanized systems, especiallyfor row crops, the units of soil compaction are long continuous bands which are highly organized andclearly defined for preferential treatment. However, because of tradition the whole field is ploughede~ch year. The overall.efforts can now be logically reduced by reducing the area treated each year and,since the extent of this area depends on the traffic system, further advances in minimum tillage areexpected from improved designs in tillage traffic systems.

1. IntroductionSoil tillage is a basic agricultural operation

characterized by complex objectives, a widechoice of methods and much diversity ofopinion. In most cases tillage practices areinfluenced by tradition and intuition rather thanby rational analysis. Kuipers' has reviewed therecent revival in soil tillage research and hasestablished the specific nomenclature used in thispaper. He regards financial profit as the finalobjective of tillage. The relationship between till­age and yield depends upon the technologicaleffects of tillage which mainly involve changes inweed populationand in the soilphysical condition.Changes in the latter generally involve a reduc­tion in soil bulk density to improve water, air,temperature and mechanical resistance relation­ships and to promote biological activity. There­fore any agency that tends to increase the bulkdensity of soil is a potential determinant of thetillage requirement of soil.

This series of four papers considers thegeneral extent to which the traffic that is asso­ciated with cultural operations may affect soilproperties and hence the tillage requirements.

• Division of Soils, The Cunningham Laboratory, CSIRO, St.Lucia, S.W.6, Queensland, Australia, formerly of Division of LandResearch and Regional Survey, CSIRO, Canberra

t Division of Land Research and Regional Survey, CSIRO,Canberra

The examples are drawn from northern Australianexperience. The first paper deals with a descrip­tion of traffic patterns for classical and moderntillage systems.

2. Traffic patterns2.1. Draught animal systems

In draught animal systems the unit of soilcompaction by traffic is a disc of rarely more than6 in dia which produces a small hydrologicallyclosed depression. These compacted discs aresparsely scattered across a face generally at leastas wide as the implement. For broadcast cropsafter several tillage operations prior to planting arandom distribution of such discs is expected anda high proportion of the total area of the field isexpected to be unaffected by traffic. For rowcrops the distribution of traffic compaction willbe partially orientated by the rows after plantingbut will still be discontinuous. In all cases thissoil compaction will be supplemented by some­what similar effects caused by human foot traffic.The net effect is a fine-grained mosaic of smalldiscs of compacted soil in a matrix of uncom­pacted soil. The distance from a compact pointto adjacent loose soil is not large for the move­ment of water or roots, and surface water tendsto be ponded rather than channelled over thecompacted portions of the field. The only

170

W. ARNDT

practical way to relieve such a distribution of soilcompaction is to plough the whole field, and thefrequency of ploughing will depend upon therate of accumulation of such compaction.

2.2. Mechanized systemsIn mechanized systems wheels replace feet as

soil compacting agents. The unit of soil com­paction is a wide band spaced at regular intervalsand serving as a long water channel rather than asmall pond. Apart from differences in absolutedegrees of compaction wheel traffic differs fromfoot traffic mainly in this high degree of organiza­tion.

2.2.1. Row CROPPING

The most highly organized form of mechanicalcultivation is found in row cropping where avariety of machines may be used in several ways.

In countries with dry climates, the tractor­implement units tend to be used soon after rainwhen the soil is most susceptible to compaction.For the reasons given in Section 3.1 it is mostrealistic to assume that inter-tillage for weedcontrol is not generally effective for shatteringthe soil compaction produced by wheel traffic.The increment in soil compaction due to eachwheel passage is expected to decrease withrepeated passages, but for the present the degreeof soil compaction can only be indicated by thesum of the wheel passages.

When row crop tractors with tyre widths of12-14 in are used the width of the compactedband of soil (traffic band) in the centre of theinter-row spaces approaches 18 in after allowingfor imperfect overlap and lateral compaction;when located in each inter-row space, thesebands are separated by bands of uncompactedsoil of similar width in which the crop is grow­ing (crop bands). When a traffic band does notoccur in an inter-row space the equivalent bandis called a blank band, since it supports neithercrop nor traffic. When land is tilled in theabsence of a crop as in a bare fallow or in thepreparation of seed beds there are no crop bandsand the blank bands vary in width usually inmultiples of 18 in. It is least confusing to retainthe unit band width of 18 in and to describe widerbands as an adjacent series of blank bands each18 in wide.

171

When conventional tractors are used, thetraffic bands are not located in the centres of theinter-row spaces and the above statements needto be revised accordingly. Since such tractors areoutdated for row cropping, such a revision is notwarranted in this paper.

Differences in the frequency and distributionof the various bands and in the accumulation oftraffic on the traffic bands following the use of avariety of row cropping systems are shown inFig. 1. When a conventional 2-track: 2-rowsystem is used for planting, a pair of traffic bandsare located side by side within each alternateinter-row space (Fig. 1A). The intensity T =f + b, where f and b are the front and backwheel traffic respectively, is quite serious, thebands being located too close to the seedlingsfor subsequent shattering by deep inter-tillage.Since these traffic bands are also the most con­venient to follow in inter-row tillage, the trafficpattern is fixed (B), producing a compaction(f + b)p + (f + b)c during the planting andcultivating runs. If, to prevent further compac­tion, the existing traffic bands are deliberatelyavoided, the system is one of staggered traffic (C).The previously uncompacted inter-row spacesare then compacted by adjacent traffic bandsand a very small proportion of the field remainsuncompacted. In cases where a row crop:2-track: 2-row: fixed traffic system of inter­tillage is superimposed on the conventional2-track: 2-row: fixed traffic system of planting(D) a central band of inter-tillage traffic withintensity (2f + 2b) is partly superimposed on theexisting and adjacent traffic bands from planting.Each alternate inter-row space is then broadlyand severely compacted.

When a row crop tractor with a wheel spacingequivalent to 2-row spacings is used for plantingwith two row planter (E), the outer track servingas the marker for sowing the next pair of rows,each alternate inter-row space has a traffic bandover which front and back wheels have passedtwice, i.e. (2f + 2b). When the inter-tillageimplement spans two row widths it is most con­venient to follow the (F), so that T = (2f + 2b)p+ (2f + 2b)c

1• Traffic effects accumulate on the

same bands when the system is used repeatedly(G) and extremely contrasting soil conditionsoccur in adjacent inter-row spaces (T + (2f+2b)p+ (2f + 2b)c 1 + (2f + 2b)c2 ) . These contrasts

172 TRAFFIC COMPACTION OF SOIL AND TILLAGE REQUIREMENTS

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Fig. 1. The distribution of traffic and crop bands for a variety of rowcropping systems

W. ARNDT

can be avoided by using staggered rather than afixed traffic scheme during the first inter-tillageoperation (H). After the second staggering oftraffic soil conditions are again uneven (I) butthese can be levelled by staggering the nextoperation. When a row crop: 2-track: 2-rowplanting system is followed by a row crop:3-track: 4-row inter-tillage system far morecomplex patterns are found. The simplest caseis the one associated with the most natural andconvenient traffic system for inter-tilling (J). Inthe first sweep the rear wheels (r) pass over theI st and 3rd inter-row space within the crop,which were previously uncompacted (T = rcJ

The dual front wheel (d) passes over one of thetraffic bands from planting, whilst the otherexisting traffic band receives no further traffic.When this is repeated over the field, all existingblank bands become traffic bands and each alter­nate traffic bands from planting further com­pacted whilst the other is not. The wide sweepallows much opportunity for staggering thetraffic but this is inconvenient with broadmachines in small fields (K). Consequently afixed traffic system is usually followed and largedifferences in traffic history can accumulate (L),e.g.T = rC, + rC2 + rcs' T = (2f + 2b)p + dCI +dc• + dcs and T = (2f + 2b)p.

For large-scale farming operations muchlarger power units and operating widths can beconsidered. The power units, whether wheeledor track-laying, are likely to be 2-track ones andoperating widths equivalent to 8 rows (24 ft)might be feasible. A hypothetical row crop:2-track: 8-row planting system is shown at M.Since the width of operation is far greater than thewheel spacing, there is much scope for staggeringthe traffic but even where this is not done (N) thefrequency of traffic bands and the total traffic ismuch lower than that in the small farm systems.

A module of traffic is defined as the minimumnumber of bands required to describe the traffichistory of the field. The span of the module isoffset from the operating width of the implementand the row spaces in order to avoid the need todescribe half-band widths at each end of themodule. The traffic history of a field is describedin terms of the module, each of the componentbands being described in serial order, e.g.

cropj(2f + 2b)p + (2f + 2b)cljcropjblank.

173

Since all bands are assumed to be 18 in wide thewidth of the module is implied by the descriptionof each of its components.

A considerable amount of soil compactiondue to wheel traffic can also be expected as a resultof the operations involved in seedbed preparationbetween ploughing and planting. Since themodules of traffic and the direction of travel mayvary in a variable number of operations (2 to 6)complex patterns and large differences in thedistribution and intensity of soil compaction canbe expected prior to planting. For the presentthese traffic effects have to be largely ignoredbecause of their hidden nature.

2.2.2. CLOSE DRILL CROPPING

Where crops are sown in closely spaced drillswith no inter-row cultivation the traffic pattern isnot as apparent as in row cropping. There willbe an organized but complex pattern resultingfrom seedbed tillage as described for row crop­ping. This pattern is hidden and the traffic fromplanting is generally similar to that shown inFig. 1M. This is not generally extensive and thehidden traffic compaction is therefore relativelymore important than it is for row cropping. ForAustralian conditions in particular, close drillcropping is associated with frequent shallow til­lage. Therefore the pre-sowing tillage traffic ispotentially more serious than it is for row cropswhere it is generally deeper and less frequent.For close drill cropping the advantage of widermodules of traffic that are usually available tendsto be offset by the greater degree of freedom towander over the field during the majority ofpassages.

3. The carryover of traffic compactionThe general level of soil compaction actually

produced by the traffic depends mainly upon thesoil type, soil water content and the dimensionsof the traction unit, but to a lesser extent also onother factors.

3.1. The efficiency of inter-row tillage

It was assumed in Section 2 that inter-tillagewas not generally effective in breaking up soilcompaction due to wheel traffic, which tended toaccumulate with frequency of passage. Evenwhere weed control by tillage is effective, excava­tions often show that severe soil compaction

174 TRAFFIC COMPACTION OF SOIL AND TILLAGE REQUIREMENTS

occurs at a shallow depth since the traffic bandsare compacted and depressed as much as 2 inbelow the general level of the loose crop andblank bands. Weed control in the traffic andblank bands depends largely upon mechanicaltillage. Since inter-tillage at depths greater than3 in causes undesirable root damage, the effectivedepth of inter-tillage in the traffic bands iscorrespondingly less, although it requires greaterloosening. Weed growth is controlled but thetraffic bands are not shattered and two additionalpassages of the tractor wheels tend to makethem more compact.

The length and the shape of the outer tinesmay be adjusted to suit the difference in surfaceelevation and hardness of the traffic bands. Thisis not always effective since difficulties areencountered in maintaining an even workingdepth of the tines, presumably because of flexingin the ends of the spring steel tool bars andbouncing of the pneumatic tyres of the tractor(G). When the traffic is staggered so that theblank bands are subjected to traffic, the contrastbetween adjacent inter-row spaces is reducedand the central and most rigid gang of tines is setto work on the harder traffic band. In thestaggered system the risk due to the spreading ofthe traffic is generally more than offset byimproved chances of breaking up the soil com­paction (H).

3.2. Other cultural operationsOther inter-row operations affect the efficiency

of inter-tillage as a soil loosening operation. Forcotton as many as 6 insecticidal spray operationsare carried out very soon after rain, generallywith row crop: 2-track: 2-row systems. Thisproduces a general level of soil compaction thatcannot be shattered except by special measuresunder ideal soil-moisture conditions. There is achoice to be made between low infiltration ratesdue to soil compaction or extensive root damagedue to deep tillage (5-6 in). Peanuts on the otherhand are not subjected to spray equipmenttraffic and are generally ridged to facilitate theirharvest. A special effort is generally made withthe tillage prior to ridging to provide the necessaryamount of loose soil in the inter-row spaces. Ifthis operation cannot be carried out during theshort period when the soil-water content is suit­able for 2-3 in tillage, the tines have to be

adjusted to rip the traffic bands to a depthapproaching 6 in. Consequently in the followingyear peanut land tends to be rather loose whilstcotton land tends to be hard. Other crops suchas sorghum, which are neither sprayed norridged, tend to leave the land in an intermediatecondition.

It is apparent that a wide variation in theresidual soil compaction can be expected afterrow cropping and this almost certainly influencessoil properties and subsequent tillage require­ments.

4. DiscussionSoil compaction by traffic is an important

determinant of the tillage requirements of soil,and changes in traffic patterns are expected tocause changes in the tillage requirements. In theclassical draught animal systems the randomdistribution of compacted soil discs was no doubtresponsible for the tradition of treating the wholefield in a uniform manner each year. However,the fields were not necessarily ploughed eachyear. In Europe thorough ploughing wasgenerally carried out once in each 3-5 year croprotation and in the Bombay dry farmingmethod" ploughing is recommended once in3 years. In the new mechanized systems thecompacted bands may be individually large butcollectively they may affect similar or lesserproportions of the field. For row crops inparticular, where the traffic bands are highlyorganized and clearly defined by the lines ofstubble, the uniform treatment of the wholefield each year especially by ploughing does notseem justified. These traffic bands could beselectively ploughed or else avoided for use ascrop bands in subsequent years until otherfactors forced the ploughing of the whole field.In the classical systems the overall expendituretillage could only be reduced by reducing thefrequency of major tillage. In the new mecha­nized systems the effort can be reduced over timeor in area and the latter is the more populartrend in modern minimum tillage systems.3

The above discussion suggests that themechanization of agriculture may have en­couraged a commonly accepted level of excessivetraffic necessitating excessivetillage. The moderntrend to reduce tillage efforts might therefore be

W. ARNDT

more correctly regarded as rational tillage.Since the potential for reducing tillage seems todepend largely upon potential reductions intraffic compaction, the design and method ofusing tillage equipment is of particular signifi­cance in further developments of minimumtillage methods.

175

REFERENCES

I Kuipers, H. The objectives ofsoil tillage. Neth. J. agrie .SeL, 1963, 11, 91

] Twenty-five years of research in dry farming in BombayState. Agric, Res . Stn, Sholapur, Bombay State, 1958

J Blake, G. R. Objectives of soil tillage related to fieldoperations and soil management. Neth . J. agric, Sci.,1963, 11, 130