IV. - The effect of traffic compaction on a number of soil properties

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    IV. The Effect of Traffic Compaction on a Number of Soil Properties

    W. ARNDT

    Large differences in bulk density, soil strength, clod size before and after heavy rain, infiltrationrate and water loss from the tillage layer were measured in adjacent crop, traffic and blank bandsafter row cropping on Tippera clay loam at Katherine, Northern Territory, Australia. These indicatelarge differences in the tillage requirements and responses in adjacent bands usually 18 in wide. Thetillage requirements may eventually be reduced to the control of soil compaction by previous traffic,which could be minimized by choosing the best traffic modules.

    1. IntroductionSeveral of the traffic systems described in Part

    I of this series are in use on Tippera clay loam, alateritic red earth at Katherine Research Station,Northern Territory, Australia. On severaloccasions a number of soil properties weremeasured in adjacent narrow bands, described astraffic, crop, and blank bands. The technologicaleffects of tillage in the various bands are dis-cussed.

    2. Methods2.1. Bulk density

    Soil cores of 3 in dia and 2 in long were taken24 h after wetting and covering the sites. Thecrop and traffic bands of bulrush millet stubbleland which had been planted and inter-tilledtwice by the row crop: 2-track: 2-row: fixedtraffic system were sampled to a depth of 6 in in3 X 2 in stages. Similar samples were takenfrom a grazed native pasture and a seedbed ingood planting condition. Samples were alsotaken at 0-2 in in peanut crop bands, in a barefallow after rain, in a traffic band after l2-tractorwheel passages without tillage, and in a fieldplot of loosened soil that was protected fromtraffic during 18 months of open exposure.

    2.2. Soil strengthMeasurements were made in the crop and

    traffic bands of the stubble of bulrush millet, 7 dafter the first rain storm for the season (1,24 in)when the initial land breaking operations werebeing done.

    The calibrated tillage tool was a vertical bar,40 in long, pivotted 10 in from the lower end, towhich a horizontal cutting edge (5 X t in) was

    fitted. This was lowered into an excavation andbrought to bear on a soil face 5 in below the sur-face. A spring balance was attached to the upperend of the bar and a manual load was appliedthrough the balance. The load on the cuttingedge at the time of soil failure was calculatedfrom the spring balance reading and the leverratio.

    The penetrometer was a simple spring-loadedblade. Readings were taken at the same time asthose with the calibrated tillage tool at horizon-tal intervals of 6 in and vertical intervals of 3 inon a pit face extending across the crop trafficand blank bands of the bulrush millet. The soilsurface level in the crop band was taken as thedatum for the verticle scale.

    2.3. Tilth after initial tillageIn the bulrush millet land, sample areas of

    crop and traffic bands were dug over by hand toa depth of 6 in with a 6 in width of cut. The dugsoil from 3 samples (18 X 18 in) in each case wasbulked, air dried and separated into clod sizeclasses with sieves of t, t, 1,2,4 and 6 in squareopenings. Adjacent sorghum land planted with arow crop: 2-track: 2-row system and inter-tilledthree times with a row crop: 3-track: 4-row:fixed traffic system was chisel ploughed. Samples,each 16 in square to the full depth of ploughing,were taken from the crop and traffic bands shownin Fig. 1. After air drying they were separatedinto clod size classes with the above sieves.

    2.4. Tilth after effective inter-tillageBlocks of land that had been cropped con-

    tinuously to peanuts for 4,9 and 14 a respectivelywere available. The sowing and first inter-tillagefollowed the row crop: 2-track: 2-row fixed

  • W. ARNDT

    Fig. 1. Banded differences in soil tilth after chisel plough-ing land cropped to sorghum

    traffic system in which alternate inter-rowspaces were subjected to a total ot four passagesof the front and back wheels (T1) . The remaininginter-row spaces were not compacted by traffic(T2) . The second inter-tillage with the sameequipment was staggered so that T1 receivedno further compaction, whilst T2 receivedtwo passages of both wheels.

    Soil moisture conditions were excellent for theinter-tillage that followed and the tilth of adjoin-ing inter-row spaces was as uniform as could everbe expected. Five samples to the full depth oftilling and 12 in square were bulked for alternatetraffic bands in the 3 blocks that differed in theirprevious cropping history. After air drying theclod size classes were determined by sieves withL 1. t and I in square openings.2.5. Soil surface condition after heavy rain

    In February 1961 adjacent plots of cotton andcowpeas in a crop rotation trial and plots ofpeanuts in a monoculture trial were inter-tilledfor the second time following a row crop: 2 track:2 row: fixed traffic system. The traffic bandswere effectively ripped up by extended tines.Elsewhere a bare fallow plot maintained bymeans of a row crop: 2 track: 4 row: fixed trafficof harrowing was ripped up with tines having thesame traffic module as the harrowing operations.

    Following heavy rain on all three sites therewere banded differences in the proportions ofsurface soil which remained as stable clodsprotruding above the general level of the sealed


    surfaces. These stable clods (> 1 in dia) werecollected from 15 ft sections in adjacent trafficand blank bands on the cotton, cowpea andpeanut lands and were weighed after air drying.The differences on the bare fallow plot wererecorded photographically.

    2.6. Infiltration ratesA double ring infiltrometer with a 12 in inner

    ring, a 22 in outer ring and a head of 07 in wasused. The infiltration into the inner ring wasrecorded after 5, 10, 15,20,40,60 and 90 min forreplicated samples of the crop traffic and blankbands of the bulrush millet area.

    2.7. Soil water storageAfter artificially wetting the crop and traffic

    bands of the bulrush millet land during the ringinfiltrometer tests, the top inch of soil wasloosened by hand tilling. After 3 d of hot dryweather the loose top inch of soil was removedand a core of soil 45 in in dia was removed fromthe 1-7 in level on each site. The cores were ovendried for the determination of the gravimetricand volumetric soil water contents.

    After 7 d of hot dry weather following the firstrain storm for the season (1,24 in) the gravi-metric soil water contents in 2 in stages to 12 inwere determined for the crop and traffic bands.The volumetric water contents were determinedusing associated bulk density determinations.

    2.8. The pattern of water loss by evaporationfrom top soil

    The water loss pattern for top soil wasexamined in cultivated field plots that weresaturated and firmed by light rolling during thedry season. Soil water contents were determinedat depths of 0-1, 1-2,2-3, 3-6 and 6-9 in after10, 25 and 65 d of open exposure.

    3. Results3.1. Bulk density

    In the traffic bands the mean bulk density for0-2 in was recorded as 13 g/cm3 (Fig. 2). Thismean does not describe the samples adequatelysince they consisted of 1 in of very loose soil over1 in of soil as compact as that in the 2-4 in layer.This occurs because the top inch of soil in atraffic band is pulverized rather than compacted


    Fig. 3. Size distribution of clods after the first breakingof the crop bands ofsorghum (1) and bulrush millet (2), andthe adjacent traffic bands of the same sorghum (3) and

    millet (4)

    3.2. Soil strengthThe loads required to shatter the soil in the

    crop and traffic bands were 117 ( 6) and 459( 123) lb respectively. A similar order of differ-ence was also found by the penetrometer readingswhich were 29 ( 10) lb for the crop band , 28( 15) lb for the blank band , and 105 ( 5) Ibfor the traffic band.


    Clod s ize, in diao

    -g 10U

    3.3. Tilth after initial tillageThe clod size distribution was remarkably

    similar for hand-dug bulrush millet land andchisel ploughed sorghum land in both crop andtraffic zones (Fig. 3). Error terms cannot begiven for the bulked samples of bulrush milletland. On the sorghum land the mean contents ofclods t in) in the crop and traffic bands were746 ( 7,0) and 345 ( 50) % respectively, the

    0> 100. 90o

    " 80E" 70ooo


    different bulk densities which subsequently needto be reduced and increased respectively bytillage to restore the seedbed conditions.

    Fig. 2 suggests that the bulk density decreasesrather than increases when a seedbed is croppedwithout traffic. Elsewhere the bulk density for0-2 in was 120 ( 0,05) g/cm3 for a bare fallowthat had been levelled by heavy rain prior toplanting and 122 ( 0,05) g/cm3 for the cropbands of a peanut crop. After 18 months ofexposure, bare soil without traffic had a bulkdensity of 1-14 ( 0'04) g/cm3 The combinedevidence suggests that this particular soil doesnot have the strong natural tendency for self-compaction that casual observation suggests.

    Since there was no significant change in thebulk density at 2-4 in when the number of wheelpassages was increased from 6 to 12, and sincethese bulk densities are similar to that underheavily grazed pasture, tractor wheels do notappear to be any more serious than animalhooves from the point of viewof soil compaction.

    The bulk density at 6 in on arable land (seed-bed, crop band and traffic band) was generallysimilar to or lower than that in grazed nativepasture land. In the seedbed, bulk density wasconstant to 6 in except for the loose surfacemulch. As the surface was approached, the bulkdensity decreased in the crop band and increasedin the traffic band except for the loose layer in thetop inch. The bulk density of the