compaction and soil structure modification by wheel traffic in the northern corn belt1

6
DIVISION S-6—SOIL AND WATER MANAGEMENT AND CONSERVATION Compaction and Soil Structure Modification by Wheel Traffic in the Northern Corn Belt 1 W. B. VOORHEES, C. G. SENST, AND W. W. NELSON 2 ABSTRACT Increasing size and weight of farm tractors is causing increasing concern about soil compaction. Controlled wheel-traffic studies in Minnesota on a silty clay loam showed that wheel traffic of normal farming operations could compact the soil to a 45-cm depth. Pen- etrometer resistance was a more sensitive indicator of soil compaction than was bulk density. Wheel traffic increased soil bulk density by 20% or less, whereas penetrometer resistance was increased by as much as 400%. Fall tillage essentially alleviated bulk compaction in the 0- to 15-cm layer. Plowing was more effective than disking or chiseling in decreasing compaction in the 15- to 30-cm layer. Compared with plowing, bulk density and penetrometer resistance values for chiseling or disking were about 5 and 40% higher, respectively. Compaction below the tillage depth was not completely ameliorated by annual freezing and thawing. Wheel-induced compaction was more persistent in individual soil structure units than in bulk soil. Strength and density of wheel tracked clods were greater and average aggregate diamter was larger than that of nontracked clods, a difference which persisted over- winter. Additional Index Words: soil strength, density, penetrometer re- sistance, compaction persistence. T HE EFFECTS of tractor wheel traffic on soil properties and plant growth have been studied extensively (Barnes et al., 1971). However, most of this research was con- ducted in the southern and southeastern United States under soil and climatic conditions quite different from those in much of the Corn Belt region. Results from earlier studies on longevity of wheel- induced compaction in the Corn Belt tended to support the contention (Gill, 1971) that compaction was not a problem north of the "hard-freeze line." Phillips and Kirkham (1962) reported no residual effects of tractor-wheel com- paction on a Colo clay soil near Ames, Iowa, after tillage, and freezing and thawing. Wittsell and Hobbs (1965) found that 1 year of freezing and thawing ameliorated a com- pacted silt loam soil in Kansas. Krumbach and White (1964) noted decreased bulk density due to freezing of Celina loam in Michigan. Kucera and Promersberger (1960) concluded that freezing and thawing removed compaction in North Dakota. More recent research results, however, tend to disagree with the above results. Blake et al. (1976) found that 9 years of cropping and freezing and thawing did not alleviate a compacted soil layer, artificially formed at the bottom of 'Contribution from the North Central Soil Conservation Research Center, North Central Region, ARS, USDA, Morris, MN 56267, in cooperation with the Minnesota Agric. Exp. Stn., Sci. Jour. Series no. 9809. Received 29 Aug. 1977. Approved 4 Jan. 1978. "Soil Scientist and Agricultural Research Technician, USDA, Morris, Minn.; and Professor, University of Minnesota, St. Paul, Minn.; respec- tively. the plow furrow in a clay loam soil in Minnesota. Bisal and Nielsen (1967) showed little effect of freezing on break- down of clay loam clods. Wheel-induced soil compaction has also persisted in Sweden (Ericksson et al., 1974) under freezing and thawing conditions similar to those in the northern Corn Belt. Recent observations of soil compaction persistence, despite freezing and thawing, may be partly due to increasing tractor weight. From 1948 to 1968, the average tractor weight had increased from about 2,700 to about 4,500 kg (McKibben, 1971). Four-wheel drive tractors now may weigh more than 15,000 kg. The Farm and Industrial Equipment Institute projected a 13% increase in the sales of 4-wheel drive tractors in 1976, along with a decrease in sales of tractors under 80 hp (Anonymous, 1976). The number and size of tires on tractors have increased to help distribute the increased weight, but this can cause more surface soil to be wheel-tracked. Depending on several management factors, agricultural fields are generally subjected to wheel traffic at least three times each growing season—during tillage, planting, and harvesting. Frequently, other operations are necessary, each potentially capable of compacting the soil. During normal farming operations, 100% of the surface soil may be subjected to the compactive forces of a wheel at least once during a season (Voorhees, 1977a). The trend towards larger and heavier farm machinery, plus indications that soil compaction may persist despite freezing and thawing, requires a reassessment of the extent and persistence of wheel-induced soil compaction from present-day farming operations in the northern Corn Belt. In this manuscript, we will discuss some soil structural modifications over a 5-year period, resulting from wheel traffic of normal row-crop farming in Minnesota. METHODS AND PROCEDURES Field experiments were initiated the spring of 1973 on a Nicollet silty clay loam (Aquic Haplodoll) at the Minnesota Southwest Experiment Station, Lamberton, Minn. The plot site had previously been uniformly and conventionally cropped to a corn-soybean rotation for 4 years. Soil phosphorus level was low (< 10 kg/ha extractable P). No attempts were made to restrict,or control wheel traffic prior to experiment initiation, but most pre- experiment wheel traffic was perpendicular to the wheel traffic imposed during the experiment. Corn stalks were plowed in the fall immediately preceding initiation of the experiment. Plots (45 m long and 18.3 m wide) were situated on a nearly level site in a factorial design, involving four replications of two phosphorus (P) fertilizer application rates (0 and 57 kg/ha P), and two kinds of primary fall tillage for each of two crops, (corn, Zea mays and soybeans, Glycine max). Primary fall tillage consisted of either plowing or tandem disking for corn, and either plowing or chiseling for soybeans. All tractor wheel traffic was controlled so it always occurred in 344

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DIVISION S-6—SOIL AND WATER MANAGEMENT ANDCONSERVATION

Compaction and Soil Structure Modification by Wheel Traffic in the Northern Corn Belt1

W. B. VOORHEES, C. G. SENST, AND W. W. NELSON2

ABSTRACTIncreasing size and weight of farm tractors is causing increasing

concern about soil compaction. Controlled wheel-traffic studies inMinnesota on a silty clay loam showed that wheel traffic of normalfarming operations could compact the soil to a 45-cm depth. Pen-etrometer resistance was a more sensitive indicator of soil compactionthan was bulk density. Wheel traffic increased soil bulk density by20% or less, whereas penetrometer resistance was increased by asmuch as 400%. Fall tillage essentially alleviated bulk compaction inthe 0- to 15-cm layer. Plowing was more effective than disking orchiseling in decreasing compaction in the 15- to 30-cm layer.Compared with plowing, bulk density and penetrometer resistancevalues for chiseling or disking were about 5 and 40% higher,respectively. Compaction below the tillage depth was not completelyameliorated by annual freezing and thawing.

Wheel-induced compaction was more persistent in individual soilstructure units than in bulk soil. Strength and density of wheeltracked clods were greater and average aggregate diamter was largerthan that of nontracked clods, a difference which persisted over-winter.

Additional Index Words: soil strength, density, penetrometer re-sistance, compaction persistence.

THE EFFECTS of tractor wheel traffic on soil propertiesand plant growth have been studied extensively (Barnes

et al., 1971). However, most of this research was con-ducted in the southern and southeastern United States undersoil and climatic conditions quite different from those inmuch of the Corn Belt region.

Results from earlier studies on longevity of wheel-induced compaction in the Corn Belt tended to support thecontention (Gill, 1971) that compaction was not a problemnorth of the "hard-freeze line." Phillips and Kirkham(1962) reported no residual effects of tractor-wheel com-paction on a Colo clay soil near Ames, Iowa, after tillage,and freezing and thawing. Wittsell and Hobbs (1965) foundthat 1 year of freezing and thawing ameliorated a com-pacted silt loam soil in Kansas. Krumbach and White(1964) noted decreased bulk density due to freezing ofCelina loam in Michigan. Kucera and Promersberger(1960) concluded that freezing and thawing removedcompaction in North Dakota.

More recent research results, however, tend to disagreewith the above results. Blake et al. (1976) found that 9years of cropping and freezing and thawing did not alleviatea compacted soil layer, artificially formed at the bottom of

'Contribution from the North Central Soil Conservation ResearchCenter, North Central Region, ARS, USDA, Morris, MN 56267, incooperation with the Minnesota Agric. Exp. Stn., Sci. Jour. Series no.9809. Received 29 Aug. 1977. Approved 4 Jan. 1978.

"Soil Scientist and Agricultural Research Technician, USDA, Morris,Minn.; and Professor, University of Minnesota, St. Paul, Minn.; respec-tively.

the plow furrow in a clay loam soil in Minnesota. Bisal andNielsen (1967) showed little effect of freezing on break-down of clay loam clods. Wheel-induced soil compactionhas also persisted in Sweden (Ericksson et al., 1974) underfreezing and thawing conditions similar to those in thenorthern Corn Belt.

Recent observations of soil compaction persistence,despite freezing and thawing, may be partly due toincreasing tractor weight. From 1948 to 1968, the averagetractor weight had increased from about 2,700 to about4,500 kg (McKibben, 1971). Four-wheel drive tractorsnow may weigh more than 15,000 kg. The Farm andIndustrial Equipment Institute projected a 13% increase inthe sales of 4-wheel drive tractors in 1976, along with adecrease in sales of tractors under 80 hp (Anonymous,1976). The number and size of tires on tractors haveincreased to help distribute the increased weight, but thiscan cause more surface soil to be wheel-tracked.

Depending on several management factors, agriculturalfields are generally subjected to wheel traffic at least threetimes each growing season—during tillage, planting, andharvesting. Frequently, other operations are necessary,each potentially capable of compacting the soil. Duringnormal farming operations, 100% of the surface soil maybe subjected to the compactive forces of a wheel at leastonce during a season (Voorhees, 1977a).

The trend towards larger and heavier farm machinery,plus indications that soil compaction may persist despitefreezing and thawing, requires a reassessment of the extentand persistence of wheel-induced soil compaction frompresent-day farming operations in the northern Corn Belt.In this manuscript, we will discuss some soil structuralmodifications over a 5-year period, resulting from wheeltraffic of normal row-crop farming in Minnesota.

METHODS AND PROCEDURESField experiments were initiated the spring of 1973 on a

Nicollet silty clay loam (Aquic Haplodoll) at the MinnesotaSouthwest Experiment Station, Lamberton, Minn. The plot sitehad previously been uniformly and conventionally cropped to acorn-soybean rotation for 4 years. Soil phosphorus level was low(< 10 kg/ha extractable P). No attempts were made to restrict,orcontrol wheel traffic prior to experiment initiation, but most pre-experiment wheel traffic was perpendicular to the wheel trafficimposed during the experiment. Corn stalks were plowed in thefall immediately preceding initiation of the experiment. Plots (45m long and 18.3 m wide) were situated on a nearly level site in afactorial design, involving four replications of two phosphorus (P)fertilizer application rates (0 and 57 kg/ha P), and two kinds ofprimary fall tillage for each of two crops, (corn, Zea mays andsoybeans, Glycine max). Primary fall tillage consisted of eitherplowing or tandem disking for corn, and either plowing orchiseling for soybeans.

All tractor wheel traffic was controlled so it always occurred in

344

VOORHEES ET AL. I SOIL MODIFICATION BY WHEEL TRAFFIC IN CORN BELT 345

Table 1—Tractor weights used in various operations.Operation Tractor weight, kgPrimary fall tillage

Disking corn stalksPlowing corn stalkstPlowing bean stubblefChiseling bean stubble

Simulated fertilizer spreaderSecondary spring tillage-herbicide incorporationPlantingSimulated combineCorn stalk chopping

3,7007,3007,3003,7007,2503,7003,7007,2503,700

T Tractor equipped with dual rear wheels; all other operations are withsingle rear wheels.

the same place for every operation. These traffic lanes alwaysoccurred midway between rows, spaced 76 cm apart. Rear tractortires were 46-cm wide. Careful driving insured that actual trafficlanes were within 5 cm of desired location. Standard two-wheeldrive tractors with a wide front end and single rear wheels wereused in all cases, except for moldboard plowing, when dual rearwheels were used along with an on-land hitch on a 5-bottom plow.This arrangement kept the rear tractor wheel out of the plowfurrow. This was the only operation for which original trafficlanes could not be followed. However, all wheel traffic stilloccurred only between the rows, and the use of dual rear wheelsminimized any soil compaction on previously untrafficked in-terrows.

Implement wheels were positioned to follow tractor wheels andnever carried the full weight of the implement. Since combine andfertilizer spreader with the proper wheel spacing to follow thetraffic lanes were not available, these operations were done byhand, and the wheel traffic was simulated with a tractor. Of the 25interrow areas in a plot, 14 had wheel traffic, 11 did not. Thevarious operations and the weight of the tractors used are listed inTable 1.

A total of five to six tractor passes, typical of row crop farmingin the northern Corn Belt, constituted the total amount of wheeltraffic on a plot per season. Areas of soil-tire contact are difficultto measure (Soane, 1970), but pressure exerted on the soil likelyranged from about 2.5 kg/cm2, during stalk chopping, to about 5kg/cm2, during simulated fertilizer and combine traffic.

Since annual precipitation at the experimental site was belowlongterm average all 5 years, wheel traffic was never imposedunder relatively wet soil conditions. The zero degree isothermcommonly extends to at least the 90-cm depth during winter.Surface layers are commonly subjected to more than one freezingand thawing cycle each winter.

Soil compaction, caused by wheel traffic, cannot be assessedwith a single measurement. Since soil compaction has manysubsequent effects on soil environment and plant growth, severalsoil parameters were measured: bulk density, penetrometer re-sistance, and aggregate size, density, and crushing strength.

Bulk densities of wheel-tracked and nontracked soil profileswere determined from undisturbed 4.76-cm diameter soil corestaken in 15-cm increments to a 90-cm depth and in 30-cmincrements to a 90- to 150-cm depth. Pairs of bulk density coreswere obtained at random from all treatments in adjacent interrows,one wheel-tracked, the other nontracked. The coefficient ofvariation (C.V.) for bulk density values was generally less than5%, with no consistent differences in C.V. between wheel-tracked and nontracked profiles, or various soil depths.

Because of soil-surface sinkage after wheel traffic, researcherssometimes use depth corrections when comparing compacted andnoncompacted soil profiles (Soane et al., 1976). In this study,however, root growth response to soil environment at an absolutesoil depth was of prime importance. Thus, the actual soil surfaceat sampling time with no depth corrections was used as a referencepoint from which to measure depths within the soil profile.Consequences of not making the sinkage depth adjustmentresulted in less than 10% overestimation of compaction depth.

As an indicator of soil strength, penetrometer resistance wasmeasured in close proximity to each bulk density measurementusing a hand-held, soil penetrometer (Soil Test, Evanston, 111.)3with a 30° conical probe, 1.9 cm in diameter, mounted on a 0.95-cm diameter shaft. The probe was slowly pushed vertically intothe soil and the maximum total point resistance encountered in 15-cm increments was recorded to a 60-cm depth. Coefficient ofvariation was about 10% for the wheel-tracked profiles, andincreased up to 50% for the 0- to 15-cm depth in the nontrackedprofile.

Besides these indicators of the bulk changes in soil structure,wheel traffic effects on individual, soil structural units were alsomeasured.

Aggregate-size distribution was obtained from bulk soil sam-ples (~ 7 kg each) taken at random from each treatment. Sampleswere taken to a depth of 15 cm from both wheel tracked andnontracked interrows as well as in the row. A rotary sieve (Chepil,1962) was modified to gently separate the air-dried bulk soil intothe following size classes: < 0.5, 0.5 to 1, 1 to 2, 2 to 3, 3 to 5, 5to 9, 9 to 12, 12 to 30 and > 30-mm diameter. The weightfraction in each size class was calculated to determine theaggregate size distribution.

The density of individual air-dried soil clods was determined bywater displacement after coating them with paraffin. Clods wereapproximately 5 cm in diameter, and a minimum of 5 clods wererandomly selected from the soil surface midway between the rowsin each treatment. Similarly collected soil clods were held at fieldmoisture content and carved into 2.5-cm diameter spheres. Thesespheres were placed in an Instron Universal Testing machine3

between two parallel plates. A load was slowly applied to oneplate with the other plate connected to a load cell. The load wasincreased until the soil sphere ruptured or failed. The load at thepoint of clod failure ("crushing strength") was recorded.

RESULTS AND DISCUSSION

Bulk Density

Measurements were made in the summer after com-pletion of all spring field operations. Bulk density valuesfrom the tillage, crop, and P treatments were compositedbecause these factors generally had no statistically sig-nificant effect on bulk density. Figures 1,2, and 3 show theyearly post-planting bulk density values for wheel-trackedand nontracked profiles at the 0- to 15-, 15- to 30-, and 30-to 45-cm depths, respectively. Primary fall tillage effectsare delineated on the graphs, although composited valueswere used for statistical testing of significance betweenwheel-tracked and nontracked bulk densities. Compositedvalues are the average of the "plow" and "chisel or disk"values in Figs. 1 to 3, with each mean an average of at least16 observations. There were no observed differences inbulk density values between chiseling and disking.

Averaged over 5 years, wheel traffic associated withspring field operations increased the bulk density of the 0-to 15-cm soil layer by 20%, and the 15- to 30-cm layer ofsoil by 10% over that of nontracked soil. This magnitude ofbulk density difference between wheel-tracked and non-tracked profiles was fairly consistent for each of the 5years. Yearly variations in values of bulk density were notapparently related to soil water content at time of wheeltraffic. The gravimetric water content during spring op-

of trade names does not imply that these items are recom-mended or endorsed by the Department of Agriculture over similarproducts of other companies not mentioned. Trade names are used here forconvenience in reference only.

346 SOIL SCI. SOC. AM. J., VOL. 42, 1978

1.7

1.6I-o>>-" '-4

I 1.3lijQ* 1.230> I.I

1.0

Wheeltracked

o-Fall chisel or disk

1973 1974 1975 1976

YEAR

1977

Fig. 1—Midsummer bulk density for 0- to 15-cm depth as affected bywheel traffic and fall tillage.

1.7

1.6

u

1-4

1.3s '•o

1.0

Wheel"̂ tracked

"-Fall plow x\.o-Fall chisel or disk •

Nontracked

1973 1974 1975 1976

YEAR1977

Fig. 2—Midsummer bulk density for 15- to 30-cm depth as affectedby wheel traffic and fall tillage.

1.7

"§ ''

1.0

Wheel'tracked

ontracked

x-Fall plowo-Fall chisel or disk

1973 1974 1979

YEAR1976 1977

Fig. 3—Midsummer bulk density for 30- to 45-cm depth as affectedby wheel traffic and fall tillage.

erations ranged from 18 to 22% which is relatively dry.Fluctuations in soil water content were not related to bulkdensity fluctuations.

For the 0- to 15-cm depth (Fig. 1), the bulk density of thewheel-track soil was statistically higher (p = 0.01) thanthat of the nontracked soil in all 5 years. This was also truefor the 15- to 30-cm depth (Fig. 2) in all years except 1973.

Table 2—Penetrometer resistance as affected by wheel traffic.Measurements made after spring tillage and planting.

Maximum total resistance

Depth

cm

0-1515-3030-4545-60

0-1515-3030-4545-60

0-1515-3030-4545-60

0-1515-3030-4545-60

0-1515-3030-4545-60

Wheel-tracked

197313.512.114.716.0

197415.211.3T12.412.7

197514.411.9$12.311.0

197622.lt19.8t19.824.1

1977

16.6t14.0J14.2$13.6

Nontracked

6.710.313.113.4

4.87.3t

12.012.8

5.37.9t

12.512.2

7.2T12.2J22.426.1

2.97.0J

12.6J13.6

Significance

*****

****

N.S.N.S.

****

N.S.N.S.

****

N.S.N.S.

******

N.S.

*,** Wheel-tracked values significantly higher than nontracked values atthe 0.05 and 0.01 levels, respectively, as determined by Student "t"test for difference between means.

t Chiseling or disking values are significantly higher (p = 0.05) thanthat for plowing.| Chiseling or disking values are significantly higher (p = 0.01) than

that for plowing.

The differences between wheel-tracked and nontracked soilat the 30- to 45-cm depth (Fig. 3) were not statisticallysignificant for any year. There were no differences in bulkdensity below the 45-cm depth.

Although the differences in post-planting bulk densitiesbetween plowing and chiseling or disking were generallynot statistically significant, their magnitude and consistencywarrant some discussion. In the surface 15 cm of soil(Fig. 1), bulk densities were slightly higher for chiseling ordisking than for plowing, although both types of tillagedisturbed the soil to a 15-cm depth. Fall plowing, underrelatively dry conditions, created a coarser soil structurethan chiseling or disking. This structure was apparentlymore resistant to weathering breakdown and reconsolida-tion and resulted in lower bulk density.

Wheel traffic-tillage interactions on bulk density weremore pronounced at the 15- to 30-cm depth (Fig. 2).Chiseling or disking resulted in significantly higher(P = 0.01) bulk density than plowing in the nontrackedarea in 1975 and 1977, and in the wheel-tracked area in1976. This effect was probably due largely to the differencein depth of tillage. The moldboard plow tilled the soil to a30-cm depth, whereas the chisel and disk penetrated to onlythe 18- to 22-cm depth. Figure 3 also shows tillage effectsat the 30- to 45-cm depth, although reasons for these effectsare not known. At all depths, wheel traffic decreased thedifference in bulk density between plowing and chiseling ordisking, as would be expected.

VOORHEES ET AL.: SOIL MODIFICATION BY WHEEL TRAFFIC IN CORN BELT 347

Table 3—Residual effects of previous year's wheel traffic onbulk density measured in spring before any additional

' spring wheel traffic.

Table 4—Residual effects of previous year's wheel traffic onpenetrometer resistance measured in spring before any

additional spring wheel traffic. 1977.

Depth

cm

0-1515-3030-4545-60

0-1515-3030-4545-60

Bulk density

Wheel-tracked Nontracked

April 19741.221.461.621.62

April 19771.321.481.551.63

1.181.401.541.57

1.241.401.581.61

Significance

N.S.**

N.S.

N.S.*

N.S.N.S.

*,** Wheel-tracked values significantly higher than nontracked values atthe 0.05 and 0.01 levels, respectively, as determined by Student "t"test for difference between means.

Penetrometer ResistanceTable 2 gives the post-planting penetrometer resistance

measurements averaged across tillage, crops, and P treat-ments. Student "t" test was used for testing differencesbetween wheel-tracked and nontracked values. Penetrome-ter resistance values in the wheel track were significantlyhigher than in the nontracked area to a depth of 30 cm forall 5 years. Except for the 1976 data, yearly variation inpenetrometer resistance values were small because of smallvariations in soil water content at time of wheel traffic. Thehigher resistances measured in 1976 were associated with a4% decrease in gravimetric water content.

Penetrometer resistance may be a more sensitive indica-tor of soil compaction than is bulk density and may also bemore meaningfully related to root growth (Voorhees et al.,1975). Whereas bulk density values increased about 20% orless due to wheel traffic, penetrometer resistance valuesincreased as much as 400%. Differences in penetrometerresistance values were statistically significant from the 0- to60-cm depth in 1973, whereas bulk density value dif-ferences were significant only for the 0- to 15-cm depth.

There were also significant differences in penetrometerresistance values between plowing and disking or chiselingwhich resembled differences in bulk density values de-lineated in Fig. 1 to 3. The statistically significant dif-ferences are indicated by "t" and "t" symbols in Table 2.At the 15- to 30-cm depth, chiseling or disking penetrome-ter resistance values were on the average 47 and 81%higher than for plowing in the wheel-tracked and non-tracked areas, respectively (data not shown).

Overwinter Persistence of Soil CompactionSince the different tillage treatments were all imposed in

the fall, the above differences in bulk density and pen-etrometer resistance (measured the following summer)imply a degree of overwinter persistence of soil com-paction.

Table 3 shows the bulk densities in April of 1974 and1977, measured before any spring wheel traffic was

Penetrometer resistance

Depth

cm0-15

15-3030-4545-6060-75

Wheel-tracked

—————— kg/cm1

5.511.514.615.716.0

Nontracked

2.78.9

13.814.115.2

Significance

N.S.*

N.S.*

N.S.

*,** Wheel-tracked values significantly higher than nontracked values atthe 0.05 and 0.01 levels, respectively, as determined by Student "t"test for difference between means.

Table 5—Effect of fall tiUage method on alleviating wheeltrack-induced soil compaction, f

Depth

cmApril 1974

0-1515-3030-4545-60

0-1515-3030-4545-60April 1976$

0-1515-3030-4545-60April 1977

0-1515-3030-4545-60

0-1515-3030-4545-60

Bulk density

Wheel-tracked Nontracked

Plow1.11 1.181.38 1.371.62 1.521.63 1.59

Chisel or disk1.33 1.191.54 1.431.62 1.561.62 1.55

Chisel or disk1.46 1.281.52 1.391.52 1.551.56 1.58

Plow1.24 1.261.42 1.381.54 1.581.64 1.62

Chisel or disk1.39 1.221.53 1.421.56 1.581.62 1.60

Significance

N.S.N.S.N.S.N.S.

N.S.*

N.S.N.S.

***#

N.S.N.S.

N.S.N.S.N.S.N.S.

*N.S.N.S.N.S.

*,** Wheel-tracked values significantly higher than nontracked values atthe 0.05 and 0.01 levels, respectively, as determined by Student "t"test for difference between means.

t All fall tillage performed in October, with bulk density being meas-ured the following April before any additional wheel traffic.

t No data available for plowing.

applied. Data from all tillage, crop, and P treatments werecomposited for values shown in this table.

Fall tillage and over-wintering decreased the bulk densityat shallow depths, so that there were no significantdifferences between wheel-tracked and nontracked soil inthe surface 15 cm the following spring. However, com-paction in the lower portion of the tilled layer persisted overwinter so that the April bulk densities of the 15- to 30- andthe 30- to 45-cm soil layers under the wheel-tracked

348 SOIL SCI. SOC. AM. J . , VOL. 42, 1978

Table 6—Density and crushing strength of wheel-tracked andnontracked soil clods.

100

Sampling date Wheel-tracked Nontracked Significance

May 1975October 1975May 1976

Clod density, g/cm"1.72 1.561.73 1.471.59 1.49

******

October 1975Crushing strength, g/cm"

572 134

*,** Wheel-tracked values significantly higher than nontracked values atthe 0.05 and 0.01 levels, respectively, as determined by Student "t"test for difference between means.

treatment were significantly higher than those of thenontracked treatment.

Similar results were observed with penetrometer re-sistance measured in the spring of 1977 before any springwheel traffic (Table 4). Wheel-tracked treatments hadhigher penetrometer resistance values to a depth of 75 cm,significantly so in the 15- to 30-cm layer and the 45- to 50-cm layer. Thus, fall tillage seems equally or more effectivethan freezing and thawing in alleviating bulk compaction inthe surface 15 cm; however, specific data needs to beobtained to evaluate the relative effectiveness of tillage andfreezing and thawing.

Effects of tillage method on ameliorating wheel-inducedsoil compaction can be delineated by separating the data inTable 3 into plowing, and chiseling or disking treatments,as shown in Table 5. Plowing to a 30-cm depth decreasedcompaction so there was no significant difference in bulkdensities between tracked and nontracked profiles. In fact,the 0- to 15-cm layer of wheel-tracked soil was less densethan that of the nontracked soil. This was likely due to arougher, more cloddy structure produced in the wheel trackafter fall plowing, a condition that persisted over winter.With chiseling or disking, however, the bulk density valueswere significantly higher in the wheel-tracked area to the30-cm depth, even in the surface 15 cm, where tillageaction would be most complete.

The zone of maximum compaction beneath a wheel is atsome depth below the soil-wheel interface, but tends toapproach the soil surface with repeated wheel traffic (Soaneet al., 1976). Thus, if wheel-induced compaction persistedacross seasons and was additive with time, compaction asindicated by the bulk density values for the top 30 cm ofsoil would then increase with time rather than go deeperthan 30 cm in the soil profile. There is no clear evidence forthis thus far; nor is there any evidence for increasing depthof wheel traffic-compaction with time. However, thisconclusion may not extrapolate to wetter soil conditionswhich would have a lower soil strength and be moreconducive to compaction.

Aggregate StructureAlthough tillage and natural weathering processes greatly

decreased overwinter persistence of compaction in thesurface 15 cm of bulk soil, individual soil structural unitswere more stable. Table 6 shows the density and crusningstrength of individual soil clods collected at randombetween the rows at various times. Nontracked clod density

EEKUJ111

<Q

Ul

Ulec.

10

0.1

Nontrackedinterrow

•—Before spring tillage—xo —— After planting —— +

A—In nontracked row zone after planting

2 10̂ 30 50 70 90 98

PERCENT UNDERSIZEFig. 4—Aggregate size distribution before spring tillage and after

planting (1975).

decreased slightly during the 1975 growing season whilewheel-tracked clods retained a density of 1.7 g/cm3 andwere significantly more dense than the nontracked clods.Tillage and overwintering decreased the density of thewheel-tracked clods (October 1975 vs. May 1976) but theyremained significantly more dense than the nontrackedclods (May 1976), Unlike bulk density values, the cloddensity of plowed plots were slightly higher than those ofchiseled or disked plots (data not shown). The morecomplete inverting action of a plow tends to bury plantresidues whereas a chisel or disk leaves more residue nearthe soil surface. This may result in higher clod densityvalues with plowing compared to chiseling or disking.

Like penetrometer resistance values, which are a moresensitive measure of bulk compaction than are bulk densityvalues, crushing strength values may be a more sensitivemeasure of clod resistance to breakdown than clod densityvalues. Wheel-tracked clods were over 300% more resis-tant to physical disruption than were nontracked clods(Table 6).

Figure 4 shows the aggregate-size distribution, withpercent undersize on a probability scale along the abscissaand aggregate size on the logarithmic ordinate. Thegeometric mean diameter (GMO) is the aggregate diameterat 50% undersize. (See Allmaras et al., 1965, for de-velopment of analysis.) Before spring tillage, the GMD ofthe wheel-tracked interrow was larger than that of thenontracked interrow (about 40 vs. 7 mm). This is furtherevidence of the extent to which soil compaction can persistover winter. The single secondary spring-tillage operationreduced the GMD of the nontracked interrow from 7 to 4mm, while the GMD of the wheel-tracked interrow re-mained unchanged. This lack of change in wheel-trackGMD was likely due to a combination of higher resistanceto breakdown by tillage (higher crushing strength) followedby reconsolidation due to wheel traffic of the plantingoperation. The aggregate size in the nontracked interrowafter planting is not much different from that in thenontracked row (4 mm in the interrow vs 3 mm in the row).

While this aggregate-size distribution may be desirable fora seedbed, such a fine structure does not fit Larson's (1964)requirements for a water-management interrow zone.

Besides the above effects of wheel traffic on aggregatesize, the plowed plots tended to have a slightly higherGMD as compared with chiseled or disked plots. Thisagreed with the previously mentioned slightly higher cloddensity with plowing.

Compaction and soil structure modifications produced bywheel traffic has several important consequences in theNorthern Corn Belt. Compaction can be harmful for plantroot growth (Voorhees, 1977b), nodule development(Voorhees et al., 1976), water intake (Blake et al., 1976),and energy conservation (Voorhees and Hendrick, 1977).High clod density may cause seedbed preparation prob-lems.

Conversely, high clod density can also offer erosioncontrol benefits (Lyles and Woodruff, 1962) and may causemore thorough root proliferation in less dense clods,resulting in increased P uptake (Voorhees et al., 1971).

Data in this experiment were obtained under relativelydry soil moisture conditions. Under wetter conditions, themagnitude of soil compaction caused by wheel trafficwould probably be greater. Increased soil water content,however, may not alleviate compaction during freezing(Blake et al., 1976).

Regardless of overwinter persistence of wheel-inducedsoil compaction, definite soil structural changes, both inbulk and individual units, are caused by spring wheeltraffic. These changes last throughout the growing seasonand may be more important agronomically in the northernCorn Belt than overwinter persistence of the previousyear's compaction.

ACKNOWLEDGMENTAppreciation is expressed to Vernon Carlson, physical science techni-

cian, ARS, for assistance in collection and analysis of data, and to RafaelUseche, former graduate assistant, University of Minnesota, for clodcrushing strength determinations.