Temporal Changes in Small Depth-Incremental Soil Bulk DensityS. D. Logsdon* and C. A. Cambardella

ABSTRACTFarmers are concerned that soil compaction will be a problem in

the first few years after conversion to strict no-till. The objective ofthis study was to determine if the changes in depth-incremental soilbulk density during transition to no-till were greater than densitychanges due to natural variation. We sampled six locations from ano-till field and six locations from a disk field of a soybean [Glycinemax (L.) Merr.]-corn (Zea mays L.) rotation and took 12 samplesat each of the 12 locations. The soil types sampled were Clarion (fine-loamy, mixed, superactive, mesic, Typic Hapludoll), Nicollet (fine-loamy, mixed, superactive, mesic, Aquic Hapludoll), and Canisteo(fine-loamy, mixed, superactive, calcareous, mesic, Typic Endoaquoll)soils. We divided each 300-mm soil sample into 15 depth-increments,and pooled the 12 samples for each location for each 20-mm depth-increment Bulk densities ranged from 0.9 to 1.3 Mg m ' in the top0 to 20 mm, and 1.2 to 1.6 Mg in ' for the depth-increments below20 nun. Comparing the first and last sampling dates for the no-tillfield, no significant changes in bulk density were detected for anydepth-increment; however, for the disk field, increases in significantbulk density were detected in 10 out of 15 depth-increments. Naturalcauses probably contributed to the bulk density changes that occurredover time in both fields. None of these bulk densities were highenough to restrict root growth. We concluded that concern aboutsoil compaction under no-till management is inappropriate for thesestructured soils.

MANY FARMERS ARE RELUCTANT TO SWITCH from diskor chisel systems to no-till practices because of

concerns about compaction, especially in the early yearsafter conversion. Bruce et al. (1990) observed that thetop 200 mm of a sandy soil after 8 yr was more densefor the no-till treatment than for the disk treatment.After 10 yr on a silt loam, the no-till system soil wasmore dense than the disk-harrow system soil in thefall and spring (Home et al., 1992). Averaging acrossrotations, soil in the no-till treatment was significantlymore dense than soil in the disk treatment of wheat(Triticum aestivum L.) for 42% of the time at the 0- to50-mm depth, 83% of the time at the 50- to 125-mmdepth, and 21% of the time at the 125- to 200-mm depth(Franzluebbers et al., 1995).

Well-structured soils and soils with macropores andfractures provide a pore network for root growth (Stypaet al., 1987; Logsdon et al., 1992) and water infiltration,often resulting in no yield reduction, even when the soilis compacted (Voorhees et al., 1989; Lowery and Schuler,1991; Logsdon et al., 1992; Kaspar et al., 1995). Otherstudies have shown that moderate compaction may bene-fit crop yield, especially during dry years (Graham et al.,1986; Johnson et al., 1990), because of better seed-soil

National Soil Tilth Laboratory, USDA-Agricultural Research Ser-vice, 2150 Pammel Dr., Ames, IA 50011. Received 5 Aug. 1998. *Cor-responding author (logsdon@nstl.gov).

Published in Soil Sci. Soc. Am. J. 64:710-714 (2000).

contact and better soil continuity contributing to capillaryrise of water to the root zone (Lipiec and Simota, 1994).

Documentation of management practices on soil den-sity is obscured by natural variations in soil density (Blakeet al., 1976; Voorhees and Lindstrom, 1984; Logsdon etal., 1992; Franzluebbers et al., 1995). Soil water contentat the time of tillage or traffic, depth of winter freezing,water content before winter freezing, shrinking and swell-ing, and action of soil fauna and roots all cause variationsin density, apart from the direct-management effect.

Small depth-incremental sampling has been used toexamine the depth distribution of soil bulk density, resi-due distribution, and organic C content (Pikul and All-maras, 1986; Allmaras et al., 1988; Staricka et al., 1991;Allmaras et al., 1996). These studies examined effectsof long-term tillage, but the small incremental techniquemight be even more valuable for examining short-termeffects of tillage system. The small depth-incrementsmight detect trends that would be diluted in larger-increment samples. Sampling below 300 mm was consid-ered unnecessary since the bulk densities are not ex-pected to be different for different tillage systems below300 mm (Logsdon et al., 1990).

There is a need for research in farmers' fields tocomplement field-plot research. Results from traffic-controlled studies at the plot-scale are hard to extrapo-late to the field- or farm-scale. Equipment dealers sim-ply do not make tractors, combines, and other fieldequipment with the same wheel spacing or swath width.We can retrofit our small plot equipment, but farmersrarely adjust their larger field equipment to have thesame wheel spacing and swath width for all operations.The nonuniformity of equipment within a farmer's fieldcauses a greater percentage of the field to be subjectto wheel-traffic compaction, compared with controlled-traffic field plots.

The primary objective of this study was to quantifythe temporal changes in soil bulk density of 20-mmdepth-increments sampled to a depth of 300 mm duringthe first 3 yr after the change to no-till management ina farmer's field. Temporal changes in soil bulk densityare also examined for a disk field to help distinguishdensity changes due to natural processes from manage-ment-influenced density changes. A second objectivewas to compare the no-till system with the disk system.

MATERIALS AND METHODSField Layout

This study was conducted on two fields in central Iowa,both on similar soils and farmed by the same operator. Thefields were located 1.6 km apart, and each field or section offield considered was about 20 ha. Both fields had been undera disk management system until 1992 and were in a corn-

Abbreviations: COLE, coefficient of linear expansion.

710

LOGSDON & CAMBARDELLA: CHANGES IN DEPTH-INCREMENTAL BULK DENSITY OVER TIME 711

soybean rotation with corn grown in even-numbered years.The two fields had been under the same management practicesby the same operator with the same soil types before thestudy began. This does not preclude minor variations in soilproperties between the two fields.

The disk management used deep disking to 180 mm in thefall after corn but not after soybean. One of the fields wasconverted to strict no-till after the corn harvest in 1992. No-till practices were maintained throughout the study until theend of the growing season in 1996. The other field was contin-ued under the disk management system. Detailed culturalhistory for the fields is included in Tables 1 and 2. The combineand tractor did not have the same wheel spacings. Since com-bine traffic is more closely spaced, combine traffic coveredadditional areas of the field not covered by previous tractortraffic. The rows were oriented the same every year, but theydid not occur at the same location each year. This resulted ina variation in traffic location from year to year. Tillage, plant-ing, and anhydrous applications caused additional soil distur-bances. The disk field in the late spring had three to six tripsthrough the field during this study compared with one to threetrips in the no-till field (Tables 1 and 2). Most of the springoperations in the disk field included soil disturbance as wellas tractor traffic, but most of the trips in the no-till field onlyinvolved tractor traffic. Disks can cause compaction belowthe depth of disking if the soil is too wet.

Small Incremental SamplingWe sampled two locations in each of three soil types for

each field, for a total of 12 general locations. The soil typeswere Clarion, Nicollet, and Canisteo soils. The Canisteo siteswere intermingled with Webster soils (fine-loamy, mixed, su-peractive, mesic, Typic Endoaquoll), which differed only inthe absence of carbonates in the surface soil. Each samplinglocation was pooled from 12 individual samples taken withinthe general location. We took each sample to a total depthof 300 mm using the technique of Pikul and Allmaras (1986).The sampling tool had a relief cutting tip of 19 mm in diam.,which screwed onto a 425-mm long cylinder (including thecutting tip). The cylinder had been welded on to a fitting thatallowed pin-attachment of a handle. Because of the reliefcutting tip, the inner diameter of the cylinder was slightlylarger than the cutting tip, which greatly reduced friction be-tween soil and tube. In addition, the sampling device wasinserted into the soil using gentle pressure after it had beensprayed with cooking spray lubricant. Inspection of the sam-ples showed that there was no compression of height comparedwith the sampling depth. We used a solid metal rod to gentlypush the sample out of the sampling cylinder on to a tray,with the top of the sample being the first to be pushed out ofthe sampling cylinder. The tray was marked in 20-mm incre-ments to aid in cutting the 300-mm soil sample into 20-mmdepth-increments. We cut each of the samples into 15 subsec-tions, each 20 mm. For each 20-mm depth-increment at eachlocation, we pooled the subsections from the 12 samples andstored them together in a soil moisture can.

We sampled the same six general locations (12 samplespooled each site) in each field on 18-20 Aug. 1993, 14 Apr.1994, 24-25 Oct. 1994, 13 Apr. 1995, 13 June 1995, 11 Apr.1996, and 16 Aug. 1996. We took additional samples on 15 Nov.1994 after the disk treatment field had been deep disked.

Sample ProcessingFor bulk density determination, we air-dried the pooled

subsections in the laboratory for 2 or 3 wk. Then we took asmall sample from each pooled subsection to oven-dry anddetermine the air-dry water content. (We did not oven-dry

Table 1. Traffic and soil disturbance for the no-till field.17 May 199317 June 199310-12 Oct.8 Nov. 1993Feb. 199423 Apr. 199424 Apr. 199428 May 199427 July 199420-23 Oct.15 May 199520 May 199519 June 199521 July 199520 Oct. 199510 Nov. 1995Feb. 199626 Apr. 199627 Apr. 199620 June 1996

Tractor trafficDrill soybean and tractor trafficCombine trafficAnhydrous application and tractor trafficTractor traffic (frozen soil)Plant corn and tractor trafficTractor trafficTractor trafficTractor trafficCombine trafficTractor trafficDrill soybean and tractor trafficTractor trafficTractor trafficCombine trafficAnhydrous application and tractor trafficTractor traffic (soil frozen)Plant corn and tractor trafficTractor trafficTractor traffic

the full pooled subsection because that would invalidate analy-sis of residue and organic C.) From the air-dry water contentwe determined the oven-dry mass; and we calculated bulkdensity from the oven-dry water content and known volumeof the pooled subsection at the time of sampling (cylinder of19-mm diam. and 20-mm length X 12 samples). Because soilwater content influences bulk density, we determined watercontent at the time of sampling by weighing the pooled subsec-tions before air-drying for the sampling dates of 13 Apr. 1995,13 June 1995,11 Apr. 1996, and 16 Aug. 1996.

For additional site characterization, we determined residuedepth distribution for each pooled subsection of the 11 June1996 sampling date, dispersed 40 g of each pooled subsectionwith sodium hexametaphosphate, and then shook them over-night on a shaker. Then we washed residue out of the dis-persed, pooled subsections as they were submerged on a 1-mmsieve and expressed residue amount on an aerial basis. Wealso determined organic C and total N for each of the eighttop-depth pooled subsections for sampling done on 16 Aug.1996. After careful subsampling, we removed carbonates with1 M H2SO4 (if necessary), and then determined organic Cand total N using dry-combustion methods in a Carlo-ErbaNA1500 NCS elemental analyzer (Haake Buchler Instru-ments, Paterson, NJ1). We expressed both organic C and totalN on an aerial basis.

Table 2. Traffic, tillage, and soil disturbance for the disk field.Nov. 1992 Deep disk and tractor traffic27 May 1993 Traffic and incorporate herbicide7 June 1993 Field cultivate and tractor traffic9 June 1993 Plant soybean and tractor traffic10-12 Oct. Combine traffic9-10 Nov. Anhydrous application and tractor trafficFeb. 1994 Tractor traffic (soil frozen)24 Apr. 1994 Field cultivate, plant corn, and tractor traffic25 Apr. 1994 Traffic and incorporate herbicide9 May 1994 Harrow and tractor traffic17 May 1994 Rotary hoe and tractor traffic10 June 1994 Row cultivate and tractor traffic20-23 Oct. Combine traffic10 Nov. 1994 Deep disk and tractor traffic19 May 1995 Tractor traffic, herbicide application, and disk twice21 May 1995 Plant soybean and tractor traffic20 June 1995 Tractor traffic17 Oct. 1995 Combine traffic28 Nov. 1995 Anhydrous application and tractor trafficFeb. 1996 Tractor traffic (soil frozen)2 May 1996 Tractor traffic, herbicide application, disk, and field

cultivate twice2-6 May 1996 Plant corn and tractor traffic

'Instrument information is provided for the benefit of the readerand does not imply endorsement by the USDA.

712 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

Table 3. Small depth-incremental residue distribution, organic C,and total N for no-till and disk fields on the spring 1996 mea-surement date.

Coarse residue

Depthmm0-2020-4040-6060-8080-100100-120120-140140-160

NTt

0.2890.2430.0990.0630.0700.0430.0190.021

DKf

0.1810.2000.1450.0980.0840.0490.0190.011

Organic CNT

———— kgm0.8440.8600.8200.8200.8360.7960.7860.784

DK-i0.7600.6700.6560.6500.6380.6360.6160.614

Organic NNT

0.0520.0600.0600.0580.0600.0560.0560.056

DK

0.0420.0520.0500.0520.0500.0500.0460.044

t NT indicates no-till system; DK indicates disk system.

Additional whole-field properties (especially C and Npools) from intensive spatial sampling for these fields beforethe no-till management was started are discussed in Cambar-della et al. (1994). The disk (pothole) field had been sampledspatially in April 1992, and the no-till field had been sampledspatially in October 1992 (Cambardella et al., 1994). Our sam-pling areas were on the other side of the no-till field from thesection sampled by Cambardella et al. (1994). In addition,the two sides of the field were in alternate sequences of thecorn-soybean rotation. The sampled areas of the fields haddifferent starting points for organic C and total N (5.4 vs.4.7 kg C m~2 and 0.45 vs. 0.42 kg N m~2) for disk and no-tillfields, respectively (Cambardella et al., 1994). Mean soil bulkdensities for the 0- to 150-mm depth were 1.32 Mg m~3 forthe disk field and 1.14 Mg m~3 for the sampled area of theno-till field; and soybean-residue cover in the other side of

60 -

40 -

20 -

60 -

40 -XI

~E 20 -I

60 -

Q.O 40 -

20 -

0 -

60 -

40 -

20 -

0 -

1993

_U

1994

J.i . l l h l . .1995

LilhlL.il

1996

Lli iJan Apr Jul Oct

Fig. 1. Rainfall patterns for April 1993 through August 1996 for themean of the two fields. Arrows indicate times of small incremen-tal sampling.

the no-till field for the fall of 1992 was 514 g m2. We did notmeasure surface residue in this study because of nonuniformdistribution. The intense rain and runoff in the spring andsummer of 1993 washed the previous year's corn residue inthe no-till field (the side sampled in this study) into pilesproducing very nonuniform residue distribution throughoutthe remainder of this study.

StatisticsThe first type of comparison was between sampling dates

within each field. Since we returned to the same six samplingareas in each field on each sampling date, we paired by sam-pling location to test for differences between sampling dates.We used the 95% confidence interval of the paired differences(Karlen and Colvin, 1992) to test for statistical significance.Confidence intervals all in the positive range would indicatesignificant increases, and confidence intervals all in the nega-tive range would indicate significant decreases. For eachdepth-increment we tested for differences between adjacentsampling dates and between the first and last sampling dates;for the disk field, we also tested between the October 1994and April 1995 sampling dates.

The second type of comparison was between tillage systems.Because the tillage systems were located in separate fields,the pairing mechanism was the three soils. We averaged thedata from the two replicates for each soil within a field, re-sulting in three soil means for each field (tillage system), depth,and date. We limited the whole-field comparisons to the firstand last sampling dates. The three soils were the same in thetwo fields, and we analyzed the pairs using the same confi-dence-interval technique (Karlen and Colvin, 1992). Logsdonand Kaspar (1995) analyzed several comparisons between twofields using a paired f-test analysis.

RESULTS AND DISCUSSIONDepth distribution of residue, organic C, and organic

N were similar for the two fields (Table 3). Previoussmall depth-incremental studies have shown that no-till,chisel, sweep, and disk management systems all resultedin most of their residue being in the top 100 mm (All-maras et al., 1988; Staricka et al., 1991; Allmaras et al.,1996). Spatial discussion of the two fields (Cambardellaet al., 1994) showed similar trends in these fields fororganic C and N.

Rainfall patterns may have affected some of the vari-ability in bulk density (Fig. 1). In 1993 for the 2 wkbefore pre-plant and planting field activities, there hadbeen 79 and 51 mm of rain, respectively, in the no-tilland disk fields. Similarly for the 2 wk before pre-plantand planting field activities in 1994, the rain amountswere 46 mm for both fields; in 1995, rain amounts were75 and 72 mm; and in 1996, they were 7 and 42 mm.Except for the 0- to 20-mm depth-increment of the diskfield, mean soil water contents in 1995 and 1996 (Table4) may have been high enough for traffic and/or tillagetools to compact the soil. Measurements on a nearbyfield indicated coefficient of linear expansion (COLE)values of 0.028, 0.050, and 0.097 m trr1 for Clarion,Nicollet, and Canisteo soils, respectively, indicatingsome shrink-swell potential. Because we measured bulkdensity (based on field-sampled volumes) rather thanclod density (based on changing volume during shrink-

LOGSDON & CAMBARDELLA-. CHANGES IN DEPTH-INCREMENTAL BULK DENSITY OVER TIME 713

Table 4. Mean small depth-incremental soil water for no-till and-disk fields on four dates.

Table 6. Small depth-incremental soil bulk densities for a diskfield on eight dates.

Depth

mm0-2020-4040-6060-8080-100100-120120-140140-160160-180180-200200-220220-240240-260260-280280-300

Apr.

NTt

0.240.320.340.330.340.360.340.370.330.330.340.340.340.340.35

1995DK

0.140.230.260.280.300.310.310.330.330.340.340.330.320.330.32

June

NT

0.250.330.350.350.360.370.370.370.380.380.360.360.360.350.36

1995

DK

— m3 m0.190.270.300.310.320.340.330.350.360.360.350.340.340.330.34

Apr.

NT-3

0.240.350.360.350.360.350.360.370.360.370.370.360.370.360.36

1996

DK

0.140.230.280.290.330.290.320.320.320.320.310.320.330.300.31

Aug.

NT

0.240.310.310.320.320.310.300.320.320.320.330.320.310.320.32

1996

DK

0.200.250.260.260.270.270.260.290.290.290.290.280.270.280.28

t NT indicates no-till system; DK indicates disk system.

age), soil volume changes during shrink-swell in thefield would only affect our measurements if the soilsurface had subsided (not measured).

There were not many significant bulk density changesfor the no-till field (Table 5). The disk field had signifi-cant bulk density increases between the summer of 1993and the spring of 1994. In the disk field (Table 6) thereduction in bulk densities due to the fall disking in1994 were not significant within the 95% confidenceinterval except for the 140- to 160-mm depth-increment.Comparing the first and last sampling dates for the no-till field (Fig. 2), there were no significant changes insoil bulk density at any depth (significance not shownfor this time-interval comparison). Comparing the firstand last sampling dates for the disk field (Fig. 2), therewere significant increases in density for the depth-incre-ments of 0 to 120, 180 to 200, 220 to 240, and 260 to300 mm.

Non-management factors probably contributed to soilbulk density changes in this study. At other sites in theno-till field, frequent surface measurements of soil watercontent (not shown) showed a mean volumetric watercontent range (maximum-minimum) throughout the

Table 5. Small depth-incremental soil bulk densities for a no-tillfield on seven dates.

Depth

mm0-2020-4040-6060-8080-100100-120120-140140-160160-180180-200200-220220-240240-260260-280280-300

8/93

1.14t1.271.311.331.381.331.351.261.30t1.281.30t1.261.261.24t1.24

4/94

0.97f1.271.321.331.341.361.311.351.291.341.301.321.261.261.30

10/94

1.101.261.311.351.361.441.361.351.261.331.311.291.271.331.39

4/95

- Mg m-3 -1.031.281.351.301.361.421.341.331.32t1.371.321.331.321.361.37

6/95

1.20t1.33f1.371.371.441.39f1.451.361.431.391.361.381.41f1.401.50

4/96

0.91f1.231.391.311.351.311.341.371.331.311.281.271.291.291.29

8/96

1.061.291.301.361.411.361.301.341.291.271.281.271.251.281.28

Depth

mm0-2020-4040-6060-8080-100100-120120-140140-160160-180180-200200-220220-240240-260260-280280-300

8/93

1.041.241.27f1.301.30t1.33f1.3St1.311.30t1.281.26f1.24f1.23t1.19f1.16f

4/94

1.071.301.401.401.43t1.46t1.441.431.421.431.401.42f1.401.401.38

10/94

0.961.271.421.401.571.531.531.47f1.381.371.361.331.371.341.38

11/94

— Mg m1.02f1.24f1.37f1.49f1.49f1.48f1.44f1.37f1.33t1.361.361.32t1.331.33t1.35

4/95-3

1.00f1.201.281.361.431.471.481.411.421.431.381.351.341.311.38

6/95

1.30f1.351.281.391.501.53t1.511.471.461.421.401.351.371.361.44

4/96

0.90t1.261.30t1.331.401.38t1.421.441.391.341.301.341.371.29t1.36

8/96

1.231.411.391.411.461.451.401.411.381.361.311.311.311.361.39

t Indicates significant change in bulk density between two adjacent sam-pling dates within the 95% confidence interval of the differences (notshown).

4 yr of this study to be 0.20 m3 m~3 for somewhat poorlydrained and well-drained soils, and 0.25 m3 m""3 forpoorly drained and very poorly drained soils. Combinedwith COLE data, shrink-swell would have been ex-pected to contribute to some of the observed densitychanges. Freeze-thaw would also have been expectedto contribute to soil density changes. Depth of freezingmeasured in this field and another field within 10 km(Sauer et al., 1998; and other data not shown) showedthat every winter, freezing depths extend at least 0.25 mfor at least 30 d, often freezing to 0.5 or 1.0 m for driersoils with less residue and snow cover. Biopores werepresent as well. Activity of surface-feeding earthworms(Lumbricus terrestris L.) was observed in sections of theno-till field (even before conversion to no-till) but notin the disk field; however, subsurface earthworms (Ap-porecteda spp.) were observed in both fields (E. Berry,personal communication, 1994).

Unger (1991) measured overwinter changes in bulkdensity for no-till soil. He observed for the 40- to 70-mmdepth that bulk density increased 1 yr, decreased 1 yr,and did not change significantly the third year. For the140- to 170-mm depth, the bulk density increased 2 yrand did not change significantly the third year.

In our study, comparing tillage system differences in1993 revealed that no bulk density differences betweenthe two fields were significant within the 95% confi-

~ 100 -

E

8/93

t Indicates significant change in bulk density between two adjacent sam-pling dates within the 95% confidence interval of the differences (notshown).

1.0 1.1 1.2 1.3 1.4 1.5 1.0 1.1 1.2 1.3 1.4 1.5

BULK DENSITY (Mg m'3)

Fig. 2. Comparison of depth-incremental bulk densities for the no-till and disk fields for the first and last sampling dates.

714 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

Table 7. 95% confidence intervals for the mean difference be-tween fields.

Depth 1993 1996mm0-202(MO40-6060-8080-100100-120120-140140-160160-180180-200200-220220-240240-260260-280280-300

——————— confidence-0.259-0.063-0.115-0.049-0.119-0.075-0.159-0.059-0.128-0.022-0.090-0.094-0.007-0.033-0.134-0.188-0.327-0.318-0.306-0.320-0.437-0.353-0.214-0.188-0.160-0.064-0.416-0.320-0.457-0.303

interval ———————-0.252-0.532-0.254-0.511-0.309-0.525-0.017-0.111-0.281-fl.345-0.087-0.251-0.082-0.278-0.175-0.315-0.004-0.094-0.033-0.253-0.080-0.160-0.026-0.140

0.021-0.115-0.062-0.222-0.003-0.199

dence interval (Table 7). Although this was after themanagement switch, only minor differences in manage-ment had occurred up to the time of sampling in Augustof 1993 (Tables 1 and 2). This again demonstrated thesimilarities of the fields at the start of the experiment.For the last sampling date, none of the bulk densitieswere significantly different between the fields withinthe 95% confidence interval except for a significantlylower bulk density for the no-till field in the depth-increment of 240 to 260 mm (Table 7). The reason thedifferences were not significant at other depths was be-cause of inconsistencies among the soils. Bulk densitiesof Nicollet and Clarion soils were numerically higherfor the disk field, but the lack of differences in theCanisteo soil resulted in no significant differences at theother depths (not shown).

None of these bulk densities were high enough tocause a compaction concern. Even if the densities hadbeen larger, the cracks and biopores present would re-duce the negative effect of compaction (Logsdon et al.,1992). Crop yields were numerically higher for the no-till field, but a direct comparison was not possible be-cause of variety differences. Soybean yields, averagedfor 1993 and 1995, were 2.8 and 2.4 Mg ha'1 for the no-till and disk fields, respectively. Corn yields, averagedfor 1994 and 1996, were 11.5 and 11.0 Mg ha'1 for theno-till and disk fields, respectively.

In summary, there were minor variations in soil densi-ties for both the no-till and disk management systems.Much of the soil-density variation was due to naturalcauses rather than by management. None of the bulkdensities were unusually high, but presence of mac-ropores would have prevented any root restriction orinfiltration problems. We conclude that concern aboutcompaction under no-till on these structured midwest-ern soils is unwarranted.

ACKNOWLEDGMENTS

We acknowledge the efforts of Gavin Simmons in data col-lection.