Tillage Effects on Bulk Density and Soil Strength of Two Mollisols1

R. L. HILL AND R. M. CRUSE2

ABSTRACTFarmers in the Midwest are concerned that continuous no-tillage

cultivation may be causing soil compaction within their fields. Astudy was conducted to determine the effects of conservation andconventional tillage on soil bulk density and soil strength of twoMollisols. Two locations, each having randomized complete-blockdesigns with three replications of continuous corn plots under no-tillage, reduced tillage, and conventional tillage were used. One lo-cation, site 1, was in its 2nd yr of tillage experimentation and theother location, site 2, was in its 8th yr of tillage experimentation.Gamma irradiation was used to determine bulk densities of undis-turbed soil cores. Soil strength was determined by using a fall conepenetrometer at soil water matric potentials of 0, —20, and —40kPa. Tillage treatments did not have a statistically significant effecton bulk density at either site. Bulk density increased significantlywith depth for the three tillage treatments at both sites. Tillagetreatments had significant effects on soil strength at site 2, but notat site 1. Strength of soil at site 2 under reduced tillage was notsignificantly different than for soils under no-tillage, but the soilsunder both these tillage systems had significantly greater soilstrengths than that observed for conventionally tilled soil. The pat-tern of differences in soil strength due to tillage treatment was thesame at site 1, but the differences were not statistically significant.Soil strength increased with decreased matric potential for the threetillage treatments of both sites. Neither bulk density nor soil strengthof the soils under any of the tillage systems for the 0 to —40 kPamatric potential range appeared to be large enough to have an ap-preciable inhibitory effect on plant root growth.

Additional Index Words: reduced tillage, no-tillage, conservationtillage, gamma irradiation, matric potential.

Hill, R.L., and R.M. Cruse. 1985. Tillage effects on bulk densityand soil strength of two Mollisols. Soil Sci. Soc. Am. J. 49:1270-1273.

CONSERVATION TILLAGE systems may result in dif-ferent soil physical properties than those nor-

mally occurring for soils under conventional tillagesystems because the soil matrix undergoes less dis-turbance with conservation tillage. Concerns exist thatcontinuous conservation tillage may cause densifica-tion of the soil, resulting in soil bulk density (Db) andstrength conditions that may be inhibitory to plantroot growth. In tilled and untilled soil, soil strengthseemed to be the main soil physical property control-ling root growth (Ehlers et al., 1983). Soil strength has

1 Journal Paper no. J-l 1573 of the Agric. and Home Econ. Exp.Stn, Ames, IA. Project no. 2462. Received 20 Sept. 1984. Approved29 Apr. 1985.2 Former Research Associate and Associate Professor, respec-tively, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. Thesenior author is presently Assistant Professor, Agronomy Dep., Univ.of Maryland.

been shown to increase with increasing bulk densityand decreasing soil matric potential but not indepen-dently (Mirreh and Ketcheson, 1973).

The results of continuous conventional and conser-vation tillage treatments on Db are not consistent andat times are contradictory. Some researchers (Gantzerand Blake, 1978; Pidgeon and Soane, 1977) have ob-served significant differences in Db between soils un-der conventional and conservation tillage treatments,whereas other researchers (Tollner et al., 1984; Blevinset al., 1977; Shear and Moschler, 1969) have not foundany significant differences. Regardless of tillagemethod, continuous tillage has been shown to resultin increased Db for tilled soils when compared withgrassland soil Db (Bauer and Black, 1981).

Tillage effects on soil strength have not been as ex-tensively studied as tillage effects on Db. A variety ofstudies has examined soil penetrometer resistance (PR)measurements in the field for soils under different til-lage systems, with soils under conservation tillagehaving consistently greater PR than soils under con-ventional tillage (Lindstrom et al., 1984; Tollner et al.,1984; Pidgeon and Soane, 1977). However, most stud-ies concerning tillage effects on PR have not consid-ered the effect of the soil water matric potential onthe soil strength at the time of measurement, althoughthe relationship of soil water matric potential to soilstrength is well documented (Ehlers et al., 1983; Mir-reh and Ketcheson, 1973; Williams and Shaykewich,1970). Because soil strength is highly dependent onsoil water matric potential, PR measurements ob-tained when soil water conditions are not controlledor closely monitored may be a result of tillage or res-idue effects on soil water conditions rather than anindependent measure of tillage effects on soil strength.

The objectives of this study were to evaluate theeffects of conservation and conventional tillage on (i)bulk density and (ii) soil strength over a range of ma-tric potential for two soils, one of which had beenexposed to continuous tillage treatments for a rela-tively brief time and, the other, for an extended timeperiod.

MATERIALS AND METHODSExperimental Measurements

The research was conducted on soils from two establishedfield sites of tillage experiments located near Ames, IA. TheLippert farm site (referred to as site 1) and the Agronomyand Agricultural Engineering Research Center site (referredto as site 2) were located 9 miles northwest and west, re-spectively, of Ames. Site 1 had been in the same continuoustillage systems for 2 yr, whereas site 2 was in its 8th yr oftillage treatments. The tillage process for each system at the

HILL & CRUSE: TILLAGE EFPECTS ON BULK DENSITY AND SOIL STRENGTH OF TWO MOLLISOLS 1271

Table 1. Selected soil physical and chemical properties for the5.0- to 12.5-cm soil depth at site 1 and site 2.

Or- Or-Soil Site Cal- Dolo- ganic ganicseries no. pH Sand Silt Clay cite mite carbon matter

gkg-Canisteo 1 7.6 35.5 36.5 28.0 7 13 35 59Nicollet 2 6.1 28.3 43.1 28.6 Trace Trace 30 52

two sites was not identical, but was consistent with practicesfollowed in conventional, reduced, and no-tillage farmingsystems. The conventional tillage at both sites was a fallmoldboard plowing followed by a spring disking. The re-duced tillage consisted of a spring disk at site 1 and a fallchisel plowing followed by a spring disk at site 2. No-tillageat site 1 utilized a flat no-till planting in previous rows, whileat site 2, a 5-to 10-cm ridge planting in previous rows wasused. Both sites were arranged in three randomized com-plete blocks of conventional, reduced, and no-tillage treat-ment plots. Individual tillage plots were 9.2 by 38 m at site1 and 27.5 by 91.5 m at site 2. The plots were in a continuouscorn rotation with 76-cm row spacing.

Mollisols of the experimental area were a Canisteo clayloam (fine-loamy, mixed [calcareous], mesic, Typic Hapla-quolls) at site 1 and a Nicollet loam (fine-loamy, mixed,mesic, Aquic Hapludolls) at site 2. Selected soil physical andchemical properties for the 5.0- to 12.5-cm soil depth aregiven in Table 1. Soil pH was determined for 1:1 soil/watersuspensions. Particle size analyses were determined by thepipette method (Walter et al., 1978). A Chittick apparatuswas used in calcite and dolomite determinations (Boellstorff,1978). Organic C was determined by the sulfuric acid-per-manganate method (Nelson and Sommers, 1982). Organicmatter content was calculated by multiplying organic C val-ues by the "Van Bemmelen factor" of 1.724.

Undisturbed soil cores 7.6-cm diam by 7.5-cm length wereobtained with a Uhland core sampler (Blake, 1965) from the5.0- to 12.5-cm depth when the corn was in the V3 growthstage (Ritchie and Hanway, 1982). Sample initiation at the5-cm depth corresponded to the depth of seed placementwhere plant root growth would be initiated. Soil cores wereobtained between adjacent corn plants and 3.75-cm perpen-dicular from the row. Six soil cores were obtained from eachtillage plot. Care was taken not to sample in rows adjacentto interrows where recent wheel traffic had occurred becausewheel traffic can significantly alter the physical structure ofthe soil (Voorhees et al., 1978) and may diminish tillageeffects (Voorhees, 1979). The soil cores were stored in plasticbags and refrigerated. Soil cores were obtained in the 1982crop year at site 1 and in the 1983 crop year at site 2.

Bulk density of the soil cores was determined by usinggamma irradiation (Gardner, 1965) at vertical distances of1.0, 2.5, 4.0, 5.5, and 7.0 cm from the top of the core, whichcorresponds to field soil depths of 6.0, 7.5, 9.0, 10.5, and12.0 cm. The effect of tillage on bulk density was evaluatedby using an analysis of variance where tillage treatment wasthe main effect and depth was the split-plot effect. Bulk den-sity was also determined by volume-weight determinationbased on the core weight and the oven-dry water content ofthe soil.

Concerns existed that friction, which might occur betweenthe soil core and the side wall of the sampler during soilcore sampling, could affect Db. A test of the core samplingmethod was done by determining Db via gamma irradiation,parallel to the vertical axis of the core, starting at the centerof the core and proceeding to the core perimeter in 0.5-cmincrements. Several cores from each tillage treatment wereexamined.

Each soil core was subsequently divided into two soil sub-cores 6.4-cm diam by 2.5-cm length corresponding to field

1.4

1.3

1.2

Site 1

conyno-tillreduced

10 12

DEPTH (cm)Fig. 1. Bulk density for the 6- to 12-cm depth as affected by tillage

treatments at site 1 and site 2.

depths of 5.0 to 7.5 cm and 10.0 to 12.5 cm. Bulk densitywas also determined by volume-weight determination basedon the subcore weight and the oven-dry water content of thesurrounding soil. The soil subcores gave soil surfaces at fielddepths of 5.0, 7.5, 10.0, and 12.5 cm.

Strength was determined at each soil surface by using afall cone penetrometer (Hansbo, 1957). Strength was eval-uated at soil water matric potentials of 0, —20, and —40kPa. The matric potentials chosen represent values at sat-uration and slightly above and below field capacity. Thesubcores were saturated by wetting from the bottom. Matricpotential values of — 20 and —40 kPa were attained by usingcompressed air and fritted glass funnels (Richards, 1965).Five determinations of soil strength were made at each soilsurface, with the mean value taken for statistical analysis.Multiple determinations were used to reduce variability thatmight be present because of the soil strength determinationbeing taken directly in a soil ped or between peds. A differentsoil subcore was used for each series of five determinationsat each depth for each soil matric potential.

Treatment effects on soil strength were evaluated by usinganalysis of variance, where tillage treatment was the maineffect, soil matric potential was the split-plot effect, and depthwas the split-split-plot effect. Least significant differences(LSD) were utilized, following a significant F-statistic, to testfor significant differences between treatment means.

RESULTS AND DISCUSSIONResults obtained from the volume-weight and

gamma irradiation method of Db determination werein close agreement; i.e., an average tillage means dif-ference of 0.05 Mg m~3 existed for the different meth-ods of Db determination. Tillage treatments did nothave a statistically significant effect on Db regardlessof which method of Db determination was used. Be-cause gamma irradiation provided the most detailedinformation about Db, results from this method willbe discussed. Each Db value used in the analysis rep-resented the mean of six multiple determinations foreach depth and tillage treatment. The average coeffi-

1272 SOIL SCI. SOC. AM. J., VOL. 49, 1985

120

100

4 0 -

Site 1

Site 2

0L

5 7 159 11DEPTH (cm)

Fig. 2. Soil strength for 5.0- to 12.5-cm depth as affected by tillagetreatments at site 1 and site 2. Points represent mean values av-eraged across replicates and matric potentials.

cient of variance was 4.1% for the analysis at the twosites. Changes in Db with depth for the three tillagetreatments are presented in Fig. 1. Soil bulk densityin the three tillage treatments at site 2 followed thepattern of reduced tillage Db > no-tillage Db > con-ventional tillage Db. Patterns in Db for different tillagetreatments were not as evident at site 1. Because site1 was only in its 2nd yr of tillage, soil under conser-vation tillage may not have had time to attain a sta-bilized value of Db. Pidgeon and Soane (1977) re-ported that 3 yr were required for soils under no-tillageto reach an equilibrium Db, after which no furtherchanges in Db were observed.

Bulk density increased with depth for the three til-lage treatments at both sites. Not surprisingly, depthhad a significant effect on Db. Means comparisonsshowing the pattern of statistical significance are givenin Table 2. Tillage by depth interactions were not sig-nificant at either site. Because Db increased with depthat both sites, regardless of the tillage treatment used,the possibility existed that some of the increased den-sity with depth might be an artifact of the samplingmethod. If this phenomenon were occurring, the soilwould be expected to have increased Db near the sleevewall compared with the Db occurring through the corecenter. Determination of Db perpendicular to the hor-izontal axis in the cores indicated that Db does notapparently change from the center to the perimeter ofthe soil cores.

Changes in soil strength with depth for the threetillage treatments are presented in Fig. 2. Tillage treat-ment had a significant effect on soil strength at site 2,but not at site 1. The coefficient of variance for theanalysis of 24.8% at site 1 and 24.7% at site 2, indi-cated the relatively high variability of the soil strengthdetermination even though five multiple determina-tions were made. Mean comparisons for the three til-

Table 2. Depth mean comparison of bulk density_________at site 1 and site 2.

Bulk density, Mg m'3

Depth, cm

6.07.59.0

10.512.5

N

99999

Sitel

Mean

1.02 A*1.10 B1.19 C1.27 D1.31 D

Site 2

Mean

1.30 A1.35 B1.39 B1.42 C1.45 C

* Different letters indicate statistical significance at the 0.05 level usingleast significant differences (LSD).

Table 3. Tillage mean comparison of soil strengthat site 1 and site 2. ____

Soil strength, kPa

Till

RNC

N

363636

Sitel

Mean

Kl U

80.5 A*77.2 A60.7 A

Site 2

Mean

91.2 A76.9 A55.5 B

* Different letters indicate statistical significance at the 0.05 level usingleast significant differences (LSD).

lage treatments are given in Table 3. Soil strength forsoils at site 2 under reduced tillage was not signifi-cantly different from that of soils under no-tillage, butthe soils under these two tillage systems had signifi-cantly greater soil strength than conventionally tilledsoil. The pattern of differences in soil strength due totillage treatment was essentially the same at both sites,but the magnitude of the differences was smaller forsoils at site 1. Matric potential and depth had signif-icant effects on soil strength at both sites. Tillage bydepth interactions were not significant at either site.

Analysis of variance at each soil depth indicatedthat tillage treatment did not have a significant effecton soil strength at any depth for site 1. At site 2, tillagetreatment had a significant effect on soil strength atthe 7.5-, 10.0-, and 12.5-cm depth. The patterns ofstatistical significance for mean comparisons are pre-sented in Table 4 for the depths at site 2 that hadsignificant .F-statistics. The pattern of tillage differ-ences was the same as previously discussed.

Soil strength generally increased with depth for thethree tillage treatments at both sites (Fig. 2). An ob-vious exception was a decrease in soil strength for soilsunder conventional tillage at both sites as depth in-creased from 10.0 to 12.5 cm. Bauder et al. (1981)reported an increase in penetrometer cone index val-ues at the 10-cm depth compared with strength aboveor below this depth, which could be interpreted as atraffic and/or tillage-induced pan. Because care wastaken to sample in nonwheeled traffic areas, the in-creased soil strength at the 10-cm depth for the con-ventionally tilled soils is likely the result of a tillage-induced pan. The pan, which appears to be formingat the 10-cm depth for the conventionally tilled soils,still has soil strength values lower than the conser-vation-tilled soils at the same depth.

Soil strength increased with decreasing matric po-tential for the three tillage treatments at both sites.

HILL & CRUSE: TILLAGE EFFECTS ON BULK DENSITY AND SOIL STRENGTH OF TWO MOLLISOLS 1273

Table 4. Tillage means comparison of soil strength for the______7.5-, 10.0-, and 12.5-cm depths at site 2 ______

Depth, cm

Table 5. Tillage means comparison of soil strength for matricpotentials of 0, - 20, and - 40 kPa at site 2.

Till

RNC

N

999

7.5

Mean

85.9 A*68.7 A45.5 B

10.0

Mean

104.8 A88.2 AB69.8 B

12.5

Mean

107.1 A87.2 A63.8 B

* Different letters indicate statistical significance at the 0.05 level usingleast significant differences (LSD).

Tillage treatments at site 2 had significant F-statisticsfor matric potentials of —20 and —40 kPa and a sig-nificant F-statistic at the 0.10 level for 0 kPa. Tillagetreatments at site 1 had a significant F-statistic at the0.05 level only at 0 kPa. Mean differences seem to bemore evident at site 2 in comparison with site 1 asmatric potential decreases (Table 5). The greatest soilstrength (130.8 kPa) resulted from soils under reducedtillage at site 2. This value of soil strength is not largeenough so that it would be expected to adversely affectplant root growth. Although no detailed comparisonshave been made between soil strength values recordedwith penetrometers and the forces that extending rootsencounter, there are indications that penetrometers cangive gross overestimates (Russell, 1977). Observationswith penetrometers suggest that soil strength has min-imal effects on root elongation unless resistance ex-ceeds 400 kPa (e.g., Mazurak and Pohlman, 1968;Greacen and Oh, 1972).

Corn belt soils under no-tillage treatments have beenshown to have significantly higher bulk densities(Lindstrom et al., 1984; Gantzer and Blake, 1978) andpenetrometer resistances (Lindstrom et al., 1984) thancomparable soils under conventional tillage. The twosimilar soils used in this study under 2 and 8 yr ofcontinuous tillage treatments have shown no signifi-cant differences in bulk density. The 8-yr site hasshown significant differences in soil strength, but eventhe maximum soil strength observed for the range ofsoil water matric potentials tested, should not be greatenough to have an appreciable inhibitory effect onplant root growth.

Matric potential, kPa

Till

RNC

N

121212

0

Mean

41.2 A*29.9 A18.1 A

-20

Mean

106.6 A84.3 AB60.8 B

-40

Mean

130.8 A116.5B87.6 C

* Different letters indicate statistical significance at the 0.05 level usingleast significant differences (LSD).