physical conditions of a lake plain soil as affected by deep tillage and wheel traffic

7
Physical Conditions of a Lake Plain Soil as Affected by Deep Tillage and Wheel Traffic B. S. Johnson,* A. E. Erickson, and W. B. Voorhees ABSTRACT Normal fall and spring tillage practices create poor physical con- ditions for crop growth on lake plain soils in Michigan. Charity clay [fine, illitic (calcareous), mesic Aerie Haplaquept] (Mahjoory and Whiteside, 1976) is an example of such a soil. It has poor internal drainage due to its naturally dense subsoil and is susceptible to physical degradation because of its unstable surface. Tillage studies were conducted from 1983 to 1985 to evaluate the potential for re- ducing the physical limitations of Charity clay. Treatments con- sisted of four combinations of a primary and secondary tillage var- iable. The primary tillage variable involved the application or omission of deep tillage using a triple-shanked subsoiler prior to moldboard plowing. The secondary tillage variable involved the presence or absence of tractor wheel traffic prior to planting. Sub- soiling improved the physical conditions of Charity clay below the B.S. Johnson and A.E. Erickson, Dep. of Crop & Soil Sciences, Michigan State Univ., E. Lansing, MI 48824-1325; and W.B. Voor- hees, USDA-ARS, Morris, MN 56267. Contribution from the Mich- igan Agric. Exp. Stn., Michigan State Univ. Partially supported by a grant from the USDA-ARS. Received 3 Nov. 1988. "Correspond- ing author. Published in Soil Sci. Soc. Am. J. 53:1545-1551 (1989). Ap horizon. Soil bulk density (p t ) was reduced by 0.05 Mg nr 3 , and pore size distribution (PSD) was altered such that the volume of pores with radii larger than 150 Mm was doubled. Preplan! wheel traffic caused subsoil compaction, increasing p b by about 0.06 Mg nr 3 , and altered all indicators of soil compaction in the Ap horizon, especially PSD and saturated hydraulic conductivity. The need to minimize wheel traffic regardless of the primary tillage practices employed was evidenced by the results of this study. M ANY MICHIGAN SOILS are characterized by nat- urally dense layers or horizons formed under physical phenomena such as those found in till and lacustrine deposits. Lake plain soils of lacustrine ori- gin are prevalent in the Saginaw Valley, an important dry bean (Phaseolus vulgaris L.) and sugar beet (Beta vulgaris L.) production area. These fine-textured soils are considered to be compact because their pores are so small that root .penetration and internal drainage is impeded. Soils with poor internal drainage lack ad- equate aeration for root growth at certain times during

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Physical Conditions of a Lake Plain Soil as Affected by Deep Tillage and Wheel TrafficB. S. Johnson,* A. E. Erickson, and W. B. Voorhees

ABSTRACTNormal fall and spring tillage practices create poor physical con-

ditions for crop growth on lake plain soils in Michigan. Charity clay[fine, illitic (calcareous), mesic Aerie Haplaquept] (Mahjoory andWhiteside, 1976) is an example of such a soil. It has poor internaldrainage due to its naturally dense subsoil and is susceptible tophysical degradation because of its unstable surface. Tillage studieswere conducted from 1983 to 1985 to evaluate the potential for re-ducing the physical limitations of Charity clay. Treatments con-sisted of four combinations of a primary and secondary tillage var-iable. The primary tillage variable involved the application oromission of deep tillage using a triple-shanked subsoiler prior tomoldboard plowing. The secondary tillage variable involved thepresence or absence of tractor wheel traffic prior to planting. Sub-soiling improved the physical conditions of Charity clay below the

B.S. Johnson and A.E. Erickson, Dep. of Crop & Soil Sciences,Michigan State Univ., E. Lansing, MI 48824-1325; and W.B. Voor-hees, USDA-ARS, Morris, MN 56267. Contribution from the Mich-igan Agric. Exp. Stn., Michigan State Univ. Partially supported bya grant from the USDA-ARS. Received 3 Nov. 1988. "Correspond-ing author.

Published in Soil Sci. Soc. Am. J. 53:1545-1551 (1989).

Ap horizon. Soil bulk density (pt) was reduced by 0.05 Mg nr3, andpore size distribution (PSD) was altered such that the volume ofpores with radii larger than 150 Mm was doubled. Preplan! wheeltraffic caused subsoil compaction, increasing pb by about 0.06 Mgnr3, and altered all indicators of soil compaction in the Ap horizon,especially PSD and saturated hydraulic conductivity. The need tominimize wheel traffic regardless of the primary tillage practicesemployed was evidenced by the results of this study.

MANY MICHIGAN SOILS are characterized by nat-urally dense layers or horizons formed under

physical phenomena such as those found in till andlacustrine deposits. Lake plain soils of lacustrine ori-gin are prevalent in the Saginaw Valley, an importantdry bean (Phaseolus vulgaris L.) and sugar beet (Betavulgaris L.) production area. These fine-textured soilsare considered to be compact because their pores areso small that root .penetration and internal drainageis impeded. Soils with poor internal drainage lack ad-equate aeration for root growth at certain times during

1546 SOIL SCI. SOC. AM. J., VOL. 53, SEPTEMBER-OCTOBER 1989

the growing season when excess water reduces thefractional volume of air-filled pores.

Physical problems caused by the prevailing farmingpractices in Michigan are more common than thosethat occur naturally (Robertson and Erickson, 1980).Many soils in the area are susceptible to wheel-in-duced compaction. Levels of organic matter neededto stabilize soil structure are difficult to maintain incrop production systems that include dry bean andsugar beet because only small quantities of residue areproduced (Lucas and Vitosh, 1978). Furthermore,conventional management for dry bean can involvefrom four to six passes over the field each spring forfield operations including secondary tillage, herbicideapplication, and planting (Christenson and Adams,1983). Under these conditions, the probability that aparticular point on the field will be affected by at leastone pass of a tractor tire is high, as most wheel trafficis randomly distributed over the field under normalfarming operations (Voorhees et al, 1979).: Few studies have been conducted in which the pri-mary objective of deep tillage was improved soil aer-ation. Subsoiling improved aeration and increasedcorn yields on a claypan soil (Woodruff and Smith,1946). Unger and Stewart (1983) reported results of aprofile-modification study on Houston black clay.Deep profile mixing caused greater and more uniformaeration throughout the profile, increased root prolif-eration, and increased cotton (Gossypium hirsutumL.) and grain sorghum [Sorghum bicolor (L.) Moench]yields.

Cannell and Jackson (1981) stated that artificialdrainage is the principal method that can alleviatepoor aeration. They stressed that artificial drainage isuseful only when continuous transmission pores existin the topsoil and subsoil and that these pores mayhave to be created by machinery. Because it is thevolume of these large pores which is affected by tillage(Cassel and Nelson, 1985), tillage of a dense soil withpoor aeration can correct temporarily the aerationproblem (Burnett and Mauser, 1968; Erickson, 1982).

The objectives of this study were to: (i) evaluate thepotential for reducing the physical limitations of a lakeplain soil with a naturally dense subsoil and an un-stable surface by subsoiling in the fall when it is dry;(ii) determine the susceptibility of the loosened topsoiland subsoil to recompaction during subsequent traffic;and (iii) determine the persistence of changes broughtabout by subsoiling.

MATERIALS AND METHODSField experiments were conducted from the fall of 1982

through 1985 on Charity clay at the Saginaw Valley Beanand Beet Research Farm, Swan Creek, MI. Charity clay hasan unstable surface and poor internal drainage when wet,but cracks moderately when dry. The site is level and arti-ficially drained with tiles spaced at 10.1 m. The field layoutwas a split-plot with fall primary tillage as main plots andspring secondary tillage as subplots, arranged in randomizedcomplete blocks and replicated four times.

Two main plot treatments, deep tillage followed by mold-board plowing (DTMP) and no deep tillage—moldboardplowing alone (NDTMP), were applied each fall from 1982through 1984 to different areas previously planted with al-falfa (Medicago saliva L.). The alfalfa was mowed periodi-

cally during the summer and sprayed with glyphosate [iso-propylamine salt of ^-(phosphonomethyOglycine] a week ortwo before the main plots were established. Deep tillage in-volved subsoiling to a depth of about 0.35 to 0.45 m usinga heavy-duty chisel plow (Model SCP-52 Chisel Plow1, Bril-lion Iron Works, Brillion, WI) that was modified for use asa triple-shanked subsoiler with shanks spaced at 0.71 m.Main plots receiving DTMP were subsoiled in two direc-tions, at right angles to each other.

Each main plot was moldboard plowed to a depth of 0.20to 0.28 m to incorporate alfalfa residue after the surface ofsubsoiled areas was subjected to one or two rainfall events.

Main plots established each year were split to include twosecondary tillage variables: conventional spring tillage (CST)and no spring tillage (NST). The CST treatment was de-signed to simulate the normal amount of preplant wheeltraffic. A seedbed was prepared in the CST plots using aspring and spike tooth harrow after the entire plot surfacewas covered by one pass of a rear tire from a tractor weigh-ing 4200 kg. Under NST, traffic was controlled by confiningwheel traffic to interrows bordering the four-row plots. Thiswas accomplished by combining planting and herbicide ap-plication into a single operation.

All plots were planted using a four-row minimum-tillageplanter except those planted with small grains. The tractorused to apply wheel traffic for CST was used during all plant-ing operations. Tractor and planter tire spacings were 2.03m. Plot dimensions were 2.0 m wide by 20.0 m in lengthduring 1983 but reduced to 10.0 m in length during 1984and 1985. Plots were wide enough to accommodate four0.50-m rows. Thus, inter-rows adjacent to the center tworows of each plot were not compacted by wheel traffic duringplanting. Where application of postemergence herbicide wasnecessary, wheel traffic was confined to the inter-rows bor-dering the plots under both CST and NST.

A sufficient number of plots existed each year to accom-modate several different crops and to maintain test areasestablished for 1983 and 1984 through the end of the study(1985). These tests areas, representing a 2nd or 3rd yr ofinvestigation, were managed as follows: (i) Deep tillage wasapplied to an area only once so that possible residual effectsof changes brought about by deep tillage could be deter-mined. Where additional fall tillage was needed for man-agement of subsequent crops, shallow chiseling or mold-board plowing was applied uniformly to these plots, (ii) Thesecondary tillage treatments were applied without reran-domization each spring so that cumulative effects of pre-plant wheel traffic could be evaluated, (iii) Crops were ro-tated as needed.

Soil physical properties were measured on undisturbedsoil cores that were 76-mm in diam. and 76-mm in length.They were obtained with a double-cylinder, hammer-drivensampler (Blake and Hartge, 1986) and weighed at the timeof sampling to determine the field water content (6f). Fivesoil cores were obtained from each plot and at each of twodepths (0.03-0.10 and 0.13-0.20 m) within the plow layerand one depth (0.27-0.34 m) below.the normal depth ofplowing. All samples were obtained from areas bounded bythe center two rows of the four-row plots.

Physical conditions of the subsoil were evaluated for the1st, 2nd, and 3rd yr after the main plot treatments wereapplied. Physical conditions of soil in the plow layer weremeasured for the 1 st and 2nd yr. Most of the samples werecollected during the latter part of June 1985, in plots plantedwith sugar beets. Soil cores taken at the 0.27- to 0.34-mdepth in plots established for a 2nd and 3rd yr were anexception. These soil cores were obtained during the earlypart of September in plots planted with small grains. Loss

'The use of a trade name in this publication does not imply en-dorsement by Michigan State Univ. of this product nor criticism ofsimilar ones not mentioned.

JOHNSON ET AL.: EFFECT OF DEEP TILLAGE AND WHEEL TRAFFIC ON SOIL PHYSICAL CONDITION 1547

of plot area available for yield measurement was avoided inthis way, as soil had to be excavated to a depth of about0.24 m to facilitate sampling below the plow layer. Samplingwas completed in 1 d for each combination of depth andyear to circumvent variable sampling conditions in terms ofef.

Each undisturbed soil core was used to determine pb, totalporosity (P), water retention in the — 1 to —100 kPa matricpotential range, PSD, and saturated hydraulic conductivity(A ,̂,). Soil cores were saturated by wetting from the bottomfor at least 48 h. Soil water retention at matric potentials of— 1, —2, —3, —4, and — 6 kPa was determined using atension table apparatus (Learner and Shaw, 1941). Volu-metric water contents corresponding to matric potentials of—10, —33.3, and —100 kPa were determined using a pres-sure plate apparatus (Klute, 1986). Soil cores were resatur-ated after removal from the last drainage step. The K^ ofthese samples was determined using the constant-headmethod (Klute and Dirksen, 1986).

Water loss between saturation and oven drying was takento represent P. Sample shrinkage over the — 1 to —100 kPamatric potential range was negligible. Therefore, air porosityat each matric potential could be determined by subtractingthe measured volumetric water content from P. The PSDwas determined using the water desorption method, and ef-fective pore sizes corresponding to various matric potentialswere estimated using the capillary rise formula (Danielsonand Sutherland, 1986).

Tillage effects on soil physical properties were evaluatedusing analysis of variance. Because our sampling schemeprecluded statistical tests of depth effects on soil physicalproperties, separate analyses were applied to each combi-

nation of sampling depth and year after establishment ofmain plots. Treatments were compared using least signifi-cant differences (Steel and Torrie, 1960). The A^, data weretransformed using Log10(ATsal) before applying analysis ofvariance and the treatment comparison procedure to elim-inate problems caused by nonnormality in the data.

RESULTS AND DISCUSSIONSubsoil Physical Conditions

Soil pb and P during the 1st yr after subsoiling areshown in Table 1. Deep tillage followed by moldboardplowing reduced pb and increased P from levels underNDTMP. Though significant, pb under DTMP aver-aged only 4% less (0.05 Mg m-3) than under NDTMP.In Canada, subsoiling reduced pb of St. Rosalie clayby a similar amount (Douglas et al., 1980) but plowingbelow conventional depths had no ameliorative effecton Brookston clay (Bolton et al., 1981).

The influence of wheel traffic associated with CSTon pb and P is also demonstrated in Table 1. Preplantwheel traffic increased pb about 0.06 Mg m-3 after oneapplication of CST (Year 1) from levels where wheeltraffic was confined to inter-rows bordering the plots(NST). Both subsoiled and nonsubsoiled areas ap-peared to be equally susceptible to compaction at the0.27- to 0.34-m depth. The combined effects of deeptillage and controlled traffic were evident during thefirst year as the pb under DTMP-NST was 0.12 Mgm-3 less than under the NDTMP-CST treatment.

Table 1. Influence of primary and secondary tillage on bulk density, porosity, and saturated hydraulic conductivity of Charity clay at the 0.27-to 0.34-m depth during the first three yr after treatment establishment.____________________________________

Tillage

Secondary

Year 1 Year 2 Year 3

Primary CSTf NSTf CST NST CST NST

Bulk density

- Mg nr!

DTMPfNDTMPfLSDP(0.05)§LSD,(0.05)

DTMPNDTMPLSDP(0.05)LSD.(0.05)

1.33 BaJ1.38 Bb

1.26Aa1.32 Ab

1.38 Aa1.39 Aa

1.33 Aa1.35 Aa

1.35 Ba1.36 Ba

1.29 Aa1.31 Aa

0.050.06

0.060.06

0.050.05

Porosity

0.50 Ab 0.54 Bb0.48 Aa 0.51 Ba

0.020.03

0.49 Aa 0.51 Ba0.49 Aa 0.50 Aa

0.020.02

0.49 Aa 0.51 Ba0.50 Aa 0.50 Aa

0.020.02

Saturated hydraulic conductivity!

DTMPNDTMPLSDpftLSD,

4.7 Aa 18 Aa4.2 Aa 17 Aa

NSNS

2.9 Aa 6.7 Ba6.2 Aa 8.4 Aa

NS*

* Significant at the 0.05 probability level.t CST = conventional spring tillage; NST = no spring tillage; DTMP = deep tillage followed by moldboard plowing; NDTMP = no deep tillage—moldboard

plowing alone.t Means in each row followed by the same upper-case letter are not different at the indicated probability level using LSD as the criterion for significance. Means

in each column followed by the same lower-case letter are not different.§ LSDP = LSD for comparison of two primary tillage means at the same or different levels of secondary tillage; LSDS = LSD for comparison of two secondary

tillage means at the same level of primary tillage.f K^ was not measured for undisturbed soil cores taken during the 1st yr at this depth.•ft LSD values for A,., are not reported because analysis of variance and treatment comparisons were applied after transforming the data using Loglo. The level

of significance for the comparisons are given in their place.

1548 SOIL SCI. SOC. AM. J., VOL. 53, SEPTEMBER-OCTOBER 1989

Bulk density is not the most sensitive indicator ofsoil compaction because it reflects only changes in P(Voorhees, 1983). Because soil aeration can beimpeded at certain times on Charity clay, the soil en-vironment for root growth is better characterized byits soil water retention properties. Tillage effects onsoil water retention and air porosity (Pa) are comparedin Fig. 1 for Year 1. Water retention for matric po-tentials ranging from — 1 to —100 kPa was not alteredby tillage (Fig. Ib). By contrast, Pa under DTMP-NSTwas consistently greater than Pa under NDTMP-CSTfor the same range of matric potentials (Fig. la). AnPH of 0.10 m3 nr3 is frequently cited as the level belowwhich soil aeration is inadequate for plant growth(Wesseling and van Wyk, 1957; Vomocil and Flocker,1961). Only subsoiling with controlled preplant wheeltraffic produced Pa greater than 0.10 m3 irr3 over theentire —2 to —10 kPa matric potential range (Fig. la).

Figure 2 compares the PSD under the various com-binations of primary and secondary tillage. Based onthe capillary model, pores with radii less than 150, 25,and 4.4 /um retain water at matric potentials of — 1,— 6, and —33.3 kPa, respectively. Pores with radiigreater than 150 nm are important in terms of soilaeration because they are air filled at the — 1 kPa ma-tric potential. Figure 2 shows that the volume of poreswith radii greater han 150 n,m was doubled by thecombination of subsoiling and controlled traffic(DTMP-NST) compared to the normal fall tillage andtraffic patterns (NDTMP-CST). The volume of poresin the remaining size classes was unaffected by tillage,but it is the large pores that facilitate soil aeration bydraining readily under gravity.

Subsoil physical properties during the 2nd and 3rdyr are also reported in Table 1. Values for pb, P, andK^ were similar under DTMP and NDTMP. Thus,changes brought about by subsoiling were not evidentdespite the fact that wheel traffic was controlled onone-half of the plots during subsequent spring tillageoperations. Burnett and Hauser (1968) suggest thatphysical changes created by deep tillage are long last-ing on fine-textured soils like Charity clay and wherethe tillage operation is drastic. They add that changesmay be transient if the deep tillage implement used isa subsoiler or chisel. Data reported in Table 1 for the2nd and 3rd yr seem to substantiate this claim.

Table 1 shows that the effects of preplant wheeltraffic were evident each year as compaction was great-est under CST. The K^ tended to be greatest wheretraffic was controlled, especially during the 2nd yr af-ter subsoiling, averaging 17.5 /urn s"1 (6.3 cm h"1) un-der NST compared to 4.4 /an s-1 (1-6 cm tr1) underCST. Soil physical conditions during the 3rd yr rep-resent the cumulative effects of preplant wheel trafficapplied during each of three consecutive seasons. Nor-mal amounts of preplant wheel traffic decreased pb byabout 0.05 Mg nr3 from no traffic. These differencesare about the same as the differences produced by theCST and NST treatments during the 1st yr where theywere applied only once. Thus, compaction caused bythree consecutive years of preplant wheel traffic wasno greater than compaction caused during the 1st yr.

The degree of subsoil compaction caused by thewheel traffic applied with CST is not surprising, as

0.20

Fig. 1. Air porosities (a) and soil water retention (b) at the 0.27- to0.34-m depth as affected by primary tillage applied the previousfall and spring secondary tillage.

0.44-,

<uEjro

oQ.

'

0.41-

0.38-

? £3 f=Z Q

(/) t/1 V) (/IO

DS

0.10-

.

0.05-

n nn_

z o zIII%

^

'/ v

i

I">

1

D(.05)

^IOE1I

l/'.LVjSM1 r/lx way \

1\ i

^N̂\ i

1$:

I

i

i

>150 150-25 25-4.4 <4.4

Pore Radius IntervalsFig. 2. Pore-size distribution of Charity clay at the 0.27- to 0.34-m

depth as affected by primary tillage applied the previous fall andspring secondary tillage.

traffic associated with normal farming operations canalter soil physical conditions to a depth of 0.45 m(Voorhees et al. 1978). However, normal farming op-erations may include the use of combines or transportvehicles that are considerably larger than the 4200 kgtractor used to apply wheel traffic in this study. InSweden, Eriksson et al. (1974) were unable to detectmeasurable changes in porosity and permeability inthe upper part of the subsoil at axle loads less than6,000 kg. In comparison, Charity clay appears to bevery sensitive to wheel-induced compaction below thenormal depth of plowing, even when soil at this depthwas not previously loosened by deep tillage.

Physical Conditions of the Ap HorizonSoil physical properties at two depths within the

plow layer are reported in Table 2 and 3. The primarytillage variables had a negligible effect on pb, P, ATsat,or 0f at either depth. Because the corresponding pri-

JOHNSON ET AL.: EFFECT OF DEEP TILLAGE AND WHEEL TRAFFIC ON SOIL PHYSICAL CONDITION 1549

Table 2. Influence of primary and secondary tillage on bulk density,porosity, saturated hydraulic conductivity, and field water contentof Charity clay at the 0.03- to 0.10-m depth during the 1st and2nd yr after treatment establishment._______________

____________________Tillage____________________

Secondary

Year 1 Year 2

Primary CSTt NSTf CST NST

Bulk density

DTMPfNDTMPf

1.291.31

"16

1.111.15

1.321.37

1.141.14

Secondary tillagemeans

LSD(0.01)§

DTMPNDTMPSecondary tillage

meansLSDfO.Ol)

DTMPNDTMPSecondary tillage

meansLSDH

DTMPNDTMPSecondary tillage

meansLSD(0.01)

1.308}: 1.13A 1.35B 1.14A0.05 0.04

Porosity—————————— m3 m"3 ——————————

0.530.52

0.580.56

0.500.49

0.570.57

0.53A 0.57B 0.49A 0.57B0.03 0.01

Saturated hydraulic conductivity

228

ISA

190150

2118

170B 20A

Field water content

200240

220B

0.370.38

0.320.34

0.380.37

0.330.31

0.38B 0.33A0.02

0.37B 0.32A0.02

** Significant at the 0.01 probability level.t CST = conventional spring tillage; NST = no spring tillage; DTMP =

deep tillage followed by moldboard plowing; NDTMP = no deep tillage—moldboard plowing alone.

J Means followed by the same upper-case letter are not different at the in-dicated probability level using LSD as the criterion for significance.

§ LSD = the least significant difference for comparison of secondary tillagemeans within a particular year.

11 LSD values for K^ are not reported because analysis of variance and treat-ment comparisons were applied after transforming the data using Logic- Thelevel of significance for the comparisons are given in their place.

mary tillage X secondary tillage interactions were alsononsignificant, treatment comparisons included inTable 2 and 3 are limited to CST and NST means.Preplant wheel traffic caused significant compactionat both depths and during each year. At the 0.03- to0.10-m depth for example (Table 2), wheel traffic as-sociated with CST increased pb by 15 and 18% duringthe 1st and 2nd yr, respectively. The same treatmentdecreased P accordingly. Values for A^t were an orderof magnitude lower on CST than on NST, as evi-denced by the fact that one application of preplantwheel traffic (Year 1) decreased A^, to an average of15 urn s-1 (5.4 cm h-1) from 170 urn s~l (61 cm h-1).Differences reported in Table 2 were similar each year.

All indicators of soil compaction at the 0.13- to0.20-m depth were influenced by the secondary tillagevariables (Table 3). The pb was 13 and 14% higher, Pwas 7 and 9% lower under CST than NST during the

Table 3. Influence of primary and secondary tillage on bulk density,porosity, saturated hydraulic conductivity, and field water contentof Charity clay at the 0.13- to 0.20-m depth during the 1st and2nd yr after treatment establishment._______________

Tillage

Primary Secondary

Year 1 Year 2

CSTf NSTf CST NST

Bulk density

DTMPfNDTMPfSecondary tillage

meansLSD(O.Ol)

DTMPNDTMPSecondary tillage

meansLSD(O.Ol)

1.271.28

1.28Bt

1.111.14

1.12A

1.421.45

1.261.25

0.051.43B 1.25A

0.08Porosity

0.530.53

0.580.57

0.510.50

0.560.57

0.53A 0.57B0.02

0.51A 0.56B0.03

Saturated hydraulic conductivity

DTMPNDTMPSecondary tillage

meansLSDH

DTMPNDTMPSecondary tillage

meansLSD(O.Ol)

2136

29A**

0.430.43

0.43B0.05

220200

210B

Field

0.380.38

0.38A

jtlH ^ ————————————————

1521

18A*

water contentm3 m~3 ———————

0.310.30

0.3 IB0.02

5864

61B

0.290.27

0.28A

*,** Significant at the 0.05 and 0.01 probability levels, respectively.f CST = conventional spring tillage; NST = no spring tillage; DTMP =

deep tillage followed by moldboard plowing; NDTMP = no deep tillage—moldboard plowing only.

t Means followed by the same upper-case letter are not different at the in-dicated probability level using LSD as the criterion for significance.

§ LSD = the least significant difference for comparison of secondary tillagemeans within a particular year.

11 LSD values for A^, are not reported because analysis of variance and treat-ment comparisons were applied after transforming the data using Log,0. Thelevel of significance for the comparisons are given in their place.

1 st and 2nd yr, respectively. Preplant wheel traffic re-duced A^t, but less extensively at this depth than atthe 0.03- to 0.10-m depth. During the first year, K^was 210 /j.m s~' (76 cm Ir1) with NST compared to 28nm S'1 (10 cm h"1) with CST. Differences were dimin-ished during the second year as Ksal averaged 61 ^ms-1 for NST and 18 nm s-1 for CST.

The 0f of soil samples obtained for physical meas-urements are included in Table 2 and 3 to illustratethe potential influence of wheel-induced compactionon wetness of the soil surface. The 0f was consistentlyand significantly higher under CST than under NST.These results are in agreement with those reported byRaghavan and McKyes (1978) where machinery trafficaltered the soil environment such that higher watercontents occurred in heavily compacted plots. The 0fdifferences seem to reflect true effects of the secondary

1550 SOIL SCI. SOC. AM. J., VOL. 53, SEPTEMBER-OCTOBER 1989

0.45n

Pore Radius Intervals

Fig. 3. Effects of one (YR1) and two (YR2) years of varying sec-ondary tillage practices on the pore size distribution of Charityclay at the 0.03- to 0.10-m depth.

0.45-,

toIE

oQ.

0.41-

0.37-

0.33-

QA5-_

0.10-

0.05-

w •

01

0.00-YR1 YR2 YR1 YR2 YR1 YR2 YR1 YR2

>150 150-25 25-4.4 <4.4

Pore Radius Intervals (ylim)

Fig. 4. Effects of one (YR1) and two (YR2) years of varying sec-ondary tillage practices on the pore size distribution of Charityclay at the 0.13- to 0.20-m depth.

tillage variables on soil physical conditions in termsof drainage and water retention. Varying water con-tents under different tillage systems have been attrib-uted at times to the impact of tillage on evaporationrates. Evaporation can be extensive in the surface 0.15m of soil but is reduced where soil is compacted bywheel traffic (Voorhees et al., 1985). This may be es-pecially true during the "falling-rate stage" of soilevaporation, when it occurs at a rate that is dependenton the hydraulic properties of the soil and less depen-dent on available energy (Philip, 1957). However, soilcores obtained for measurements reported in Table 2and those obtained for Year 1 in Table 3 were takenimmediately following a 13-d period in which rainfalltotaled 47 mm. It is reasonable to assume that differ-ential evaporation between CST and NST was dimin-ished, as evaporation rates during this period wereprobably dominated by available energy. Thus, the 0fdifferences caused by wheel traffic can be attributedprimarily to altered drainage and soil water retentionunder CST.

The PSD of Charity clay at depths of 0.03 to 0.10m and 0.13 to 0.20 m, with pore volumes averagedfor DTMP and NDTMP, are shown in Fig. 3 and 4,

roIE

oa.

roI

roE

0.30

0.20-

0.30-100

Fig. 5. Air porosity (a) and soil water retention (b) of Charity clayat the 0.03- to 0.10-m depth as affected by 1 yr of varying sec-ondary tillage practices.

respectively. Wheel traffic associated with CST in-creased the volume of small pores at the expense ofthe volume of large pores. The volume of pores in theintermediate size classes (4.4-150 jum) was relativelyunaffected by 1 or 2 yr of preplant wheel traffic.

The Pa and water retention curves for the first yearat the 0.03- to 0.10-m depth are given in Fig. 5 toillustrate the potential impact of wheel traffic on soilaeration. Conventional spring tillage increased waterretention and decreased Pa over the — 1 to —33.3 kPamatric potential range. Bullock et al. (1985) showedthat wheel-induced compaction can decrease macro-porosity, Pa at the —6 kPa matric potential, by morethan half. Changes caused by wheel traffic were com-parable in this study. At a depth of 0.03 to 0.10 m,one application of wheel traffic reduced Pa at the — 6kPa matric potential from 0.18 m3 m"3 under NST to0.08 m3 m~3 (Fig. 5a). Macroporpsity is an indicatorof the soil aeration status following a heavy rain be-cause it approximates the volume of air-filled porespace near field capacity. Controlling preplant wheeltraffic improved soil aeration as occurrences of criti-cally low P3 were probably less frequent and shorterin duration under NST than under CST.

CONCLUSIONSNormal fall and spring tillage operations on Charity

clay create unfavorable physical conditions for cropgrowth, as internal drainage is poor and soil aerationmay be impeded at certain critical times. Physical con-ditions of Charity clay were improved below the nor-mal depth of plowing by subsoiling in the fall whenthe soil was relatively dry. Results of this investigationindicate that changes brought about by subsoiling maypersist through only 1 yr despite efforts to controlpostsubsoiling traffic.

Wheel traffic associated with conventional springtillage recompacted soil loosened by deep tillage, butalso increased the density of the subsoil where normalfall tillage was applied. Effects of wheel traffic on phys-

JOHNSON ET AL.: EFFECT OF DEEP TILLAGE AND WHEEL TRAFFIC ON SOIL PHYSICAL CONDITION 1551

ical properties below the normal depth of plowingwere evident during the 1st, 2nd, and 3rd yr after sub-soiling but were not cumulative when applied 3 yr insuccession.

The unstable surface of Charity clay proved to beextremely susceptible to wheel-induced compaction.The most sensitive indicators of compaction in theAp horizon were K^ and PSD. Subsoiling followedby controlled preplant wheel traffic produced the mostfavorable environment for root growth based on im-proved internal drainage and soil aeration.