ELSEVIER Soil & Tillage Research 38 (19%) 35-46

Effect of subsoiling and subsequent tillage on soil bulk density, soil moisture, and corn yield ’

S.D. Evans a* * , M.J. Lindstrom b, W.B. Voorhees b, J.F. Moncrief ‘, G.A. Nelson a

a West Central Experiment Station, State Hwy. 329, P.O. Box 329, Morris, MN 56267, USA

b USDA-Agricultural Research Service CARS), North Central Soil Conservation Research Laboratory, Morris, MN 56267, USA

’ Department of Soil, Water, and Climate, University of Minnesota. St. Paul. MN 55108, USA

Accepted 19 February 1996

Abstract

Many producers use subsoilers periodically to alleviate suspected compaction caused by traffic from tillage, planting, and harvesting equipment. In the fall of 1988 a study was initiated in the upper Midwest region of the USA near Morris, Minnesota to study the effects of a one-time subsoiling and its interaction with four subsequent primary tillage systems (fall moldboard plowing, fall chisel plowing, spring disking, and no-tilling) on soil compaction, soil moisture, penetrometer resistance, and corn (Zea mays L.) growth and grain yield. The experiment was established on a Hamerly clay loam (Aeric Calciaquoll)-Aastad clay loam (Pachic Udic Hap- loboroll) complex. Subsoiling was performed in the fall of 1988 and the study was cropped to continuous corn from 1989 to 1991 on a site that had been farmed many years by normal 6-row, 76-cm row width equipment. Results show that subsoiling had very little effect on plant growth and no effect on grain yield over three cropping seasons following the subsoiling operation. Subsoiling had significant effects on soil bulk density and volumetric soil moisture content in 1989, but by 1990-1991 these effects were not significant. Volumetric soil moisture content generally increased in relation to soil bulk density increases. Tillage impacted surface residue accumulation, but did not affect soil bulk density, volumetric soil moisture, or grain yield. Results from this study indicate that subsoiling soils does not necessarily result in better yields or better soil moisture availability, particularly if compaction problem are not evident.

Keywords: Subsoiling; Compaction; Soil moisture; Bulk density; Penetrometer resistance

* Corresponding author. Tel.: (612) S89- 17 1 I; fax: (612) 589-4870. ’ Contribution from the Minnesota Agricultural Experiment Station Journal Series, Paper No. 21,172.

0167.1987/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO167-1987(96)01020-3

36 S.D. Evuns et al./Soil & Tillage Research 38 (1996) 35-44

1. Intr~uction

Soil compaction has been shown to reduce crop yields in the northern corn belt of the USA and Canada (Adams et al., 1960; Raghavan et al., 1979). Modem row crop production requires many trips over the field and as a result much of the field area is subjected to field traffic, some areas many times. The degree of soil compaction caused by field traffic is determined by the surface contact pressure of the tires while the depth that compaction extends into the soil is determined by axle load (Soehne, 1958). Controlled wheel-traffic studies by Voorhees et al. (1978) showed that normal row crop farming operations with equipment weight limited to 4.5 Mg per axle could result in soil compaction below 30 cm.

Compaction in the surface layer can be ameliorated to a limited extent during the winter season by freezing and thawing. This process was shown to be limited to the surface 10 cm by Voorhees (1983). A combination of shallow surface tillage and freezing and thawing was more effective than freezing and thawing alone, but to completely eliminate soil compaction in the surface 20 cm zone disruptive tillage such as moldboard plowing is required.

Compaction below the normal tillage depth presents a unique problem. High axle loads can cause compaction zones to develop below 30 cm and may extend to 50 cm or greater (Voorhees et al., 1986; Lowery and Schuler, 1991; Gameda et al., 1985). Axle loads ranged from 8 to 20 Mg which is not excessive when a modem four-wheel drive tractor may weigh 12-16 Mg. Large combine harvesters can have a loaded weight of 24 Mg with 75% of the weight on the front axle, and large grain carts can carry loads of 20-36 Mg on a single axle. The increased bulk density and soil strength below normal tillage depths are only slowly ameliorated by natural forces such as freezing and thawing or wetting and drying (Blake et al., 1976; Voorhees et al., 1986).

Increased awareness of the problems associated with soil compaction and particularly subsoil compaction has generated an interest in the possible benefits that could be obtained from a deep tillage such as subsoiling. Subsoiling trials in the northern corn belt, however, have not been shown to be effective in changing soil properties for extended periods. Improvements in soil properties that have occurred with subsoiling were rapidly negated by subsequent wheel traffic (Duval et al., 1985; Johnson et al., 1989). Subsoiling also requires high energy input and to be economically effective, the benefits from subsoiling should last for several years. Despite these problems, many farmers are subsoiling because of increased concerns about crop yield reductions attributed to subsoil compaction and to the permanent nature of compaction below the normal tillage depth.

This study was established to investigate the effects of a one-time subsoiling on soil bulk density, penetrometer resistance, soil moisture, and corn growth and yield in a field that had been in continuous corn grain or silage production for many years. The maximum axle loads on the experimental site over the past 10 years were estimated to be no more than 9 Mg. The objectives of this study were twofold: first to determine if subsoiling would improve soil properties on a production field, farmed with modem equipment with intensive field traffic and secondly to monitor soil properties after subsoiling as influenced by tillage and wheel-track variables.

S.D. Evans et al./ Soil & Tillage Research 38 (1996) 35-46 37

2. Materials and methods

2.1. Tillage, planting, and harvest

This study was conducted at the West Central Experiment Station located at Morris, MN (45”35’N, 95”55’W) on a Hamerly clay loam (fine-loamy, mixed, frigid Aeric Calciaquoll) and Aastad clay loam (fine-loamy, mixed Pachic Udic Haploboroll) com- plex. The Ap horizon had an organic matter content of 5.1% (w/w) and the profile particle size analyses are shown in Table 1. The area had been in continuous corn farmed with normal 6-row, 76-cm row width equipment with primary tillage systems of either fall moldboard plowing or fall chisel plowing. The study was conducted from the fall of 1988 through the fall of 1991 to study the effects of a one-time subsoiling on crop growth, soil compaction, and soil moisture in four primary tillage systems: fall mold- board plowing, fall chisel plowing, spring disking, and no-tilling. The experiment was cropped to continuous corn. Tillages were main plots (18.3 m X 30.5 m) and subsoiling treatments were subplots (9.1 m X 30.5 m). Primary tillage systems were split by subsoiling creating the following treatments: fall moldboard plow, subsoiled (MSS); fall moldboard plow, no subsoiling (MNS); fall chisel plow, subsoiled (CSS); fall chisel plow, no subsoiling (CNS); no-till, subsoiled (NSS); no-till, no subsoiling (NNS); spring disk, subsoiled (DSS); and spring disk, no subsoiling (DNS). The fall and spring tillage treatments by year are shown in Table 2. The experimental plot area was established in the fall of 1988. The 1988 crop was corn harvested as silage. The entire experimental area was amended with a broadcast application of 73 kg P ha- ’ and 11.2 kg zinc ha- ’ in the fall of 1988 prior to any tillage. The experimental design was a split-plot randomized complete block with four replications with tillage as main plots and subsoiling vs. no subsoiling as subplots. One-half of each main plot was subsoiled in the same direction as the existing corn rows on 14 October 1988. A subsoiler was used to establish the MSS, CSS, and NSS treatments and a paraplow to establish the DSS treatment. The MNS treatment was moldboard plowed and CNS treatment was chisel plowed in the fall of 1988. All subsequent tillages except the June anhydrous ammonia application and row cultivation are shown in Table 2.

All wheel traffic from tractors during the tillage secondary operations was confined to the position to be used by the tractor pulling the corn planter. All subsequent wheel traffic was also confined to corn planter tractor wheel tracks. The plots were seeded to hybrids ‘Pioneer 3772’, ‘Pioneer 3788’, and ‘Pioneer 3788’ corn on 12 May 1989, 14

Table 1 Particle size analysis

fiPh Clay km) (o/o, w/w)

O-15 36 15-30 38

30-45 42 45-60 44

Silt Sand

(%, w/w) (%, w/w)

34 30 34 28

32 26 30 26

38 S.D. Evans et al./ Soil R Tillage Reseurch 38 (19%) 35-46

Table 2 Primary and secondary tillage applications a

Treatment

MSS MNS css CNS DSS DNS NSS NSS

Fall 1988

ss MB ss CH PP

ss -

Spring 1989

DH DH DH DH DH DH

Fall 1989

MB MB CH CH

-

Spring 1990

DH DH DH DH DH DH

Fall 1990

MB MB CH CH

Spring 1991

FC FC FC FC DH DH - -

a SS, subsoiler, five teeth on 76 cm spacing, 41 cm depth; MB, moldboard plow, onland hitch with six bottoms on 46 cm spacing, 20 cm depth; CH, chisel plow, two rows of lo-cm-width twisted points on 38 cm spacing, 15 cm depth; PP, paraplow, four bottoms on 53 cm spacing, 33 cm depth; DH, disk harrow, two rows of 56-cm-diameter blades on 23 cm spacing with front row notched, 10 cm depth; FC, field cultivator, three rows of 23-cm-width sweeps on 53 cm spacing, 10 cm depth.

May 1990 and 15 May 1991, respectively. Plots were seeded with a B-row planter in 76-cm rows. Either terbufos 15G (S-[[ 1 ,l-dimethlyethyl)thio]methyl]O,O-diethyl phos- phorodithioate) or chlorpyrifos f5G (O,O-diethyl O-(3,5,6-trichloro-Z-pyridinyl)phos- phoro~ioate) at a rate of 11.2 kg ha- ’ (1.68 kg ha-’ active ingredient (a.i.)) was applied at seeding to control corn rootworm (Diaborotica longicornis Say.). No starter fertilizer was used. Alachlor (2-chloro-N-(2,6-diethyphenyl)“W(methoxymethy1) ac- etamide) + cyanazine (2[[4-chloro-6-(ethylamino)- 1,3,5-triazin-2-yl]amino&2-methyl propanenitrile) (3.4 + 2.5 kg ha-’ a-i. respectively) was broadcast pre-emergence on 15 May 1989, 15/16 May 1990, and 15 May 1991 for weed control. A~y~ous ~monia (82-O-O) was knife injected midway between the rows to a depth of 8 cm on 16 June 1989, 25 June 1990, and 17 June 1991 to provide 134 kg N ha-‘. All plots including no-till were row cultivated to a depth of 5 cm with a 41-cm-wide sweep cultivator on 16 June 1989,25 June 1990, and 17 June 1991.

2.2. Plant measurements

Crop residue measurements were taken in the spring prior to spring tillage and then again immediately after corn planting each year by the line transect method (Sloneker and Molde~auer, 1977). After corn emerged in 1989 and 1990, two 3-m rows in each plot were staked and corn plant heights measured at 2 week intervals to record rate of growth. Plots were harvested for grain with a plot combine on 4 October 1989, 5 October 1990, and 30 September 1991. Harvest weight and grain moisture were recorded. Grain yields were adjusted to I55 g kg- ’ moisture content.

2.3. Soil sampling

In the fall of 1988 soil samples were taken for determination of soil bulk density (pb) and volumetric soil moisture on a dry basis (01, prior to subsoiljng and tillage. Three

S.D. &am et al./&2 & Wage Research 38 lf996J 35-46 39

soil cores were taken to a depth of 60 cm in Z-cm increments with a tractor-mounted hydraulic probe from each plot. The three cores were combined for p,, and 0 determination. Each spring and fall beginning in 1989 through the fall of 1991, pb and 0 were dete~ined to a depth of 60-cm from two soil cores in tractor wheel traffic areas and two soil cores in non-wheel traffic areas of each plot. Soil bulk density and 0 were determined in 30-cm increments on 15 May 1989 and in 15-cm increments on 11 October 1989, 21 May 1990, 10 October 1990, and 3 October 1991. No soil cores were taken in the spring of 1991 due to extremely wet conditions. No attempt was made to keep track of the subsoiler slots, so some sampling may have coincided with the slots and some sampling would have occurred in areas between subsoiler slots. The soil samples were dried at 105°C for soil moisture and bulk density determinations.

Soil penetrometer resistance measurements were taken at the same time as the p,, and 0 in all years with a hand-operated recording penetrometer with a 30” conical probe, 1.9 cm in diameter, mounted on a 0.95 cm diameter shaft. A series of three penetrometer readings were taken at 2.5 cm increments to 30 cm depth, 5.0 cm increments to 40 cm depth, and a final reading at 50 cm depth from the wheel-tracked and non-wheel-tracked interrows from all treatments.

3. Results and discussion

Rainfall for the April through August period was nearly normal in 1989, below normal in 1990, and above normal in 1991. Rainfall in September and October was slightly above normal in 1989 and near normal in 1990 and 1991. Tem~ra~res during the 1989 and 1990 growing season were near normal, but were above normal in 1991, especially during May and June.

3.1. Soil bulk density

The analysis of variance values for each sampling date for pb as influenced by tillage (T), subsoiling (SS), wheel traffic (WT), and depth (D) for the spring of 1989 through the fall of 1991 are given in Table 3. The average pb taken in 1988 before subsoiling were 0.83 Mg rne3, 1.33 Mg rnm3, 1.39 Mg me3, and 1.45 Mg rnw3 in the O-15 cm, 15-30 cm, 30-45 cm, and 45-60 cm depth increments, respectively. Soil bulk density was significantly affected throughout the study by WT, D, with a WT X D interaction, as would be expected. ‘Ihe T effects were not significant at any time during the study. Subsoiling and the SS X WT interaction were significant early in the study and the effects disappeared after one growing season. In the spring of 1989 following treatment, subsoiling (averaged over all other variables) decreased pb by 0.06 Mg rnm3. In contrast, p,, at the same time was 1.24 Mg mm3 in the non-wheel track area vs. 1.38 Mg m.-3 in the tracked area. Therefore, the wheel traffic from tillage, planting, and spraying increased pa by twice as much (0.14 Mg m- 3 > as SS decreased it.

The SS X WT X D interaction was significant in the fall of 1989 (Table 3). Signifi- cant differences in pi, were observed in the O-15, 15-301 and 30-45 cm soil increments (Table 4). There was a small effect of SS in decreasing pb in the non-wheel track area

40 S.D. Euans et al. / Soil & Tillage Reseurch 38 f 1996) 35-46

Table 3 Analysis of variance for pb to a depth of 60 cm in spring 1989, fall 1989, spring 1990, fall 1990, and fall 1991

Variable a Spring 1989 Fall 1989 Spring 1990 Fall 1990 Fall 1991 --

Tillage (T) NS NS NS NS NS Subsoiling (SS) ** ** NS NS NS TxSS NS NS NS NS NS Wheel traffic (WT) * * ** ** ** ** TXWT NS * NS NS NS SSXWT ** ** NS NS NS TXSSXWT NS NS NS NS NS Depth CD) ** ** ** +* ** TxD NS NS NS NS NS SSXD NS ** NS NS NS WTXD * ** ** ** ** TXSSXD NS NS NS NS NS TXWTXD NS NS NS NS NS SSXWTXD NS ** NS NS NS TxSSXWTXD NS NS NS NS NS

cv (o/o) 10.2 5.5 7.5 5.0 5.4

a T, fall moldboard plow, fall chisel plow, disk harrow, and no-till; SS, subsoiled and non-subsoiled; WT, wheel track and non-wheel track areas; D, four depth increments (O-15, 1.5-30, 30-45, and 45-60 cm), except spring 1989, when only two depth increments were used (O-30 and 30-60 cm). * P < 0.05; l * P < 0.01; NS, non-signrficant.

and a large effect of WT in increasing pi, that same year. In the 15-30 cm zone the effect of both WT (Table 5) and SS (data not shown) are evident ~rou~out the study, even though the subsoiting effects were significant only in the fall of 1989.

Wheel traffic effects on pb were evident throughout the study (Table 5). There was a significant WT X D interaction as evidenced by the large differences in pi, between tracked and non-tracked zones in the O-15 cm and 15-30 cm soil zones. By the fall of 1991 all effects in the O-15 and 15-30 cm zones were due to wheel traffic.

3.2. Volumetric sail moisture

The analysis of variance values for each sampling date for 0 as influenced by T, SS, WT, and D for the fall of 1989 through the fall of 1991 are given in Table 6. Volume~ic

Table 4 Effects of subsoiling, wheel traffic, and depth traRic on Ph (Mg rnm3) in fat] 1989

Depth

(cm)

Subsoiled

No wheel trafftc wheel traffic

Not subsoiled

No wheel traffic

LSD (0.05)

WheeI traffic

O-15 0.63 1.20 0.80 1.17 0.08 15-30 1.20 1.40 1.35 1.44 0.04 30-45 1.38 1.42 1.41 1.46 0.04 45-60 1.48 1.51 I .45 1.48 NS

S.D. Evans et al./Soil & Tillage Research 38 (1996) 35-46 41

Table 5 Effects of wheel traffic and depth on pb (Mg mm3; averaged over subsoiled and not subsoiled) in spring 1989, fall 1989, spring 1990, fall 1990, and fall 1991

Wheel traffic Depth Spring 1989 Fall 1989 spring 1990 Fall 1990 Fall 1991 (cm)

No Yes

No

Yes

No

Yes

No Yes

45-60 - I .47 45-60 1 so

F-value 4.24 132.81

Significance ** **

O-15 O-15

15-30 - 1.28

15-30 - 1.42

30-45 1.38 b 1.40 30-45 1.47 b 1.44

1.11 a 0.75 0.81 0.97 0.96

1.30 a 1.19 1.07 1.28 1.34

I .34 1.36 1.39

1.41 1.42 1.46

1.41 1.41 1.49

1.44 1.40 1.49

1.51 1.46 1.53 1.51 1.47 1.56

23.09 19.14 29.97 ** ** **

a O-30 cm depth increment. b 30-60 cm depth increment. ** P <O.Ol.

Table 6 Analysis of variance for 8 to a depth of 60 cm in spring 1989, fall 1989, spring 1990, fall 1990, and fall 1991

Variable a Spring 1989 Fall 1989 Spring 1990 Fall 1990 Fall 1991

Tillage (T) Subsoiling (SS) TXSS Wheel traffic (WTj TXWT SSxWT TXSSXWT Depth (D) TxD SSXD WTXD TXSSXD TXWTXD SSXWTXD TxSSXWTXD

cv (%o)

NS **

NS ** *

**

NS NS NS *

**

NS

NS NS NS

8.6

NS *

NS ** *

**

NS **

NS **

** **

NS *

NS

7.1

NS

NS NS **

NS NS

NS ** **

NS **

NS

NS NS NS

10.6

NS NS NS **

NS NS

NS ** *

NS **

NS NS NS

NS

6.3

NS NS NS **

*

NS

NS **

NS NS **

NS NS NS

NS

9.6

a SS, subsoiler, five teeth on 76 cm spacing, 41 cm depth; MB, moldboard plow, onland hitch with six bottoms on 46 cm spacing, 20 cm depth; CH, chisel plow, two rows of IO-cm-width twisted points on 38 cm spacing, 15 cm depth; PP, paraplow, four bottoms on 53 cm spacing, 33 cm depth; DH, disk harrow, two rows of 56-cm-diameter blades on 23 cm spacing with front row notched, IO cm depth; FC, field cultivator, three rows of 23-cm-width sweeps on 53 cm spacing, 10 cm depth. li P < 0.05; * l P < 0.01; NS, non-signiticant.

42 S.D. Evans et al./Soil L Tiilage Reseurch 38 (19%) 3.5-46

Table 7 Effects of subsoiling and wheel traffic (averaged over four depths) on 0 (m3 m - ‘) in spring 1989, fall t 989. spring 1990, fall 1990, and fall 1991

Subsoiling Wheel traffic Spring 1989 Fall 1989 Spring 1990 Fall 1990 Fall 1991

No No 0.330 0.308 0.356 0.313 0.369 No Yes 0.351 0.338 0.384 0.346 0.404

Yes Yes

F-vague Signi~c~ce

No 0.298 0.284 0.367 0.310 0.377 Yes 0.341 0.339 0.377 0.344 0.408

14.64 34.94 2.50 0.02 0.25 ** ** NS NS NS

* ’ P < 0.01; NS, non-significant.

soil moisture was closely related to pb. A trend for reduced 0 for subsoiled versus non-subsoiled plots in non-wheel track areas appears in the spring and fall of 1989 with little effect later (Table 7). Regardless of subsoiling, 0 tended to be higher in wheel track than in non-wheel track areas.

3.3. F~ne~o~eter readings

Penetrometer resistance readings showed differences due to tillage in the spring with the no-till non-tracked treatment having a higher penetrometer resistance than the moldboard plow and chisel plow non-tracked treatments within the zone of tillage as would be expected (data not shown). These tillage differences were not present when wheel-tracked. Fall penetrometer resistance measurements showed no difference due to tillage. A highly significant difference due to wheel traffic was observed (Fig. 1). There were also highly significant differences in penetrometer resistance due to subsoiling in the non-tracked areas below the IS-em depth in the fall of 1989, while in tracked areas the subsoiling effects were small. By the fall of 1991, the effect of subsoiling had largely disappeared. In general, resistance increased significantly with soil depth in the non-tracked areas in both 1989 and 1991. In the tracked areas, resistance did not increase much with depth in the fall of 1989, but there was a small resistance increase with depth in the fall of 199 1.

Soil moisture interactions with penetrometer resistance measurements as affected by tillage or subsoiling were not considered to be a compounding factor (Table 6). In general, the moisture content was at or near field capacity at the time of measurement but when differences in penetrometer resistance were observed, 0 was greater for the wheel-tracked than for the non wheel-tracked variable (Table 7).

3.4. Residue cover

Surface residue cover measurements were made prior to spring tillage and after corn planting in 1989-1991, Tillage by subsoiling measurement indicate that all chisel, disk,

S.D. Evans et al./ Soil & Tillage Research 38 (1996) 35-46 43

1991 H 0 1 2 3 0 1 2 3

Penetrometer resistance (MPa)

Fig. 1. Effect of subsoiling and wheel traftic on soil penetrometer resistance in the fall of 1989 and 1991 after

harvest, but before fall tillage.

and no-till plots that had been subsoiled in the fall of 1988 had lower amounts of plant residue in 1989, both before spring tillage and after corn planting, than non-subsoiled plots (Table 8). In 1990 and 1991 there were no differences between subsoiled and

Table 8 Effects of tillage and subsoiling on percent plant residue cover from 1989 to 1991

Treatment 1989 1990 1991

BTa AP b BT AP BT

MNS 10 8 8 12 14

MSS ’ 28 8 7 13 15

CNS 39 15 52 41 59

css 24 9 55 42 59

DNS 58 19 88 68 82

DSS d 42 16 89 64 80

NNS 49 28 93 82 81

ss 23 16 93 81 82

AP

15 15

53 55

60

58

80 81

a BT, before spring tillage. b AP, after planting. ’ MSS was not fall plowed in 1988, only subsoiled.

d DSS was not fall chiseled in 1988, only subsoiled.

44 S.D. Evans et al./ Soil & Tillage Research 38 (1996) 35-46

Table 9 Corn plant height (cm) as affected by tillage, subsoiling, and wheel traffic in 1990

Treatment Date

20 June 6 July 19 July 3 August

Moldboard plow 41 114 184 243 Chisel plow 37 101 168 232 Spring disk 34 93 156 217

No-till 33 88 148 207

Main plots Significance

LSD (0.05)

cv (%o)

* ** **

6 10 10

5.5 6.0 4.2

Subsoiled 37 100 166 Not subsoiled 36 97 161

** 12

3.1

228 221

Subplots Significance NS NS NS **

Tillage X subsoilinginteraction Significance NS NS NS NS

* P < 0.05; * * P < 0.01; NS, non-significant,

non-subsoiled plots. The average residue cover after planting for 19!W- 1991 was 14%, 47%, 62%, and 81% for moldboard plow, chisel plow, spring disk, and no-till treat- ments, respectively.

Table 10

Corn grain yield (Mg ha-‘) as affected by tillage, subsoiling, and wheel trafftc in 1989, 1990, and 1991

Treatment 1989 1990 1991 Average

Moldboard plow 10.05 6.23 9.43 8.57

Chisel plow 9.96 5.81 8.98 8.25

Spring disk 9.85 6.8 1 9.35 8.67 No-till 9.64 6.99 8.84 8.49

Main plots Significance NS NS NS LSD (0.05) - -

cv (%) 4.0 6.7 5.4

Subsoiled 9.88 6.42 9.03 8.44

Not subsoiled 9.87 6.5 1 9.27 8.56

Subplots Significance NS NS NS

Tillage X subsoilinginteraction Significance NS NS NS

NS, non-significant.

S.D. Evans er al./Soil & Tillage Research 38 (1996) 35-46 45

3.5. Plant growth and yield

The rate of growth of corn plants, measured by corn plant height, was monitored in 1989 and 1990. Corn plant heights were not affected by subsoiling or tillage in 1989 (data not shown). Plant height was significantly influenced by tillage at all measurement dates in 1990, by subsoiling only on the 3 August, and there was no subsoiling by tillage interaction (Table 9). Grain yield was not influenced by subsoiling, tillage, or their interaction in any year (Table 10).

4. Summary

In the spring of 1989 the subsoiled areas had lower pb and 0 values than non-subsoiled areas. By the fall of 1989 the 0 differences had almost disappeared, but the pb differences remained. Compacted areas resulting from wheel traffic during planting, cultivating, and harvesting repacked the soil to its original density. The upper 30 cm of non-wheel traffic areas continued to show reduced p,, due to subsoiling through the fall of 1991. A three-way interaction of SS X WT X D was significant in 1989 and showed reduced pb due to subsoiling in the upper 45 cm. This zone is within the depth of subsoiling implements. Penetrometer measurements show large effects of wheel traffic, but very small effects of subsoiling, regardless of tillage system. Volumet- ric soil moisture was not influenced by subsoiling in 1990. In 1991, a SS X WT interaction was significant for 0 but most effects seem to be from wheel traffic, not subsoiling. Plant height was influenced by tillage and subsoiling in 1990 but not in 1989. There were no effects of tillage or subsoiling on corn grain yield in any year.

In conclusion, a one-time subsoiling of a soil that had been farmed with normal 6-row equipment had very little effect on plant growth and no effect on grain yield during three cropping seasons following the subsoiling operation. Subsoiling had significant effects on pb and 0, but these effects decreased quickly with time. The effects of normal wheel traffic following the subsoiling operation showed that the soil quickly returned to the same pb and 0 values as those of the non-subsoiled areas. There were no significant effects of the four primary tillage systems on pb, 0, or grain yield. Therefore, this is not a recommended soil management practice with pb in the range observed in this study even though the study area had been subjected to intensive wheel traffic over several years.

References

Adams, E.P., Blake, G.R., Martin, W.P. and Boelter, D.H., 1960. Influences of soil compaction on crop

growth and development. Trans. 7th Int. Congress on Soil Science, Madison, WI, Int. Sot. Soil Sci.. Vienna, Austria, Vol. 1, pp. 607-615.

Blake, G.R., Nelson, W.W. and Allmaras, R.R., 1976. Persistence of subsoil compaction in a Mollisol. Soil Sci. Sot. Am. J., 40: 943-948.

Duval, J., Raghavan, G.S.V., Mehuys, G.R. and Gameda, S., 1985. Residual effects of compaction and tillage

on the soil profile characteristics of a clay-textured soil. Can. J. Soil Sci., 69: 417-423.

46 S.D. Eunns et al./ Soil & Tillage Research 38 (1996) 35-46

Gameda, S., Raghavan, G.S.V., Theriault, R. and M&yes, E., 1985. High axle load compaction and corn yields. Trans. ASAE, 28: 175991765.

Johnson, B.S., Erickson, A.E. and Voorhees, W.B., 1989. Physical conditions of a lake plain soil as affected by deep tillage and wheel traffic. Soil Sci. Am. J., 53: 1545-1551.

Lowery, B. and Schuler, R.T., 1991. Temporal effects of subsoil compaction on soil strength and plant growth. Soil Sci. Am. J., 55: 216-223.

Raghavan, G.S.V., McKyes, E., Taylor, F., Richard, P. and Watson, A., 1979. The relationship between machinery traffic and corn yield reductions in successive years. Trans. ASAE, 22: 1256 1259.

Sloneker, L.L. and Moldenhauer, W.C., 1977. Measuring the amount of crop residue remaining after tillage. J. Soil Water Conserv., 32: 231-236.

Soehne, W., 1958. Fundamentals of pressure distribution and soil compaction under tractor tires. Agric. Eng., 39: 279-28 1,290.

Voorhees, W.B., 1983. Relative effectiveness of tillage and natural forces in aileviating wheel-induced soii compaction. Soil Sci. Sot. Am. J., 47: 129-133.

Voorhees, W.B., Senst, C.G. and Nelson, W.W., 1978. Compaction and soil structure modification by wheel traffic in the northern corn belt. Soil Sci. Am. J., 42: 344-349.

Voorhees, W.B., Nelson, W.W. and Randall, G.W., 1986. Extent and persistance of subsoil compaction caused by heavy axle loads. Soil Sci. Sac. Am. J., 50: 428-433.