Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol55.Issue1-2.May2000:
Soil & Tillage Research 55 (2000) 31±42
Soil and maize response to plow and no-tillage after alfalfa-to-maize
conversion on a clay loam soil in New York
U. Karunatilake, H.M. van Es*, R.R. Schindelbeck
Department of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853-1901, USA
Received 10 March 1999; received in revised form 11 January 2000; accepted 27 January 2000
Abstract
No-tillage in association with row crop production is generally believed to be poorly adapted to ®ne-textured soils,
especially in temperate humid climates. The relative success of conservation tillage may be impacted by changes in soil
structure. The objective of this study is to evaluate the performance of reduced tillage systems after rotation from a perennial
sod crop. An experiment involving spring and fall moldboard plow till (PT), no-till (NT)/zone till (ZT), and ridge till (RT)
under maize (Zea mays L.) production following alfalfa (Medicago sativa L.) was conducted on a Kingsbury clay loam soil
(Gleyic Luvisol) in Northern New York. Soil water content, strength and temperature, plant height, leaf area and number, leaf,
stem and root biomass, and root distribution were measured during the 1992 and 1993 growing seasons for spring PT and NT,
while from 1994 to 1999 only yield measurements were made. Tillage in 1992 occurred under adequately dry conditions, but
in 1993 under partially plastic consistency state, resulting in an underconsolidated plow layer. Soil water contents were
generally higher for NT than PT in 1992, but equal in 1993. Root proliferation was good in the subsoil although soil strengths
were generally above the 2 MPa level, suggesting that penetrometer measurements are not a good indicator of rooting
potential in a well-structured soil. Soil strength was higher in both years under NT, and under both tillage treatments was
negatively related to soil water content, except in the surface layer where soil penetrability appears mostly affected by
aggregate arrangement. NT recorded higher plant heights, leaf area index and leaf numbers in 1993, while PT recorded higher
per plant leaf area, stem and root biomass. Roots were generally more abundant under PT than NT at all depths, and were
reduced in traf®cked inter-row areas. Maize yield was signi®cantly higher under PT in 1992, but similar to NT in 1993. Further
yield data from 1994 to 1999 indicate that reduced tillage systems can perform equally or better compared to fall PT on this
soil type. Spring PT generally yields lower than fall PT, NT/ZT, and RT. In general, long-term use of reduced tillage systems is
economical on well-structured clay loam soils if adequate consideration is given to maintaining soil structure. # 2000
Elsevier Science B.V. All rights reserved.
Keywords: No-tillage; Zone tillage; Ridge tillage; Roots; Soil compaction; Soil quality; Clay loam; Maize
1. Introduction
*
Corresponding author. Tel.: 1-607-255-5629;
fax: 1-607-255-6143.
E-mail address: [email protected] (H.M. van Es)
Perennial crops are generally considered to improve
soil structure and may help prevent land degradation.
Conversion to conventional annual row cropping,
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 0 9 6 - 9
32
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
however, may degrade soil structure, and thereby
negatively affect soil physical processes, crop growth
potential and yield (Unger, 1975; Hermawan and
Cameron, 1993). Use of conservation tillage is generally believed to reduce these negative aspects of row
crop production by maintaining soil structure and
limiting erosion. The speci®c effects of conservation
tillage practices on soil properties may nevertheless be
contradictory due to variations in soil and environmental conditions. For example, yield differences
between conventional and conservation tillage are
known to vary greatly among soil types with ®netextured soils generally being less suitable for reduced
and no-tillage (Cox et al., 1990a). Some researchers
reported higher soil strength under conservation tillage than under conventional (plow) tillage (e.g.,
Douglas, 1986; Braim et al., 1992; Horne et al.,
1992), while others reported no differences, or even
the opposite pattern (Mielke et al., 1984; Packer et al.,
1984). A need for more site-speci®c tillage research is
therefore warranted for the proper management of
soils at any location. Generally, conventionally tilled
soils tend to have lower water contents than no-tilled
plots for shallower depths, as higher macroporosity
and low residue levels increase atmospheric water
losses (Unger and Fulton, 1990). Also, inadequate
aeration generally persists longer during wet periods
with untilled soils as they may not have suf®cient
drainable porosity from an inadequate volume of
macropores (although this may also occur in soils
that received intensive secondary tillage and packing).
Soil strength measured as penetration resistance has
been correlated with root penetrability (Chaney et al.,
1985; Braim et al., 1992; Liebig et al., 1993). It can be
measured easily, rapidly, and inexpensively and is
widely used for assessment of tillage effects on the
rooting environment. Soil strength increases with
higher soil bulk density and lower water potential
(Douglas, 1986). Hill (1990) found 2±5 times higher
penetration resistances within the 0.16 m depth under
continuous no-till cultivation compared to conventional plow tillage. This ratio became smaller with
the drying of soil although the absolute differences in
strength increased. Reports of penetration resistances
under conservation tillage that are the same or lower
than under conventional tillage (e.g., Mielke et al.,
1984; Packer et al., 1984; Simmons and Cassel, 1989;
Hermawan and Cameron, 1993) usually involved
structurally unstable soils in which tillage increased
dispersion, disaggregation and settling after heavy
rainfall.
Delayed early growth of maize under conservation
tillage compared to conventional tillage may be
caused by higher mechanical impedance (Hughes
et al., 1992). Braim et al. (1992) reported a depression
in ®nal barley (Hordeum spp.) shoot mass by 20%
under direct drilling and 10% under shallow cultivation compared to conventional tillage. Carefoot et al.
(1990) reported cereal grain yields in a semi-arid
region under no-tillage to be greater than those under
conventional till on medium and ®ne-textured soils but
not on coarse-textured soils. Conversely, Cox et al.
(1990a) in a humid region reported better adaptability
of reduced and no-tillage systems to coarse-textured
soils, especially during wet years. Hughes et al. (1992)
observed a 22% forage yield reduction in maize grown
under no-till compared to conventional till in drier
years, but not during wet years. In general, the relative
success of reduced and no-tillage systems in the North
Central and Northeastern USA is strongly affected by
weather and soil type, with ®ne-textured and poorly
drained soils generally posing the greatest challenge to
their adoption (Johnson and Lowery, 1985; Grif®th
et al., 1986; Lal et al., 1989; Cox et al., 1990b). It is
postulated that this apparent lack of adaptability of
reduced tillage is in part related to soil structural
problems. The objectives of this study were to evaluate
the effects of conventional and reduced tillage on soil
physical behavior (water content, temperature, and
strength), and maize response (root distribution, shoot
growth, and yield) on a clay loam soil after conversion
from a perennial sod crop, and to determine the longerterm effects of these tillage practices on crop yield.
2. Materials and methods
2.1. Experimental site
The experiment was conducted at the Cornell University Experimental Farm at Willsboro, NY
(44.228N, 73.268W). The soil at the site is a Kingsbury
clay loam (Gleyic Luvisol, FAO; ®ne, illitic, mesic,
Aeric Ochraqualf, USDA) with 400±600 g kgÿ1 clay
within the soil pro®le which originated from glaciolacustrine deposits. The site has a slope of 2±3%.
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
Drainage at the experimental site has been improved
with subsurface drains at 0.91 m depth and 18.3 m
spacing. The site was under alfalfa sod for 5 years
prior to this study.
A spatially balanced randomized complete block
design (van Es and van Es, 1993) with four replicates
was used for allocation of treatments to plots. The
experimental design was based on four tillage treatments, no-tillage (NT), spring plow tillage (PT), fall
PT, and ridge tillage (RT). Only two treatments, spring
PT and NT, were used for intensive measurements
during 1992 and 1993 as they represent extremes in
terms of soil disturbance. The tillage experiment was
continued from 1994 to 1999, but only yield measurements were made. The plots were 27.518.3 m2 in
size. Alfalfa was sprayed with glyphosate in the fall of
1991 and again immediately after planting in 1992.
Moldboard plowing was performed to a depth of
20 cm in PT plots using a four-bottom moldboard
plow on 21 May 1992 and 25 May 1993. Secondary
tillage was performed on the same dates using offset
double disks. Plots were disked a second time on the
following day in 1993 to ameliorate cloddy seedbed
conditions.
Maize (variety Pioneer 3751) was planted on 21
May 1992 and 26 May 1993 at 0.76 m row spacing and
target populations of 68 000 seeds haÿ1 using a fourrow Buffalo planter (Fleischer Manufacturing, Lincoln, NE). The average depth of seed placement was
0.05 m. Fertilizer and herbicide applications were
based on Cornell University recommendations and
were identical for all tillage treatments. No nutrient
de®ciencies or pest pressure was observed during the
course of the experiment. Planting and harvesting
were performed using four-row equipment with controlled traf®c. Ridges were built in late spring on RT
plots using a Buffalo ridger-cultivator. Starting in
1995, the NT plots were converted to zone tillage
(ZT) using a four-row Kinze zone-tillage planter
(Kinze Manufacturing, Williamsburg, IA). It includes
three ¯uted coulter blades and spider row cleaners to
create a 15 cm wide and 10 cm deep loosened and
residue-free strip around the plant row.
2.2. Weather and soil measurements
Precipitation and maximum and minimum temperatures were measured daily at an automated weather
33
station (located within 200 m from the research site)
managed by the Northeast Regional Climate Center
(Ithaca, NY). In both years, instruments for soil
measurements were installed in the fourth and ®fth
non-traf®cked inter-row from the plot border within 5
days of planting. Soil water content was measured at
0.15 m depth increments to a depth of 1.2 m using a
®eld-calibrated neutron moisture gauge (Model
503DR Hydroprobe, Campbell Paci®c Nuclear, Martinez, CA). Custom-made thermistors were installed at
0.02, 0.05, 0.10, 0.15. 0.20, 0.30, 0.45, and 0.75 m
depths. Digital multimeter (Radio Shack, Fort Worth,
TX) readings were converted to soil temperature using
individual calibration curves for each thermistor. Soil
water content and temperature measurements were
made in each plot during the growing season from
May to October at 2-day intervals during the early part
of the growing season, and less frequently during the
latter part. Penetrometer measurements were taken for
every 0.035 m increment to a depth of 0.49 m using a
digital recording Bush soil penetrometer (Findley
Irvine, Penicuik, Scotland) with a 308 angle, 12 mm
diameter cone. Four measurements were made in
the non-traf®cked inter-rows and in the row zones
in each plot. Measurements were made seven times
during the season in both years and converted to
cone index values (Cassel, 1983). Estimates for
0.15, 0.30 and 0.45 m depths were obtained by
averaging three readings obtained from the nearest
depths for each, and were used for comparison with
soil water content estimates from the neutron moisture
gauge.
2.3. Plant measurements
Plant measurements were made only in 1993.
Plant population was determined at 32 days after
tillage (DAT) by counting the number of plants in
a 302.28 m2 area in each plot covering three
adjacent rows. Crop residue cover was measured
at 32 DAT using the line transact method (Sloneker
and Moldenhauer, 1977). Five plants per plot were
marked randomly at the beginning of the growing
season and used throughout the season to measure
plant growth parameters. Plant height was measured
from the ground level to the base of the last fully
opened leaf (when ligules were visible), eight times
during the season. Leaf area was estimated by
34
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
measuring leaf length from the base to the tip and
leaf width at the widest place for the same ®ve
plants and on the same dates as the plant height
measurements. A factor to convert length and width
measurements to actual leaf areas was developed at
the beginning of the season from leaf measurements
of 30 plants. Leaf area index (LAI) was estimated
by dividing leaf areas by the equivalent surface
area for each plant at the measurement location.
Plant biomass was determined by cutting aboveground plant parts from ®ve plants per plot. The
leaves and stems were determined separately for 23,
32, 39, 51, 60, 70 and 77 DAT after drying at 658C
for 4 days.
Root weight was determined for 26, 36, 42, 53, 66,
74 and 81 DAT by excavating the entire root system
and surrounding soil down to 1 m, for the same ®ve
plants used for shoot weight measurements. Root and
soil material were saturated for 2±3 days prior to
washing with the use of a low pressure water jet.
Oven dry weight was determined after drying at 658C
for 4 days. The trench pro®le method (BoÈhm, 1979)
was used to map root distributions in two replicates at
45 and 85 DAT. Trenches were dug perpendicular to
maize rows to include the planting row, non-traf®cked
inter-row, and traf®cked inter-row positions. Trench
walls (2 m width1 m depth) were leveled and
washed, and a plastic sheet and 55 cm2 iron mesh
grid were ®xed onto the wall. Visible roots were
marked on the plastic sheet.
Maize grain yield for each plot was determined by
harvesting ears from each of ®ve rows of 6.1 m length.
Water content of a subsample of 20 ears was used to
adjust plant grain yields to 15.5% moisture.
Analyses of variance were performed using the SAS
GLM procedure (SAS Institute, 1985). Error mean
squares were estimated based on repeated measurements in both time and space. The data were analyzed
as from a series of experiments as described by
Cochran and Cox (1957). The SAS VARCOMP
procedure was used to estimate variance components
and proper error mean square values. The method
suggested by Satterthwaite (1946) was used to
obtain approximations to F-values. Duncan's multiple
range test and least signi®cant differences were
carried out to compare means. Unless indicated
otherwise, statistical signi®cance was based on the
a0.05 error level.
3. Results and discussion
3.1. Soil aggregation
The 1993 growing season (1 May±30 September)
was slightly warmer than 1992, with average temperatures of 18.5 and 17.28C, respectively. The 1992
and 1993 growing seasons received 272.5 and
255.3 mm rainfall, respectively (Fig. 1) compared to
an estimated 30-year mean growing season rainfall of
390 mm. In 1992, only 6 mm was received during the
10-day period prior to tillage compared to 28 mm in
1993. In 1993, tillage was delayed by 5 days compared
to 1992 due to wetter soil conditions. In 1992, tillage
of the killed alfalfa sod occurred under friable soil
consistency and resulted in a seedbed with coarse
angular aggregates of 10±20 mm diameter. In 1993,
soil aggregates in the surface layer had reduced to
3 mm size (unpublished data) and were also affected
by compaction and smearing due to tillage when the
lower part of the plow layer had plastic consistency.
For the PT treatment, disking was therefore performed
twice for seedbed preparation in 1993 to address
underconsolidation problems, compared to only once
Fig. 1. Precipitation at the research site in 1992 and 1993.
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
35
NT had consistently higher soil water contents
(expressed here as depth equivalent, i.e., volumetric
water contentsoil layer thickness) for 0.075±
0.675 m depth in 1992 (Fig. 2). Signi®cant differences
(a0.05) were only recorded for 0.075±0.225 m depth
interval (25 out of 46 measurement dates), where
differences in aggregate arrangement between PT
and NT were greatest. This may be explained for
the surface horizon by a higher volume fraction of
large pores providing greater gravitational drainage
under PT compared to NT (Coote and Malcolm-
McGovern, 1989; Hayhoe et al., 1993). In addition,
higher surface residue cover for NT than PT (78 and
17%, respectively) may have contributed to wetter soil
at shallower depths, as also measured by Nyborg and
Malhi (1989). Ohiri and Ezumah (1990) reported
increased soil drying rates and decreased water contents after tillage due to vapor movement being
enhanced by increased macroporosity within the plowed layer. This process likely played a major role in
the plow layer in this study because the coarse aggregate arrangement resulting from tillage promoted
penetration of air currents into inter-aggregate cavities. Total pro®le water content under NT was consistently higher than under PT, although these
differences were only signi®cant for 12 out of 46
measurement dates (Fig. 2).
In contrast to 1992, the water contents in 1993
(Fig. 3) did not show differences between tillage
treatments at individual measurement depths nor for
Fig. 2. Soil water content in 1992 for PT and NT by depth interval
and days after tillage (*indicates dates with signi®cant differences
at a0.05).
Fig. 3. Soil water content in 1993 for PT and NT by depth interval
and days after tillage (*indicates dates with signi®cant differences
at a0.05).
in 1992. Most differences in measured soil properties
between 1992 and 1993 were related to different soil
aggregate size distributions, apparently as a result of
these varying soil water and consistency conditions at
the time of tillage.
3.2. Soil water
36
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
the total pro®le. The measurements in 1993 were for a
shorter time period and may not have been suf®cient to
observe differences in water uptake later in the season.
Apparently, higher surface residue cover under NT
was not very effective in conserving soil moisture.
Drying patterns of deeper soil layers during a
growing season generally follow the root growth
pattern (Hamblin, 1982). In both years, soil water at
depths between 0.225 and 0.825 m decreased by the
end of the season. Drying was initiated at 0.225±
0.375, 0.375±0.525, 0.525±0.675 and 0.675±
0.825 m depths on 73, 75, 78 and 90 days after tillage
(DAT) in 1992 (Fig. 2) and 48, 53, 57 and 70 DAT in
1993 (Fig. 3), respectively. A similar pattern of soil
drying was measured for PT and NT in 1992 above
0.525 m, (Figs. 2 and 3), but soil water extraction at
0.525±0.675 and 0.675±0.825 m depth intervals was
higher under PT than NT after 90 DAT, indicating
deeper and more active root growth. Under both
treatments, however, apparent root activity was minimal below 0.675 m depth, especially in 1993.
The estimated pro®le water losses during drying
periods in both 1992 and 1993 (Table 1) showed
similar water extraction for NT and PT in the early
season (21±33 DAT in 1992 and 29±46 DAT in 1993).
In the absence of well-developed root systems at this
time, soil evaporation is considered to have been the
dominant process of water loss and higher surface
residue cover for NT was apparently less important in
reducing evaporation. The pro®le water loss under PT
was signi®cantly higher in the latter part of the season
(76±103 DAT) in 1992, presumably due to water
uptake resulting from a more extensive root system
(Fig. 2). In contrast, a drying period between 49 and 56
DAT in 1993 did not show signi®cant tillage treatment
differences in root water uptake (Fig. 3), although it
may have been too early in the season to show root
growth differences.
3.3. Soil temperature
Soil temperature data (Figs. 4 and 5) generally show
equal or higher values at shallow depths for the NT
than the PT treatment. These were signi®cant in 1992
for 32 out of 50 and 18 out of 50 measurement dates
for the 0.02 and 0.05 m depths, respectively, and in
1993 for 2 out of 45 and 3 out of 45 dates at the 0.02
and 0.05 m depth, respectively. This is generally in
contradiction to results by others from similar climates
(e.g., Wall and Stobbe, 1983; Carefoot et al., 1990;
Cox et al., 1990a), who reported lower soil temperatures for NT. In our study, soil temperatures were
measured at daily intervals during the early part of the
morning. At that time, higher soil temperatures may
occur under NT due to the insulating effect of surface
Table 1
Estimated pro®le water losses from 0.075 to 1.125 m depth during
soil drying periods
Year
Days after
tillage
Tillage
Profile water
loss (mm)
1992
21±33
Plow till
No-till
Plow till
No-till
13.2aa
14.2a
35.8a
30.2b
Plow till
No-till
Plow till
No-till
15.8a
16.9a
23.8a
25.7a
76±103
1993
29±46
49±56
a
Values with same letter in a column are not signi®cantly
different between tillage treatments within each drying period at
a0.05.
Fig. 4. Soil temperature in 1992 for PT and NT by depth interval
and days after tillage.
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
Fig. 5. Soil temperature in 1993 for PT and NT by depth interval
and days after tillage.
residue during cooler nights. In addition, enhanced air
circulation in the coarsely aggregated surface layer
under PT may also have increased night-time soil
cooling (Wierenga et al., 1982). The soil temperature
data in this study have not been made with suf®cient
temporal frequencies to evaluate the diurnal ¯uctuations, which are essential to relate them to crop
growth.
3.4. Soil strength
Tillage can affect soil strength in various ways
(Cassel et al., 1995). Fig. 6 depicts penetrometermeasured soil strength (averaged for row and nontraf®cked inter-row positions) as a function of soil
water content for multiple measurement dates. The
general trend was an increase in soil strength as the
season progressed and the soil dried out. At 0.15 m,
soil strength values were below the 2 MPa critical
level above which root growth is generally considered
to be slow (Taylor and Ratliff, 1969; Bengough and
Mullins, 1990) for most of the growing season. Only
37
Fig. 6. Soil-strength±soil-water content relationship in 1992 and
1993 for three depths (lines follow a time sequence, generally from
wet to dry).
the NT treatment recorded values above this level
during the middle and latter part of the season. For
the 0.30 and 0.45 m depths, soil strength values for
both tillage treatments were generally above the
2 MPa level, except during wet periods of the early
1993 season. Pikul et al. (1993) showed soil strength
in the surface layer under NT exceeding that under
PT by about 1 MPa in a poorly aggregated soil. In
our study, soil strength differences between tillage
treatments were smaller, which may be attributed
to the large number of failure zones in the wellaggregated soil. Also, higher water contents in NT
may have masked tillage-induced differences (Hill,
1990). Positional effects (row vs. inter-row) were
generally non-signi®cant.
Although soil strengths above 2 MPa are generally
considered to be the limit for root growth, Warnaars
and Eavis (1972) reported that not to be the case for
soils with a high degree of structural development and
where roots can explore weaker failure planes between
aggregates. This is supported by the data on soil water
content and root distribution in this study which
showed active root exploration in the subsoil despite
high strengths.
38
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
Table 2
Relationship between soil strength (y in MPa) and soil water
content (x in m3 mÿ3) using the model yabx
Year
1992
Depth
0.15
0.30
0.45
1993
0.15
0.30
0.45
Tillage
Model parameters
r2
a
b
Plow till
No-till
Plow till
No-till
Plow till
No-till
1.29
1.49
4.02
12.67
5.21
11.37
0.48
0.42
ÿ4.98
ÿ28.62
ÿ8.16
ÿ24.66
0.002nsa
0.001nsa
0.058nsa
0.416***
0.158*
0.273***
Plow till
No-till
Plow till
No-till
Plow till
No-till
7.59
7.79
21.18
13.33
13.33
22.14
ÿ18.45
ÿ17.71
ÿ53.77
ÿ31.71
ÿ46.58
ÿ55.84
0.589***
0.443***
0.794***
0.431***
0.368***
0.636***
a
Non-signi®cant.
Signi®cant at a0.1.
***
Signi®cant at a0.01.
*
The
soil-strength±water-content
relationship
(Fig. 6) of a soil may change subsequent to tillage
due to soil settling resulting from wetting and drying
cycles. Douglas (1986) reported well-de®ned relationships (r2>0.9) between soil strength and water content
at undisturbed, deeper depths, but very erratic patterns
for shallower, disturbed depths under both plowed and
fallow conditions in a well-structured soil. In our
study, well-de®ned negative relationships between
soil strength and water content were observed for
undisturbed depths (0.30 and 0.45 m) in both seasons
and also for the 0.15 m depth in 1993 when the soil
structural elements were more consolidated than in
1992 (Fig. 6 and Table 2). Apparently, the wetness±
strength relation at the 0.15 m depth in 1992 was
poorly de®ned despite wide ranges in water contents.
This is presumably the result of penetration resistance
being primarily de®ned by the frictional forces
between individual aggregates (as the soil yields to
the penetrometer force by moving aggregates), rather
than the cohesive forces exerted by intra-aggregate
water menisci (which provide most of the strength
to aggregates). An analysis of covariance showed
that only a small fraction (10%) of tillage-induced
differences in soil strength could be explained by
variable soil wetness, compared to 96% as reported
by Weaich et al. (1992). In both years, soil strength
at the 0.15 m depth increased from the ®rst to the
second measurement date despite increasing water
contents, suggesting soil settling. Besides post-tillage
soil consolidation, the non-unique relationship between water content and soil strength may be the result
of hysteresis, because soil strength within aggregates
is physically related to cohesive forces associated with
soil water potential rather than to soil water content,
as de®ned by Coulomb's equation. It is also notable
that the wetness±strength relationship is better de®ned
and has higher r2 values for 1993 than 1992 due to
observations from a wider range of soil water conditions (Fig. 6 and Table 2).
3.5. Maize growth and yield
Plant density at 32 DAT in 1993 was signi®cantly
higher under NT than PT (66 193 and 57 559 plants
haÿ1), which is dissimilar to results of several other
studies (e.g., Wall and Stobbe, 1983; Schneider and
Gupta, 1985). This may be attributed to low moisture
levels (from increased air circulation) and poor seed±
soil contact in the cloddy surface layer under PT.
NT generally maintained higher plant heights in
1993 than PT throughout the season, although this was
only signi®cant during the early part of the season
(Table 3). Higher plant heights of maize under NT
compared to PT is inconsistent with many other
research efforts. Arora et al. (1991) reported that better
plant growth of NT maize occurred only in coarsetextured soils as a result of modi®ed root growth. In
this study, higher water losses from increased air
circulation in the cloddy seedbeds under PT may be
responsible for lower plant growth in the early growing season. The maximum difference in plant height
between tillage treatments occurred at 35 DAT when
plant height under NT was 60% greater than that under
PT. This coincided with the low amount of rainfall
received during the period immediately before measurements (
Soil and maize response to plow and no-tillage after alfalfa-to-maize
conversion on a clay loam soil in New York
U. Karunatilake, H.M. van Es*, R.R. Schindelbeck
Department of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853-1901, USA
Received 10 March 1999; received in revised form 11 January 2000; accepted 27 January 2000
Abstract
No-tillage in association with row crop production is generally believed to be poorly adapted to ®ne-textured soils,
especially in temperate humid climates. The relative success of conservation tillage may be impacted by changes in soil
structure. The objective of this study is to evaluate the performance of reduced tillage systems after rotation from a perennial
sod crop. An experiment involving spring and fall moldboard plow till (PT), no-till (NT)/zone till (ZT), and ridge till (RT)
under maize (Zea mays L.) production following alfalfa (Medicago sativa L.) was conducted on a Kingsbury clay loam soil
(Gleyic Luvisol) in Northern New York. Soil water content, strength and temperature, plant height, leaf area and number, leaf,
stem and root biomass, and root distribution were measured during the 1992 and 1993 growing seasons for spring PT and NT,
while from 1994 to 1999 only yield measurements were made. Tillage in 1992 occurred under adequately dry conditions, but
in 1993 under partially plastic consistency state, resulting in an underconsolidated plow layer. Soil water contents were
generally higher for NT than PT in 1992, but equal in 1993. Root proliferation was good in the subsoil although soil strengths
were generally above the 2 MPa level, suggesting that penetrometer measurements are not a good indicator of rooting
potential in a well-structured soil. Soil strength was higher in both years under NT, and under both tillage treatments was
negatively related to soil water content, except in the surface layer where soil penetrability appears mostly affected by
aggregate arrangement. NT recorded higher plant heights, leaf area index and leaf numbers in 1993, while PT recorded higher
per plant leaf area, stem and root biomass. Roots were generally more abundant under PT than NT at all depths, and were
reduced in traf®cked inter-row areas. Maize yield was signi®cantly higher under PT in 1992, but similar to NT in 1993. Further
yield data from 1994 to 1999 indicate that reduced tillage systems can perform equally or better compared to fall PT on this
soil type. Spring PT generally yields lower than fall PT, NT/ZT, and RT. In general, long-term use of reduced tillage systems is
economical on well-structured clay loam soils if adequate consideration is given to maintaining soil structure. # 2000
Elsevier Science B.V. All rights reserved.
Keywords: No-tillage; Zone tillage; Ridge tillage; Roots; Soil compaction; Soil quality; Clay loam; Maize
1. Introduction
*
Corresponding author. Tel.: 1-607-255-5629;
fax: 1-607-255-6143.
E-mail address: [email protected] (H.M. van Es)
Perennial crops are generally considered to improve
soil structure and may help prevent land degradation.
Conversion to conventional annual row cropping,
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 0 9 6 - 9
32
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
however, may degrade soil structure, and thereby
negatively affect soil physical processes, crop growth
potential and yield (Unger, 1975; Hermawan and
Cameron, 1993). Use of conservation tillage is generally believed to reduce these negative aspects of row
crop production by maintaining soil structure and
limiting erosion. The speci®c effects of conservation
tillage practices on soil properties may nevertheless be
contradictory due to variations in soil and environmental conditions. For example, yield differences
between conventional and conservation tillage are
known to vary greatly among soil types with ®netextured soils generally being less suitable for reduced
and no-tillage (Cox et al., 1990a). Some researchers
reported higher soil strength under conservation tillage than under conventional (plow) tillage (e.g.,
Douglas, 1986; Braim et al., 1992; Horne et al.,
1992), while others reported no differences, or even
the opposite pattern (Mielke et al., 1984; Packer et al.,
1984). A need for more site-speci®c tillage research is
therefore warranted for the proper management of
soils at any location. Generally, conventionally tilled
soils tend to have lower water contents than no-tilled
plots for shallower depths, as higher macroporosity
and low residue levels increase atmospheric water
losses (Unger and Fulton, 1990). Also, inadequate
aeration generally persists longer during wet periods
with untilled soils as they may not have suf®cient
drainable porosity from an inadequate volume of
macropores (although this may also occur in soils
that received intensive secondary tillage and packing).
Soil strength measured as penetration resistance has
been correlated with root penetrability (Chaney et al.,
1985; Braim et al., 1992; Liebig et al., 1993). It can be
measured easily, rapidly, and inexpensively and is
widely used for assessment of tillage effects on the
rooting environment. Soil strength increases with
higher soil bulk density and lower water potential
(Douglas, 1986). Hill (1990) found 2±5 times higher
penetration resistances within the 0.16 m depth under
continuous no-till cultivation compared to conventional plow tillage. This ratio became smaller with
the drying of soil although the absolute differences in
strength increased. Reports of penetration resistances
under conservation tillage that are the same or lower
than under conventional tillage (e.g., Mielke et al.,
1984; Packer et al., 1984; Simmons and Cassel, 1989;
Hermawan and Cameron, 1993) usually involved
structurally unstable soils in which tillage increased
dispersion, disaggregation and settling after heavy
rainfall.
Delayed early growth of maize under conservation
tillage compared to conventional tillage may be
caused by higher mechanical impedance (Hughes
et al., 1992). Braim et al. (1992) reported a depression
in ®nal barley (Hordeum spp.) shoot mass by 20%
under direct drilling and 10% under shallow cultivation compared to conventional tillage. Carefoot et al.
(1990) reported cereal grain yields in a semi-arid
region under no-tillage to be greater than those under
conventional till on medium and ®ne-textured soils but
not on coarse-textured soils. Conversely, Cox et al.
(1990a) in a humid region reported better adaptability
of reduced and no-tillage systems to coarse-textured
soils, especially during wet years. Hughes et al. (1992)
observed a 22% forage yield reduction in maize grown
under no-till compared to conventional till in drier
years, but not during wet years. In general, the relative
success of reduced and no-tillage systems in the North
Central and Northeastern USA is strongly affected by
weather and soil type, with ®ne-textured and poorly
drained soils generally posing the greatest challenge to
their adoption (Johnson and Lowery, 1985; Grif®th
et al., 1986; Lal et al., 1989; Cox et al., 1990b). It is
postulated that this apparent lack of adaptability of
reduced tillage is in part related to soil structural
problems. The objectives of this study were to evaluate
the effects of conventional and reduced tillage on soil
physical behavior (water content, temperature, and
strength), and maize response (root distribution, shoot
growth, and yield) on a clay loam soil after conversion
from a perennial sod crop, and to determine the longerterm effects of these tillage practices on crop yield.
2. Materials and methods
2.1. Experimental site
The experiment was conducted at the Cornell University Experimental Farm at Willsboro, NY
(44.228N, 73.268W). The soil at the site is a Kingsbury
clay loam (Gleyic Luvisol, FAO; ®ne, illitic, mesic,
Aeric Ochraqualf, USDA) with 400±600 g kgÿ1 clay
within the soil pro®le which originated from glaciolacustrine deposits. The site has a slope of 2±3%.
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
Drainage at the experimental site has been improved
with subsurface drains at 0.91 m depth and 18.3 m
spacing. The site was under alfalfa sod for 5 years
prior to this study.
A spatially balanced randomized complete block
design (van Es and van Es, 1993) with four replicates
was used for allocation of treatments to plots. The
experimental design was based on four tillage treatments, no-tillage (NT), spring plow tillage (PT), fall
PT, and ridge tillage (RT). Only two treatments, spring
PT and NT, were used for intensive measurements
during 1992 and 1993 as they represent extremes in
terms of soil disturbance. The tillage experiment was
continued from 1994 to 1999, but only yield measurements were made. The plots were 27.518.3 m2 in
size. Alfalfa was sprayed with glyphosate in the fall of
1991 and again immediately after planting in 1992.
Moldboard plowing was performed to a depth of
20 cm in PT plots using a four-bottom moldboard
plow on 21 May 1992 and 25 May 1993. Secondary
tillage was performed on the same dates using offset
double disks. Plots were disked a second time on the
following day in 1993 to ameliorate cloddy seedbed
conditions.
Maize (variety Pioneer 3751) was planted on 21
May 1992 and 26 May 1993 at 0.76 m row spacing and
target populations of 68 000 seeds haÿ1 using a fourrow Buffalo planter (Fleischer Manufacturing, Lincoln, NE). The average depth of seed placement was
0.05 m. Fertilizer and herbicide applications were
based on Cornell University recommendations and
were identical for all tillage treatments. No nutrient
de®ciencies or pest pressure was observed during the
course of the experiment. Planting and harvesting
were performed using four-row equipment with controlled traf®c. Ridges were built in late spring on RT
plots using a Buffalo ridger-cultivator. Starting in
1995, the NT plots were converted to zone tillage
(ZT) using a four-row Kinze zone-tillage planter
(Kinze Manufacturing, Williamsburg, IA). It includes
three ¯uted coulter blades and spider row cleaners to
create a 15 cm wide and 10 cm deep loosened and
residue-free strip around the plant row.
2.2. Weather and soil measurements
Precipitation and maximum and minimum temperatures were measured daily at an automated weather
33
station (located within 200 m from the research site)
managed by the Northeast Regional Climate Center
(Ithaca, NY). In both years, instruments for soil
measurements were installed in the fourth and ®fth
non-traf®cked inter-row from the plot border within 5
days of planting. Soil water content was measured at
0.15 m depth increments to a depth of 1.2 m using a
®eld-calibrated neutron moisture gauge (Model
503DR Hydroprobe, Campbell Paci®c Nuclear, Martinez, CA). Custom-made thermistors were installed at
0.02, 0.05, 0.10, 0.15. 0.20, 0.30, 0.45, and 0.75 m
depths. Digital multimeter (Radio Shack, Fort Worth,
TX) readings were converted to soil temperature using
individual calibration curves for each thermistor. Soil
water content and temperature measurements were
made in each plot during the growing season from
May to October at 2-day intervals during the early part
of the growing season, and less frequently during the
latter part. Penetrometer measurements were taken for
every 0.035 m increment to a depth of 0.49 m using a
digital recording Bush soil penetrometer (Findley
Irvine, Penicuik, Scotland) with a 308 angle, 12 mm
diameter cone. Four measurements were made in
the non-traf®cked inter-rows and in the row zones
in each plot. Measurements were made seven times
during the season in both years and converted to
cone index values (Cassel, 1983). Estimates for
0.15, 0.30 and 0.45 m depths were obtained by
averaging three readings obtained from the nearest
depths for each, and were used for comparison with
soil water content estimates from the neutron moisture
gauge.
2.3. Plant measurements
Plant measurements were made only in 1993.
Plant population was determined at 32 days after
tillage (DAT) by counting the number of plants in
a 302.28 m2 area in each plot covering three
adjacent rows. Crop residue cover was measured
at 32 DAT using the line transact method (Sloneker
and Moldenhauer, 1977). Five plants per plot were
marked randomly at the beginning of the growing
season and used throughout the season to measure
plant growth parameters. Plant height was measured
from the ground level to the base of the last fully
opened leaf (when ligules were visible), eight times
during the season. Leaf area was estimated by
34
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
measuring leaf length from the base to the tip and
leaf width at the widest place for the same ®ve
plants and on the same dates as the plant height
measurements. A factor to convert length and width
measurements to actual leaf areas was developed at
the beginning of the season from leaf measurements
of 30 plants. Leaf area index (LAI) was estimated
by dividing leaf areas by the equivalent surface
area for each plant at the measurement location.
Plant biomass was determined by cutting aboveground plant parts from ®ve plants per plot. The
leaves and stems were determined separately for 23,
32, 39, 51, 60, 70 and 77 DAT after drying at 658C
for 4 days.
Root weight was determined for 26, 36, 42, 53, 66,
74 and 81 DAT by excavating the entire root system
and surrounding soil down to 1 m, for the same ®ve
plants used for shoot weight measurements. Root and
soil material were saturated for 2±3 days prior to
washing with the use of a low pressure water jet.
Oven dry weight was determined after drying at 658C
for 4 days. The trench pro®le method (BoÈhm, 1979)
was used to map root distributions in two replicates at
45 and 85 DAT. Trenches were dug perpendicular to
maize rows to include the planting row, non-traf®cked
inter-row, and traf®cked inter-row positions. Trench
walls (2 m width1 m depth) were leveled and
washed, and a plastic sheet and 55 cm2 iron mesh
grid were ®xed onto the wall. Visible roots were
marked on the plastic sheet.
Maize grain yield for each plot was determined by
harvesting ears from each of ®ve rows of 6.1 m length.
Water content of a subsample of 20 ears was used to
adjust plant grain yields to 15.5% moisture.
Analyses of variance were performed using the SAS
GLM procedure (SAS Institute, 1985). Error mean
squares were estimated based on repeated measurements in both time and space. The data were analyzed
as from a series of experiments as described by
Cochran and Cox (1957). The SAS VARCOMP
procedure was used to estimate variance components
and proper error mean square values. The method
suggested by Satterthwaite (1946) was used to
obtain approximations to F-values. Duncan's multiple
range test and least signi®cant differences were
carried out to compare means. Unless indicated
otherwise, statistical signi®cance was based on the
a0.05 error level.
3. Results and discussion
3.1. Soil aggregation
The 1993 growing season (1 May±30 September)
was slightly warmer than 1992, with average temperatures of 18.5 and 17.28C, respectively. The 1992
and 1993 growing seasons received 272.5 and
255.3 mm rainfall, respectively (Fig. 1) compared to
an estimated 30-year mean growing season rainfall of
390 mm. In 1992, only 6 mm was received during the
10-day period prior to tillage compared to 28 mm in
1993. In 1993, tillage was delayed by 5 days compared
to 1992 due to wetter soil conditions. In 1992, tillage
of the killed alfalfa sod occurred under friable soil
consistency and resulted in a seedbed with coarse
angular aggregates of 10±20 mm diameter. In 1993,
soil aggregates in the surface layer had reduced to
3 mm size (unpublished data) and were also affected
by compaction and smearing due to tillage when the
lower part of the plow layer had plastic consistency.
For the PT treatment, disking was therefore performed
twice for seedbed preparation in 1993 to address
underconsolidation problems, compared to only once
Fig. 1. Precipitation at the research site in 1992 and 1993.
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
35
NT had consistently higher soil water contents
(expressed here as depth equivalent, i.e., volumetric
water contentsoil layer thickness) for 0.075±
0.675 m depth in 1992 (Fig. 2). Signi®cant differences
(a0.05) were only recorded for 0.075±0.225 m depth
interval (25 out of 46 measurement dates), where
differences in aggregate arrangement between PT
and NT were greatest. This may be explained for
the surface horizon by a higher volume fraction of
large pores providing greater gravitational drainage
under PT compared to NT (Coote and Malcolm-
McGovern, 1989; Hayhoe et al., 1993). In addition,
higher surface residue cover for NT than PT (78 and
17%, respectively) may have contributed to wetter soil
at shallower depths, as also measured by Nyborg and
Malhi (1989). Ohiri and Ezumah (1990) reported
increased soil drying rates and decreased water contents after tillage due to vapor movement being
enhanced by increased macroporosity within the plowed layer. This process likely played a major role in
the plow layer in this study because the coarse aggregate arrangement resulting from tillage promoted
penetration of air currents into inter-aggregate cavities. Total pro®le water content under NT was consistently higher than under PT, although these
differences were only signi®cant for 12 out of 46
measurement dates (Fig. 2).
In contrast to 1992, the water contents in 1993
(Fig. 3) did not show differences between tillage
treatments at individual measurement depths nor for
Fig. 2. Soil water content in 1992 for PT and NT by depth interval
and days after tillage (*indicates dates with signi®cant differences
at a0.05).
Fig. 3. Soil water content in 1993 for PT and NT by depth interval
and days after tillage (*indicates dates with signi®cant differences
at a0.05).
in 1992. Most differences in measured soil properties
between 1992 and 1993 were related to different soil
aggregate size distributions, apparently as a result of
these varying soil water and consistency conditions at
the time of tillage.
3.2. Soil water
36
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
the total pro®le. The measurements in 1993 were for a
shorter time period and may not have been suf®cient to
observe differences in water uptake later in the season.
Apparently, higher surface residue cover under NT
was not very effective in conserving soil moisture.
Drying patterns of deeper soil layers during a
growing season generally follow the root growth
pattern (Hamblin, 1982). In both years, soil water at
depths between 0.225 and 0.825 m decreased by the
end of the season. Drying was initiated at 0.225±
0.375, 0.375±0.525, 0.525±0.675 and 0.675±
0.825 m depths on 73, 75, 78 and 90 days after tillage
(DAT) in 1992 (Fig. 2) and 48, 53, 57 and 70 DAT in
1993 (Fig. 3), respectively. A similar pattern of soil
drying was measured for PT and NT in 1992 above
0.525 m, (Figs. 2 and 3), but soil water extraction at
0.525±0.675 and 0.675±0.825 m depth intervals was
higher under PT than NT after 90 DAT, indicating
deeper and more active root growth. Under both
treatments, however, apparent root activity was minimal below 0.675 m depth, especially in 1993.
The estimated pro®le water losses during drying
periods in both 1992 and 1993 (Table 1) showed
similar water extraction for NT and PT in the early
season (21±33 DAT in 1992 and 29±46 DAT in 1993).
In the absence of well-developed root systems at this
time, soil evaporation is considered to have been the
dominant process of water loss and higher surface
residue cover for NT was apparently less important in
reducing evaporation. The pro®le water loss under PT
was signi®cantly higher in the latter part of the season
(76±103 DAT) in 1992, presumably due to water
uptake resulting from a more extensive root system
(Fig. 2). In contrast, a drying period between 49 and 56
DAT in 1993 did not show signi®cant tillage treatment
differences in root water uptake (Fig. 3), although it
may have been too early in the season to show root
growth differences.
3.3. Soil temperature
Soil temperature data (Figs. 4 and 5) generally show
equal or higher values at shallow depths for the NT
than the PT treatment. These were signi®cant in 1992
for 32 out of 50 and 18 out of 50 measurement dates
for the 0.02 and 0.05 m depths, respectively, and in
1993 for 2 out of 45 and 3 out of 45 dates at the 0.02
and 0.05 m depth, respectively. This is generally in
contradiction to results by others from similar climates
(e.g., Wall and Stobbe, 1983; Carefoot et al., 1990;
Cox et al., 1990a), who reported lower soil temperatures for NT. In our study, soil temperatures were
measured at daily intervals during the early part of the
morning. At that time, higher soil temperatures may
occur under NT due to the insulating effect of surface
Table 1
Estimated pro®le water losses from 0.075 to 1.125 m depth during
soil drying periods
Year
Days after
tillage
Tillage
Profile water
loss (mm)
1992
21±33
Plow till
No-till
Plow till
No-till
13.2aa
14.2a
35.8a
30.2b
Plow till
No-till
Plow till
No-till
15.8a
16.9a
23.8a
25.7a
76±103
1993
29±46
49±56
a
Values with same letter in a column are not signi®cantly
different between tillage treatments within each drying period at
a0.05.
Fig. 4. Soil temperature in 1992 for PT and NT by depth interval
and days after tillage.
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
Fig. 5. Soil temperature in 1993 for PT and NT by depth interval
and days after tillage.
residue during cooler nights. In addition, enhanced air
circulation in the coarsely aggregated surface layer
under PT may also have increased night-time soil
cooling (Wierenga et al., 1982). The soil temperature
data in this study have not been made with suf®cient
temporal frequencies to evaluate the diurnal ¯uctuations, which are essential to relate them to crop
growth.
3.4. Soil strength
Tillage can affect soil strength in various ways
(Cassel et al., 1995). Fig. 6 depicts penetrometermeasured soil strength (averaged for row and nontraf®cked inter-row positions) as a function of soil
water content for multiple measurement dates. The
general trend was an increase in soil strength as the
season progressed and the soil dried out. At 0.15 m,
soil strength values were below the 2 MPa critical
level above which root growth is generally considered
to be slow (Taylor and Ratliff, 1969; Bengough and
Mullins, 1990) for most of the growing season. Only
37
Fig. 6. Soil-strength±soil-water content relationship in 1992 and
1993 for three depths (lines follow a time sequence, generally from
wet to dry).
the NT treatment recorded values above this level
during the middle and latter part of the season. For
the 0.30 and 0.45 m depths, soil strength values for
both tillage treatments were generally above the
2 MPa level, except during wet periods of the early
1993 season. Pikul et al. (1993) showed soil strength
in the surface layer under NT exceeding that under
PT by about 1 MPa in a poorly aggregated soil. In
our study, soil strength differences between tillage
treatments were smaller, which may be attributed
to the large number of failure zones in the wellaggregated soil. Also, higher water contents in NT
may have masked tillage-induced differences (Hill,
1990). Positional effects (row vs. inter-row) were
generally non-signi®cant.
Although soil strengths above 2 MPa are generally
considered to be the limit for root growth, Warnaars
and Eavis (1972) reported that not to be the case for
soils with a high degree of structural development and
where roots can explore weaker failure planes between
aggregates. This is supported by the data on soil water
content and root distribution in this study which
showed active root exploration in the subsoil despite
high strengths.
38
U. Karunatilake et al. / Soil & Tillage Research 55 (2000) 31±42
Table 2
Relationship between soil strength (y in MPa) and soil water
content (x in m3 mÿ3) using the model yabx
Year
1992
Depth
0.15
0.30
0.45
1993
0.15
0.30
0.45
Tillage
Model parameters
r2
a
b
Plow till
No-till
Plow till
No-till
Plow till
No-till
1.29
1.49
4.02
12.67
5.21
11.37
0.48
0.42
ÿ4.98
ÿ28.62
ÿ8.16
ÿ24.66
0.002nsa
0.001nsa
0.058nsa
0.416***
0.158*
0.273***
Plow till
No-till
Plow till
No-till
Plow till
No-till
7.59
7.79
21.18
13.33
13.33
22.14
ÿ18.45
ÿ17.71
ÿ53.77
ÿ31.71
ÿ46.58
ÿ55.84
0.589***
0.443***
0.794***
0.431***
0.368***
0.636***
a
Non-signi®cant.
Signi®cant at a0.1.
***
Signi®cant at a0.01.
*
The
soil-strength±water-content
relationship
(Fig. 6) of a soil may change subsequent to tillage
due to soil settling resulting from wetting and drying
cycles. Douglas (1986) reported well-de®ned relationships (r2>0.9) between soil strength and water content
at undisturbed, deeper depths, but very erratic patterns
for shallower, disturbed depths under both plowed and
fallow conditions in a well-structured soil. In our
study, well-de®ned negative relationships between
soil strength and water content were observed for
undisturbed depths (0.30 and 0.45 m) in both seasons
and also for the 0.15 m depth in 1993 when the soil
structural elements were more consolidated than in
1992 (Fig. 6 and Table 2). Apparently, the wetness±
strength relation at the 0.15 m depth in 1992 was
poorly de®ned despite wide ranges in water contents.
This is presumably the result of penetration resistance
being primarily de®ned by the frictional forces
between individual aggregates (as the soil yields to
the penetrometer force by moving aggregates), rather
than the cohesive forces exerted by intra-aggregate
water menisci (which provide most of the strength
to aggregates). An analysis of covariance showed
that only a small fraction (10%) of tillage-induced
differences in soil strength could be explained by
variable soil wetness, compared to 96% as reported
by Weaich et al. (1992). In both years, soil strength
at the 0.15 m depth increased from the ®rst to the
second measurement date despite increasing water
contents, suggesting soil settling. Besides post-tillage
soil consolidation, the non-unique relationship between water content and soil strength may be the result
of hysteresis, because soil strength within aggregates
is physically related to cohesive forces associated with
soil water potential rather than to soil water content,
as de®ned by Coulomb's equation. It is also notable
that the wetness±strength relationship is better de®ned
and has higher r2 values for 1993 than 1992 due to
observations from a wider range of soil water conditions (Fig. 6 and Table 2).
3.5. Maize growth and yield
Plant density at 32 DAT in 1993 was signi®cantly
higher under NT than PT (66 193 and 57 559 plants
haÿ1), which is dissimilar to results of several other
studies (e.g., Wall and Stobbe, 1983; Schneider and
Gupta, 1985). This may be attributed to low moisture
levels (from increased air circulation) and poor seed±
soil contact in the cloddy surface layer under PT.
NT generally maintained higher plant heights in
1993 than PT throughout the season, although this was
only signi®cant during the early part of the season
(Table 3). Higher plant heights of maize under NT
compared to PT is inconsistent with many other
research efforts. Arora et al. (1991) reported that better
plant growth of NT maize occurred only in coarsetextured soils as a result of modi®ed root growth. In
this study, higher water losses from increased air
circulation in the cloddy seedbeds under PT may be
responsible for lower plant growth in the early growing season. The maximum difference in plant height
between tillage treatments occurred at 35 DAT when
plant height under NT was 60% greater than that under
PT. This coincided with the low amount of rainfall
received during the period immediately before measurements (