Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol83.Issue3-4.March2000:
Scientia Horticulturae 83 (2000) 187±204
Tillage alters root distribution in a mature
asparagus planting
Daniel Drost*, Darlene Wilcox-Lee
Department of Fruit and Vegetable Science, Cornell University, Ithaca, NY 14853, USA
Accepted 22 July 1999
Abstract
Only a limited number of studies have been conducted on the root growth of asparagus. Soil
cores from six sample depths (0.15, 0.3, 0.45, 0.6, 0.75, and 0.9 m) in two tillage systems (till and
no-till) were separated into ®brous (Fi) and ¯eshy (Fl) roots and root length density (RLD)
determined. Harvest dates for roots began on 22 March and continued at three-week intervals until 8
November. Since analysis of variance was unable to identify signi®cant tillage system by harvest
date by sample depth interaction, the data were re-analyzed using the additive main effects and
multiplicative interactions (AMMI) statistical model. AMMI identi®ed signi®cant interactions
between tillage dates (tillage systems by harvest dates) and sample depth for Fi-RLD and Fl-RLD.
Fi- and Fl-RLD increased in all depths before spear harvest in March and April, decreased during
the spear harvest period of May to June, then increased after fern establishment in July before
declining late in the growing season. Regardless of sample depth or harvest date, RLD for ®brous
and ¯eshy roots were greater in no-till than till. RLD were greatest in the 0.3 and 0.45 m depths and
tended to decrease as depth increased for both tillage systems. In general, RLD were greater for
®brous compared to ¯eshy roots in all depths. A better understanding of the changes in root growth
may be useful for improving asparagus yields through better crop management. # 2000 Elsevier
Science B.V. All rights reserved.
Keywords: Asparagus of®cinalis L.; No-till; Root length density
*
Corresponding author. Present address: Department of Plants, Soils, and Biometeorology, Utah
State University, 4820 University Hill, Logan, UT 84322-4820, USA. Tel.: 1-435-797-2258;
fax: 1-435-797-3376.
E-mail address: dand@ext.usu.edu (D. Drost).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 9 2 - 8
188
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
1. Introduction
Asparagus growth studies have investigated changes in fern and root dry weight
and bud numbers during the ®rst year after plant establishment (Dufault and
Greig, 1983; Fisher, 1982; Haynes, 1987). Only recently have growth patterns in
a mature asparagus planting over a complete growing season been studied
(Wilcox-Lee and Drost, 1991). There is, however, only limited information
available on the root distribution patterns of ®eld grown asparagus (Reijmerink,
1973). Neither of these studies monitored the changes that occur over a whole
growing season. The root system of asparagus consists of an underground rhizome,
¯eshy storage roots, and ®brous feeding roots. Fleshy roots are generally unbranched, vary in diameter from 2 to 6 mm and grow to lengths of 1±2 m over
several growing seasons (Blasberg, 1932; Reijmerink, 1973). Fibrous roots may
be branched or unbranched and up to 2 mm in diameter. Fleshy roots remain functional, as carbohydrate storage organs for up to six years (Scott et al., 1939) while
®brous roots appear to grow for only one year (Reijmerink, 1973; Tiedjens, 1926).
Cultivation of asparagus beds often results in damage to both the crown and
emerging spears (Putnam, 1972; Tiedjens, 1926). This tillage damage reduces
crown growth (Wilcox-Lee and Drost, 1991), delays fern development and
ultimately lowers yields (Putnam, 1972; Wilcox-Lee and Drost, 1991). No-till
systems minimize these deleterious effects which ensures high productivity
(Putnam, 1972; Wilcox-Lee and Drost, 1991). In mature asparagus beds, ®brous
root growth in the tillage layer was reduced and numbers increased with depth as
the plants matured (Reijmerink, 1973). Tillage effects on ®brous and ¯eshy
asparagus root growth patterns over time have not been described.
Meaningful root data often requires a large number of samples (Bohm, 1979),
excessive amounts of labor and time to collect the data (Bohm, 1979; Taylor,
1986) and novel statistical analyses to accurately interpret the results (Zobel,
1990). The additive main effects and multiplicative interactions (AMMI) model
has been used successfully for analysis and interpretation of yield trials from
many different locations (Zobel et al., 1988), to help understand the in¯uences of
photoperiod and temperature on ¯owering in beans (Wallace et al., 1991), and for
determining temperatures role in altering basal and lateral root numbers (Zobel,
1990). AMMI analysis has been shown to improve both the postdictive and
predictive success of yield trials (Gauch and Zobel, 1989) by ®ltering the noise
(random variation) from the data pattern, thereby improving predictive accuracy
(Gauch and Zobel, 1988). Root data is generally dif®cult to analyze due to the
spatial variability of the soil around a plant, changes in root length that occur over
time (harvest dates) and space (depth and location in the soil pro®le) as well as
variation from sample to sample (Bohm, 1979; Taylor, 1986). The objectives of
this study were to characterize the effects of tillage systems on ®brous and ¯eshy
roots over a complete growing season in a mature asparagus planting.
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
189
2. Materials and methods
2.1. Plant materials and general experimental conditions
Six-year-old asparagus (Asparagus of®cinalis L. cultivar `Centennial') plants,
grown on a Riverhead sandy loam (mixed, mesic, Typic Dystrochrept) were
destructively sampled to assess shoot, bud, and root growth during the 1989
growing season (Wilcox-Lee and Drost, 1991). The Riverhead soil is
characterized as a dry ®ne sandy loam with less than 5% gravel to a depth of
0.8 m. Below 0.8 m is the BC horizon, which may have up to 30% gravel. In
addition to the plant growth and yield responses, asparagus root growth dynamics
were also evaluated. The experimental design was a randomized complete block
with four replications. Test plots consisting of asparagus rows 9.1 m long
separated from the adjacent treatments by a buffer row. All rows were spaced
1.7 m center to center, with 0.3 m between plants with crowns planted
approximately 15±25 cm below the soil surface.
Herbicide and tillage treatments were initiated in 1984 and consisted of: (1) (T)
plus 4-amino-6-tert-butyl-3(methylthio)-as-triazin-5(4H)-one (metribuzin), (2)
tilled (T) plus metribuzin and 2-(a-naphthoxy)-N,N-diethyl propionamide
(napropamide), (3) no-till (NT) plus metribuzin, and (4) no-till (NT) plus
metribuzin and napropamide. Metribuzin was applied to T and NT at 1.12 kg a.i.
haÿ1 and napropamide at 1.68 kg a.i. haÿ1 before harvest in April and again after
harvest in June. In NT plots, after mowing the fern in the spring, the plant debris
was left on the soil surface and herbicides were applied over the top of the beds.
In tilled plots, fern was mowed and the debris incorporated using a rotary hoe set
to cut to a depth of 8 cm before the ®rst herbicide application. Due to weed
pressure after harvest, herbicides were reapplied to T and NT plots and an
additional tillage operation similar to the earlier one was applied to the T plots in
June. Irrigation (sprinkler) was supplied at a rate of 2.5 cm per week to
supplement natural rainfall throughout the year. Soil temperatures were
monitored in two replications of the T and NT at the six root sampling depths
throughout the year. Air temperatures at 0.15 and 1.8 m above the soil surface
within the row were also recorded. Temperatures were recorded hourly and
averaged for each day and depth.
2.2. Root sampling and measurements
Root sampling began on 22 March and continued at three-week intervals until 8
November 1989. No samples were collected, however, on 3 May. Due to the
limited plot size (9.1 m) only six harvests each consisting of 1.5 m of the plot
length were made for whole plant evaluations (Wilcox-Lee and Drost, 1991).
Therefore, root sampling alternated between the metribuzin and metribuzin plus
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D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
napropamide in the T and NT treatments throughout the growing season. Roots
were assessed at six soil depths (0±0.15, 0.15±0.30, 0.30±0.45, 0.45±0.60, 0.60±
0.75, 0.75±0.90 m) and two locations (0.4 or 0.8 m from the row center) on each
harvest date. Soil cores, 15 cm long and 7.5 cm in diameter were collected with a
hand auger on the 11 sampling dates throughout the year. Root growth under the
row was not sampled due to the dif®culty of taking cores through the asparagus
crown. Samples were collected from four replications until 5 September and from
two replications thereafter.
Soil cores were stored in polyethylene bags at 48C for up to 10 weeks before
root separation and evaluation (Bohm, 1979). Soil cores were manually broken up
and ¯eshy roots were removed, prior to ®brous root extraction. Fibrous asparagus
roots were extracted from the soil by a modi®ed hydroelutriator (Smucker et al.,
1982). Soil samples were soaked for 4 min in water, then washed for 4 min on to
a 1 mm2 mesh screen to collect the ®brous roots and ¯oating organic matter from
the soil. Fibrous roots were then manually separated from organic matter and both
root types were stored in 15% ethanol (v/v) at 48C until root lengths and dry
weights were measured (Bohm, 1979).
Fibrous root lengths were measured with a Delta-T Length/Area meter (DeltaT Devices, Cambridge, UK) attached to an Ikegami ITC 510 video camera
(Ikegami Electronics (USA), Maywood, NJ) ®tted with a 50 mm camera lens and
monitored on a Ikegami PM205 screen. The camera focal length was 0.45 m at 4
f-stop. All root measurements were made with ambient light supplied by
overhead ¯uorescent lamps. The imaging system was calibrated against known
lengths of No. 50 white sewing thread, cut into 1 cm segments which were
randomly distributed onto 13 cm diameter velveteen disks. From these data, a
zero intercept model for root length (y 1.5919(x); r2 0.996) was developed
and used as a calibration constant for the ®brous roots.
Stored ®brous root samples were measured after pouring them onto the
velveteen disks in a buchner funnel, teasing them apart and vacuum ®ltering off
the alcohol. The disk and root sample was divided into quarters by imaginary
lines and each quarter measured in one orientation. The disk was rotated 908,
measured a second time and the average of the lengths was used to calculate
®brous root length density (Fi-RLD) as
Fi-RLD L X=V m=m3 ;
(1)
where L is the average ®brous root length (m), X the calibration constant (1.5919)
and V is the volume of the soil core. Fleshy roots were measured by hand with a
ruler and ¯eshy root length density (Fl-RLD) calculated as
Fl-RLD L=V m=m3 ;
(2)
where L is the ¯eshy root length and V is the volume of the soil core. A random
sample of ten ®brous and ¯eshy roots from the 27 May harvest date were
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
191
measured under a dissecting microscope and average root diameter determined
for all depths, locations and tillage systems.
2.3. Statistics
Fibrous and ¯esh RLD were analyzed by standard analysis of variance to
determine main effects and interactions of tillage and herbicide systems, soil
depths and locations, and harvest dates. Due to the unbalanced nature of the
experimental design (herbicides sampled on alternate dates, unequal replications
and missing data), the general linear models procedure (SAS, 1985) was used for
analysis of variance. While analysis of variance is useful for describing the main
effects, it may yield misleading information about the interactions (Snedecor and
Cochran, 1980). The AMMI model was then used to further analyze the data
paying particular attention to the interactions. AMMI statistical model has been
successfully used to understand the results that arises when interaction terms
contain large degrees of freedom (Gauch, 1988; Zobel et al., 1988). AMMI uses
standard analysis of variance to compute the main effects and then applies
principle components analysis (PCA) to the residual to analyze interactions
(Zobel, 1990; Zobel et al., 1988). The primary restriction to the use of AMMI
model is it requires two way data tables (Bradu and Gabriel, 1978). This requirement was satis®ed by creating two way data tables by combining tillage systems
(T and NT) and harvest dates (1±11) to create 22 tillage dates (T1±T11 and NT1±
NT11) to be tested at the six sample depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m).
A detailed description of the AMMI statistical model and associated calculations
have been described elsewhere (Gauch and Zobel, 1989; Zobel et al., 1988).
3. Results
Table 1 presents the complete analysis of variance for all variables (tillage
systems, herbicides, sampling depths, harvest dates, and locations) entered into
the model for Fi-RLD and Fl-RLD. Several interactions had large degrees of
freedom (df) with small sums of squares (SS) which resulted in non-signi®cant P
values. Tillage system, sampling depth, and harvest date main effects and several
of the two-way interactions were highly signi®cant for the root parameters
measured. However, the analysis of variance gave little insight into changes that
occurred in the three-way or greater interactions. Location (0.4 or 0.8 m) and
herbicide (Sencor or Sencor Metribuzin) effects were generally non-signi®cant
for most main effects and interactions. Since location's contribution to the sums
of squares was small (Table 1) values were averaged before the terms were
removed from the model. Removal of location from the analysis would reduce the
model df by one-half. However, due to the association of herbicides with harvest
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D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Table 1
Analysis of variance of ®brous (Fi) and ¯eshy (Fl) asparagus root length density (RLD) with tillage
systems (T), herbicide (H), harvest dates (HD), sampling depth (SD), and sampling location (L)
Source of variation
Total
Model
Replications (reps)
Tillage systems (T)
Herbicides (H)
T*H
T*H*reps (error a)
Harvest dates (HD)
T*HD
H*HD
T*H*HD
Sampling depth (SD)
T*SD
H*SD
T*H*SD
HD*SD
T*HD*SD
H*HD*SD
T*H*HD*SD
Sampling location (L)
T*L
H*L
T*H*L
HD*L
T*HD*L
H*HD*L
T*H*HD*L
SD*L
T*SD*L
H*SD*L
T*H*SD*L
HD*SD*L
T*HD*SD*L
H*HD*SD*L
T*H*HD*SD*L
Error b
a
Degrees of freedom
849
275
3
1
1
1
9
5
5
4
4
5
5
5
5
25
25
20
20
1
1
1
1
5
5
4
4
5
5
5
5
25
25
20
20
574
Nonsigni®cant.
Signi®cant at P < 0.05.
**
Signi®cant at P < 0.01.
***
Signi®cant at P < 0.001.
*
Sums of squares
Fi-RLD
Fl-RLD
33.209
16.404***
0.631
1.744**
1.005**
0.062a
0.527a
0.687**
0.370*
0.769***
0.303*
3.684***
0.285a
0.572**
0.101a
1.335**
0.395a
0.646a
0.248a
0.003a
0.008a
0.001a
0.011a
0.211a
0.071a
0.176a
0.106a
0.311a
0.090a
0.047a
0.064a
0.230a
0.496a
0.374a
0.840a
16.804
4.011
2.059***
0.032
0.104*
0.010a
0.002a
0.110a
0.111***
0.016a
0.017a
0.032*
0.755***
0.052**
0.028a
0.009a
0.172**
0.099a
0.077a
0.019a
0.006a
0.006a
0.000a
0.001a
0.039**
0.011a
0.005a
0.013a
0.076**
0.006a
0.006a
0.006a
0.040a
0.097a
0.022a
0.081a
1.952
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
193
Table 2
Standard analysis of variance for ®brous (Fi) and ¯eshy (Fl) asparagus root length density (RLD)
with 22 tillage dates (TDa) in six sampling depths (SD)
Source of variation
Total
Model
Tillage dates (TDa)
Sampling depth (SD)
TDa*SD
Error
df
431
131
21
5
105
300
Sums of squares
Fi-RLD
Fl-RLD
11.643
6.359***
2.635***
1.968***
1.755a
5.283
1.391
0.765***
0.149***
0.390***
0.226a
0.626
a
Nonsigni®cant.
Signi®cant at P < 0.05.
***
Signi®cant at P < 0.001.
*
dates and tillage systems, removal of this variable would not alter the df but
would assign more SS to the residual.
In an attempt to better understand the interaction between harvest dates, tillage
systems and sampling depths, these variables were combined to test seasonal root
changes. The creation of tillage dates (TDa) from the two tillage systems and 11
harvest dates which were compared to the six sampling depth (SD) were to satisfy
the requirements for two-way data tables when using the AMMI analysis. Table 2
shows the standard analysis of variance for the reduced model with the combined
TDa and SD. Results from analysis of variance indicate that for the Fi-RLD and
Fl-RLD variables, the interaction between TDa and SD was not signi®cant
(Table 2). The lack of a signi®cant interaction is not surprising considering the
large df (105) associated with this interaction. However, even if the interaction of
TDa SD was signi®cant, differences presented by the data would be dif®cult to
describe and interpret.
While the standard analysis of variance indicated that there were no signi®cant
interactions between TDa and SD (Table 2), visual observation of the data
suggested that there were distinct changes in RLD during the year. The changes
for Fi-RLD are presented in Fig. 1 and will be used to illustrate these differences.
Fi-RLD appeared to be greater in NT than T in depths 0.15±0.6 m over much of
the growing season, while in the 0.75 and 0.9 m depths, Fi-RLD were similar for
T and NT. There also appeared to be distinct seasonal changes in Fi-RLD during
the growing season. In T, Fi-RLD were low in the 0.15 m depth throughout most
of the year, while in NT, Fi-RLD at this depth were highest early in the year then
declined during the summer and fall. Fi-RLD increased during April in the 0.3±
0.75 m depths in T and NT then declined during May and June before increasing
again from July to September. Few changes in Fi-RLD occurred after full canopy
development in August in the 0.45±0.75 m depths. Furthermore, Fi-RLD was
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D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Fig. 1. Changes in ®brous root length density in till (T) and no-till (NT) asparagus as in¯uenced by
harvest date for six sampling depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m). Values are the means of
the replications the standard errors.
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
195
Table 3
Additive main effects and multiplicative interactions (AMMI) analysis of variance for ®brous (Fi)
and ¯eshy (Fl) asparagus root length density (RLD) with 22 tillage dates (TDa) at six sampling
depths (SD)
Source of variation
df
Sums of squares
Fi-RLD
Total
Model
Tillage dates (TDa)
Sampling depth (SD)
TDa*SD
I-PCA axis
Residual
Error
431
131
21
5
105
25
80
300
12.304
7.059***
2.760***
2.278***
2.022a
1.407***
0.615a
5.245
Fl-RLD
1.484
0.857***
0.180***
0.402***
0.274a
0.169***
0.107a
0.627
a
Nonsigni®cant.
Signi®cant at P < 0.05.
***
Signi®cant at P < 0.001.
*
generally low in the 0.75 and 0.9 m depths during the spring but increased during
the summer and fall.
Since TDa and SD appeared to interact with each other (Fig. 1), the data were
re-analyzed by the AMMI model in an attempt to gain more meaningful insights
into the possible interactions. The AMMI analysis is presented in Table 3 and the
SS can be compared to those of analysis of variance listed in Table 2 since the
AMMI model uses analysis of variance to calculate the main effects. Exact
duplication of SS for the main effects of TDa and SD by standard analysis of
variance and the AMMI models was not achieved due to the presence of missing
data points although the values were very similar. The AMMI analysis clearly
showed the existence of a signi®cant TDa SD interaction, which was not
apparent when testing the data by analysis of variance (Table 2). The ®rst PCA
axis from the analysis of the interaction for the root parameters measured
separated out 70% and 60% of the interaction SS in 24% of the interaction df for
the Fi-RLD and Fl-RLD, respectively. The reduction in df from 105 to 25 now
allowed easier interpretation of the interactions that occur between TDa and SD.
Fig. 2 presents the biplot for the ®rst statistically signi®cant PCA axis and the
means (main effects) for Fi-RLD. The means of the main effects (TDa and SD)
and the grand mean are presented along the x-axis while any deviation of the
grand mean through the interaction effects (TDa by SD) are illustrated in the yaxis. For any combination of TDa and SD, the main effects equals the TDa mean
plus SD mean minus the grand mean (RLD 2267) while the interaction is the
TDa I-PCA score times the SD I-PCA score. For example, NT2 (RLD 4422) at
the 0.3 m (RLD 2579) depth has a main effect RLD of (4422 2579) ±
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D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Fig. 2. Biplot of the AMMI model for ®brous root length density of asparagus roots with 22 tillage dates (till (T) and no-till (NT), dates 1±11) in six
sampling depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m). The grand mean was 2300 m/m3.
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
197
2267 4734 m/m3 and an interaction of 54.1 49.5 2678 m/m3. Thus, the
AMMI model estimates Fi-RLD to be 7412 m/m3 (4734 2678) for NT2 at the
0.3 m depth. This was close to the observed Fi-RLD of 7490 m/m3 noted in
Fig. 1. TDa and SD with the same sign on the I-PCA axis have a positive
interaction (mean Fi-RLD will increase) while if different, their interactions are
negative (mean Fi-RLD will decrease). Values of I-PCA close to zero have only
small interaction effects (main effects change only). For example, 0.15 and 0.6 m
depths (I-PCA close to zero) differ only in main effects where 0.3 and 0.75 m
depths differ in interaction since the I-PCA scores differ greatly but not the mean
Fi-RLD (Fig. 2).
From Fig. 2, differences in Fi-RLD occurred between T and NT at most harvest
dates supporting the differences illustrated in Fig. 1. Harvest dates signi®cantly
affected Fi-RLD, since early harvest dates (2 and 3) produce greater root lengths
than late harvest dates (9±11). The decline in Fi-RLD during the harvest period in
T and NT are illustrated by a decreasing mean Fi-RLD from date 3±6 (Fig. 2). FiRLD increased again on dates 7 and 8 before declining late in the year (dates 9±
11). Sample depth will in¯uence the Fi-RLD for the different TDa. Sample depths
0.3 and 0.45 m are on the positive side of the y-axis while 0.75 and 0.9 m are on
the negative side. Thus, Fi-RLD in the upper regions of the soil pro®le will be
greater than at deeper depths.
Although there was no inherent interaction between TDa and SD for ¯eshy root
length from the analysis of variance (Table 1), there were distinct patterns of FlRLD growth occurring throughout the season (Fig. 3). In general, Fl-RLD were
greater in NT than T in the shallow depths with little difference in root length
occurring at depths below 0.6 m. Root lengths decreased as depth in the soil
pro®le increased with the highest Fl-RLD occurring at 0.3 m and the lowest at
0.75 and 0.9 m.
Visual observations of the ¯eshy root data suggest that seasonal differences in
RLD were also apparent (Fig. 3). Fl-RLD was greater in NT than T in the 0.15 m
depth from March to May and again from July to September. While root growth
at 0.15 m declined in NT during the harvest period, Fl-RLD increased slowly in
T. In both T and NT, Fl-RLD increased late in the year in the 0.15 m depth. FlRLD were greatest in April in the 0.3 and 0.45 m depths then declined during
May and June. Fl-RLD continued to decline in T compared to NT during the fern
growth period of July before stabilizing in August (Fig. 3). There were few
changes in Fl-RLD during the year at 0.6±0.9 m depths.
Fig. 4 presents the biplot for the ®rst statistically signi®cant PCA axis and the
means (main effects) for Fl-RLD, which shows the signi®cant TDa by SD
interaction (Table 2). Fl-RLD was greater in NT than T on most harvest dates.
Early harvests generally had greater root lengths than late harvests. Depth in the
soil pro®le plays a signi®cant role in the interactions partitioned out in the I-PCA
axis. Root lengths were greatest in at 0.3 m (large mean Fl-RLD and positive
198
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Fig. 3. Changes in ¯eshy root length density in till (T) and no-till (NT) asparagus as in¯uenced by
harvest date in six sampling depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m). Values are the means of
the replications the standard errors.
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
199
Fig. 4. Biplot of the AMMI model for ¯eshy root length density of asparagus roots with 22 tillage dates (till (T) and no-till (NT), dates 1±11) in six
sampling depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m). The grand mean was 520 m/m3.
200
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
I-PCA score) and least at 0.9 m (low mean Fl-RLD and negative I-PCA score).
There were few differences in mean Fl-RLD between the 0.15, 0.45 and 0.6 m
depths. Low root lengths early (T1, T2, NT1 and NT2) and late (T10, NT10, T11
and NT11) in the season in the deeper depths (0.75 and 0.9 m) are illustrated by
low mean Fl-RLD and negative I-PCA scores. Fl-RLD declined from dates 3 to 6
in T but remained unchanged or increased slightly in NT during the same period.
Root lengths increased again on dates 7 and 8 (increasing mean RLD) in both T
and NT with little change occurring after date 9 (constant mean Fl-RLD).
Tillage system, sampling depth, or location did not in¯uence ®brous root
diameters when measured in late May. Fibrous root diameters ranged from 0.40 to
0.58 mm with an average diameter of 0.52 mm. There was a signi®cant
interaction between tillage system, sampling location and sampling depth for
¯eshy root diameters. Fleshy root diameters were generally greatest near the soil
surface at 0.40 m distance from the row and decreased as depth in the soil pro®le
increased in both T and NT. At the 0.80 m distance (between adjacent rows),
¯eshy root diameters were similar in both T and NT between the 0.15 and 0.45 m
depths, then decreased as sampling depth increased. Fleshy root diameters varied
from 2.5 to 4.0 mm at depths above 0.45 m. At 0.6 m and below, ¯eshy root
diameters ranged from 1.3 to 2.0 mm.
Soil temperatures were similar for the T and NT at all time during the growing
season (data not shown). Temperature varied with time of the year and depth in the
soil pro®le. Soil temperatures were greatest near the soil surface and least below
0.6 m. There was considerable oscillations in temperature at 0.15 m with temperatures ranging from 58C early and late in the year to greater than 258C during
the summer. As depth in the soil pro®le increased, temperature remained more
stable and averaged between 158C and 258C during most of the growing season.
4. Discussion
There was a considerable variation in asparagus RLD during the sampling
period resulting in the failure of analysis of variance to detect differences between
the interactions of tillage systems, harvest dates, and sample depths. In contrast,
the AMMI analysis gave a reliable indication of differences in ®brous and ¯eshy
RLD in T and NT systems and to changes in root development during the growing
season. The AMMI model with the biplot display allowed overall patterns in
®brous and ¯eshy RLD to be visualized (differences between T and NT, changes
over time and depth) as well as speci®c TDa SD interactions.
From our study, it is evident that even shallow tillage operations signi®cantly
reduce root growth near the soil surface. Asparagus root reductions near the soil
surface have been attributed to aging of the primary (¯eshy) roots, soil compaction, and structural breakdown associated with tillage operations (Reijmerink,
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
201
1973). Tillage operations in asparagus have been used historically to control
weeds, loosen the soil prior to spear emergence, and to incorporate fern residues
(Putnam, 1972). During asparagus establishment, tillage operations are frequently
used to ®ll in the planting furrow. Injury to young roots caused by tillage in newly
established plantings would create a root free zone which regular tillage
operations in the future would maintain. Frequency and depth of tillage have been
associated with a root free zone close to the soil surface in grapes (Van Huyssteen
and Weber, 1980). With asparagus crown depths at 0.15±0.25 m, proper
equipment adjustment must be maintained to avoid additional crown injury.
Damage to crowns and the roots near the crown will reduce ¯eshy root extension
and ®brous root growth, since ®brous roots are initiated from ¯eshy roots
(Blasberg, 1932). While tillage can change the physical characteristics of the soil,
RLD were not believed to be signi®cantly modi®ed by these structural changes.
Decreases in ®brous and ¯eshy root lengths during and after harvest (May±
July) were not limited to the top 0.15 m of the soil pro®le. Fibrous and ¯eshy
RLD reductions occurred at most depths early in the season and are believed to be
associated with strong sink demands by the developing spears and fern resulting
in low carbohydrate availability for continued root growth. Root growth
recovered when fern growth slowed in July in NT but continued to decline in
T. Tillage operations have been shown to delay fern development and contribute
to yield reductions in asparagus (Putnam, 1972; Wilcox-Lee and Drost, 1991).
Several things may contribute to the extended decline in RLD in the tilled system.
The early tillage operation would damage emerging spears and ®brous root near
the soil surface. This would decrease yield. The additional tillage operation after
harvest would delay canopy development, which would limit leaf area, decrease
the photosynthetic potential of the plant, contribute to disease spread and limit
new bud development ultimately weakening the plant. In addition, this would also
delay root growth resumption. In asparagus summer root growth is important for
nutrient absorption, water uptake, and the creation of additional carbohydrate
storage. Slowing these processes will ultimately impact growth and in¯uence longterm productivity of this perennial plant (Putnam, 1972; Wilcox-Lee and Drost,
1991). It is not clear from this study what role tillage operations have on root growth
at depths greater than 0.9 m. Root growth reductions deeper in the soil may be
related to the continued decline in stored carbohydrates (Dufault and Greig, 1983;
Haynes, 1987; Shelton and Lacy, 1980) or to the smaller pool of carbohydrates
available in tilled systems (Haynes, 1987; Wilcox-Lee and Drost, 1991).
Roots of grape (Freeman and Smart, 1976), apple and plum (Head, 1967), and
spruce (Deans and Ford, 1986) show marked seasonal changes in root growth.
Root growth often begins before shoot growth in the spring (Dell and Wallace,
1983; Head, 1967) then decreases with the onset of shoot growth (Head, 1967)
before resuming during the summer or fall (Dell and Wallace, 1983; Freeman and
Smart, 1976). Similar growth patterns were evident for ®brous asparagus roots at
202
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
most soil depths while seasonal changes in ¯eshy root growth were less apparent.
Since ¯eshy roots grow for many years (Scott et al., 1939), the seasonal changes
in RLD noted in our study are believed to be related more to sampling and soil
variability than to actual increases or decreases in Fl-RLD (Bohm, 1979). Since
tillage is known to affect root development, early tillage (March) would impact
®brous root growth (Figs. 1 and 2), while late tillage operations (July) may affect
¯eshy root growth (Figs. 3 and 4). Haynes (1987) reported that in young
asparagus plantings, ¯eshy root numbers increased after fern establishment in the
summer though he says nothing about ®brous roots. Therefore, tillage operations
that reduce ¯eshy root growth will have long-term effects on crown development
and plant growth.
Maximum root growth of ®brous and ¯eshy roots occurred at 0.3 m, which is
immediately adjacent to the crown, and generally declined as depth in the soil
pro®le. Uniform root distribution throughout the soil pro®le will improve both
water and nutrient utilization thereby minimizing potential stresses that can
in¯uence plant growth. High yields in asparagus are determined in part by rooting
depth (Reijmerink, 1973). The Riverhead soil is fairly uniform to a depth of
0.8 m. The abrupt change in soil structure at 0.8 m, may be partially responsible
for the low RLD at depth in the pro®le. Soil pro®le changes can contribute to
differences in asparagus rooting pattern by altering water holding capacity,
porosity, and penetration resistance (Reijmerink, 1973).
Soil temperatures changes can also in¯uence root growth (Bohm, 1979; Dell
and Wallace, 1983). Temperature records from this study showed that soil
temperatures were similar in T and NT throughout the year though temperature
did change with depth. Other studies have shown that temperature differences do
occur between T and NT. The lack of difference in temperature in this study may
be due to the lack of a signi®cant quantity of mulch present on the soil surface in
the NT plots. Most studies that show temperature depression in NT occur when
large quantities of mulch are present. With less mulch on the surface, less
depression would occur resulting in similar temperatures near the surface in T and
NT asparagus. More important for root growth is the damage that occurs to those
roots as a result of the tillage operations both before and after harvest. Since
differences in soil temperature during the year were not great, it does not appear
to contribute to the differences in root growth noted.
5. Conclusions
Several conclusions can be drawn from these studies. First, since roots are
dif®cult to access, novel procedures are needed to analyze them. The AMMI
statistical model did a good job assessing differences between tillage systems,
harvest dates and sampling depth. Second, changes in ®brous root length density
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
203
were easier to detect than ¯eshy root length densities. Since ®brous roots grow for
only one year, changes over time and depth would be expected. These variations
in root growth suggest that management strategies (nutrient and water application
practices) need to take into account these changes. In addition, since ¯eshy roots
grow for several years, variation in RLD over time is due more to sampling and
soil variability rather than unique differences in RLD. However, the differences
between T and NT illustrate that cultural practices will alter ¯eshy root numbers
which ultimately affect plant performance. Finally, almost nothing is known
about the role of root age, rooting depth and root type on nutrient and water
uptake by the plant. Additional work in this area is required to better understand
the yield physiology of asparagus.
References
Blasberg, C.H., 1932. Phases of the anatomy of Asparagus of®cinalis. Botanical Gazette 94, 206±
214.
Bohm, W., 1979. Methods of Studying Root Systems. Springer, Berlin, BRD.
Bradu, D., Gabriel, K.R., 1978. The biplot as a diagnostic tool for models of two-way tables.
Technometrics 20, 47±68.
Deans, J.D., Ford, E.D., 1986. Seasonal patterns of radial root growth and starch dynamics in
plantation-grown sitka spruce trees of different ages. Tree Physiol. 1, 241±251.
Dell, B., Wallace, I.M., 1983. Periodicity of ®ne root growth in Jarrah (Eucalyptus marginata Donn
ex Sm.). Aust. J. Bot. 31, 247±254.
Dufault, R.J., Greig, J.K., 1983. Dynamic growth characteristics in seedling asparagus. J. Am. Soc.
Hort. Sci. 108, 1026±1030.
Freeman, B.M., Smart, R.E., 1976. A root observation laboratory for studies with grapevines. Am.
J. Enol. Viticult. 27, 36±39.
Fisher, K.J., 1982. Comparisons of the growth and development of young asparagus plants
established from seedling transplants and by direct seeding. N.Z.J. Expt. Agric. 10, 405±408.
Gauch, H.G., 1988. Model selection and validation for yield trials with interaction. Biometrics 44,
705±715.
Gauch, H.G., Zobel, R.W., 1988. Predictive and postdictive success of statistical analyses of yield
trials. Theor. Appl. Genet. 76, 1±10.
Gauch, H.G., Zobel, R.W., 1989. Accuracy and selection success in yield trial analysis. Theor.
Appl. Genet. 77, 473±481.
Haynes, R.J., 1987. Accumulation of dry matter and changes in storage carbohydrate and amino
acid content in the ®rst two years of asparagus growth. Scientia Horticulturae 32, 17±23.
Head, G.C., 1967. Effects of seasonal changes in shoot growth on the amount of unsuberised root on
apple and plum trees. J. Hort. Sci. 42, 169±180.
Putnam, A.R., 1972. Ef®cacy of a zero-tillage cultural system for asparagus produced from seed
and crowns. J. Am. Soc. Hort. Sci. 97, 621±624.
Reijmerink, A., 1973. Microstructure, soil strength, and root development of asparagus on loamy
sands in the Netherlands. Neth. J. Agric. Sci. 21, 24±43.
SAS Institute Inc., 1985. SAS User's Guide: Statistics, 5th ed. SAS Institute Inc., Cary, NC.
Scott, L.E., Mitchell, J.H., McGinty, R.A., 1939. Effects of certain treatments on the carbohydrate
reserves of asparagus crowns. S. Carolina Agric. Expt. Stn. Bull. 321.
204
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Shelton, D.L., Lacy, M.L., 1980. Effects of harvest duration on yield and depletion of storage
carbohydrates in asparagus roots. J. Am. Soc. Hort. Sci. 105, 332±335.
Smucker, A.J.M., McBurney, S.L., Srivastava, A.K., 1982. Quantitative separation of roots from
compacted soil by the hydropneumatic elutriation system. Agron. J. 74, 500±503.
Snedecor, G.W., Cochran, W.B., 1980. Statistical Methods, 7th ed. Iowa State Univ., Ames, IA.
Taylor, H.M., 1986. Methods of studying root systems in the ®eld. In: Root Systems of Vegetable
Crops, vol. 21. Proceedings of the Symposium on HortScience, pp. 952±956.
Tiedjens, V.A., 1926. Some observations on roots and crown bud formation in Asparagus of®cinalis.
Proc. Am. Soc. Hort. Sci. 23, 189±195.
Van Huyssteen, L., Weber, H.W., 1980. The effect of selected minimum and conventional tillage
practices in vineyard cultivation on vine performance. South. Afr. J. Enol. Viticult 1, 77±83.
Wallace, D.H., Gniffke, P.A., Massaya, P.N., Zobel, R.W., 1991. Photoperiod, temperature, and
genotype interaction effects on days, and nodes required for ¯owering of bean. J. Am. Soc. Hort.
Sci. 116, 534±543.
Wilcox-Lee, D., Drost, D., 1991. Tillage reduces yield and crown, fern, and bud growth in a mature
asparagus planting. J. Am. Soc. Hort. Sci. 116, 937±941.
Zobel, R.W., 1990. A powerful statistical model for understanding genotype-by-environment
interaction. In: Lang, M.S. (Ed.), Genotype-by-Environment interaction and Plant Breeding.
Louisiana State University, Baton Rouge, LA, pp. 126±140.
Zobel, R.W., Wright, M.J., Gauch, H.G., 1988. Statistical analysis of yield trial. Agron. J. 80, 388±
393.
Tillage alters root distribution in a mature
asparagus planting
Daniel Drost*, Darlene Wilcox-Lee
Department of Fruit and Vegetable Science, Cornell University, Ithaca, NY 14853, USA
Accepted 22 July 1999
Abstract
Only a limited number of studies have been conducted on the root growth of asparagus. Soil
cores from six sample depths (0.15, 0.3, 0.45, 0.6, 0.75, and 0.9 m) in two tillage systems (till and
no-till) were separated into ®brous (Fi) and ¯eshy (Fl) roots and root length density (RLD)
determined. Harvest dates for roots began on 22 March and continued at three-week intervals until 8
November. Since analysis of variance was unable to identify signi®cant tillage system by harvest
date by sample depth interaction, the data were re-analyzed using the additive main effects and
multiplicative interactions (AMMI) statistical model. AMMI identi®ed signi®cant interactions
between tillage dates (tillage systems by harvest dates) and sample depth for Fi-RLD and Fl-RLD.
Fi- and Fl-RLD increased in all depths before spear harvest in March and April, decreased during
the spear harvest period of May to June, then increased after fern establishment in July before
declining late in the growing season. Regardless of sample depth or harvest date, RLD for ®brous
and ¯eshy roots were greater in no-till than till. RLD were greatest in the 0.3 and 0.45 m depths and
tended to decrease as depth increased for both tillage systems. In general, RLD were greater for
®brous compared to ¯eshy roots in all depths. A better understanding of the changes in root growth
may be useful for improving asparagus yields through better crop management. # 2000 Elsevier
Science B.V. All rights reserved.
Keywords: Asparagus of®cinalis L.; No-till; Root length density
*
Corresponding author. Present address: Department of Plants, Soils, and Biometeorology, Utah
State University, 4820 University Hill, Logan, UT 84322-4820, USA. Tel.: 1-435-797-2258;
fax: 1-435-797-3376.
E-mail address: dand@ext.usu.edu (D. Drost).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 9 2 - 8
188
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
1. Introduction
Asparagus growth studies have investigated changes in fern and root dry weight
and bud numbers during the ®rst year after plant establishment (Dufault and
Greig, 1983; Fisher, 1982; Haynes, 1987). Only recently have growth patterns in
a mature asparagus planting over a complete growing season been studied
(Wilcox-Lee and Drost, 1991). There is, however, only limited information
available on the root distribution patterns of ®eld grown asparagus (Reijmerink,
1973). Neither of these studies monitored the changes that occur over a whole
growing season. The root system of asparagus consists of an underground rhizome,
¯eshy storage roots, and ®brous feeding roots. Fleshy roots are generally unbranched, vary in diameter from 2 to 6 mm and grow to lengths of 1±2 m over
several growing seasons (Blasberg, 1932; Reijmerink, 1973). Fibrous roots may
be branched or unbranched and up to 2 mm in diameter. Fleshy roots remain functional, as carbohydrate storage organs for up to six years (Scott et al., 1939) while
®brous roots appear to grow for only one year (Reijmerink, 1973; Tiedjens, 1926).
Cultivation of asparagus beds often results in damage to both the crown and
emerging spears (Putnam, 1972; Tiedjens, 1926). This tillage damage reduces
crown growth (Wilcox-Lee and Drost, 1991), delays fern development and
ultimately lowers yields (Putnam, 1972; Wilcox-Lee and Drost, 1991). No-till
systems minimize these deleterious effects which ensures high productivity
(Putnam, 1972; Wilcox-Lee and Drost, 1991). In mature asparagus beds, ®brous
root growth in the tillage layer was reduced and numbers increased with depth as
the plants matured (Reijmerink, 1973). Tillage effects on ®brous and ¯eshy
asparagus root growth patterns over time have not been described.
Meaningful root data often requires a large number of samples (Bohm, 1979),
excessive amounts of labor and time to collect the data (Bohm, 1979; Taylor,
1986) and novel statistical analyses to accurately interpret the results (Zobel,
1990). The additive main effects and multiplicative interactions (AMMI) model
has been used successfully for analysis and interpretation of yield trials from
many different locations (Zobel et al., 1988), to help understand the in¯uences of
photoperiod and temperature on ¯owering in beans (Wallace et al., 1991), and for
determining temperatures role in altering basal and lateral root numbers (Zobel,
1990). AMMI analysis has been shown to improve both the postdictive and
predictive success of yield trials (Gauch and Zobel, 1989) by ®ltering the noise
(random variation) from the data pattern, thereby improving predictive accuracy
(Gauch and Zobel, 1988). Root data is generally dif®cult to analyze due to the
spatial variability of the soil around a plant, changes in root length that occur over
time (harvest dates) and space (depth and location in the soil pro®le) as well as
variation from sample to sample (Bohm, 1979; Taylor, 1986). The objectives of
this study were to characterize the effects of tillage systems on ®brous and ¯eshy
roots over a complete growing season in a mature asparagus planting.
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
189
2. Materials and methods
2.1. Plant materials and general experimental conditions
Six-year-old asparagus (Asparagus of®cinalis L. cultivar `Centennial') plants,
grown on a Riverhead sandy loam (mixed, mesic, Typic Dystrochrept) were
destructively sampled to assess shoot, bud, and root growth during the 1989
growing season (Wilcox-Lee and Drost, 1991). The Riverhead soil is
characterized as a dry ®ne sandy loam with less than 5% gravel to a depth of
0.8 m. Below 0.8 m is the BC horizon, which may have up to 30% gravel. In
addition to the plant growth and yield responses, asparagus root growth dynamics
were also evaluated. The experimental design was a randomized complete block
with four replications. Test plots consisting of asparagus rows 9.1 m long
separated from the adjacent treatments by a buffer row. All rows were spaced
1.7 m center to center, with 0.3 m between plants with crowns planted
approximately 15±25 cm below the soil surface.
Herbicide and tillage treatments were initiated in 1984 and consisted of: (1) (T)
plus 4-amino-6-tert-butyl-3(methylthio)-as-triazin-5(4H)-one (metribuzin), (2)
tilled (T) plus metribuzin and 2-(a-naphthoxy)-N,N-diethyl propionamide
(napropamide), (3) no-till (NT) plus metribuzin, and (4) no-till (NT) plus
metribuzin and napropamide. Metribuzin was applied to T and NT at 1.12 kg a.i.
haÿ1 and napropamide at 1.68 kg a.i. haÿ1 before harvest in April and again after
harvest in June. In NT plots, after mowing the fern in the spring, the plant debris
was left on the soil surface and herbicides were applied over the top of the beds.
In tilled plots, fern was mowed and the debris incorporated using a rotary hoe set
to cut to a depth of 8 cm before the ®rst herbicide application. Due to weed
pressure after harvest, herbicides were reapplied to T and NT plots and an
additional tillage operation similar to the earlier one was applied to the T plots in
June. Irrigation (sprinkler) was supplied at a rate of 2.5 cm per week to
supplement natural rainfall throughout the year. Soil temperatures were
monitored in two replications of the T and NT at the six root sampling depths
throughout the year. Air temperatures at 0.15 and 1.8 m above the soil surface
within the row were also recorded. Temperatures were recorded hourly and
averaged for each day and depth.
2.2. Root sampling and measurements
Root sampling began on 22 March and continued at three-week intervals until 8
November 1989. No samples were collected, however, on 3 May. Due to the
limited plot size (9.1 m) only six harvests each consisting of 1.5 m of the plot
length were made for whole plant evaluations (Wilcox-Lee and Drost, 1991).
Therefore, root sampling alternated between the metribuzin and metribuzin plus
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D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
napropamide in the T and NT treatments throughout the growing season. Roots
were assessed at six soil depths (0±0.15, 0.15±0.30, 0.30±0.45, 0.45±0.60, 0.60±
0.75, 0.75±0.90 m) and two locations (0.4 or 0.8 m from the row center) on each
harvest date. Soil cores, 15 cm long and 7.5 cm in diameter were collected with a
hand auger on the 11 sampling dates throughout the year. Root growth under the
row was not sampled due to the dif®culty of taking cores through the asparagus
crown. Samples were collected from four replications until 5 September and from
two replications thereafter.
Soil cores were stored in polyethylene bags at 48C for up to 10 weeks before
root separation and evaluation (Bohm, 1979). Soil cores were manually broken up
and ¯eshy roots were removed, prior to ®brous root extraction. Fibrous asparagus
roots were extracted from the soil by a modi®ed hydroelutriator (Smucker et al.,
1982). Soil samples were soaked for 4 min in water, then washed for 4 min on to
a 1 mm2 mesh screen to collect the ®brous roots and ¯oating organic matter from
the soil. Fibrous roots were then manually separated from organic matter and both
root types were stored in 15% ethanol (v/v) at 48C until root lengths and dry
weights were measured (Bohm, 1979).
Fibrous root lengths were measured with a Delta-T Length/Area meter (DeltaT Devices, Cambridge, UK) attached to an Ikegami ITC 510 video camera
(Ikegami Electronics (USA), Maywood, NJ) ®tted with a 50 mm camera lens and
monitored on a Ikegami PM205 screen. The camera focal length was 0.45 m at 4
f-stop. All root measurements were made with ambient light supplied by
overhead ¯uorescent lamps. The imaging system was calibrated against known
lengths of No. 50 white sewing thread, cut into 1 cm segments which were
randomly distributed onto 13 cm diameter velveteen disks. From these data, a
zero intercept model for root length (y 1.5919(x); r2 0.996) was developed
and used as a calibration constant for the ®brous roots.
Stored ®brous root samples were measured after pouring them onto the
velveteen disks in a buchner funnel, teasing them apart and vacuum ®ltering off
the alcohol. The disk and root sample was divided into quarters by imaginary
lines and each quarter measured in one orientation. The disk was rotated 908,
measured a second time and the average of the lengths was used to calculate
®brous root length density (Fi-RLD) as
Fi-RLD L X=V m=m3 ;
(1)
where L is the average ®brous root length (m), X the calibration constant (1.5919)
and V is the volume of the soil core. Fleshy roots were measured by hand with a
ruler and ¯eshy root length density (Fl-RLD) calculated as
Fl-RLD L=V m=m3 ;
(2)
where L is the ¯eshy root length and V is the volume of the soil core. A random
sample of ten ®brous and ¯eshy roots from the 27 May harvest date were
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
191
measured under a dissecting microscope and average root diameter determined
for all depths, locations and tillage systems.
2.3. Statistics
Fibrous and ¯esh RLD were analyzed by standard analysis of variance to
determine main effects and interactions of tillage and herbicide systems, soil
depths and locations, and harvest dates. Due to the unbalanced nature of the
experimental design (herbicides sampled on alternate dates, unequal replications
and missing data), the general linear models procedure (SAS, 1985) was used for
analysis of variance. While analysis of variance is useful for describing the main
effects, it may yield misleading information about the interactions (Snedecor and
Cochran, 1980). The AMMI model was then used to further analyze the data
paying particular attention to the interactions. AMMI statistical model has been
successfully used to understand the results that arises when interaction terms
contain large degrees of freedom (Gauch, 1988; Zobel et al., 1988). AMMI uses
standard analysis of variance to compute the main effects and then applies
principle components analysis (PCA) to the residual to analyze interactions
(Zobel, 1990; Zobel et al., 1988). The primary restriction to the use of AMMI
model is it requires two way data tables (Bradu and Gabriel, 1978). This requirement was satis®ed by creating two way data tables by combining tillage systems
(T and NT) and harvest dates (1±11) to create 22 tillage dates (T1±T11 and NT1±
NT11) to be tested at the six sample depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m).
A detailed description of the AMMI statistical model and associated calculations
have been described elsewhere (Gauch and Zobel, 1989; Zobel et al., 1988).
3. Results
Table 1 presents the complete analysis of variance for all variables (tillage
systems, herbicides, sampling depths, harvest dates, and locations) entered into
the model for Fi-RLD and Fl-RLD. Several interactions had large degrees of
freedom (df) with small sums of squares (SS) which resulted in non-signi®cant P
values. Tillage system, sampling depth, and harvest date main effects and several
of the two-way interactions were highly signi®cant for the root parameters
measured. However, the analysis of variance gave little insight into changes that
occurred in the three-way or greater interactions. Location (0.4 or 0.8 m) and
herbicide (Sencor or Sencor Metribuzin) effects were generally non-signi®cant
for most main effects and interactions. Since location's contribution to the sums
of squares was small (Table 1) values were averaged before the terms were
removed from the model. Removal of location from the analysis would reduce the
model df by one-half. However, due to the association of herbicides with harvest
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D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Table 1
Analysis of variance of ®brous (Fi) and ¯eshy (Fl) asparagus root length density (RLD) with tillage
systems (T), herbicide (H), harvest dates (HD), sampling depth (SD), and sampling location (L)
Source of variation
Total
Model
Replications (reps)
Tillage systems (T)
Herbicides (H)
T*H
T*H*reps (error a)
Harvest dates (HD)
T*HD
H*HD
T*H*HD
Sampling depth (SD)
T*SD
H*SD
T*H*SD
HD*SD
T*HD*SD
H*HD*SD
T*H*HD*SD
Sampling location (L)
T*L
H*L
T*H*L
HD*L
T*HD*L
H*HD*L
T*H*HD*L
SD*L
T*SD*L
H*SD*L
T*H*SD*L
HD*SD*L
T*HD*SD*L
H*HD*SD*L
T*H*HD*SD*L
Error b
a
Degrees of freedom
849
275
3
1
1
1
9
5
5
4
4
5
5
5
5
25
25
20
20
1
1
1
1
5
5
4
4
5
5
5
5
25
25
20
20
574
Nonsigni®cant.
Signi®cant at P < 0.05.
**
Signi®cant at P < 0.01.
***
Signi®cant at P < 0.001.
*
Sums of squares
Fi-RLD
Fl-RLD
33.209
16.404***
0.631
1.744**
1.005**
0.062a
0.527a
0.687**
0.370*
0.769***
0.303*
3.684***
0.285a
0.572**
0.101a
1.335**
0.395a
0.646a
0.248a
0.003a
0.008a
0.001a
0.011a
0.211a
0.071a
0.176a
0.106a
0.311a
0.090a
0.047a
0.064a
0.230a
0.496a
0.374a
0.840a
16.804
4.011
2.059***
0.032
0.104*
0.010a
0.002a
0.110a
0.111***
0.016a
0.017a
0.032*
0.755***
0.052**
0.028a
0.009a
0.172**
0.099a
0.077a
0.019a
0.006a
0.006a
0.000a
0.001a
0.039**
0.011a
0.005a
0.013a
0.076**
0.006a
0.006a
0.006a
0.040a
0.097a
0.022a
0.081a
1.952
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
193
Table 2
Standard analysis of variance for ®brous (Fi) and ¯eshy (Fl) asparagus root length density (RLD)
with 22 tillage dates (TDa) in six sampling depths (SD)
Source of variation
Total
Model
Tillage dates (TDa)
Sampling depth (SD)
TDa*SD
Error
df
431
131
21
5
105
300
Sums of squares
Fi-RLD
Fl-RLD
11.643
6.359***
2.635***
1.968***
1.755a
5.283
1.391
0.765***
0.149***
0.390***
0.226a
0.626
a
Nonsigni®cant.
Signi®cant at P < 0.05.
***
Signi®cant at P < 0.001.
*
dates and tillage systems, removal of this variable would not alter the df but
would assign more SS to the residual.
In an attempt to better understand the interaction between harvest dates, tillage
systems and sampling depths, these variables were combined to test seasonal root
changes. The creation of tillage dates (TDa) from the two tillage systems and 11
harvest dates which were compared to the six sampling depth (SD) were to satisfy
the requirements for two-way data tables when using the AMMI analysis. Table 2
shows the standard analysis of variance for the reduced model with the combined
TDa and SD. Results from analysis of variance indicate that for the Fi-RLD and
Fl-RLD variables, the interaction between TDa and SD was not signi®cant
(Table 2). The lack of a signi®cant interaction is not surprising considering the
large df (105) associated with this interaction. However, even if the interaction of
TDa SD was signi®cant, differences presented by the data would be dif®cult to
describe and interpret.
While the standard analysis of variance indicated that there were no signi®cant
interactions between TDa and SD (Table 2), visual observation of the data
suggested that there were distinct changes in RLD during the year. The changes
for Fi-RLD are presented in Fig. 1 and will be used to illustrate these differences.
Fi-RLD appeared to be greater in NT than T in depths 0.15±0.6 m over much of
the growing season, while in the 0.75 and 0.9 m depths, Fi-RLD were similar for
T and NT. There also appeared to be distinct seasonal changes in Fi-RLD during
the growing season. In T, Fi-RLD were low in the 0.15 m depth throughout most
of the year, while in NT, Fi-RLD at this depth were highest early in the year then
declined during the summer and fall. Fi-RLD increased during April in the 0.3±
0.75 m depths in T and NT then declined during May and June before increasing
again from July to September. Few changes in Fi-RLD occurred after full canopy
development in August in the 0.45±0.75 m depths. Furthermore, Fi-RLD was
194
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Fig. 1. Changes in ®brous root length density in till (T) and no-till (NT) asparagus as in¯uenced by
harvest date for six sampling depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m). Values are the means of
the replications the standard errors.
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
195
Table 3
Additive main effects and multiplicative interactions (AMMI) analysis of variance for ®brous (Fi)
and ¯eshy (Fl) asparagus root length density (RLD) with 22 tillage dates (TDa) at six sampling
depths (SD)
Source of variation
df
Sums of squares
Fi-RLD
Total
Model
Tillage dates (TDa)
Sampling depth (SD)
TDa*SD
I-PCA axis
Residual
Error
431
131
21
5
105
25
80
300
12.304
7.059***
2.760***
2.278***
2.022a
1.407***
0.615a
5.245
Fl-RLD
1.484
0.857***
0.180***
0.402***
0.274a
0.169***
0.107a
0.627
a
Nonsigni®cant.
Signi®cant at P < 0.05.
***
Signi®cant at P < 0.001.
*
generally low in the 0.75 and 0.9 m depths during the spring but increased during
the summer and fall.
Since TDa and SD appeared to interact with each other (Fig. 1), the data were
re-analyzed by the AMMI model in an attempt to gain more meaningful insights
into the possible interactions. The AMMI analysis is presented in Table 3 and the
SS can be compared to those of analysis of variance listed in Table 2 since the
AMMI model uses analysis of variance to calculate the main effects. Exact
duplication of SS for the main effects of TDa and SD by standard analysis of
variance and the AMMI models was not achieved due to the presence of missing
data points although the values were very similar. The AMMI analysis clearly
showed the existence of a signi®cant TDa SD interaction, which was not
apparent when testing the data by analysis of variance (Table 2). The ®rst PCA
axis from the analysis of the interaction for the root parameters measured
separated out 70% and 60% of the interaction SS in 24% of the interaction df for
the Fi-RLD and Fl-RLD, respectively. The reduction in df from 105 to 25 now
allowed easier interpretation of the interactions that occur between TDa and SD.
Fig. 2 presents the biplot for the ®rst statistically signi®cant PCA axis and the
means (main effects) for Fi-RLD. The means of the main effects (TDa and SD)
and the grand mean are presented along the x-axis while any deviation of the
grand mean through the interaction effects (TDa by SD) are illustrated in the yaxis. For any combination of TDa and SD, the main effects equals the TDa mean
plus SD mean minus the grand mean (RLD 2267) while the interaction is the
TDa I-PCA score times the SD I-PCA score. For example, NT2 (RLD 4422) at
the 0.3 m (RLD 2579) depth has a main effect RLD of (4422 2579) ±
196
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Fig. 2. Biplot of the AMMI model for ®brous root length density of asparagus roots with 22 tillage dates (till (T) and no-till (NT), dates 1±11) in six
sampling depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m). The grand mean was 2300 m/m3.
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
197
2267 4734 m/m3 and an interaction of 54.1 49.5 2678 m/m3. Thus, the
AMMI model estimates Fi-RLD to be 7412 m/m3 (4734 2678) for NT2 at the
0.3 m depth. This was close to the observed Fi-RLD of 7490 m/m3 noted in
Fig. 1. TDa and SD with the same sign on the I-PCA axis have a positive
interaction (mean Fi-RLD will increase) while if different, their interactions are
negative (mean Fi-RLD will decrease). Values of I-PCA close to zero have only
small interaction effects (main effects change only). For example, 0.15 and 0.6 m
depths (I-PCA close to zero) differ only in main effects where 0.3 and 0.75 m
depths differ in interaction since the I-PCA scores differ greatly but not the mean
Fi-RLD (Fig. 2).
From Fig. 2, differences in Fi-RLD occurred between T and NT at most harvest
dates supporting the differences illustrated in Fig. 1. Harvest dates signi®cantly
affected Fi-RLD, since early harvest dates (2 and 3) produce greater root lengths
than late harvest dates (9±11). The decline in Fi-RLD during the harvest period in
T and NT are illustrated by a decreasing mean Fi-RLD from date 3±6 (Fig. 2). FiRLD increased again on dates 7 and 8 before declining late in the year (dates 9±
11). Sample depth will in¯uence the Fi-RLD for the different TDa. Sample depths
0.3 and 0.45 m are on the positive side of the y-axis while 0.75 and 0.9 m are on
the negative side. Thus, Fi-RLD in the upper regions of the soil pro®le will be
greater than at deeper depths.
Although there was no inherent interaction between TDa and SD for ¯eshy root
length from the analysis of variance (Table 1), there were distinct patterns of FlRLD growth occurring throughout the season (Fig. 3). In general, Fl-RLD were
greater in NT than T in the shallow depths with little difference in root length
occurring at depths below 0.6 m. Root lengths decreased as depth in the soil
pro®le increased with the highest Fl-RLD occurring at 0.3 m and the lowest at
0.75 and 0.9 m.
Visual observations of the ¯eshy root data suggest that seasonal differences in
RLD were also apparent (Fig. 3). Fl-RLD was greater in NT than T in the 0.15 m
depth from March to May and again from July to September. While root growth
at 0.15 m declined in NT during the harvest period, Fl-RLD increased slowly in
T. In both T and NT, Fl-RLD increased late in the year in the 0.15 m depth. FlRLD were greatest in April in the 0.3 and 0.45 m depths then declined during
May and June. Fl-RLD continued to decline in T compared to NT during the fern
growth period of July before stabilizing in August (Fig. 3). There were few
changes in Fl-RLD during the year at 0.6±0.9 m depths.
Fig. 4 presents the biplot for the ®rst statistically signi®cant PCA axis and the
means (main effects) for Fl-RLD, which shows the signi®cant TDa by SD
interaction (Table 2). Fl-RLD was greater in NT than T on most harvest dates.
Early harvests generally had greater root lengths than late harvests. Depth in the
soil pro®le plays a signi®cant role in the interactions partitioned out in the I-PCA
axis. Root lengths were greatest in at 0.3 m (large mean Fl-RLD and positive
198
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Fig. 3. Changes in ¯eshy root length density in till (T) and no-till (NT) asparagus as in¯uenced by
harvest date in six sampling depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m). Values are the means of
the replications the standard errors.
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
199
Fig. 4. Biplot of the AMMI model for ¯eshy root length density of asparagus roots with 22 tillage dates (till (T) and no-till (NT), dates 1±11) in six
sampling depths (0.15, 0.3, 0.45, 0.6, 0.75 and 0.9 m). The grand mean was 520 m/m3.
200
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
I-PCA score) and least at 0.9 m (low mean Fl-RLD and negative I-PCA score).
There were few differences in mean Fl-RLD between the 0.15, 0.45 and 0.6 m
depths. Low root lengths early (T1, T2, NT1 and NT2) and late (T10, NT10, T11
and NT11) in the season in the deeper depths (0.75 and 0.9 m) are illustrated by
low mean Fl-RLD and negative I-PCA scores. Fl-RLD declined from dates 3 to 6
in T but remained unchanged or increased slightly in NT during the same period.
Root lengths increased again on dates 7 and 8 (increasing mean RLD) in both T
and NT with little change occurring after date 9 (constant mean Fl-RLD).
Tillage system, sampling depth, or location did not in¯uence ®brous root
diameters when measured in late May. Fibrous root diameters ranged from 0.40 to
0.58 mm with an average diameter of 0.52 mm. There was a signi®cant
interaction between tillage system, sampling location and sampling depth for
¯eshy root diameters. Fleshy root diameters were generally greatest near the soil
surface at 0.40 m distance from the row and decreased as depth in the soil pro®le
increased in both T and NT. At the 0.80 m distance (between adjacent rows),
¯eshy root diameters were similar in both T and NT between the 0.15 and 0.45 m
depths, then decreased as sampling depth increased. Fleshy root diameters varied
from 2.5 to 4.0 mm at depths above 0.45 m. At 0.6 m and below, ¯eshy root
diameters ranged from 1.3 to 2.0 mm.
Soil temperatures were similar for the T and NT at all time during the growing
season (data not shown). Temperature varied with time of the year and depth in the
soil pro®le. Soil temperatures were greatest near the soil surface and least below
0.6 m. There was considerable oscillations in temperature at 0.15 m with temperatures ranging from 58C early and late in the year to greater than 258C during
the summer. As depth in the soil pro®le increased, temperature remained more
stable and averaged between 158C and 258C during most of the growing season.
4. Discussion
There was a considerable variation in asparagus RLD during the sampling
period resulting in the failure of analysis of variance to detect differences between
the interactions of tillage systems, harvest dates, and sample depths. In contrast,
the AMMI analysis gave a reliable indication of differences in ®brous and ¯eshy
RLD in T and NT systems and to changes in root development during the growing
season. The AMMI model with the biplot display allowed overall patterns in
®brous and ¯eshy RLD to be visualized (differences between T and NT, changes
over time and depth) as well as speci®c TDa SD interactions.
From our study, it is evident that even shallow tillage operations signi®cantly
reduce root growth near the soil surface. Asparagus root reductions near the soil
surface have been attributed to aging of the primary (¯eshy) roots, soil compaction, and structural breakdown associated with tillage operations (Reijmerink,
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
201
1973). Tillage operations in asparagus have been used historically to control
weeds, loosen the soil prior to spear emergence, and to incorporate fern residues
(Putnam, 1972). During asparagus establishment, tillage operations are frequently
used to ®ll in the planting furrow. Injury to young roots caused by tillage in newly
established plantings would create a root free zone which regular tillage
operations in the future would maintain. Frequency and depth of tillage have been
associated with a root free zone close to the soil surface in grapes (Van Huyssteen
and Weber, 1980). With asparagus crown depths at 0.15±0.25 m, proper
equipment adjustment must be maintained to avoid additional crown injury.
Damage to crowns and the roots near the crown will reduce ¯eshy root extension
and ®brous root growth, since ®brous roots are initiated from ¯eshy roots
(Blasberg, 1932). While tillage can change the physical characteristics of the soil,
RLD were not believed to be signi®cantly modi®ed by these structural changes.
Decreases in ®brous and ¯eshy root lengths during and after harvest (May±
July) were not limited to the top 0.15 m of the soil pro®le. Fibrous and ¯eshy
RLD reductions occurred at most depths early in the season and are believed to be
associated with strong sink demands by the developing spears and fern resulting
in low carbohydrate availability for continued root growth. Root growth
recovered when fern growth slowed in July in NT but continued to decline in
T. Tillage operations have been shown to delay fern development and contribute
to yield reductions in asparagus (Putnam, 1972; Wilcox-Lee and Drost, 1991).
Several things may contribute to the extended decline in RLD in the tilled system.
The early tillage operation would damage emerging spears and ®brous root near
the soil surface. This would decrease yield. The additional tillage operation after
harvest would delay canopy development, which would limit leaf area, decrease
the photosynthetic potential of the plant, contribute to disease spread and limit
new bud development ultimately weakening the plant. In addition, this would also
delay root growth resumption. In asparagus summer root growth is important for
nutrient absorption, water uptake, and the creation of additional carbohydrate
storage. Slowing these processes will ultimately impact growth and in¯uence longterm productivity of this perennial plant (Putnam, 1972; Wilcox-Lee and Drost,
1991). It is not clear from this study what role tillage operations have on root growth
at depths greater than 0.9 m. Root growth reductions deeper in the soil may be
related to the continued decline in stored carbohydrates (Dufault and Greig, 1983;
Haynes, 1987; Shelton and Lacy, 1980) or to the smaller pool of carbohydrates
available in tilled systems (Haynes, 1987; Wilcox-Lee and Drost, 1991).
Roots of grape (Freeman and Smart, 1976), apple and plum (Head, 1967), and
spruce (Deans and Ford, 1986) show marked seasonal changes in root growth.
Root growth often begins before shoot growth in the spring (Dell and Wallace,
1983; Head, 1967) then decreases with the onset of shoot growth (Head, 1967)
before resuming during the summer or fall (Dell and Wallace, 1983; Freeman and
Smart, 1976). Similar growth patterns were evident for ®brous asparagus roots at
202
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
most soil depths while seasonal changes in ¯eshy root growth were less apparent.
Since ¯eshy roots grow for many years (Scott et al., 1939), the seasonal changes
in RLD noted in our study are believed to be related more to sampling and soil
variability than to actual increases or decreases in Fl-RLD (Bohm, 1979). Since
tillage is known to affect root development, early tillage (March) would impact
®brous root growth (Figs. 1 and 2), while late tillage operations (July) may affect
¯eshy root growth (Figs. 3 and 4). Haynes (1987) reported that in young
asparagus plantings, ¯eshy root numbers increased after fern establishment in the
summer though he says nothing about ®brous roots. Therefore, tillage operations
that reduce ¯eshy root growth will have long-term effects on crown development
and plant growth.
Maximum root growth of ®brous and ¯eshy roots occurred at 0.3 m, which is
immediately adjacent to the crown, and generally declined as depth in the soil
pro®le. Uniform root distribution throughout the soil pro®le will improve both
water and nutrient utilization thereby minimizing potential stresses that can
in¯uence plant growth. High yields in asparagus are determined in part by rooting
depth (Reijmerink, 1973). The Riverhead soil is fairly uniform to a depth of
0.8 m. The abrupt change in soil structure at 0.8 m, may be partially responsible
for the low RLD at depth in the pro®le. Soil pro®le changes can contribute to
differences in asparagus rooting pattern by altering water holding capacity,
porosity, and penetration resistance (Reijmerink, 1973).
Soil temperatures changes can also in¯uence root growth (Bohm, 1979; Dell
and Wallace, 1983). Temperature records from this study showed that soil
temperatures were similar in T and NT throughout the year though temperature
did change with depth. Other studies have shown that temperature differences do
occur between T and NT. The lack of difference in temperature in this study may
be due to the lack of a signi®cant quantity of mulch present on the soil surface in
the NT plots. Most studies that show temperature depression in NT occur when
large quantities of mulch are present. With less mulch on the surface, less
depression would occur resulting in similar temperatures near the surface in T and
NT asparagus. More important for root growth is the damage that occurs to those
roots as a result of the tillage operations both before and after harvest. Since
differences in soil temperature during the year were not great, it does not appear
to contribute to the differences in root growth noted.
5. Conclusions
Several conclusions can be drawn from these studies. First, since roots are
dif®cult to access, novel procedures are needed to analyze them. The AMMI
statistical model did a good job assessing differences between tillage systems,
harvest dates and sampling depth. Second, changes in ®brous root length density
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
203
were easier to detect than ¯eshy root length densities. Since ®brous roots grow for
only one year, changes over time and depth would be expected. These variations
in root growth suggest that management strategies (nutrient and water application
practices) need to take into account these changes. In addition, since ¯eshy roots
grow for several years, variation in RLD over time is due more to sampling and
soil variability rather than unique differences in RLD. However, the differences
between T and NT illustrate that cultural practices will alter ¯eshy root numbers
which ultimately affect plant performance. Finally, almost nothing is known
about the role of root age, rooting depth and root type on nutrient and water
uptake by the plant. Additional work in this area is required to better understand
the yield physiology of asparagus.
References
Blasberg, C.H., 1932. Phases of the anatomy of Asparagus of®cinalis. Botanical Gazette 94, 206±
214.
Bohm, W., 1979. Methods of Studying Root Systems. Springer, Berlin, BRD.
Bradu, D., Gabriel, K.R., 1978. The biplot as a diagnostic tool for models of two-way tables.
Technometrics 20, 47±68.
Deans, J.D., Ford, E.D., 1986. Seasonal patterns of radial root growth and starch dynamics in
plantation-grown sitka spruce trees of different ages. Tree Physiol. 1, 241±251.
Dell, B., Wallace, I.M., 1983. Periodicity of ®ne root growth in Jarrah (Eucalyptus marginata Donn
ex Sm.). Aust. J. Bot. 31, 247±254.
Dufault, R.J., Greig, J.K., 1983. Dynamic growth characteristics in seedling asparagus. J. Am. Soc.
Hort. Sci. 108, 1026±1030.
Freeman, B.M., Smart, R.E., 1976. A root observation laboratory for studies with grapevines. Am.
J. Enol. Viticult. 27, 36±39.
Fisher, K.J., 1982. Comparisons of the growth and development of young asparagus plants
established from seedling transplants and by direct seeding. N.Z.J. Expt. Agric. 10, 405±408.
Gauch, H.G., 1988. Model selection and validation for yield trials with interaction. Biometrics 44,
705±715.
Gauch, H.G., Zobel, R.W., 1988. Predictive and postdictive success of statistical analyses of yield
trials. Theor. Appl. Genet. 76, 1±10.
Gauch, H.G., Zobel, R.W., 1989. Accuracy and selection success in yield trial analysis. Theor.
Appl. Genet. 77, 473±481.
Haynes, R.J., 1987. Accumulation of dry matter and changes in storage carbohydrate and amino
acid content in the ®rst two years of asparagus growth. Scientia Horticulturae 32, 17±23.
Head, G.C., 1967. Effects of seasonal changes in shoot growth on the amount of unsuberised root on
apple and plum trees. J. Hort. Sci. 42, 169±180.
Putnam, A.R., 1972. Ef®cacy of a zero-tillage cultural system for asparagus produced from seed
and crowns. J. Am. Soc. Hort. Sci. 97, 621±624.
Reijmerink, A., 1973. Microstructure, soil strength, and root development of asparagus on loamy
sands in the Netherlands. Neth. J. Agric. Sci. 21, 24±43.
SAS Institute Inc., 1985. SAS User's Guide: Statistics, 5th ed. SAS Institute Inc., Cary, NC.
Scott, L.E., Mitchell, J.H., McGinty, R.A., 1939. Effects of certain treatments on the carbohydrate
reserves of asparagus crowns. S. Carolina Agric. Expt. Stn. Bull. 321.
204
D. Drost, D. Wilcox-Lee / Scientia Horticulturae 83 (2000) 187±204
Shelton, D.L., Lacy, M.L., 1980. Effects of harvest duration on yield and depletion of storage
carbohydrates in asparagus roots. J. Am. Soc. Hort. Sci. 105, 332±335.
Smucker, A.J.M., McBurney, S.L., Srivastava, A.K., 1982. Quantitative separation of roots from
compacted soil by the hydropneumatic elutriation system. Agron. J. 74, 500±503.
Snedecor, G.W., Cochran, W.B., 1980. Statistical Methods, 7th ed. Iowa State Univ., Ames, IA.
Taylor, H.M., 1986. Methods of studying root systems in the ®eld. In: Root Systems of Vegetable
Crops, vol. 21. Proceedings of the Symposium on HortScience, pp. 952±956.
Tiedjens, V.A., 1926. Some observations on roots and crown bud formation in Asparagus of®cinalis.
Proc. Am. Soc. Hort. Sci. 23, 189±195.
Van Huyssteen, L., Weber, H.W., 1980. The effect of selected minimum and conventional tillage
practices in vineyard cultivation on vine performance. South. Afr. J. Enol. Viticult 1, 77±83.
Wallace, D.H., Gniffke, P.A., Massaya, P.N., Zobel, R.W., 1991. Photoperiod, temperature, and
genotype interaction effects on days, and nodes required for ¯owering of bean. J. Am. Soc. Hort.
Sci. 116, 534±543.
Wilcox-Lee, D., Drost, D., 1991. Tillage reduces yield and crown, fern, and bud growth in a mature
asparagus planting. J. Am. Soc. Hort. Sci. 116, 937±941.
Zobel, R.W., 1990. A powerful statistical model for understanding genotype-by-environment
interaction. In: Lang, M.S. (Ed.), Genotype-by-Environment interaction and Plant Breeding.
Louisiana State University, Baton Rouge, LA, pp. 126±140.
Zobel, R.W., Wright, M.J., Gauch, H.G., 1988. Statistical analysis of yield trial. Agron. J. 80, 388±
393.