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®cinalisL.; 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).
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 un-branched, 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 func-tional, 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.
2. Materials and methods
2.1. Plant materials and general experimental conditions
Six-year-old asparagus (Asparagus of®cinalisL. 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. haÿ1and napropamide at 1.68 kg a.i. haÿ1before 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
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 mm2mesh 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 (Delta-T Devices, Cambridge, UK) attached to an Ikegami I(Delta-TC 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 (y1.5919(x); r20.996) 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 LX=V m=m3; (1)
whereLis the average ®brous root length (m),Xthe calibration constant (1.5919) andVis 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-RLDL=V m=m3; (2)
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 require-ment 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
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 Degrees of freedom Sums of squares
Fi-RLD Fl-RLD
Total 849 33.209 4.011
Model 275 16.404*** 2.059***
Replications (reps) 3 0.631 0.032
Tillage systems (T) 1 1.744** 0.104*
Herbicides (H) 1 1.005** 0.010a
T*H 1 0.062a 0.002a
Sampling depth (SD) 5 3.684*** 0.755***
T*SD 5 0.285a 0.052**
Sampling location (L) 1 0.003a 0.006a
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 TDaSD 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
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 df Sums of squares
Fi-RLD Fl-RLD
Total 431 11.643 1.391
Model 131 6.359*** 0.765***
Tillage dates (TDa) 21 2.635*** 0.149***
Sampling depth (SD) 5 1.968*** 0.390***
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 TDaSD 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 they -axis. For any combination of TDa and SD, the main effects equals the TDa mean plus SD mean minus the grand mean (RLD2267) while the interaction is the TDa I-PCA score times the SD I-PCA score. For example, NT2 (RLD4422) at the 0.3 m (RLD2579) depth has a main effect RLD of (44222579) ±
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 Fl-RLD
Total 431 12.304 1.484
Model 131 7.059*** 0.857***
Tillage dates (TDa) 21 2.760*** 0.180***
Sampling depth (SD) 5 2.278*** 0.402***
TDa*SD 105 2.022a 0.274a
I-PCA axis 25 1.407*** 0.169***
Residual 80 0.615a 0.107a
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.
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22674734 m/m3 and an interaction of 54.149.52678 m/m3. Thus, the AMMI model estimates Fi-RLD to be 7412 m/m3(47342678) 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 RLD from date 3±6 (Fig. 2). Fi-RLD 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 they-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 Fl-RLD growth occurring throughout the season (Fig. 3). In general, Fl-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, RLD increased late in the year in the 0.15 m depth. Fl-RLD 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. 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.
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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 tem-peratures 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 TDaSD interactions.
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 long-term 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).
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
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.
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