Pflanzenbauwissenschaften, 13 (2). S. 49–59, 2009, ISSN 1431-8857. © Verlag Eugen Ulmer, Stuttgart
Relations between soil structural properties and sugar beet yield on
a Luvisol
Beziehungen zwischen Kenngrößen der Bodenstruktur und dem Ertrag von Zuckerrüben auf
einem Luvisol
H.-J. Koch
Institut für Zuckerrübenforschung, Göttingen
Summary
Field studies elucidating the relation between soil structural properties and sugar beet (Beta vulgaris L.) growth are
scarce. To close this gap for temperate climate in central
Europe, soil and crop data from a long-term field study on
a Luvisol with different soil tillage and wheeling treatments were subject to analyses of correlation and variance/covariance. In spring 2004-2006, penetration resistance (PR), bulk density (BD), volume of pores >50 µm
(air filled pore volume, AFPV) and air permeability (AP)
were determined in 5-10, 18-23, 30-35, 40-45, and
55-60 cm soil depth. Sugar beet yield and fibrous root
growth was measured at several dates during the growing
season. Variation of soil structural properties was largest in
the 18-23 cm depth, while in all other layers variances
were substantially smaller. Sugar beet yield correlated significantly to data from 18-23 cm depth but not from other
layers. Surprisingly, at May harvest PR and BD showed a
positive correlation to yield, while AFPV and AP were negatively related to yield. From July onward these relations
changed fundamentally: now, PR and BD were negatively
correlated to yield, while the correlation was positive for
AFPV and AP. Introducing PR from 18-23 cm depth as a covariate to the ANOVA of yield data from September harvest
decreased treatment (tillage x wheeling) variance by -76%
compared to -31-52% due to the other soil parameters. Fibrous root length density from 15-30 cm depth in July
showed only a weak correlation to soil properties
(18-23 cm). In contrast, increasing PR significantly increased fibrous root diameter (July) and taproot fanginess
(September). From this study it can be concluded that elevated soil strength in topsoil layers may impede sugar beet
taproot formation even though nutrient and water supply
are not limited by soil strength. Soil tillage and other management operations have to assure a spring PR of
0.5-1 MPa and a BD of 1.35-1.45 Mg m-3 in the topsoil
down to at least 20 cm depth for high yielding sugar beet
crops on Luvisols.
Boden- und Pflanzendaten eines Langzeitversuches auf einem Luvisol mit differenzierter Bodenbearbeitung und Befahrung einer Auswertung mittels Korrelations- und Varianz-/Kovarianzanalyse
unterzogen.
Im
Frühjahr
2004-2006 wurden Eindringwiderstand (PR), Lagerungsdichte (BD), Porenvolumen >50 µm (luftgefülltes Porenvolumen, AFPV) und Luftpermeabilität (AP) in 5-10, 18-23,
30-35, 40-45 und 55-60 cm Bodentiefe gemessen. Zuckerrübenertrag und Faserwurzelwachstum wurden zu verschiedenen Terminen während der Vegetationszeit erhoben. Die Variation der Bodenstrukturparameter war am
größten in 18-23 cm Tiefe, während in allen anderen
Schichten die Datenvariabilität erheblich geringer war.
Der Zuckerrübenertrag korrelierte nur mit Daten aus
18-23 cm Tiefe signifikant. Überraschender Weise waren
zur Maiernte PR und BD positiv mit dem Ertrag korreliert,
während die Korrelationen für AFPV und AP negativ waren. Von Juli an änderte sich dies grundlegend: PR und BD
waren nun negativ und AFPV und AP positiv mit dem Ertrag korreliert. Die Berücksichtigung des PR aus 18-23 cm
Tiefe als Kovariable in der Varianzanalyse der Ertragsdaten aus der Septemberernte verminderte den Behandlungseffekt (Bodenbearbeitung x Befahrung) um -76% gegenüber -31-52% bei Integration anderer Bodenparameter. Die Wurzellängendichte aus 15-30 cm Tiefe (Juli) war
nur gering mit den Bodeneigenschaften aus 18-23 cm
Tiefe korreliert. Demgegenüber stiegen Wurzeldurchmesser (Juli) und Beinigkeit (September) mit steigendem PR
an. Die Studie weist darauf hin, dass eine erhöhte Bodenfestigkeit in der Krume die Ausbildung des Rübenkörpers
von Zuckerrüben beeinträchtigt, auch wenn die Nährstoffund Wasserversorgung dadurch nicht limitiert werden.
Insbesondere die Bodenbearbeitung muss sicherstellen,
dass die Frühjahrswerte von PR und BD 0,5-1 MPa und
1,35-1,45 Mg m-3 in der Krume bis etwa 20 cm Tiefe nicht
übersteigen.
Schlüsselworte: Bodenstruktur, Lagerungsdichte, Eindringwiderstand, Grobporen, Luftpermeabilität
Key words: soil structure, dry bulk density, penetration
resistance, coarse pores, air permeability
Zusammenfassung
Feldstudien zur Beziehung zwischen Kenngrößen der Bodenstruktur und dem Wachstum von Zuckerrüben (Beta
vulgaris L.) sind selten. Um diese Lücke für die gemäßigten
Klimabedingungen Mitteleuropas zu schließen, wurden
Pflanzenbauwissenschaften 2/2009
Introduction
During recent decades, sugar beet yield substantially increased in central Europe. Progress in cultivar performance and crop management (MÄRLÄNDER et al. 2003),
and moreover, climate change related increases in temperature and atmospheric CO2 level can be regarded as the
main causes for this effect (CHMIELEWSKI et al. 2004, JAGGARD et al. 2007). Simultaneously, the weight of machines
50
H.-J. Koch: Relations between soil structural properties and sugar beet yield on a Luvisol
used for field crop management considerably increased
(VDI 2007). Moreover, reduced tillage practices characterised by decreased tillage depth and intensity were increasingly applied for sugar beet cultivation (MERKES et al.
2003). Such developments may have enhanced the risk of
soil structure degradation (BOIZARD et al. 2002, KOCH et al.
2008, KOCH et al. 2009) thereby limiting the yield of arable
crops (BOONE & VEEN 1994). Unfavourable soil structural
properties may particularly restrict the growth of sugar
beet compared to many other crops due to the intimate
contact between the taproot and the adjacent topsoil
(0-30 cm depth). Moreover, expeditious leaf canopy development securing a maximum radiation interception requires rapid formation of an extended fibrous root system
during early growth to meet the crops’ demand for adequate nutrient and water supply (PIDGEON et al. 2001). In
contrast to autumn sown crops, retarded early growth of
spring sown sugar beet cannot be fully compensated for
later in the growing season.
Several soil physical parameters are applicable to characterise structural properties relevant for crop growth:
Penetration resistance (PR) and soil bulk density (BD)
were frequently used to quantify soil mechanical strength
affecting root elongation (GREACEN & OH 1972, TAYLOR &
BRAR 1991, ATWELL 1993), thereby influencing the acquisition of water and nutrients (CANNELL 1977, BOONE & VEEN
1982, BARRACLOUGH & WEIR 1988, AMATO & RITCHIE 2002).
Moreover, the volume of pores >50 µm in diameter (air
filled pore volume = AFPV) was used to characterise soil
physical quality. Such pores are filled with air except during limited time periods following heavy rainfall, when soil
water tension may drop below 6 kPa. After winter, large
AFPV provides rapid warming up of the topsoil (RICHARD &
CELLIER 1998). Furthermore, AFPV reflects the soil's capability to provide oxygen (MOLDRUP et al. 2000) required for
optimum root growth and functioning (BOONE et al. 1994).
This aspect is also covered by air permeability (AP) as a
conductivity parameter: However, AP is regarded much
more sensitive to soil structural changes compared to AFPV, because it also refers to continuity, connectivity and
tortuosity of pores (GROENEVELT et al. 1984, ROSEBERG &
MCCOY 1990, BLACKWELL et al. 1990). By this parameter, saturated water flux properties are properly described as well
(IVERSEN et al. 2001). Although all these soil structural parameters are linked to each other more or less closely, the
questions arise if (i) there is a principal key factor limiting
root and shoot growth, and (ii) the factor is constant over
the whole growing season or changes due to changing environmental conditions and crop demands.
Field studies on the relation between soil structural
properties and crop growth and yield have been conducted
for e.g. cereals, soybean, maize, potatoes and peas (BOONE
1986, BOONE et al. 1987, LIPIEC & SIMOTA 1994, WHALLEY et
al. 1995, FLOWERS & LAL 1998, ARVIDSSON 1999, CZYZ 2004,
TRIPATHI et al. 2005, WHALLEY et al. 2006). For sugar beet,
recent field studies elucidating the relation between soil
structural quality and sugar beet growth are not available
for high yielding conditions on loessial soil under temperate climate. Hence, our study aimed to illuminate the relations between soil physical parameters from various
depths of a Luvisol and sugar beet yield at several dates of
the growing season. Soil parameters revealing a close correlation to yield might allow the identification of the main
cause-and-effect pathway(s) linking soil structure and
sugar beet growth in different time periods of the growing
season under field conditions.
In this study, soil and plant data from a long-term tillage
and wheeling experiment were subject to analyses of correlation and variance/covariance. Details about the treat-
ment effects on soil structure have been previously presented by KOCH et al. (2008).
Material and Methods
Site description, machine properties, experimental layout
In 1992, a long-term field experiment was established in
Harste near Goettingen (Lower Saxony, Germany,
51°30’ N, 9°56’ E) in order to study the effects of tillage
systems on soil properties and plant growth. The wheeling
treatments were superimposed from 2001 onward to investigate the effects of heavy machinery. The soil of the experimental site was a Stagnic Luvisol derived from loess
with a colluvial history of the topsoil (ANONYMOUS 2005).
The clay and silt content of the 0-10 cm soil horizon ranged
from 120-150 g kg-1 and 830-860 g kg-1, respectively. The
content of organic carbon accounted for 10-12 g kg-1 and
about 8 g kg-1 in 0-30 cm and 30-50 cm soil depth, respectively. The average water filled pore volume at a water tension of 6.2 kPa (field capacity) was about 37 m3 100 m-3.
Long-term (1952-2004) annual rainfall and air temperature were 601 mm and 8.8°C, respectively.
Before 1992, the soil was uniformly mouldboard
ploughed for many years, and sugar beet, winter wheat
(Triticum aestivum L.) and winter barley (Hordeum vulgare
L.) were grown in a rotation. In autumn 1992, the experimental site (4.8 ha) was divided into three equally sized
fields. Sugar beet, winter wheat and winter barley were
cultivated in a rotation having each crop in one of the fields
every year. Following winter barley, white mustard was
grown as a catch crop. After harvest, all crop residues remained on the field.
Tillage treatments consisted of annual mouldboard
ploughing (MP, 30 cm deep), and shallow-mixing conservation tillage (SM, ≤10 cm deep) with a wing-bladed rigid
tine cultivator. Additionally, a light disc or tine harrow and
a rotary harrow were used in both treatments operating
5 cm deep at maximum to conduct stubble tillage and
seedbed preparation.
In October 2001, the first wheeling pass with heavy machinery (wheel load 7.8–11.7 Mg, average ground contact
pressure 100-145 kPa) was carried out on the respective
field grown with sugar beet (Tab. 1, Field 1) after regular
beet harvest with a light 1- or 2-row harvester. At this additional wheeling, half of every tillage plot was completely
covered by wheel ruts. In 2002 and 2003, cereals were
grown on Field 1, and heavy loading was deliberately repeated on the same half-plots in late autumn several
months after cereal harvest with a customary combine. In
autumn 2002 and 2003, the sequence of three times
wheeling was started on Field 2 (2002) and Field 3 (2003,
Tab. 1), respectively.
Prior to wheeling, sprinkler irrigation (2-5 doses of
about 25 mm each) was applied if necessary to secure high
soil moisture at the machine pass (90-100% of field capacity in most cases). For wheeling, a six-row self propelled
sugar beet tanker harvester (SBH 6, Holmer TerraDos with
two axles, wheel load 7.8-11.7 Mg) was used operating
track-by-track. Complete cover of the wheeled plots required four single passes per plot, whereby the tracks of
the second and the fourth pass were positioned in the unwheeled interspaces right between the tracks of the first
and the third pass. More details of the machine properties
and wheeling conditions were given by KOCH et al. (2008).
After each wheeling, the soil was tilled according to MP
or SM management, and cereals, cereals and sugar beet
were grown after the first, second and third wheeling, rePflanzenbauwissenschaften 2/2009
H.-J. Koch: Relations between soil structural properties and sugar beet yield on a Luvisol
51
Tab. 1: Field management and sampling dates (month/year) during the study period, Harste 2004-2006
Termine der Anbaumaßnahmen und Probennahmen (Monat/Jahr) im Versuchszeitraum, Harste 2004-2006
1. Wheeling
2. Wheeling
3. Wheeling
Mouldb. ploughing / Shallow mixing
Sowing sugar beet
Soil sampling
Plant sampling (incl. roots)
spectively. Cultivation of crops followed regional standards. Sugar beet was cultivated with a plant density of
about 90,000 plants ha-1 (manually thinned from 7.7 to
23 cm in-row plant distance) at a row width of 45 cm and
fertilised with 110 kg N ha-1.
On each field a randomised split-plot design with four
replicates was permanently established. Soil tillage was
applied on main plots (16.2 m x 14 m) and wheeling with
the SBH 6 on sub plots (8.1 m x 14 m).
Samplings and measurements
For this study, effects on soil properties and crop growth
were investigated in sugar beet grown as the regular rotational crop after the third wheeling conducted in the years
2003 (Field 1), 2004 (Field 2) and 2005 (Field 3, Tab. 1).
Soil data
On all plots soil PR was measured to a depth of 65 cm in
April/May after sugar beet sowing at a soil water tension
close to field capacity. The penetrologger (Eijkelkamp,
Giesbeek, Netherlands) was equipped with a cone having
a cross-sectional area of 0.1 cm2 and an angle of 60°. Measurements were performed with ten internal replicates per
plot. Mean values per plot were calculated for the layers
5-10, 18-23, 30-35, 40-45 and 55-60 cm.
Undisturbed soil core samples and disturbed samples
were taken in April/May 2004 (Field 1, two replicates),
2005 (Field 2, four replicates) and 2006 (Field 3, three replicates, Tab. 1) from 5-10, 18-23, 30-35, 40-45 and
55-60 cm depth. The number of sampled replicates
changed during the years/fields due to the working capacity available at any one time. In order to take samples manually, a pit was dug in each plot. Soil cores of 250 cm3 volume (80 mm in diameter) were taken vertically from each
layer with 8-10 internal replicates per plot and depth.
In the lab, the soil cores were capillary wetted to saturation on a sand bed and then drained to a water tension of
6.2 kPa. Subsequently, the core weight was recorded and
evidently continuous pores ≥1 mm in diameter were sealed
with silicone. Vertical pneumatic conductivity was measured with the apparatus PL 300 (Umwelt Geräte Technik
GmbH, Müncheberg, Germany) and used to calculate AP
according to KMOCH & HANUS (1965).
Subsequently, the cores were oven-dried to constant
weight. BD was derived from cylinder dry weight and volume. Particle density was measured on disturbed samples
using an Accupyc 1330 gas pycnometer (Micromeritics
Instr. Corp., Norcross, GA, USA) according to BLAKE &
HARTGE (1986). Total pore volume was calculated from BD
Pflanzenbauwissenschaften 2/2009
1
Field
2
3
10/01
11/02
11/03
11/03
04/04
05/04
05/04-09/04
11/02
11/03
11/04
11/05
04/05
04/05
05/05-09/05
11/03
11/04
11/05
11/05
04/06
05/06
05/06-09/06
and particle density (not shown). Volume of pores >50 µm
equivalent diameter, i. e. AFPV, was calculated as the difference between total pore volume and the volume of water retained in the cylinder at 6.2 kPa water tension.
Plant data
Sugar beet were harvested at the mid of May, June, July,
August, and finally, at the end of September (Tab. 1). At
May harvest, the plants had 6-8 true leaves starting to
touch leaves of their in-row neighbours. Surplus plants
(leaves, initial taproot, adhering fibrous roots) exceeding
the target number of 90,000 plants ha-1 were carefully
removed from rows destined for September harvest.
Thereby, a uniform stand with an almost equal in-row
plant distance of 23 cm was obtained. The number of harvested plants per plot in May was about 150. At all subsequent dates, four rows of sugar beet were harvested
from each plot at a length of either 4.8 or 5.75 m
(8.64 m2 or 10.35 m2 accounting for 75-90 plants per
plot). Leaves and tops were manually cut off the taproot
and weighed in the field. A representative sub-sample
(∼15 tops with leaves attached per plot) was taken to the
lab for dry matter determination. Except at September
harvest (machine harvest) storage roots were manually
dug out, washed and weighed. Grinded sub-samples of
plant material were weighed, oven-dried at 105°C for
24 h and weighed again. Data were used for the calculation of dry matter yield of the total plant neglecting fibrous roots (biological yield). At May harvest, the single
plant weight was calculated from dry weight divided by
plant number. The yield per hectare was obtained by multiplication of single plant weight with the population density of 90,000 plants ha-1.
Sugar beet fibrous roots were sampled in June, July and
August with a root auger (7,5 cm in diameter) from 0-15,
15-30, 30-45 and 45-60 cm soil depth (45-60 cm depth
was solely sampled in July and August). Samples were taken in six replicates per plot along a diagonal line between
plant rows. Immediately after sampling, roots were carefully washed out from the cores. After manual separation
of mineral and organic residues the clean roots were
scanned (Epson Expression 1680 Pro 1600x3200 dpi,
Meerbusch, Germany). Root length was determined by a
picture processing computer program (PicColor NT Image
analysis, Version 4.0, FIBUS, Germany). Data were used to
calculate root length density (km m-3), and root diameter
averaged over total length of fibrous roots.
At September harvest, fanging of taproots was visually
scored. Fangy beets are given as the percentage of all taproots.
52
H.-J. Koch: Relations between soil structural properties and sugar beet yield on a Luvisol
Statistical analysis
Results
Soil physical properties
BD was selected as a representative to display essential effects of tillage and wheeling on soil structure (Fig. 1). The
SM tillage system significantly decreased BD in 5-10 and
40-45 cm, but substantially increased values in 18-23 cm
depth compared to MP treatment (Tab. 2). Wheeling significantly increased BD values of the 5-10, 18-23 and
40-45 cm layer, and moreover, in the 55-60 cm horizon of
SM tillage treatment. Treatment effects obtained for PR,
AFPV, and AP were well in line with those for BD (KOCH et
al. 2008).
Within each sampling depth, the coefficients of Spearman’s rank correlation revealed significant correlations between BD, AFPV and AP, either positively (AFPV vs. AP) or
negatively (BD vs. AFPV and AP, Tab. 3). In the 18-23 cm
1,7
1,6
Bulk density [Mg m-3]
For parameters measured with plot-internal replicates, the
mean value was calculated for each plot as arithmetic
means either from original data or, when not normally distributed, as retransformed mean after logarithmic transformation. For the plot mean, normal distribution of residues was tested (Shapiro-Wilk-test, α ≤0.1) with the
UNIVARIATE procedure (SAS V 8.01, Cary, NY, USA) according to DUFNER et al. (2002). Not normally distributed
data sets were transformed to obtain normal distribution
using the Box-Cox macro (ANONYMOUS 2007) or the natural
logarithm.
The ANOVA of soil and plant data was performed with
the MIXED procedure setting field/year as random effect
and the treatments tillage and wheeling as fixed effects.
The model was a split-plot-design with tillage as main plots
and wheeling as subplots. Effects were regarded significant at α≤0.05 (F-test). For more detailed information,
probability of F-values at α≤0,1 is given as well. Significant
interactions between field/year x tillage, field/year x
wheeling and field/year x tillage x wheeling did not occur
and thus, were not included in statistical information given
in tables and figures. Data were processed separately for
each soil depth and plant harvest date.
For presentation, arithmetic means or retransformed
means were used if normal distribution was lacking. In figures, means of tillage x wheeling interaction are displayed
for more detailed information. The Least Square Means
procedure was applied to compare individual means.
The relations among soil physical parameters over depth
and moreover, between soil properties and crop yield were
evaluated by Spearman’s rank correlation (PROC CORR).
Selected soil parameters revealing a close correlation to
yield were submitted to regression analysis (PROC REG).
For both calculations, yield data centred for field/year effects were used to eliminate yield variation caused by
field/year. In contrast, original data were applied for soil
parameters.
Centred taproot yield of September harvest was submitted to an ordinary linear ANOVA (PROC GLM), whereby
the two-factorial structure of the data was suspended to
examine combined treatment effects. In a second step, soil
parameters with close correlation to September taproot
yield were integrated into the ANOVA model. In the model,
type I sums of squares were applied to fit quantitative covariates before adding the qualitative treatment variable.
Finally, the relative change of the treatment sum of squares
caused by covariate integration was calculated.
MP unwheeled
MP wheeled
SM unwheeled
SM wheeled
a ab a
b
1,5
1,4
1,3
0,0
5-10
18-23
30-35
40-45
55-60
Soil depth [cm]
Fig. 1: Effect of repeated wheeling with heavy load and soil
tillage on soil bulk density in different depths, Harste
2004-2006; in case of significant interaction (cf. Tab. 2) different letters above columns indicate significant differences at
α ≤ 0.05; MP=Mouldboard ploughing, SM=Shallow mixing
Einfluss von wiederholter Befahrung mit schweren Maschinen
und Bodenbearbeitung auf die Lagerungsdichte in verschiedenen
Tiefen, Harste 2004-2006; bei signifikanter Wechselwirkung (s.
Tab. 2) zeigen unterschiedliche Buchstaben über den Säulen signifikante Unterschiede mit α ≤ 0,05 an; MP=Pflugbearbeitung,
SM=Flach mischende Bearbeitung
layer the coefficients were highest and close to one. Additionally, only in this horizon PR significantly correlated to
all the other parameters.
Plant parameters
Total plant yield increased from 0.015 Mg ha-1 in May to
17-19 Mg ha-1 in September (Fig. 2). SM significantly increased yield in May compared to MP, but from July onward the effect of tillage reversed and total plant yield and
taproot yield were diminished by SM compared to MP
(Fig. 2, Tab. 4). Wheeling significantly decreased the total
plant and/or taproot yield in June-August but not in May
and September. No significant interactions occurred.
In July, root length density of sugar beet plants decreased with depth from about 15 to 4 km m-3 (Fig. 3). In
0-15 cm depth, wheeling significantly decreased root
length density in MP but not in SM. In 15-30 cm depth, the
effect of wheeling was significant on average across tillage,
while in the deeper layers neither tillage nor wheeling significantly affected root length density (Tab. 5). Results
from samplings in June and August partly differed in the
absolute level of root length density from those obtained in
July, but treatment effects were very similar across all sampling dates (not shown).
Root diameter substantially increased from 0-15 cm
sampling depth to the 30-60 cm layers in all treatments
(Fig. 3, Tab. 5), and were significantly higher in SM compared to MP tillage in all depths (Fig. 3). Wheeling significantly increased root diameter in the topmost layer only
(Tab. 5). Very similar results were obtained from the samplings in June and August (not shown).
Taproot fanginess at September harvest was substantially increased by SM compared to MP, and moreover, in the
wheeled compared to the unwheeled treatment (Fig. 4).
Interactions between tillage and wheeling were not detectable.
Pflanzenbauwissenschaften 2/2009
H.-J. Koch: Relations between soil structural properties and sugar beet yield on a Luvisol
53
Tab. 2: Probability level of F-values (α < 0.1) for dry bulk density, Harste 2004-2006; ns=not significant
Irrtumswahrscheinlichkeit der F-Werte (α < 0,1) für die Lagerungsdichte, Harste 2004-2006; ns=nicht signifikant
Tillage (T)
Wheeling (W)
TxW
5-10
18-23
Soil depth [cm]
30-35
40-45
55-60
0.0033
0.0044
ns
< 0.0001
< 0.0001
ns
0.0676
ns
0.0535
0.0356
0.0016
ns
ns
ns
0.0266
Tab. 3: Coefficients of Spearman’s rank correlation between penetration resistance (PR), bulk density (BD), air filled pore volume
(AFPV) and air permeability (AP), Harste 2004-2006; coefficients significant at α ≤ 0.05 are underlined, n = 36
Rangkorrelationskoeffizienten nach Spearman zwischen Eindringwiderstand (PR), Lagerungsdichte (BD), luftführendem Porenvolumen (AFPV) und Luftpermeabilität (AP), Harste 2004-2006; signifikante Koeffizienten (α ≤ 0,05) sind unterstrichen, n = 36
Depth [cm]
5-10
18-23
30-35
40-45
55-60
PR
BD
AFPV
AP
BD
AFPV
AP
BD
AFPV
AP
BD
AFPV
AP
BD
AFPV
AP
0.17
–0.27
–0.13
0.80
–0.66
–0.73
0.48
–0.16
0.21
0.44
–0.10
0.13
0.66
–0.61
–0.26
BD
AFPV
–0.87
–0.72
0.81
–0.96
–0.91
0.89
–0.87
–0.52
0.79
–0.82
–0.57
0.79
–0.87
–0.58
0.62
Tab. 4: Probability level of F-values (α < 0.1) for total plant and taproot yield, Harste 2004-2006; ns = not significant
Irrtumswahrscheinlichkeit der F-Werte (α < 0,1) für den Gesamtpflanzen- und Rübenertrag, Harste 2004-2006; ns = nicht signifikant
Total plant yield
Tillage (T)
Wheeling (W)
TxW
Taproot yield
Tillage (T)
Wheeling (W)
TxW
May
June
Harvest date
July
August
September
0.0035
ns
ns
ns
0.0454
ns
0.0179
0.0737
ns
0.0018
0.0466
ns
0.0017
ns
ns
–*
–
–
ns
0.0616
ns
0.0268
0.0008
ns
0.0058
0.0025
ns
0.0017
ns
ns
* not determined
Correlations between crop growth and soil physical parameters
The coefficients of Spearman’s rank correlation revealed
significant relations between plant yield and soil physical
properties only for data from the 18-23 cm layer. In all other depths, significant correlations occurred only eventually
(not shown). Therefore, this analysis continues with exclusively focussing on the 18-23 cm layer.
Pflanzenbauwissenschaften 2/2009
At May harvest, total plant yield was positively correlated with PR and BD, and negatively related to AFPV and AP
(Tab. 6). In June, this general scheme continued, but coefficients decreased and became insignificant for three out of
four parameters. Contrastingly, from July harvest onwards
correlation coefficients became negative for PR and BD,
and positive for AFPV and AP. At all harvest dates, coefficients were highest for PR. A similar pattern was detected
for the relation between taproot yield and soil structural
H.-J. Koch: Relations between soil structural properties and sugar beet yield on a Luvisol
20,0
16,0
Leaves and tops
12,0
8,0
Total plant yield
4,0
[Mg ha-1]
Taproot
0,02
0,01
0,00
May
June
July
August
Harvest date
September
1,2
0,6
0,0
Fig. 2: Effect of repeated wheeling with heavy load and soil
tillage on total plant and taproot dry matter yield of sugar
beet, Harste 2004-2006; MP = Mouldboard ploughing, SM =
Shallow mixing
Einfluss von wiederholter Befahrung mit schweren Maschinen
und Bodenbearbeitung auf den Gesamtpflanzen- und Rübentrockenmasseertrag von Zuckerrüben, Harste 2004-2006;
MP=Pflugbearbeitung, SM=Flach mischende Bearbeitung
properties (Tab. 6). The highest coefficient of correlation
observed was -0.65 for the relation between taproot yield
and PR in September.
Fibrous root length density at July harvest revealed only
weak correlations to soil parameters, whereas the root diameter was positively related to PR and BD, but negatively
correlated with AFPV and AP (Tab. 6). Similar relations
were detected for taproot fanginess at September.
The dependencies between PR and taproot yield, root
diameter and fanginess are depicted in Fig. 5A-C. PR values varied from 0.5-2 MPa. The relations were described
best by linear regression with coefficients of determination
ranging from 0.21-0.35. Consistently, regression analysis
of all other combinations of crop vs. soil parameters calculated for each harvest date gave no indication for deviation
from linearity of relations (not shown).
Introducing soil data from the 18-23 cm layer as covariates into the fixed model analysis of variance/covariance
of taproot yield in September substantially diminished the
treatment sum of squares (Tab. 7). Application of AFPV,
BD, AP and PR decreased values by 31, 48, 52 and 76%, respectively. Combining two or more soil parameters as co-
Root length density [km m-3]
0
10
20
30
Root diameter [mm]
0,0
0,1
0,2
0,3
0,4
b
0-15
Soil depth [cm]
MP unwheeled
MP wheeled
SM unwheeled
SM wheeled
Taproot yield, Total plant yield [Mg ha-1]
54
a
ab
ab
15-30
30-45
45-60
MP unwheeled
MP wheeled
SM unwheeled
SM wheeled
Fig. 3: Effect of repeated wheeling with heavy load and soil
tillage on sugar beet fibrous root length density and diameter
in different soil depths at July harvest, Harste 2004-2006; in
case of significant interaction (cf. Tab. 2) different letters
above columns indicate significant differences at α ≤ 0.05;
MP=Mouldboard ploughing, SM=Shallow mixing
Einfluss von wiederholter Befahrung mit schweren Maschinen
und Bodenbearbeitung auf die Längendichte und den Durchmesser der Faserwurzeln im Juli, Harste 2004-2006; bei signifikanter
Wechselwirkung (s. Tab. 2) zeigen unterschiedliche Buchstaben
an den Säulen signifikante Unterschiede mit α ≤ 0,05 an;
MP=Pflugbearbeitung, SM=Flach mischende Bearbeitung
variates did not substantially decrease the residual treatment sum of squares compared to PR alone (not shown).
Discussion
In this study, BD was used for basic quantitative characterisation of soil structural properties of the experimental
fields. Across treatments and depths, values mostly ranged
from 1.4-1.6 Mg m-3, which is classified as a medium level
according to ANONYMOUS (2005). Variation of BD within a
single layer was largest in 18-23 cm depth (Fig. 1), clearly
reflecting residual effects of wheeling treatments, and
moreover, differences in working depth between tillage
systems (MEYER et al. 1997, RASMUSSEN 1999). Results from
Tab. 5: Probability level of F-values (α < 0.1) for fibrous root length density and diameter at July harvest, Harste 2004-2006,
ns = not significant
Irrtumswahrscheinlichkeit der F-Werte (α < 0,1) für die Längendichte und den Durchmesser der Faserwurzeln im Juli, Harste
2004-2006; ns = nicht signifikant
Soil depth [cm]
Root length density
Tillage (T)
Wheeling (W)
TxW
Root diameter
Tillage (T)
Wheeling (W)
TxW
0-15
15-30
30-45
45-60
ns
0.0449
0.0226
ns
0.0177
ns
ns
0.0553
ns
ns
ns
ns
0.0268
0.0059
ns
0.0016
ns
ns
0.0166
ns
ns
0.0532
ns
ns
Pflanzenbauwissenschaften 2/2009
H.-J. Koch: Relations between soil structural properties and sugar beet yield on a Luvisol
Tillage (T)
Wheeling (W)
TxW
Taproot fanginess [% fangy beet]
40
0,0258
0,0552
ns
30
20
10
ed
el
el
w
SM
he
w
un
he
ed
ed
el
he
w
P
M
SM
M
P
un
w
he
el
ed
0
Fig. 4: Effect of repeated wheeling with heavy load and soil
tillage on taproot fanginess of sugar beet in September, Harste
2004-2006; the table gives probabilities of F-values (α < 0,1);
MP=Mouldboard ploughing, SM=Shallow mixing, ns=not significant
Einfluss von wiederholter Befahrung mit schweren Maschinen
und Bodenbearbeitung auf die Beinigkeit von Zuckerrüben im
September, Harste 2004-2006; die Tabelle enthält die Irrtumswahrscheinlichkeit der F-Werte bei α ≤ 0,1; MP=Pflugbearbeitung, SM=Flach mischende Bearbeitung, ns=nicht signifkant
this depth are regarded as representative for the entire layer between 15 and 30 cm depth, which is sustained by the
more detailed analysis of PR profiles over depth conducted
on our study sites by HEUER et al. (2006).
Significant correlations among BD, AFPV and AP within
one sampling depth (Tab. 3) indicate inherent systematic
relations between soil physical parameters: Increase in BD
preferentially decreases the volume of wide pores (AFPV)
and AP. Obviously, AP is directly related to AFPV, while
pore diameter, and tortuosity and continuity characteristics may modify this correlation (BALL et al. 1988, LIPIEC &
HATANO 2003). The strongest correlations between r = 0.89
and 0.96 were detected in the 18-23 cm layer, which was
presumably due to the broad variation of data in this layer.
AP largely depends on the presence of continuous
macropores, which make measurements highly variable
(STEPNIEWSKI et al. 1994). In our study, artificial closure of
vertical biopores ≥1 mm has likely reduced the variation of
AP data, and moreover, allowed to detect even small
changes in the remaining system of wide and medium
sized pores (KOCH et al. 2008). Therefore, AP values obtained in our study may not represent the conditions prevalent in the bulk soil, but are significant for O2 supply of fibrous roots except those invading continuous macropores.
PR characterising soil strength strongly depends on soil
water content, texture and bulk density (BRADFORD et al.
1986). In our study, PR measurements were conducted at
more or less invariable conditions with regard to soil water
status (field capacity) and clay content. Therefore, variation of bulk density was likely the main cause for variation
of PR data resulting in close correlation between PR and
BD as observed in four of five depths (Tab. 3).
To conclude, soil structural data derived from identical
core samples such as BD, AFPV and AP usually reveal a
Pflanzenbauwissenschaften 2/2009
55
close linear relation. BD and PR, even if derived from independent measurements, may also correlate closely because
both properties are related to the packing density of soil
solids.
Significant correlations between soil physical parameters and crop parameters occurred only when data from
the 18-23 cm layer, representing 15-30 cm depth, were
used (Tab. 6). This clearly points to the importance of the
few upper soil decimetres for crop growth. In this layer, absolute values of PR and AFPV varied from 0.5-2.0 MPa and
2-10 m3 100 m-3, respectively (KOCH et al. 2008). BD variation was from 1.35-1.6 Mg m-3 (Fig. 1). In the other layers
values varied much less, which may have caused the less
close relations to crop parameters. Nevertheless, structural
properties of the uppermost horizon (0-15 cm) and also
soil layers deeper than 30 cm (plough pan, subsoil) are
well known to affect crop yield (IDE et al. 1987, ALAKUKKU
2000, VOORHEES 2000, NEVENS & REHEUL 2003), but have to
be excluded from the further discussion.
As far as we know, our study presents for the first time
correlations of early sugar beet yield with soil physical
properties for field conditions: At May harvest sugar beet
yield positively correlated with PR and BD, and revealed a
negative relation to AFPV and AP in 18-23 cm depth
(Tab. 6). In contrast, HOFFMANN & JUNGK (1995) reported
from pot experiments, that yield of 21 days old sugar beet
plants grown on loessial soil substantially decreased, when
BD and PR exceeded 1.5 Mg m-3 and 0.65 MPa, respectively. Increase of early sugar beet yield due to SM tillage has
been previously reported by TOMANOVA et al. (2006) for the
period 1993-2001 on the same experimental field. Data revealed that early plant growth was clearly enhanced by increased leaf K concentration in the SM treatment, which
obviously was caused by an elevated plant available K level
in the uppermost 10 cm soil layer. Derived from these findings it can be assumed for our study, that improved K supply may have been the main cause for enhanced early sugar beet growth in the SM treatment, which was not yet nullified by the increased PR in parts of the topsoil.
During summer, TOMANOVA et al. (2006) did not detect
any shortage in nutrient uptake limiting sugar beet yield
formation, and thus, shortages in nutrient supply were not
likely to limit crop growth during summer in our study.
Moreover, no differences in plant wilting occurred during
drought periods indicating that water supply did not differ
between treatments. Just small differences in root length
density between tillage and wheeling treatments during
June-August give support to this assumption.
For June harvests, Spearman’s coefficients of correlation
between soil physical properties from 18-23 cm depth and
crop yield were low and mostly insignificant (Tab. 6). At
July to September harvests, coefficients substantially increased, but signs reversed compared to May and June relations: Both, PR and BD were negatively related to total
plant and taproot yield, while AFPV and AP correlated positively to yield (Tab. 6). Regression analysis revealed linear
relations for all parameter combinations tested. Theoretically, soil properties are expected to relate squared or exponentially to crop yield describing an optimum value or
range (ARVIDSSON & HAKANSSON 1991, BOONE et al. 1994).
The optimum range may change depending on (i) specific
demands of the crop species under consideration, (ii) soil
and weather conditions prevalent for the total growing period or parts of it, and (iii) interactions with the state of
other relevant parameters (BOONE 1988).
Consistently, earlier studies on sugar beet revealed quadratic and exponential functions to fit best: Sugar yield
substantially decreased when BD exceeded 1.4 Mg m-3 on
loessial soil (KOCH et al. 2009) and 1.5 Mg m-3 on sandy
56
H.-J. Koch: Relations between soil structural properties and sugar beet yield on a Luvisol
Tab. 6: Coefficients of Spearman’s rank correlation between
sugar beet total plant yield (n = 36), taproot yield (n = 36), taproot fanginess (n = 36), length density and diameter of fibrous
roots in 15-30 cm depth (n = 32) and penetration resistance
(PR), bulk density (BD), air filled pore volume (AFPV) and air permeability (AP); field effects were excluded from plant data, soil
data were taken from 18-23 cm depths, Harste 2004-2006; correlation coefficients significant at α ≤ 0.05 are underlined
Rangkorrelationskoeffizienten nach Spearman zwischen Gesamtpflanzenertrag (n = 36), Rübenertrag (n = 36), Beinigkeit
(n = 36) sowie Längendichte und Durchmesser der Faserwurzeln
(n = 32) von Zuckerrüben und Eindringwiderstand (PR), Lagerungsdichte (BD), luftführendem Porenvolumen (AFPV) sowie
Luftpermeabilität (AP); Pflanzendaten bereinigt um den Feldeffekt, Bodendaten aus 18-23 cm Tiefe, Harste 2004-2006; signifikante Koeffizienten (α ≤ 0,05) sind unterstrichen; n = 36
Harvest date
PR
Total plant yield
May
0.53
June
0.26
July
–0.51
August
–0.57
September
–0.62
Taproot yield
June
0.26
July
–0.48
August
–0.48
September
–0.65
Fibrous root length density
July
–0.11
Fibrous root diameter
July
0.66
Taproot fanginess
September
0.41
DBD
AFPV
AP
0.37
0.28
–0.46
–0.57
–0.43
–0.26
–0.27
0.40
0.53
0.28
–0.42
–0.34
0.38
0.54
0.39
0.26
–0.56
–0.51
–0.47
–0.24
0.55
0.51
0.34
–0.31
0.42
0.41
0.45
–0.29
0.36
0.33
0.58
–0.48
–0.55
0.49
–0.45
–0.43
and clayey loam (DRAYCOTT et al. 1970, JAGGARD 1977) in at
least parts of the topsoil down to 30 cm. CZERATZKI (1972)
and PETELKAU et al. (1999) identified BD values ranging
from 1.35 to 1.48 Mg m-3 as optimum for sugar beet
growth on loessial/loamy soils. Both, lower and higher values were shown to reduce yield. In the topsoil of our experimental site, BD data lower than the optimum of about
1.4 Mg m-3 were not measured (Fig. 1). This might explain
that we only found linear relations between soil and crop
data but no quadratic functions.
PR values exceeding 2 MPa were reported to diminish fibrous root growth of sugar beet, maize and wheat, and
moreover, other growth characteristics of various crops
(ATWELL 1993). In our experiment, the absolute values of
PR measured in topsoil layers at field capacity in spring did
not exceed 2 MPa (KOCH et al. 2008). Root length density
revealed no correlation to PR (Tab. 6), even if root measurements were conducted later in the growing season,
when PR had presumably raised due to soil drying. For
AFPV, values from several of our treatments (KOCH et al.
2008) substantially fell below the critical threshold value
of 10 m3 100 m-3 reported by CZERATZKI (1972) for loamy
topsoil. However, this comparison may be biased, because
CZERATZKI (1972) measured AFPV at a water tension of
10 kPa while our data were obtained at 6.2 kPa. Nevertheless, yield at the July and August harvests was closely relat-
ed to AFPV (Tab. 6). High rainfall occurring in July and August of two of the three experimental seasons (2004, 2005;
not shown) may have caused high water content in the topsoil. Thereby, low oxygen supply may have limited plant
growth, especially taproot formation. STOCKFISCH et al.
(2002) found that the taproot growth rate from July to
September was negatively correlated to mean daily rainfall
in this period.
For AP, no information on threshold values describing
limiting or optimum conditions for plant growth are available. Derived from distribution of data solely, values below
10 x 10-12 m2 were evaluated as very low by KMOCH & HANUS (1965). In our study, AP of the 18-23 cm layer varied
from 0.15 to 3 x 10-12 m2 (KOCH et al. 2008), which must
be regarded as extremely low. But in contrast to our data,
KMOCH & HANUS (1965) refer to samples derived from bulk
soil including biopores.
At September harvest, PR data from 18-23 cm depth
(measured in spring) revealed the closest correlation with
total plant and taproot yield detected in the whole data set
(Tab. 6). Variation of PR explained taproot yield variance
by 34% (Fig. 5A). Integration of PR as a covariate into
ANOVA of taproot yield decreased the treatment sum of
squares by 76%, whereas BD, AFPV and AP accounted for
-48, -31, and -52% only (Tab. 7). These results indicate
that variation of soil strength measured by spring PR substantially affected sugar beet growth during summer and
early autumn. Due to soil drying PR regularly increases
over summer, which likely exaggerates relatively small differences measured at high soil moisture in spring (VAZ et
al. 2001, LAPEN et al. 2004). Under such conditions secondary thickening growth proceeds and results in the formation of the taproot to be filled up with assimilates provided
by the leaf canopy. It is well accepted that soil strength may
affect crop growth by signalling from root to shoot: Root
caps are able to detect high soil strength causing an elevated level of abscisic acid in the leaves, which in turn causes
stomatal closure and reduction of the net photosynthesis
rate (TARDIEU et al. 1991). Such parameters were not measured in our study, but increasing PR from 0,5-1 MPa to
about 2 MPa significantly increased fibrous root diameter
(Fig. 5B), clearly revealing the occurrence of mechanical
stress (ATWELL 1993). However, mechanical stress appeared to be only small, because root length density was
affected only slightly and correlation with PR was not significant.
Moreover, secondary thickening of the sugar beet taproot may involve specific growth processes that are sensitive to elevated soil strength. GLIEMEROTH (1953) and RYDBERG (1992) reported that sugar beet react to unfavourable
soil structure with increased fanging of the taproot. Likewise, increasing PR enhanced taproot fanginess in our
study (Fig. 5C). HANUS & BERNARD (1964) found that the
portion of fangy taproots rose with decreasing AP, which is
confirmed by our study (Tab. 6). Enhanced fanginess is
likely to increase root breakage at sugar beet harvest,
which may contribute to lower yield coming along with elevated soil strength. In this study, the breakage loss attributed to taproot fanginess has not been determined, but
based on principal considerations combined with results
from other investigations (KROMER et al. 1998) breakage
loss can be assumed to account for less than 3% of taproot
yield differences compared to a total yield variation >10%
observed in our study (Fig. 2).
Finally it must be emphasized, that unambiguous conclusions on the cause-and-effect mechanism(s) between
soil structure and sugar beet yield formation cannot be
drawn from our data due to the close inter-relations among
the soil physical parameters included. Moreover, soil strucPflanzenbauwissenschaften 2/2009
H.-J. Koch: Relations between soil structural properties and sugar beet yield on a Luvisol
Degrees
of freedom
Sum of
squares
Without covariate
Treatments
3
18.3
Replicates(Field)
8
8.8
Error
24
14.1
With covariate Penetration resistance
Penetration resistance
Pr > F
0.0001
0.1147
14.1
< 0.0001
Treatments
3
Replicates(Field)
8
Error
23
With covariate Bulk density
4.3
8.7
14.0
0.0968
0.1324
9.0
0.0009
Treatments
3
9.5
Replicates(Field)
8
8.7
Error
23
14.1
With covariate Air filled pore volume
0.0073
0.1368
Air filled pore volume
1
Taproot yield [Mg ha-1]
13
12
11
Field 1
Field 2
Field 3
10
0,0 0,5
0,40
1
5.8
0.0049
12.6
8.9
13.8
0.0016
0.1178
0,28
0,24
Field 1
Field 2
Field 3
0,20
Air permeability
1
9.7
0.0006
Treatments
Replicates(Field)
Error
3
8
23
8.8
8.6
14.1
0.0098
0.1377
–52
tural quality for sugar beet growth may not be defined by
only one but several factors, which may change in importance over the growing season as the environmental conditions and, additionally, the crop’s demand may change.
The soil parameters included in this study showed substantial variation only in 15-30 cm depth. Therefore, the effect
of soil structural properties in other layers could not be
evaluated.
Conclusions
Our study emphasises the significance of soil structural
properties affecting sugar beet yield on Luvisol. The data
indicate that during summer and early autumn elevated
soil strength in 15-30 cm depth was the most important
factor to impede taproot yield formation and increase taproot fanginess and fibrous root diameter. In contrast, fibrous root length was not decreased and thus, nutrient and
y = 0,231 + 0,044 x
r2 = 0,35***
N = 32
1,0
1,5
2,0
C
40
30
20
y = 18,61 + 7,88 x
r2 = 0,21*
N = 36
10
0
–31
2,0
0,32
50
–48
1,5
B
0,00
–76
1,0
0,36
0,0 0,5
Treatments
3
Replicates(Field)
8
Error
23
With covariate Air permeability
Pflanzenbauwissenschaften 2/2009
y = 13,98 - 1,31 x
r2 = 0,34***
N = 36
0,0 0,5
1
Dry bulk density
Change
of Sum of
squares
[%]
A
14
0
Taproot fanginess [%]
Source
15
Fibrous root diameter [mm]
Tab. 7: Splitting up sums of squares for sugar beet taproot
yield at September (Mg ha-1, field effects excluded) by analysis
of variance without covariate compared to analysis of covariance with soil parameters from 18-23 cm depth as covariates
(fixed effects model, tillage and wheeling were combined to
“treatments”)
Aufspaltung der Quadratsummen für den Rübenertrag von Zuckerrüben im September (Mg ha-1, um Feldeffekte bereinigt) in der
Varianzanalyse ohne Kovariable gegenüber der Varianzanalyse
mit Bodenparametern aus 18-23 cm Tiefe als Kovariablen (Modell mit fixen Effekten, Bodenbearbeitung und Befahrung zu Behandlungen kombiniert)
57
1,0
1,5
Penetration resistance [MPa]
2,0
Fig. 5: Relation between penetration resistance in 18-23 cm
soil depth and dry matter yield of taproot at September harvest (A), fibrous root diameter in 15-30 cm depth at July harvest (B), and fanginess of taproot at September harvest (C);
field effects excluded from all plant data, Harste 2004-2006; r2
significant at α < 0.05 (*), α < 0.001 (***)
Beziehung zwischen dem Eindringwiderstand in 18-23 cm Bodentiefe und dem Rübentrockenmasseertrag im September (A),
Faserwurzeldurchmesser in 15-30 cm Tiefe im Juli (B) und Beinigkeit des Rübenkörpers im September (C); alle Pflanzendaten bereinigt um Feldeffekte, Harste 2004-2006; r2 signifikant bei
α < 0,05 (*), α < 0,001 (***)
water supply were likely not limiting. Additionally, root
oxygen supply may have been limitative for taproot
growth. However, unambiguous elucidation of physiological cause-and-effect mechanisms could not be achieved in
our field trials but needs controlled environment experiments. In soil layers deeper than 30 cm variation of soil
physical properties was too small to detect significant relations to sugar beet yield.
Conclusively, maximum utilisation of site specific sugar
beet yield potential on Luvisol requires favourable soil
structural conditions in the topsoil down to at least 20 cm
depth. In this layer, tillage and other management operations affecting soil structure (e.g. machinery use) have to
assure a limited soil strength, which can be roughly characterized by a spring PR of 0.5-1 MPa and a BD of
1.35-1.45 Mg m-3. Online sensoring of soil structure and
using the information obtained to control working quality
58
H.-J. Koch: Relations between soil structural properties and sugar beet yield on a Luvisol
of tillage implements could contribute to improve crop
yield. But this probably requires the design of new sensing
techniques.
Acknowledgement
The author gratefully acknowledges the excellent technical
assistance of Ines Wiese, Wilfried Hübener and Manfred
Jordan. I also express gratitude to the “Bayerische Landesanstalt für Landwirtschaft” for support in performing measurements on soil samples. Data acquisition for this study
was funded by Holmer Maschinenbau GmbH, ROPA Fahrzeug- und Maschinenbau GmbH and Grimme Landmaschinenfabrik GmbH & Co. KG.
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Received on September 25, 2009;
accepted on November 30, 2009
Address of the author: Dr. Heinz-Josef Koch, Institut für Zuckerrübenforschung, Holtenser Landstr. 77, D-37079 Göttingen,
E-mail: [email protected]