Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol57.Issue1-2.Sept2000:
Soil & Tillage Research 57 (2000) 69±82
Soil strength and soil pore characteristics for
direct drilled and ploughed soils
Per Schjùnninga,*, Karl J. Rasmussenb
a
Danish Institute of Agricultural Sciences, Department of Crop Physiology and Soil Science,
Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark
b
Danish Institute of Agricultural Sciences, Department of Agricultural Engineering, Research Centre Bygholm,
PO Box 536, DK-8700 Horsens, Denmark
Received 13 December 1999; received in revised form 17 July 2000; accepted 28 July 2000
Abstract
Direct drilling has often been reported to increase density and strength and to affect pore continuity and tortuosity of the
upper soil layers. In this study these aspects were studied for three texturally differing soils 4±6 years after initiation of
continuous trials with direct drilling and mouldboard ploughing. The soils studied were a coarse sandy soil (Korntved, 5%
clay), a sandy loam (Ballum, 8% clay) and a silty loam (Hùjer, 19% clay). The crop rotation at Korntved was spring barley and
winter rye while at Ballum and Hùjer it was spring barley and winter wheat. Both crops were grown every year. All ®elds had
been mouldboard ploughed for decades prior to the trial period. The ploughed treatment (PL) was imposed in the autumn and
the seedbed preparation and drilling were performed with an S-tined seedbed harrow and a traditional drill. The direct drilled
(DD) treatment received no tillage other than the drilling which was performed by a triple-disc drill. Straw and stubble were
burned. In the 4th, 5th and 6th years of the trial period, minimally disturbed soil cores were taken from the 4 to 8, 14 to 18 and
24 to 28 cm depths, i.e. two horizons above the ploughing depth of 20 cm, and one horizon below this depth. Longer cores
were sampled in the 18±27 cm depth in order to include this transition layer. Furthermore, in the 4th year of the trial period
shear strength was measured in the ®eld at 2-week intervals in the spring with a vane shear tester in the two upper layers
mentioned. All samplings and measurements took place in the ®eld grown with spring barley. In the laboratory air diffusivity
and air permeability were measured at ®eld-sampled water content and again when the soil cores were drained to a matric
potential of ÿ100 hPa. Cone penetration resistance was measured with a 2 mm diameter penetrometer. Separate core samples
from the 14 to 18 cm depth of the Korntved and Hùjer soils were used for estimating soil cohesion and soil internal friction by
a shear annulus method at ®eld-sampled water content.
For all soils, DD increased soil bulk density in the two upper soil layers. The shear vane tester also generally estimated
higher shear strength for the DD compared to the PL treatment. The shear annulus measurements in the laboratory revealed no
differences between tillage treatments for the Korntved soil, while a tendency of higher cohesion and internal soil friction was
found for the DD treatment on the Hùjer soil. The cone penetration measurements indicated a stronger top-soil and fewer highstrength soil elements in the 24±28 cm horizon for the DD than for the PL treatment. Generally the DD treatment had lower
volume of macropores (i.e. pores > 30 mm) in the 4±8 and 14±18 cm depths than the PL treatment. This was re¯ected in
reduced air diffusivities and air permeabilities for these horizons. An exception was the 14±18 cm depth of the Ballum soil,
where increased air diffusivity and air permeability was measured at ®eld-sampled water content. Continuity indices
*
Corresponding author. Tel.: 45-8999-1766; fax: 45-8999-1719.
E-mail address: [email protected] (P. Schjùnning).
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 4 9 - 5
70
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
calculated from air diffusivity and air permeability measurements showed that the DD soil from the 4 to 8 and 14 to 18 cm
depths had less continuous and more tortuous macropores than the ploughed soil. # 2000 Elsevier Science B.V. All rights
reserved.
Keywords: Direct drilling; Ploughing; Soil strength; Soil pore characteristics; Pore continuity; Plough pan
1. Introduction
Tillage has been performed since ancient times in
order to ful®l three major requirements: (i) to improve
soil tilth, (ii) to combat weeds and (iii) to incorporate
plant residues and organic manures. For most soil
management systems and crop rotations, all three
requirements are still valid. Mouldboard and disc
ploughing have been the most obvious way of meeting
these demands and are still the most widely used
primary tillage methods in many countries including
Denmark (Riley et al., 1994; Rasmussen, 1999). However, the mechanical turnover of the top 20±25 cm
layer of soil is an energy-demanding procedure, it
brings new weed seeds to the top layer ready for
germination, and it kills a lot of soil fauna active in
the turnover of organic matter (AndreÂn and LagerloÈf,
1983). Furthermore, several investigations have
reported a signi®cant densi®cation of the soil layer
just below the ploughing depth (e.g. Ehlers, 1973;
Rydberg, 1987; Francis et al., 1987). The continued
ploughing of many soils with tractors of ever-increasing weight and power has created critical conditions
for soil processes such as air exchange (Teiwes and
Ehlers, 1987; Schjùnning, 1989) and water movement
(Comia et al., 1994; Ball et al., 1998). Direct drilling
or shallow tillage have been shown to affect the soil
characteristics of importance to these soil functions
for the horizon just below the equivalent depth of
ploughing, compared to continuously ploughed soil.
For this critical soil layer conversion to direct drilling
or shallow tillage has been shown to increase the
volume of macropores (e.g. Ehlers, 1973; Comia
et al., 1994) and to improve air conductivity by
diffusion as well as by convection (Douglas and Goss,
1987; Schjùnning, 1989; Comia et al., 1994).
On the other hand, repeated direct drilling or shallow tillage will increase the density of the non-cultivated soil layers compared to ploughed soil (Rydberg,
1987; Douglas and Goss, 1987) and decrease the
volume of macropores (Douglas et al., 1980; Schjùnning, 1989). Schjùnning (1989) found a signi®cant
reduction in air diffusivity for shallow tilled topsoil
layers compared to ploughed soil and modelled
this effect as being of even higher importance to
the air exchange of the soil pro®le than the low
diffusivity found in the dense pan below the ploughing
depth in a ploughed soil. Water in®ltration has been
found to increase for reduced tilled and direct drilled
(DD) soils as compared to ploughing (Ehlers, 1997)
which probably is due to increased numbers of
persistent surface-connected macrochannels created
by earthworms (Ehlers, 1975; Ehlers and Claupein,
1994) or by roots. In contrast, for some soils the
saturated hydraulic conductivity has been found to
be higher in ploughed soil than in DD soil (Ball et al.,
1994, 1998).
Also soil strength has been shown to be signi®cantly
different in ploughed and DD soil. Graham et al.
(1986) reported restricted root growth due to a high
mechanical strength at 15 cm depth of a DD silt loam
soil. Braim et al. (1992) found considerably higher
penetration resistances for DD soil than for ploughed
soil. Schjùnning and Rasmussen (1989) considered the
spatial variation of soil strength below tillage depth
(5±10 cm) for a shallow-tilled soil and found a high
frequency of areas with a high penetration resistance
of >2 MPa. At the 25±30 cm depth, on the contrary,
the shallow tillage system had facilitated a reduction
in the mechanical strength compared to the continuously ploughed soil.
The effects of direct drilling and mouldboard
ploughing on crop growth characteristics and yield
were investigated in three Danish ®eld trials and
reported by Rasmussen (1988). The effects upon
earthworm and weed populations for these same trials
were reported by Andersen (1987a,b). Utilising these
trials again, the purpose of the present investigation
was to study the tillage effects on soil pore characteristics and soil mechanical properties.
71
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
2. Materials and methods
2.1. Soils and trial treatments
Field trials with repeated mouldboard ploughing
and direct drilling were performed at three locations in
Denmark for 6 years. The soils ranged from a coarse
sandy soil at Korntved, a sandy loam at Ballum and a
silt loam at Hùjer (Table 1). The Korntved and Ballum
soils originated from morainic material, while the
Hùjer soil derived from marine sediments. Prior to
the experiment all soils had been in a conventional
tillage system with annual ploughing to a depth of
20 cm. The crop rotation at Korntved was spring
barley and winter rye, while at Ballum and Hùjer it
was spring barley and winter wheat. Straw residues
were burned. Both crops were grown every year but all
investigations were performed in the ®eld with the
spring barley crop. Conventional tillage included
ploughing in the autumn to 20 cm depth and seedbed
preparation by shallow harrowing in the spring prior to
drilling. The DD plots were subject to no other tillage
operation but drilling by a triple-disc drill. The two
tillage treatments were compared in a randomised,
complete block design with three blocks at the Hùjer
location and four blocks at the two other locations.
The tillage plots measured 15:00 m 10:25 m at the
Korntved and Ballum locations and 20:00 m 10:50 m
at the Hùjer location.
2.2. Soil sampling and ®eld measurements
In years 4±6 following the start of the continuous
trial treatments, soil was sampled for physical measurements in the laboratory. In each year, minimally
disturbed soil cores were collected at the time of
plant emergence from the soil depths 4±8, 14±18
and 24±28 cm. In year 4, an extra sampling took
place approximately 4 weeks after the ®rst one. The
soil cores were collected randomly from a crosssection of the tilled soil; i.e. no attempt was made
to identify the location of each core relative to the
drilled spots. The soil cores were retrieved in metal
cylinders (diameter 6:10 cm, height 3:42 cm,
volume 100:0 cm3 ) forced into the soil by means
of a hammer. The cylinders were held in position by a
special ¯ange ensuring a vertical downward movement into the soil. After careful removal of the soil®lled cylinder, the end surfaces were trimmed with a
knife. At each sampling, four replicate cores were
taken from each combination of trial plot and sampling depth, which means that a total of 1056 cores
were collected for the Measuring Procedure I.
In year 4, additional 250 cm3 soil cores (diameter
6:10 cm, height 8:55 cm) were sampled at all locations, 18±27 cm depth, with the technique described
above. Three replicate cores were taken at plant
emergence and again 4 weeks later for each location
and trial plot (except at Hùjer where sampling was
only at plant emergence). This gave a total of 114
cores, which were used in the Measuring Procedure II.
Furthermore, at the Hùjer and Korntved locations, six
replicate 100 cm3 cores were collected at each plot
from the 14 to 18 cm layer at those two sampling
dates, yielding a total of 192 minimally disturbed soil
cores for the Measuring Procedure III.
In year 4, at plant emergence and again approximately 2, 4 and 6 weeks later, 100 cm3 soil cores were
collected for determination of dry soil bulk density at
the 4±8 and 14±18 cm soil depths. On the same dates,
vane shear strength was measured in the ®eld at the 4±
6.5 and 14±16.5 cm depths using the method of Schaffer (1960). Eight replicate measurements were made
for each combination of plot and measuring depth.
Table 1
Soil texture of the 0±20 cm layer of the soils investigated
Location
Textural
classa
Organic matter
(g/(100 g))
Clay, 30 mm was found in this
horizon. No signi®cant effects were found for the silt
loam at Hùjer.
The parameter, CG DS =D0 =e, relating the relative diffusivity DS/D0 and the air-®lled pore space, e, is
an expression of the effectiveness of the air-®lled pore
space in conducting air by diffusion, i.e. an index of
continuity of the soil pore system (Gradwell, 1961).
For the coarse sandy soil at Korntved, a signi®cant
reduction in the CG continuity index was found for the
DD soil at the 4±8 and 14±18 cm depths (Table 5). The
same was true at the 4±8 cm depth for the loamy sand
79
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
Table 5
Volume of pores with tube-equivalent diameter >30 mm and selected model estimates of soil pore characteristics as averaged for all sampling
dates in years 4±6 of continuous tillage treatmenta
Location
Korntved
Soil depth (cm)
4±8
14±18
24±28
Ballum
4±8
14±18
24±28
Hùjer
4±8
14±18
24±28
Pores > 30 mm
(m3/(100 m3))
Effective pore
diameter, DB (mm)
Continuity indices
CG (mÿ3 10ÿ2 )
log(PO, mm2)
PL
DD
LSD0.05
PL
DD
LSD0.05
PL
DD
LSD0.05
32.3
27.5
1.6 (2)
28.1
27.8
NS (1)
28.4
28.2
NS (0)
118
99
7 (1)
112
106
6 (0)
105
105
NS (0)
26.3
21.2
1.5 (2)
23.4
21.5
1.4 (1)
24.2
23.0
NS (0)
2.05
1.81
0.06 (2)
1.95
1.87
0.06 (0)
1.92
1.90
NS (0)
PL
DD
LSD0.05
PL
DD
LSD0.05
PL
DD
LSD0.05
12.6
13.4
NS (2)
12.6
14.6
1.5 (2)
12.2
14.1
1.7 (2)
181
161
16 (2)
188
141
19 (3)
156
138
14 (1)
10.3
8.7
1.3 (0)
10.1
11.2
NS (0)
8.6
9.1
NS (0)
2.01
1.80
0.11 (2)
2.00
1.80
0.14 (1)
1.79
1.71
NS (2)
PL
DD
LSD0.05
PL
DD
LSD0.05
PL
DD
LSD0.05
7.5
5.2
NS (0)
7.0
4.9
NS (0)
4.6
6.2
NS (1)
130
140
NS (0)
203
201
NS (0)
170
181
NS (0)
5.7
5.4
NS (0)
10.7
9.2
NS (0)
7.9
8.1
NS (0)
1.32
1.28
NS (0)
1.93
1.92
NS (0)
1.70
1.83
NS (0)
Tillage system
a
The number in parentheses indicates the number of individual sampling events (out of the maximum possible of four) with signi®cant
trend (P 0:05) similar to the one found for the average of all four samplings.
at Ballum, while no signi®cant effects were observed
for the Hùjer soil.
The index, PO K=e (Groenevelt et al., 1984),
relating air permeability to the air-®lled pore space
can also be taken as an expression of continuity of the
soil pore system and is often quoted as pore organisation (PO) (Blackwell et al., 1990). In accordance with
the CG-index, a reduced continuity of the coarse sandy
soil at Korntved was found when converting from
ploughing to direct drilling (Table 5). A lower POindex was also found for DD soil in the 4±8 cm layer
of the loamy sand at Ballum. The silty loam at Hùjer
displayed no signi®cant effects for this parameter.
However, although not statistically signi®cant, it
should be noted that for the 24±28 cm horizon there
is a corresponding increase in both effective pore
diameter and the two continuity indices (Table 5).
Generally, the effects of tillage on the soil pore
characteristics are similar to the results obtained by
Ball (1981b), Schjùnning (1985b, 1989), Ball et al.
(1989) and Ball and Robertson (1994). In other
words, when draining soil pores with necks in the
range 30±50 mm, the pores in DD or shallow tilled
soil appear to be less continuous and more tortuous
than in ploughed soil. However, as found by Ball
(1981b) and Schjùnning (1985b, 1989), the situation
may well be the opposite when pores of sizes 150±
200 mm only are drained of water. From that evidence,
DD soils appear to exhibit larger, channel-like pores
than ploughed soil.
80
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
Fig. 4. Relative air diffusivity, DS/D0, and log air permeability (mm2) measured in 100 cm3 soil cores at a matric water potential of ÿ100 hPa.
The air permeability values indicated at 20 cm depth derive from longer, 250 cm3 soil cores sampled in the 18±27 cm depth. (*) Ploughed,
PL; (*) direct drilled, DD.
4. Conclusions
When compared with continuously ploughed soil,
4±6 years with direct drilling caused the upper soil
layer (0±20 cm) to:
be more dense and display higher strength, and for
the silt loam also a tendency of higher cohesion and
soil internal friction;
exhibit a reduced volume of macropores (>30 mm)
for the sandy soil and the silt loam, whereas the
opposite was found for the sandy loam;
display a reduced continuity of pores when drained
to a matric potential of ÿ100 hPa.
For the soil below 20 cm the DD soil appeared to:
exhibit a reduced frequency of high-strength spots
in the silt loam,
display increased volumes of macropores (>30 mm)
in the sandy loam and the silt loam,
display a higher continuity of macropores for the
silt loam.
The results also showed that the pore system in the
two soil layers in the sandy soil and the sandy loam
was well connected whether the soils were ploughed
or directly drilled. On the contrary, the ploughing had
on the silt loam introduced a limiting permeability,
which was only partly eliminated by 4 years of direct
drilling. No general recommendation concerning tillage system can be given from the studies. However,
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
the results highlight the bene®ts and drawbacks
for both tillage systems in terms of soil physical
properties.
Acknowledgements
We thank Dr. K. Kristensen and Dr. B. Hansen of
the Biometry Research Unit of the Danish Institute of
Agricultural Sciences for invaluable statistical advice.
References
Andersen, A., 1987a. Regnorme uddrevet med strùm i forsùg med
direkte saÊning og plùjning. Dan. J. Plant Soil Sci. 91, 3±14.
Andersen, A., 1987b. Ukrudts¯oraen i forsùg med direkte saÊning
og plùjning ved forskellige kvñlstofniveauer. Dan. J. Plant Soil
Sci. 91, 243±254.
AndreÂn, O., LagerloÈf, J., 1983. Soil fauna (Microarthropods,
Enchytraeids, Nematodes) in Swedish agricultural cropping
systems. Acta Agric. Scand. 33, 33±52.
Ball, B.C., 1981a. Modelling of soil pores as tubes using gas
permeabilities, gas diffusivities and water release. J. Soil Sci.
32, 465±481.
Ball, B.C., 1981b. Pore characteristics of soils from two cultivation
experiments as shown by gas diffusivities and permeabilities
and air-®lled porosities. J. Soil Sci. 32, 483±498.
Ball, B.C., O'Sullivan, M.F., 1982. Soil strength and crop
emergence in direct drilled and ploughed cereal seedbeds in
seven ®eld experiments. J. Soil Sci. 33, 609±622.
Ball, B.C., Robertson, E.A.G., 1994. Soil structural and transport
properties associated with poor growth of oil-seed rape in soil
direct drilled when wet. Soil Till. Res. 31, 119±133.
Ball, B.C., Lang, R.W., O'Sullivan, M.F., Franklin, M.F., 1989.
Cultivation and nitrogen requirements for continuous winter
barley on a gleysol and a cambisol. Soil Till. Res. 13, 333±352.
Ball, B.C., Lang, R.W., Robertson, E.A.G., Franklin, M.F., 1994.
Crop performance and soil conditions on imperfectly drained
loams after 20 and 25 years of conventional tillage or direct
drilling. Soil Till. Res. 31, 97±118.
Ball, B.C., TebruÈgge, F., Sartori, L., GiraÂldez, J.V., GonzaÂlez, P.,
1998. In¯uence of no-tillage on physical, chemical and
biological soil properties. In: TebruÈgge, F., BoÈhrnsen, A.
(Eds.), Experience with the Applicability of No-tillage Crop
Production in the West-European Countries. Final Report.
Concerted Action No. AIR 3-CT 93-1464, pp. 7±27.
Blackwell, P.S., Graham, J.P., Armstrong, J.V., Ward, M.A.,
Howse, K.R., Dawson, C.J., Butler, A.R., 1986. Compaction
of a silt loam soil by agricultural vehicles. I. Effects upon soil
conditions. Soil Till. Res. 7, 97±116.
Blackwell, P.S., Ringrose-Voase, A.J., Jayawardane, N.S., Olsson,
K.A., McKenzie, D.C., Mason, W.K., 1990. The use of air®lled porosity and intrinsic permeability to air to characterize
81
structure of macropore space and saturated conductivity of clay
soils. J. Soil Sci. 41, 215±228.
Blake, G.R., Hartge, K.H., 1986. Particle density. In: Klute, A.
(Ed.), Methods of Soil Analysis: Part 1. Physical and
Mineralogical Methods. Agronomy Monographs No. 9. American Society of Agronomy, Madison WI, pp. 377±382.
Braim, M.A., Chaney, K., Hodgson, D.R., 1992. Effects of
simpli®ed cultivation on the growth and yield of spring barley
on a sandy loam soil. 2. Soil physical properties and root
growth; root:shoot relationships, in¯ow rates of nitrogen; water
use. Soil Till. Res. 22, 173±187.
Comia, R.A., Stenberg, M., Nelson, P., Rydberg, T., HaÊkansson, I.,
1994. Soil and crop responses to different tillage systems. Soil
Till. Res. 29, 335±355.
Dexter, A.R., 1987. Mechanics of root growth. Plant Soil 98, 303±
312.
Douglas, J.T., Goss, M.J., 1987. Modi®cation of porespace by
tillage in two stagnogley soils with contrasting management
histories. Soil Till. Res. 10, 303±317.
Douglas, J.T., Goss, M.J., Hill, D., 1980. Measurements of pore
characteristics in a clay soil under ploughing and direct drilling,
including use of a radioactive tracer (144Ce) technique. Soil
Till. Res. 1, 11±18.
È nderung der
Ehlers, W., 1973. Strukturzustand und zeitliche A
Wasser- und Luftgehalte waÈhrend einer Vegationsperiode in
unbearbeiteter und bearbeiteter LoÈss-Parabraunerde. Z. AckerP¯anzenbau 137, 213±232.
Ehlers, W., 1975. Observations on earthworm channels and
in®ltration on tilled and untilled loess soil. Soil Sci. 119,
242±249.
Ehlers, W., 1997. Optimising the components of the soil water
balance by reduced and no-tillage. In: TebruÈgge, F., BoÈhrnsen,
A. (Eds.), Experience with the Applicability of No-tillage Crop
Production in the West-European countries. Proceedings of the
Workshop III, Evora, Portugal, 1996. Concerted Action No.
AIR 3-CT 93-1464, pp. 107±118.
Ehlers, W., Claupein, W., 1994. Approaches towards conservation
tillage in Germany. In: Carter, M.R. (Ed.), Conservation Tillage
in Temperate Agroecosystems. Lewis Publishers, Boca Raton, FL.
FAO, 1986. Guidelines for the Coding of Soil Data (draft) Proposal
for an International Soil Data Bank. FAO, Rome.
Fountaine, E.R., Brown, N.J., 1959. Shearing resistance of topsoils
under small normal loads. J. Agric. Eng. Res. 4, 53±59.
Francis, G.S., Cameron, K.C., Swift, R.S., 1987. Soil physical
conditions after six years of direct drilling or conventional
cultivation on a silt loam soil in New Zealand. Aust. J. Soil Res.
25, 517±529.
Gradwell, M.W., 1961. A laboratory study of the diffusion of
oxygen through pasture topsoils. NZ J. Sci. 4, 250±270.
Graham, J.P., Blackwell, P.S., Armstrong, J.V., Christian, D.G.,
Howse, K.R., Dawson, C.J., Butler, A.R., 1986. Compaction of
a silt loam by wheeled agricultural vehicles. II. Effects on
growth and yield of direct-drilled winter wheat. Soil Till. Res.
7, 189±203.
Groenevelt, P.H., Kay, B.D., Grant, C.D., 1984. Physical assessment of a soil with respect to rooting potential. Geoderma 34,
101±114.
82
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
Grover, B.L., 1955. Simpli®ed air permeameters for soil in place.
Soil Sci. Soc. Am. Proc. 19, 414±418.
Rasmussen, K.J., 1988. Plùjning, direkte saÊning og reduceret
jordbearbejdning til korn. Dan. J. Plant Soil Sci. 92, 233±248.
Rasmussen, K.J., 1999. Impact of ploughless soil tillage on yield
and soil quality: a Scandinavian review. Soil Till. Res. 53, 3±14.
Riley, H., Bùrresen, T., Ekeberg, E., Rydberg, T., 1994. Trends in
reduced tillage research and practice in Scandinavia. In: Carter,
M.R. (Ed.), Conservation Tillage in Temperate Agroecosystems. Lewis Publishers, Boca Raton, FL, pp. 23±45.
Rydberg, T., 1987. Studies in ploughless tillage in Sweden 1975±
1986. Division of Soil Management Uppsala, Sweden, Report
No. 76, 35 pp.
SAS, 1996. SAS/STATTM Software: Changes and Enhancements
through Release 6.11. SAS Institute, Cary, NC, 1104 pp.
Schaffer, G., 1960. Eine Methode der Abscherwiderstandsmessung
bei AckerboÈden zur Beurteilung ihrer Strukturfestigkeit im
Felde. Landwirtsch. Forsch. 13, 24±33.
Schjùnning, P., 1985a. En laboratoriemetode til maÊling af
luftdiffusion i jord. Special Publication No. S1773. The Danish
Institute of Plant and Soil Science, Copenhagen.
Schjùnning, P., 1985b. Porùsitetsforhold i landbrugsjord. II. Effekt
af halmnedmuldning og jordbearbejdning. Dan. J. Plant Soil
Sci. 89, 425±433.
Schjùnning, P., 1986. Shear strength determination in undisturbed
soil at controlled water potential. Soil Till. Res. 8, 171±179.
Schjùnning, P., 1989. Long-term reduced tillage. II. Soil pore
characteristics as shown by gas diffusivities and permeabilities
and air-®lled porosities. Soil Till. Res. 15, 91±103.
Schjùnning, P., Rasmussen, K.J., 1989. Long term reduced
cultivation. I. Soil strength and stability. Soil Till. Res. 15,
79±90.
Searle, S.R., Casella, G., McCulloch, C., 1992. Variance Components. Wiley, New York, 501 pp.
Taylor, S.A., 1949. Oxygen diffusion as a measure of soil aeration.
Soil Sci. Soc. Am. Proc. 14, 55±60.
Taylor, H.M., 1971. Effects of soil strength on seedling emergence,
root growth and crop yield. In: Barnes, K.K., Carleton, W.M.,
Taylor, H.M., Throckmorton, R.I., Vanden Berg, G.E. (Eds.),
Compaction of Agricultural Soils. American Society of
Agricultural Engineers Monograph, pp. 292±305.
Teiwes, K., Ehlers, W., 1987. Der Ein¯uss der Bodenbearbeitung
auf Porenraumgliederung, ungesaÈttigte Wasserleitung und
Gasdiffusion. II. Porenraumgliederung und Gasdiffusion.
Mitteilgn. Dtsch. Bodenkdl. Gesellsch. 53, 479±484.
Whiteley, G.M., Utomo, W.H., Dexter, A.R., 1981. A comparison
of penetrometer pressures and the pressure exerted by roots.
Plant Soil 61, 351±364.
Soil strength and soil pore characteristics for
direct drilled and ploughed soils
Per Schjùnninga,*, Karl J. Rasmussenb
a
Danish Institute of Agricultural Sciences, Department of Crop Physiology and Soil Science,
Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark
b
Danish Institute of Agricultural Sciences, Department of Agricultural Engineering, Research Centre Bygholm,
PO Box 536, DK-8700 Horsens, Denmark
Received 13 December 1999; received in revised form 17 July 2000; accepted 28 July 2000
Abstract
Direct drilling has often been reported to increase density and strength and to affect pore continuity and tortuosity of the
upper soil layers. In this study these aspects were studied for three texturally differing soils 4±6 years after initiation of
continuous trials with direct drilling and mouldboard ploughing. The soils studied were a coarse sandy soil (Korntved, 5%
clay), a sandy loam (Ballum, 8% clay) and a silty loam (Hùjer, 19% clay). The crop rotation at Korntved was spring barley and
winter rye while at Ballum and Hùjer it was spring barley and winter wheat. Both crops were grown every year. All ®elds had
been mouldboard ploughed for decades prior to the trial period. The ploughed treatment (PL) was imposed in the autumn and
the seedbed preparation and drilling were performed with an S-tined seedbed harrow and a traditional drill. The direct drilled
(DD) treatment received no tillage other than the drilling which was performed by a triple-disc drill. Straw and stubble were
burned. In the 4th, 5th and 6th years of the trial period, minimally disturbed soil cores were taken from the 4 to 8, 14 to 18 and
24 to 28 cm depths, i.e. two horizons above the ploughing depth of 20 cm, and one horizon below this depth. Longer cores
were sampled in the 18±27 cm depth in order to include this transition layer. Furthermore, in the 4th year of the trial period
shear strength was measured in the ®eld at 2-week intervals in the spring with a vane shear tester in the two upper layers
mentioned. All samplings and measurements took place in the ®eld grown with spring barley. In the laboratory air diffusivity
and air permeability were measured at ®eld-sampled water content and again when the soil cores were drained to a matric
potential of ÿ100 hPa. Cone penetration resistance was measured with a 2 mm diameter penetrometer. Separate core samples
from the 14 to 18 cm depth of the Korntved and Hùjer soils were used for estimating soil cohesion and soil internal friction by
a shear annulus method at ®eld-sampled water content.
For all soils, DD increased soil bulk density in the two upper soil layers. The shear vane tester also generally estimated
higher shear strength for the DD compared to the PL treatment. The shear annulus measurements in the laboratory revealed no
differences between tillage treatments for the Korntved soil, while a tendency of higher cohesion and internal soil friction was
found for the DD treatment on the Hùjer soil. The cone penetration measurements indicated a stronger top-soil and fewer highstrength soil elements in the 24±28 cm horizon for the DD than for the PL treatment. Generally the DD treatment had lower
volume of macropores (i.e. pores > 30 mm) in the 4±8 and 14±18 cm depths than the PL treatment. This was re¯ected in
reduced air diffusivities and air permeabilities for these horizons. An exception was the 14±18 cm depth of the Ballum soil,
where increased air diffusivity and air permeability was measured at ®eld-sampled water content. Continuity indices
*
Corresponding author. Tel.: 45-8999-1766; fax: 45-8999-1719.
E-mail address: [email protected] (P. Schjùnning).
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 4 9 - 5
70
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
calculated from air diffusivity and air permeability measurements showed that the DD soil from the 4 to 8 and 14 to 18 cm
depths had less continuous and more tortuous macropores than the ploughed soil. # 2000 Elsevier Science B.V. All rights
reserved.
Keywords: Direct drilling; Ploughing; Soil strength; Soil pore characteristics; Pore continuity; Plough pan
1. Introduction
Tillage has been performed since ancient times in
order to ful®l three major requirements: (i) to improve
soil tilth, (ii) to combat weeds and (iii) to incorporate
plant residues and organic manures. For most soil
management systems and crop rotations, all three
requirements are still valid. Mouldboard and disc
ploughing have been the most obvious way of meeting
these demands and are still the most widely used
primary tillage methods in many countries including
Denmark (Riley et al., 1994; Rasmussen, 1999). However, the mechanical turnover of the top 20±25 cm
layer of soil is an energy-demanding procedure, it
brings new weed seeds to the top layer ready for
germination, and it kills a lot of soil fauna active in
the turnover of organic matter (AndreÂn and LagerloÈf,
1983). Furthermore, several investigations have
reported a signi®cant densi®cation of the soil layer
just below the ploughing depth (e.g. Ehlers, 1973;
Rydberg, 1987; Francis et al., 1987). The continued
ploughing of many soils with tractors of ever-increasing weight and power has created critical conditions
for soil processes such as air exchange (Teiwes and
Ehlers, 1987; Schjùnning, 1989) and water movement
(Comia et al., 1994; Ball et al., 1998). Direct drilling
or shallow tillage have been shown to affect the soil
characteristics of importance to these soil functions
for the horizon just below the equivalent depth of
ploughing, compared to continuously ploughed soil.
For this critical soil layer conversion to direct drilling
or shallow tillage has been shown to increase the
volume of macropores (e.g. Ehlers, 1973; Comia
et al., 1994) and to improve air conductivity by
diffusion as well as by convection (Douglas and Goss,
1987; Schjùnning, 1989; Comia et al., 1994).
On the other hand, repeated direct drilling or shallow tillage will increase the density of the non-cultivated soil layers compared to ploughed soil (Rydberg,
1987; Douglas and Goss, 1987) and decrease the
volume of macropores (Douglas et al., 1980; Schjùnning, 1989). Schjùnning (1989) found a signi®cant
reduction in air diffusivity for shallow tilled topsoil
layers compared to ploughed soil and modelled
this effect as being of even higher importance to
the air exchange of the soil pro®le than the low
diffusivity found in the dense pan below the ploughing
depth in a ploughed soil. Water in®ltration has been
found to increase for reduced tilled and direct drilled
(DD) soils as compared to ploughing (Ehlers, 1997)
which probably is due to increased numbers of
persistent surface-connected macrochannels created
by earthworms (Ehlers, 1975; Ehlers and Claupein,
1994) or by roots. In contrast, for some soils the
saturated hydraulic conductivity has been found to
be higher in ploughed soil than in DD soil (Ball et al.,
1994, 1998).
Also soil strength has been shown to be signi®cantly
different in ploughed and DD soil. Graham et al.
(1986) reported restricted root growth due to a high
mechanical strength at 15 cm depth of a DD silt loam
soil. Braim et al. (1992) found considerably higher
penetration resistances for DD soil than for ploughed
soil. Schjùnning and Rasmussen (1989) considered the
spatial variation of soil strength below tillage depth
(5±10 cm) for a shallow-tilled soil and found a high
frequency of areas with a high penetration resistance
of >2 MPa. At the 25±30 cm depth, on the contrary,
the shallow tillage system had facilitated a reduction
in the mechanical strength compared to the continuously ploughed soil.
The effects of direct drilling and mouldboard
ploughing on crop growth characteristics and yield
were investigated in three Danish ®eld trials and
reported by Rasmussen (1988). The effects upon
earthworm and weed populations for these same trials
were reported by Andersen (1987a,b). Utilising these
trials again, the purpose of the present investigation
was to study the tillage effects on soil pore characteristics and soil mechanical properties.
71
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
2. Materials and methods
2.1. Soils and trial treatments
Field trials with repeated mouldboard ploughing
and direct drilling were performed at three locations in
Denmark for 6 years. The soils ranged from a coarse
sandy soil at Korntved, a sandy loam at Ballum and a
silt loam at Hùjer (Table 1). The Korntved and Ballum
soils originated from morainic material, while the
Hùjer soil derived from marine sediments. Prior to
the experiment all soils had been in a conventional
tillage system with annual ploughing to a depth of
20 cm. The crop rotation at Korntved was spring
barley and winter rye, while at Ballum and Hùjer it
was spring barley and winter wheat. Straw residues
were burned. Both crops were grown every year but all
investigations were performed in the ®eld with the
spring barley crop. Conventional tillage included
ploughing in the autumn to 20 cm depth and seedbed
preparation by shallow harrowing in the spring prior to
drilling. The DD plots were subject to no other tillage
operation but drilling by a triple-disc drill. The two
tillage treatments were compared in a randomised,
complete block design with three blocks at the Hùjer
location and four blocks at the two other locations.
The tillage plots measured 15:00 m 10:25 m at the
Korntved and Ballum locations and 20:00 m 10:50 m
at the Hùjer location.
2.2. Soil sampling and ®eld measurements
In years 4±6 following the start of the continuous
trial treatments, soil was sampled for physical measurements in the laboratory. In each year, minimally
disturbed soil cores were collected at the time of
plant emergence from the soil depths 4±8, 14±18
and 24±28 cm. In year 4, an extra sampling took
place approximately 4 weeks after the ®rst one. The
soil cores were collected randomly from a crosssection of the tilled soil; i.e. no attempt was made
to identify the location of each core relative to the
drilled spots. The soil cores were retrieved in metal
cylinders (diameter 6:10 cm, height 3:42 cm,
volume 100:0 cm3 ) forced into the soil by means
of a hammer. The cylinders were held in position by a
special ¯ange ensuring a vertical downward movement into the soil. After careful removal of the soil®lled cylinder, the end surfaces were trimmed with a
knife. At each sampling, four replicate cores were
taken from each combination of trial plot and sampling depth, which means that a total of 1056 cores
were collected for the Measuring Procedure I.
In year 4, additional 250 cm3 soil cores (diameter
6:10 cm, height 8:55 cm) were sampled at all locations, 18±27 cm depth, with the technique described
above. Three replicate cores were taken at plant
emergence and again 4 weeks later for each location
and trial plot (except at Hùjer where sampling was
only at plant emergence). This gave a total of 114
cores, which were used in the Measuring Procedure II.
Furthermore, at the Hùjer and Korntved locations, six
replicate 100 cm3 cores were collected at each plot
from the 14 to 18 cm layer at those two sampling
dates, yielding a total of 192 minimally disturbed soil
cores for the Measuring Procedure III.
In year 4, at plant emergence and again approximately 2, 4 and 6 weeks later, 100 cm3 soil cores were
collected for determination of dry soil bulk density at
the 4±8 and 14±18 cm soil depths. On the same dates,
vane shear strength was measured in the ®eld at the 4±
6.5 and 14±16.5 cm depths using the method of Schaffer (1960). Eight replicate measurements were made
for each combination of plot and measuring depth.
Table 1
Soil texture of the 0±20 cm layer of the soils investigated
Location
Textural
classa
Organic matter
(g/(100 g))
Clay, 30 mm was found in this
horizon. No signi®cant effects were found for the silt
loam at Hùjer.
The parameter, CG DS =D0 =e, relating the relative diffusivity DS/D0 and the air-®lled pore space, e, is
an expression of the effectiveness of the air-®lled pore
space in conducting air by diffusion, i.e. an index of
continuity of the soil pore system (Gradwell, 1961).
For the coarse sandy soil at Korntved, a signi®cant
reduction in the CG continuity index was found for the
DD soil at the 4±8 and 14±18 cm depths (Table 5). The
same was true at the 4±8 cm depth for the loamy sand
79
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
Table 5
Volume of pores with tube-equivalent diameter >30 mm and selected model estimates of soil pore characteristics as averaged for all sampling
dates in years 4±6 of continuous tillage treatmenta
Location
Korntved
Soil depth (cm)
4±8
14±18
24±28
Ballum
4±8
14±18
24±28
Hùjer
4±8
14±18
24±28
Pores > 30 mm
(m3/(100 m3))
Effective pore
diameter, DB (mm)
Continuity indices
CG (mÿ3 10ÿ2 )
log(PO, mm2)
PL
DD
LSD0.05
PL
DD
LSD0.05
PL
DD
LSD0.05
32.3
27.5
1.6 (2)
28.1
27.8
NS (1)
28.4
28.2
NS (0)
118
99
7 (1)
112
106
6 (0)
105
105
NS (0)
26.3
21.2
1.5 (2)
23.4
21.5
1.4 (1)
24.2
23.0
NS (0)
2.05
1.81
0.06 (2)
1.95
1.87
0.06 (0)
1.92
1.90
NS (0)
PL
DD
LSD0.05
PL
DD
LSD0.05
PL
DD
LSD0.05
12.6
13.4
NS (2)
12.6
14.6
1.5 (2)
12.2
14.1
1.7 (2)
181
161
16 (2)
188
141
19 (3)
156
138
14 (1)
10.3
8.7
1.3 (0)
10.1
11.2
NS (0)
8.6
9.1
NS (0)
2.01
1.80
0.11 (2)
2.00
1.80
0.14 (1)
1.79
1.71
NS (2)
PL
DD
LSD0.05
PL
DD
LSD0.05
PL
DD
LSD0.05
7.5
5.2
NS (0)
7.0
4.9
NS (0)
4.6
6.2
NS (1)
130
140
NS (0)
203
201
NS (0)
170
181
NS (0)
5.7
5.4
NS (0)
10.7
9.2
NS (0)
7.9
8.1
NS (0)
1.32
1.28
NS (0)
1.93
1.92
NS (0)
1.70
1.83
NS (0)
Tillage system
a
The number in parentheses indicates the number of individual sampling events (out of the maximum possible of four) with signi®cant
trend (P 0:05) similar to the one found for the average of all four samplings.
at Ballum, while no signi®cant effects were observed
for the Hùjer soil.
The index, PO K=e (Groenevelt et al., 1984),
relating air permeability to the air-®lled pore space
can also be taken as an expression of continuity of the
soil pore system and is often quoted as pore organisation (PO) (Blackwell et al., 1990). In accordance with
the CG-index, a reduced continuity of the coarse sandy
soil at Korntved was found when converting from
ploughing to direct drilling (Table 5). A lower POindex was also found for DD soil in the 4±8 cm layer
of the loamy sand at Ballum. The silty loam at Hùjer
displayed no signi®cant effects for this parameter.
However, although not statistically signi®cant, it
should be noted that for the 24±28 cm horizon there
is a corresponding increase in both effective pore
diameter and the two continuity indices (Table 5).
Generally, the effects of tillage on the soil pore
characteristics are similar to the results obtained by
Ball (1981b), Schjùnning (1985b, 1989), Ball et al.
(1989) and Ball and Robertson (1994). In other
words, when draining soil pores with necks in the
range 30±50 mm, the pores in DD or shallow tilled
soil appear to be less continuous and more tortuous
than in ploughed soil. However, as found by Ball
(1981b) and Schjùnning (1985b, 1989), the situation
may well be the opposite when pores of sizes 150±
200 mm only are drained of water. From that evidence,
DD soils appear to exhibit larger, channel-like pores
than ploughed soil.
80
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
Fig. 4. Relative air diffusivity, DS/D0, and log air permeability (mm2) measured in 100 cm3 soil cores at a matric water potential of ÿ100 hPa.
The air permeability values indicated at 20 cm depth derive from longer, 250 cm3 soil cores sampled in the 18±27 cm depth. (*) Ploughed,
PL; (*) direct drilled, DD.
4. Conclusions
When compared with continuously ploughed soil,
4±6 years with direct drilling caused the upper soil
layer (0±20 cm) to:
be more dense and display higher strength, and for
the silt loam also a tendency of higher cohesion and
soil internal friction;
exhibit a reduced volume of macropores (>30 mm)
for the sandy soil and the silt loam, whereas the
opposite was found for the sandy loam;
display a reduced continuity of pores when drained
to a matric potential of ÿ100 hPa.
For the soil below 20 cm the DD soil appeared to:
exhibit a reduced frequency of high-strength spots
in the silt loam,
display increased volumes of macropores (>30 mm)
in the sandy loam and the silt loam,
display a higher continuity of macropores for the
silt loam.
The results also showed that the pore system in the
two soil layers in the sandy soil and the sandy loam
was well connected whether the soils were ploughed
or directly drilled. On the contrary, the ploughing had
on the silt loam introduced a limiting permeability,
which was only partly eliminated by 4 years of direct
drilling. No general recommendation concerning tillage system can be given from the studies. However,
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
the results highlight the bene®ts and drawbacks
for both tillage systems in terms of soil physical
properties.
Acknowledgements
We thank Dr. K. Kristensen and Dr. B. Hansen of
the Biometry Research Unit of the Danish Institute of
Agricultural Sciences for invaluable statistical advice.
References
Andersen, A., 1987a. Regnorme uddrevet med strùm i forsùg med
direkte saÊning og plùjning. Dan. J. Plant Soil Sci. 91, 3±14.
Andersen, A., 1987b. Ukrudts¯oraen i forsùg med direkte saÊning
og plùjning ved forskellige kvñlstofniveauer. Dan. J. Plant Soil
Sci. 91, 243±254.
AndreÂn, O., LagerloÈf, J., 1983. Soil fauna (Microarthropods,
Enchytraeids, Nematodes) in Swedish agricultural cropping
systems. Acta Agric. Scand. 33, 33±52.
Ball, B.C., 1981a. Modelling of soil pores as tubes using gas
permeabilities, gas diffusivities and water release. J. Soil Sci.
32, 465±481.
Ball, B.C., 1981b. Pore characteristics of soils from two cultivation
experiments as shown by gas diffusivities and permeabilities
and air-®lled porosities. J. Soil Sci. 32, 483±498.
Ball, B.C., O'Sullivan, M.F., 1982. Soil strength and crop
emergence in direct drilled and ploughed cereal seedbeds in
seven ®eld experiments. J. Soil Sci. 33, 609±622.
Ball, B.C., Robertson, E.A.G., 1994. Soil structural and transport
properties associated with poor growth of oil-seed rape in soil
direct drilled when wet. Soil Till. Res. 31, 119±133.
Ball, B.C., Lang, R.W., O'Sullivan, M.F., Franklin, M.F., 1989.
Cultivation and nitrogen requirements for continuous winter
barley on a gleysol and a cambisol. Soil Till. Res. 13, 333±352.
Ball, B.C., Lang, R.W., Robertson, E.A.G., Franklin, M.F., 1994.
Crop performance and soil conditions on imperfectly drained
loams after 20 and 25 years of conventional tillage or direct
drilling. Soil Till. Res. 31, 97±118.
Ball, B.C., TebruÈgge, F., Sartori, L., GiraÂldez, J.V., GonzaÂlez, P.,
1998. In¯uence of no-tillage on physical, chemical and
biological soil properties. In: TebruÈgge, F., BoÈhrnsen, A.
(Eds.), Experience with the Applicability of No-tillage Crop
Production in the West-European Countries. Final Report.
Concerted Action No. AIR 3-CT 93-1464, pp. 7±27.
Blackwell, P.S., Graham, J.P., Armstrong, J.V., Ward, M.A.,
Howse, K.R., Dawson, C.J., Butler, A.R., 1986. Compaction
of a silt loam soil by agricultural vehicles. I. Effects upon soil
conditions. Soil Till. Res. 7, 97±116.
Blackwell, P.S., Ringrose-Voase, A.J., Jayawardane, N.S., Olsson,
K.A., McKenzie, D.C., Mason, W.K., 1990. The use of air®lled porosity and intrinsic permeability to air to characterize
81
structure of macropore space and saturated conductivity of clay
soils. J. Soil Sci. 41, 215±228.
Blake, G.R., Hartge, K.H., 1986. Particle density. In: Klute, A.
(Ed.), Methods of Soil Analysis: Part 1. Physical and
Mineralogical Methods. Agronomy Monographs No. 9. American Society of Agronomy, Madison WI, pp. 377±382.
Braim, M.A., Chaney, K., Hodgson, D.R., 1992. Effects of
simpli®ed cultivation on the growth and yield of spring barley
on a sandy loam soil. 2. Soil physical properties and root
growth; root:shoot relationships, in¯ow rates of nitrogen; water
use. Soil Till. Res. 22, 173±187.
Comia, R.A., Stenberg, M., Nelson, P., Rydberg, T., HaÊkansson, I.,
1994. Soil and crop responses to different tillage systems. Soil
Till. Res. 29, 335±355.
Dexter, A.R., 1987. Mechanics of root growth. Plant Soil 98, 303±
312.
Douglas, J.T., Goss, M.J., 1987. Modi®cation of porespace by
tillage in two stagnogley soils with contrasting management
histories. Soil Till. Res. 10, 303±317.
Douglas, J.T., Goss, M.J., Hill, D., 1980. Measurements of pore
characteristics in a clay soil under ploughing and direct drilling,
including use of a radioactive tracer (144Ce) technique. Soil
Till. Res. 1, 11±18.
È nderung der
Ehlers, W., 1973. Strukturzustand und zeitliche A
Wasser- und Luftgehalte waÈhrend einer Vegationsperiode in
unbearbeiteter und bearbeiteter LoÈss-Parabraunerde. Z. AckerP¯anzenbau 137, 213±232.
Ehlers, W., 1975. Observations on earthworm channels and
in®ltration on tilled and untilled loess soil. Soil Sci. 119,
242±249.
Ehlers, W., 1997. Optimising the components of the soil water
balance by reduced and no-tillage. In: TebruÈgge, F., BoÈhrnsen,
A. (Eds.), Experience with the Applicability of No-tillage Crop
Production in the West-European countries. Proceedings of the
Workshop III, Evora, Portugal, 1996. Concerted Action No.
AIR 3-CT 93-1464, pp. 107±118.
Ehlers, W., Claupein, W., 1994. Approaches towards conservation
tillage in Germany. In: Carter, M.R. (Ed.), Conservation Tillage
in Temperate Agroecosystems. Lewis Publishers, Boca Raton, FL.
FAO, 1986. Guidelines for the Coding of Soil Data (draft) Proposal
for an International Soil Data Bank. FAO, Rome.
Fountaine, E.R., Brown, N.J., 1959. Shearing resistance of topsoils
under small normal loads. J. Agric. Eng. Res. 4, 53±59.
Francis, G.S., Cameron, K.C., Swift, R.S., 1987. Soil physical
conditions after six years of direct drilling or conventional
cultivation on a silt loam soil in New Zealand. Aust. J. Soil Res.
25, 517±529.
Gradwell, M.W., 1961. A laboratory study of the diffusion of
oxygen through pasture topsoils. NZ J. Sci. 4, 250±270.
Graham, J.P., Blackwell, P.S., Armstrong, J.V., Christian, D.G.,
Howse, K.R., Dawson, C.J., Butler, A.R., 1986. Compaction of
a silt loam by wheeled agricultural vehicles. II. Effects on
growth and yield of direct-drilled winter wheat. Soil Till. Res.
7, 189±203.
Groenevelt, P.H., Kay, B.D., Grant, C.D., 1984. Physical assessment of a soil with respect to rooting potential. Geoderma 34,
101±114.
82
P. Schjùnning, K.J. Rasmussen / Soil & Tillage Research 57 (2000) 69±82
Grover, B.L., 1955. Simpli®ed air permeameters for soil in place.
Soil Sci. Soc. Am. Proc. 19, 414±418.
Rasmussen, K.J., 1988. Plùjning, direkte saÊning og reduceret
jordbearbejdning til korn. Dan. J. Plant Soil Sci. 92, 233±248.
Rasmussen, K.J., 1999. Impact of ploughless soil tillage on yield
and soil quality: a Scandinavian review. Soil Till. Res. 53, 3±14.
Riley, H., Bùrresen, T., Ekeberg, E., Rydberg, T., 1994. Trends in
reduced tillage research and practice in Scandinavia. In: Carter,
M.R. (Ed.), Conservation Tillage in Temperate Agroecosystems. Lewis Publishers, Boca Raton, FL, pp. 23±45.
Rydberg, T., 1987. Studies in ploughless tillage in Sweden 1975±
1986. Division of Soil Management Uppsala, Sweden, Report
No. 76, 35 pp.
SAS, 1996. SAS/STATTM Software: Changes and Enhancements
through Release 6.11. SAS Institute, Cary, NC, 1104 pp.
Schaffer, G., 1960. Eine Methode der Abscherwiderstandsmessung
bei AckerboÈden zur Beurteilung ihrer Strukturfestigkeit im
Felde. Landwirtsch. Forsch. 13, 24±33.
Schjùnning, P., 1985a. En laboratoriemetode til maÊling af
luftdiffusion i jord. Special Publication No. S1773. The Danish
Institute of Plant and Soil Science, Copenhagen.
Schjùnning, P., 1985b. Porùsitetsforhold i landbrugsjord. II. Effekt
af halmnedmuldning og jordbearbejdning. Dan. J. Plant Soil
Sci. 89, 425±433.
Schjùnning, P., 1986. Shear strength determination in undisturbed
soil at controlled water potential. Soil Till. Res. 8, 171±179.
Schjùnning, P., 1989. Long-term reduced tillage. II. Soil pore
characteristics as shown by gas diffusivities and permeabilities
and air-®lled porosities. Soil Till. Res. 15, 91±103.
Schjùnning, P., Rasmussen, K.J., 1989. Long term reduced
cultivation. I. Soil strength and stability. Soil Till. Res. 15,
79±90.
Searle, S.R., Casella, G., McCulloch, C., 1992. Variance Components. Wiley, New York, 501 pp.
Taylor, S.A., 1949. Oxygen diffusion as a measure of soil aeration.
Soil Sci. Soc. Am. Proc. 14, 55±60.
Taylor, H.M., 1971. Effects of soil strength on seedling emergence,
root growth and crop yield. In: Barnes, K.K., Carleton, W.M.,
Taylor, H.M., Throckmorton, R.I., Vanden Berg, G.E. (Eds.),
Compaction of Agricultural Soils. American Society of
Agricultural Engineers Monograph, pp. 292±305.
Teiwes, K., Ehlers, W., 1987. Der Ein¯uss der Bodenbearbeitung
auf Porenraumgliederung, ungesaÈttigte Wasserleitung und
Gasdiffusion. II. Porenraumgliederung und Gasdiffusion.
Mitteilgn. Dtsch. Bodenkdl. Gesellsch. 53, 479±484.
Whiteley, G.M., Utomo, W.H., Dexter, A.R., 1981. A comparison
of penetrometer pressures and the pressure exerted by roots.
Plant Soil 61, 351±364.