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Vol. 11 (2002): 219–231.

Effects of traffic and fertilization levels on grass
yields in northern Norway

Birger Volden
Planteforsk, The Norwegian Crop Research Institute, Vågønes Research Station, N-8010 Bodø, Norway,

e-mail: birger.volden@planteforsk.no

Tore E. Sveistrup, Marit Jørgensen
Planteforsk, The Norwegian Crop Research Institute, Holt Research Centre, N-9292 Tromsø, Norway

Trond K. Haraldsen
Jordforsk, Norwegian Centre for Soil and Environmental Research, Frederik A. Dahls vei 20, N-1432 Ås, Norway

Experiments with various traffic regimes in grassland (no traffic, light and medium tractor with two
mechanisation lines; forage harvester and two-step harvesting) and three fertilization levels (N 120,
180, 240 kg ha–1) were carried out in five fields on sandy soils and peat soils in Bodø, northern
Norway. Increased tractor weight caused significant decrease in yields on the peat soils with both
mechanisation lines. When the peat soils were exposed to the light tractor treatments, DM yields
increased significantly as the fertilization level rose from N 120 kg ha–1 to N 180 kg ha–1, but not for
further increase to N 240 kg ha–1. With the medium tractor treatments, there was no significant yield
increase from N 120 kg ha–1 to N 180 kg ha–1, but a further increase to N 240 kg ha–1 gave significant-
ly higher yields. Use of lighter machines on peat soils may therefore lead to reduced fertilizer costs
for farmers, and decrease the risk of leaching from the soil. On the sandy soils, yields were signifi-
cantly lower at the medium tractor treatment with two-step harvest compared to no traffic. With the
exception of decreased air-filled porosity at 1–5 cm depth of the peat soils no significant change in
soil physical properties of the plough layer were measured. Therefore, the negative impact of traffic
on plant growth was more likely due to plant injury than impaired soil conditions.

Key words: compaction, fertilizers, grasslands, peat, sand, tracks

Introduction

Grassland occupies more than 90% of the culti-
vated land in northern Norway. Due to a short
growing season, and unstable and wet weather

conditions, harvesting is often carried out on wet
soils. The grassland yields may vary considera-
bly, both due to winter damage caused by ice
cover or ponding of the soil surface, and soil
compaction and damage of the grass sward
caused by agricultural traffic. Based on traffic

mailto:birger.volden@planteforsk.no

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Volden, B. et al. Effects of traffic and fertilization on grassland yields

experiments on various soils in different parts
of Sweden Håkansson et al. (1990) concluded
that the yield reduction in leys was due primari-
ly to mechanical injury of the plants rather than
to soil compaction. Haraldsen et al. (1995)
showed that yield loss due to soil compaction in
grassland in northern Norway varied between
soils. On soils susceptible to soil compaction
(loamy and clayey soils) yield loss was observed
already in the first year leys, while in peat soils
yield loss due to soil compaction increased with
the age of the leys. Thomas and Evans (1975)
concluded that low aeration of the surface layer
of the soil, due to compaction by vehicles and
livestock, was a major obstacle to better grass-
land production in England and Wales. Increased
fertilizer application may partly compensate for
poor root development and supply sufficient
nutrients to the plants when heavy machinery is
used. Such effects have been reported by Wope-
reis et al. (1990).

Previous investigations on soil compaction
in northern Norway have been carried out by
driving over the field with a tractor in such a
way that tracks covered from 125% to 600% of
the surface area (Haraldsen et al. 1995, Sveis-
trup and Haraldsen 1997). Håkansson et al.
(1990) made use of two traffic intensities at each
of three harvest times in their Swedish studies.
However, results from experiments with a traf-
fic regime similar to farming practice combined

with different fertilizer levels have been lack-
ing. The two-step harvest system implies sever-
al passes in the same tracks and may reduce the
surface area covered by tracks compared with
the use of a forage harvester. This could reduce
the area affected by soil compaction and reduce
the direct effects on the plants. Therefore, field
experiments with different harvesting systems
using three levels of fertilizer were established
on five fields in 1998 at Vågønes, Bodø, north-
ern Norway. The aim of the study was to exam-
ine the effects of traffic intensity under varying
fertilizer levels, and to test whether the two-step
harvest system could be less harmful to soils and
plants than the forage harvest systems.

Material and methods

Soils and botanical composition in the
experimental fields

Haraldsen and Grønlund (1989) and Haraldsen
and Sveistrup (1996) have described the soils in
the experimental fields. Basic soil information
for the fields is shown in Table 1. The soil in the
three fields with mineral soil was relatively ho-
mogenous, well sorted, dominated by fine and
medium sand (80–92% in the fraction 0.06–

Table 1. Basic soil information for the experimental fields.

Type of Seeding Soil type Drainage class pH Ignition loss
grassland year (FAO 1990) 0–20 cm g/100 g

0–20 cm

Pasture 1982 Fine sand Well 5.0/5.41 06.4

Established ley 1996 Fine sand/loamy sand Moderate 6.7 05.0

New ley 1998 Fine sand/loamy sand Moderate 5.4 05.0

Established ley 1994 Moderately to strongly Poorly 6.8 40.1
decomposed peat

New ley 1998 Moderately to strongly Very poorly/ 6.1 60.4
decomposed peat poorly

1 0–5 cm/5–20 cm

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0.6 mm), and classified as Aquic Cryumbrept ac-
cording to Soil Survey Staff (1975). In the fields
with peat, the degree of decomposition was mod-
erate to strong (von Post H5–H8), and the soils
were classified as Hemic Terric Borosaprists
according to Soil Survey Staff (1975). In the field
with established ley, the peat layer was 40–100
cm deep. The drainage with construction of a
graded surface towards open ditches had caused
a partial mixing of mineral soil from below the
peat layer into the plough layer. In the field with
new ley, the peat soil was 80–120 cm deep and
was drained by plastic pipes.

Traffic regimes and fertilizer levels had no
significant effects on the botanical composition.
At the start of the experiment all fields with leys
were dominated by timothy (Phleum pratense L.)
(Table 2). The old pasture was dominated by
common bent (Agrostis capillaris L.) and smooth
meadow grass (Poa pratensis L.). In the estab-
lished leys on sandy soil (sown in 1996) the con-
tent of couch grass (Elytrigia repens (L.) Nevski)
increased rapidly during the experimental years.

Treatments
Five experimental fields were established in
spring 1998 at Vågønes research station, Bodø,

northern Norway (67°17'N, 14°26'E). Three
fields had established grass swards: an old pas-
ture seeded in 1982 on a sandy soil, one ley seed-
ed in 1996 on a sandy soil and one seeded in
1994 on a peat soil with graded surface towards
open ditches. The traffic and fertilization treat-
ments were applied in these three fields during
the seasons 1998–2000. Two fields, one on a
sandy soil and one on a peat soil, were seeded
with timothy (P. pratense) in spring 1998, and
30 Mg ha–1 aerated cattle slurry was applied be-
fore sowing. Experimental fertilization and traf-
fic treatments were applied in 1999 and 2000 in
the two fields sown in 1998.

Treatments with three replicates were as follows:

A. Main plot: Traffic intensity at harvest
• No tractor, NT
• Light tractor (axle load 2.8 Mg) with for-

age harvester and trailer, LTFH
• Light tractor (axle load 2.8 Mg) with two-

step harvesting and round baling, LTRB
• Medium tractor (axle load 4.3 Mg) with

forage harvester and trailer, MTFH
• Medium tractor (axle load 4.3 Mg) with

two-step harvesting and round baling,
MTRB

Table 2. Dominating grass species in the experimental fields (% of dry matter yields).

Peat soils Sandy soils

New ley Old ley Pasture Old ley New ley

1998 P. pratense P. pratense A. capillaris P. pratense P. pratense
sown (66%) (61%), (75%), sown

P. pratensis E. repens
(29%) (17%)

1999 P. pratense P. pratense A. capillaris P. pratense P. pratense
(90%) (65%) (68%), (64%), (96%)

P. pratensis E. repens
(18%) (26%)

2000 P. pratense P. pratense A. capillaris P. pratense P. pratense
(70%) (42%) (70%), (45%), (86%)

P. pratensis E. repens
(16%) (35%)

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Volden, B. et al. Effects of traffic and fertilization on grassland yields

B. Split-plot: N fertilization
• N 80 kg ha–1 in spring and N 40 kg ha–1

after first cut
• N 120 kg ha–1 in spring and N 60 kg ha–1

after first cut
• N 160 kg ha–1 in spring and N 80 kg ha–1

after first cut

In May, each spring 20 Mg ha–1 aerated cat-
tle slurry was applied to all plots. In addition,
NPK fertilizer (18% N – 3% P – 15% K) was
applied in spring and after the first cut giving
three levels of nitrogen application. The middle
fertilization level (N 180 kg ha–1 per season) is
recommended on mineral and peat soils for farm-
ing practice with two cuts in this region of north-
ern Norway. The first cut was taken in early July,
and the second cut was taken in September (Ta-
ble 3).

Traffic on the plots was performed by simu-
lating the equipment for fertilizing and harvest-
ing used in common farming practice with the
two mechanisation lines:

• NPK fertilizer was distributed by a light ex-
perimental spreader (Varo). A three point
linked centrifugal distributor was simulated
by attaching a 0.4 Mg weight (half load) to
the three point linkage of light (2.8 Mg) and
medium (4.3 Mg) tractors. One tour across the
plots was driven with traffic in spring and af-
ter first cut.

• Cattle slurry was spread on the plots by a liq-
uid manure spreader alongside the fields. On
the MTFH and MTRB plots this was simulat-
ed by passing across once in spring with a me-
dium-sized tractor and trailer with 3000 kg
total weight (tyres: 400 mm breadth). Light

traffic (LTFH and LTRB) was not simulated,
as the slurry would in this system be spread
by irrigation.

• The forage harvester simulated had 1.35 m
broad cutting width. Two-step harvesting was
simulated using disc harvester (5 discs, 2.5 m)
and round bale press. The simulation of light
traffic was performed by a Ford 4600 tractor
with tyres; 190 mm (in front) and 345 mm
breadth (rear) and trailer with 1.5 Mg total
weight (half load). Medium traffic was simu-
lated by a Valmet 405 tractor with tyres;
315 mm breadth (in front) and 430 mm
breadth (rear) and trailer with 2.5 Mg total
weight.

Forage harvesting was simulated by passing
the plot at 95 cm sideways distance for LTFH,
and at 80 cm sideways distance for MTFH. At
the second cut the wheels tracks were displaced
sideways compared to the first cut, 35 cm for
LTFH and 40 cm for MTFH. In the two-step sys-
tem (LTRB and MTRB) there were two passes
in same tracks at each harvest at a distance of
230 cm. The traffic was concentrated to these
tracks all years. The difference in wheel track
cover by the harvesting traffic simulation is il-
lustrated by Figure 1a (forage harvester) and
Figure 1b (two-step harvesting).

The trailer used was the same for simulation
of slurry spreading, and for light and medium
harvesting. At light load the tyre pressure was
110 kPa and the pressure was 170 kPa at medi-
um load. This was calculated to correspond to
maximum 20% deformation of the tyres at full
load for a small and medium trailer.

Yield measurements
All plots were cut twice each year at a 5–6 cm
stubble height with a Haldrup 1500 plot harvester
(load 1.2 Mg). Dry matter (DM) yields were
measured after drying yield samples (60°C,
48 h). Yield measurements in wheel tracks were
carried out in 2000 on the NT, LTRB and MTRB
treatments in the fields on peat soils and in the

Table 3. Dates of spring fertilizer application and harvest-
ing times in 1998–2000.

Year Spring fertilizer 1st cut 2nd cut
application

1998 20–22 May 7 Jul 7 Sep
1999 18 May 6–8 Jul 15–17 Sep
2000 14 May 4–6 Jul 4–8 Sep

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new ley on the sandy soil. The yields were re-
corded at first and second cut with a frame of
25 cm × 100 cm in these tracks. As these yield
measurements were rather laborious, they were
conducted on two replicates only.

Weather and climate
Bodø has a mean annual air temperature of
4.3°C, and of 12.1°C in the period June–August
(Aune 1993). Mean annual precipitation in Bodø
is 1020 mm, and around 50 mm per month dur-
ing April–June. In July–August, the monthly
precipitation is 90 mm (Førland 1993) (Table 4).
During the experimental period, there were no
problems related to ice cover on soil surface in
the winter period, and no problems with winter
damage on the grassland. Monthly air tempera-

ture and precipitation in the growing season in
the experimental period is shown in Table 4.

Soil analyses
Intact soil cores (volume 100 cm3, diameter
5.8 cm) for physical analyses (three soil cores
for each layer) were collected from two repli-
cates for each harvesting treatment in all fields
in October 2000. Pore size distribution was
measured using ceramic pressure plates (Rich-
ards 1947, 1948). Water content was measured
at saturation, –0.5, –2, –5, –10 and –100 kPa
water potential. Bulk density was calculated af-
ter the soil had been dried at 105°C. Air-filled
porosity was defined as the difference between
total porosity and water content at –10 kPa. Soil
moisture was examined at traffic time (first and

Fig. 1. Simulation of harvesting
traffic with medium heavy tractor
and trailer. a) Forage harvester.
b) Two-step harvesting. Photo af-
ter 2nd cut, 15 September 2000 at
peat soil, 2nd year ley (Photos: B.
Volden).

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second cut) on all experimental fields in 1999
and 2000, representing the actual soil moisture
before traffic started. Composite soil samples
(12–15 sub samples), representing each replicate,
were taken from the plough layer (0–20 cm),
weighed and air dried at 105°C. Soil moisture on
volume basis was calculated by using the mea-
sured soil moisture results and bulk densities.

Statistical analyses
Analyses of variance (ANOVA) were performed
according to the split plot factorial design to test
the effects on yields. Significance level of P <
0.05 was used. Separate analyses were carried
out for sandy soils and peat soils. The two fields,
which were experimentally treated and harvest-
ed only in 1999 and 2000, were computed as
missing data (least squares technique) in 1998
when grouping after soil type. The Fisher’s least-
significant-differences (LSD) test was used for
multiple comparisons. Influence of the tractor
treatments on soil physical properties was test-
ed by means of ANOVA with two replicates.

Results

Water content and soil properties
at traffic time

At traffic time (1st and 2nd cut) the volumetric
water content in the plough layer was lower in

the sandy soils than in the peat soils. In the sandy
soils, the moisture in the plough layer was close
to field capacity (–100 kPa water potential). The
water content in the plough layer of the peat soils
was between or close to –2 and –10 kPa water
potential (Table 5). The peat soil with graded
surface (new leys) was drier than the pipe drained
peat soil (established leys), especially in 2000.

Effects of traffic on DM yields
The MTRB treatment had significantly lower
DM yields than the NT and LTFH treatments,
on average of the three fields on sandy soils (Ta-
ble 6). In the peat soils, all treatments with traf-
fic had lower yield than the NT treatment (Ta-
ble 7). The medium weight equipment caused
larger yield loss than the light equipment, and
there was no significant difference between the
forage harvester system and two-step harvest-
ing with round baling.

Effects of fertilization levels on DM yields
Fertilizer application above N 120 kg ha–1 sig-
nificantly increased DM yields both on mineral
soils and peat soils (Tables 7 and 8). Neverthe-
less, the increments in yield were rather small,
only 15–16% after doubling the fertilization. In
the old pasture field, there was no yield increase
for the largest dose of fertilizer, while the other
experiments on mineral soil showed significant
or nearly significant increase also at this level
(Table 8).

Table 4. Precipitation and temperatures during the growing seasons 1998–2000 (Vågønes Research Station, Bodø), and
normals (1961–90) at Bodø [Aune (1993) and Førland (1993)].

Year Precipitation, mm Temperature, °C

Apr May Jun Jul Aug Sep Apr May Jun Jul Aug Sep

1998 28 1170 53 68 70 89 2.2 5.8 11.8 15.1 12.9 10.4
1999 80 82 76 1320 78 88 3.7 6.7 11.8 12.0 10.6 12.1
2000 73 88 1060 28 1060 1150 2.3 7.8 09.0 13.0 11.3 10.1
Normal 52 46 54 92 88 1230 2.5 7.2 10.4 12.5 12.3 09.0

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Table 5. Volumetric water content in topsoil (0–20 cm) at traffic time (mean of 1st and 2nd cut in 1999 and 2000), and water
content at –2 kPa and –10 kPa water potential.

Experiment Year Water content, m3 m–3

At traffic time

Average Range

Pasture, seeded 1982, 1999 0.34 0.30–0.38 0.44 0.28
sandy soil 2000 0.26 0.23–0.29

Established ley, seeded 1996, 1999 0.26 0.24–0.27 0.42 0.29
sandy soil 2000 0.22 0.20–0.24

New ley, seeded 1998, 1999 0.30 0.28–0.33 0.43 0.30
sandy soil 2000 0.28 0.24–0.33

Established ley, seeded 1994, 1999 0.69 0.66–0.72 0.72 0.67
peat soil 2000 0.53 0.52–0.53

New ley, seeded 1998, 1999 0.76 0.73–0.79 0.78 0.72
peat soil 2000 0.76 0.72–0.80

At –10
kPa

At –2
kPa

Table 6. Dry matter (DM) yields (kg ha–1) in three experiments on sandy soils after different traffic treatments. Average of
three years (new ley, two years).

Tractor Experiment Average

Established ley New ley Old pasture Rel.
— — — — — — — — DM, kg ha–1 — — — — — — — — yield

No tractor NT 7 980 9 790 6 880 8 020 100
Light tractor LTFH1 7 850 9 630 6 810 7 900 099
Light tractor LTRB2 7 890 9 620 6 250 7 700 096
Medium tractor MTFH1 7 970 9 360 6 600 7 810 097
Medium tractor MTRB2 7 700 9 270 6 080 7 480 093

LSD0.05 ns ns ns 0 333

LSD0.05 = Least significant difference at P < 0.05; ns = not significant
1 FH = forage harvester and trailer
2 RB = two-step harvesting and round baling

Treatment
code

Interaction between traffic treatments
and fertilization

On the peat soils, there was a significant inter-
action between traffic and fertilization on DM
yields (Table 7). Increased fertilizer application
from N 120 to 180 kg ha–1 gave high yield in-
crease, for the NT, LTFH and LTRB treatments.
Further increase from N 180 to 240 kg ha–1 gave
much less yield increase. The yield response with

the MTFH and MTRB treatments was low when
N-fertilization increased from N 120 to 180 kg
ha–1. However, a further increase in fertilizer
from N 180 to 240 kg ha–1 gave a large yield
increase. For the treatments with light traffic
(LTFH and LTRB), the yields at N 180 kg ha–1

were approximately equal to the yields of the
MTFH and MTRB treatments at N 240 kg ha–1

(Table 7). There was no significant interaction
between traffic and fertilization on the sandy

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Volden, B. et al. Effects of traffic and fertilization on grassland yields

soils probably due to the limited effect of traffic
treatments in these soils.

DM yields in tracks
The yield reduction in the tracks was significant
in the peat soils (Table 9). In these soils, the
heaviest traffic load also gave significantly less
yield in the tracks compared to light traffic load.
With the highest traffic load and application of
N 240 kg ha–1 the yield in the tracks was less
than in uncompacted soil where N 120 kg ha–1

was applied. The yields in tracks with N 180 kg
ha–1 were approximately equal to the yields of
uncompacted soil where N 120 kg ha–1 was ap-
plied. In the sandy soils, the yield in the tracks
after medium or light traffic was 71 to 75% of
the yield in uncompacted soil, but the difference
was not statistically significant.

Effects of traffic treatments on soil
physical parameters

No statistically significant differences in bulk
density and total porosity in the plough layer
were measured between the traffic treatments.
The only statistically significant effect of the
traffic treatments was decreased air-filled poros-
ity (–10 kPa water potential) in the layer 1–5 cm
of the peat soils (Table 10). Similarly, air-filled
porosity (–10 kPa water potential) in the layer
1–5 cm in the sandy soil decreased by tractor
traffic from 0.187 m3 m–3 in the NT treatment to
0.146 m3 m–3 in the MTRB treatment. The dif-
ference was not statistically significant because
of lack of replicates. The bulk density of fields
with the sandy soils was 1.28–1.32 Mg m–3 in
the layer 1–5 cm, and 1.35–1.37 Mg m–3 at 10–
14 cm depth. Bulk density of the peat soil with

Table 7. Dry matter yields (kg ha–1) in two experiments on peat soils. Average of three years (new ley, two years).

Tractor Treatment Fertilization level, N kg ha–1 Average, traffic
code

120 180 240 DM kg ha–1 Rel. yield

No tractor NT 8 300 9 340 9 630 9 090 100
Light tractor LTFH1 7 400 8 480 8 550 8 140 90
Light tractor LTRB2 7 760 8 760 8 780 8 430 93
Medium tractor MTFH1 7 070 7 390 8 400 7 620 84
Medium tractor MTRB2 7 560 7 600 8 400 7 850 86
Average, fertilization 7 620 8 310 8 750
LSD0.05: traffic 605, fertilization 863, interaction traffic × fertilization 953

LSD0.05 = Least significant difference at P < 0.05
1 FH = forage harvester and trailer
2 RB = two-step harvesting and round baling

Table 8. Yields in three experiments on sandy soils after different fertilization levels, DM kg ha–1. Average of three years
(new ley, two years).

Nitrogen fertilization level Experimental field type
N kg ha–1

Established ley New ley Old pasture Average

120 7 280 8 860 5 960 7 170
180 7 950 9 530 6 740 7 890
240 8 400 10 230 6 860 8 290

LSD0.05 0 349 0 757 0 714 0 363

LSD0.05 = Least significant difference at P < 0.05

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Table 9. Yields in (tractor) wheel tracks. Registrations in three experiments in year 2000.

Tillage treatment Fertilization, DM yield, kg ha–1

N kg ha–1
New ley, sand Established New ley, peat

ley, peat

No traffic 120 9 900 8 860 10 580
(no tracks) 180 9 780 9 720 13 320

240 10 260 12 160 15 860

Light tractor, 120 6 560 7 180 8 940
two-step harvesting 180 7 960 9 140 10 140

240 7 830 9 580 11 680

Medium tractor, 120 5 760 5 180 4 560
two-step harvesting 180 6 640 5 780 6 920

240 8 640 6 480 9 620

LSD0.05 ns 1 100 ns

No traffic1 9 980 10 250 13 250
Light, two-step1 7 450 8 630 10 250
Medium, two-step1 7 010 5 810 7 000

LSD0.05 ns 560 261

1202 7 410 7 070 8 030
1802 8 130 8 210 10 090
2402 8 910 9 410 12 390

LSD0.05 ns 1 500 1 350

LSD0.05 = Least significant difference at P < 0.05; ns = not significant
1 Means are averaged of three traffic levels
2 Means are averaged of three fertilization treatments

Table 10. Influence of traffic treatments on porosity of the plough layer.

Soil depth, cm Treatment code Total porosity, m3 m–3 Air-filled porosity, m3 m–3

Sandy soils Peat soils Sandy soils Peat soils

1–5 NT 0.475 0.800 0.187 0.136
1–5 LTFH 0.470 0.811 0.189 0.082
1–5 MTFH 0.459 0.748 0.155 0.093
1–5 LTRB 0.463 0.803 0.170 0.099
1–5 MTRB 0.475 0.798 0.146 0.075
LSD0.05 ns ns ns 0.041

10–14 NT 0.447 0.796 0.177 0.101
10–14 LTFH 0.453 0.809 0.186 0.073
10–14 MTFH 0.451 0.739 0.163 0.083
10–14 LTRB 0.465 0.777 0.173 0.079
10–14 MTRB 0.444 0.801 0.170 0.078
LSD0.05 ns ns ns ns

LSD0.05 = Least significant difference at P < 0.05; ns = not significant
NT = no tractor; LTFH = light tractor, forage harvester and trailer; MTFH = medium tractor, forage harvester and trailer;
LTRB = light tractor, two-step harvesting and round baling; MTRB = medium tractor, two-step harvesting and round baling

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graded surface was 0.41 Mg m–3 in 1–5 cm, and
0.47 Mg m–3 at 10–14 cm depth. The peat soil
with plastic pipes had uniform bulk density in
the plough layer, 0.34–0.35 Mg m–3.

Discussion

The effects of traffic on the yield of leys in these
experiments were partly related to the moisture
conditions in the soils. The well sorted sandy
soils had a moisture content close to field ca-
pacity at the time of traffic. Such soils are re-
sistant to light and medium soil compaction.
Sandy soils with similar sorting are by Karlsson
(1988) found to be suitable as root zone materi-
al for turf grass at sports fields. According to
Douglas (1994), a large number of studies show
that the largest yield loss occurs when compac-
tion is carried out under wet conditions in spring.
The same compaction on drier soil and with more
developed plants may give less yield reduction.
Zhezmer et al. (1990) found that the water con-
tent of peat soils was of great importance for the
yield decrease due to compaction. Repeated
passes with moderately heavy machinery could
completely destroy the perennial grasses on wet
peat soils, while more heavy and efficient agri-
cultural machines could be used on drier peat
soils without severe yield loss. Myhr and Njøs
(1983) found no significant yield decrease due
to compaction in fields with one annual cut on
peat soils in northern Norway, while compac-
tion caused decreased yields in fields with two
cuts on peat soils. This is in accordance with the
results at the first cut in these experiments, as
the limited traffic in spring will give small
damage directly on plants and as compaction of
the soil.

The yields in all fields were relatively high
at the basic level of fertilization, and the increase
in yields due to fertilization was significant, but
relatively small. The high yields may be related
to good growth conditions without severe growth
restrictions related to wet soils. In the compact-

ed treatments of the peat soils, the air-filled po-
rosity was less than 0.1 m3 m–3 (Table 10). The
limit for sufficient aeration of different crops has
been found to be in the range 0.08–0.12 m3 m–3

(Grable and Siemer 1968), and air-filled porosi-
ty < 0.1 m3 m–3 is characteristic of deficient aer-
ation according to Grable (1971). Without com-
paction the air-filled porosity in the upper part
of the plough layer was above the critical range.
After trafficking the air-filled porosity was
around the lower limit for satisfactory aeration
according to Grable and Siemer (1968). In years
when the peat soil is drier than field capacity
(–10 kPa water potential) in large parts of the
growing season, the decreased air-filled porosi-
ty due to compaction will be of less importance
for the grass growth and yield since actual air-
filled porosity will increase. This was the case
for the peat soil with graded surface and open
ditches. In the peat soil with plastic drainage
pipes, the soil moisture in the plough layer was
around field capacity or even wetter in periods,
and soil compaction may cause the plants grown
in this soil to suffer from poor aeration because
of excessive wetness.

In the present investigation the tractor traf-
fic treatments only caused significant reduction
of air-filled porosity in the 1–5 cm layer of the
peat soil. Sveistrup and Haraldsen (1997) found
that tractor traffic did not always cause signifi-
cant decreases in air-filled porosity in the plough
layer of peat soils in northern Norway. The mois-
ture conditions of the soil when traffic takes
place, is of great importance for how strongly
compaction reduces air-filled porosity. Both
Myhr and Njøs (1983) and Sveistrup and Ha-
raldsen (1997) found a difference in air-filled
porosity between a no tractor and a tractor treat-
ment, but no significant differences in air-filled
porosity between treatments with different trac-
tor loads. In an other investigation (Rasmussen
and Møller 1981) increasing tractor loads in-
creased bulk density and decreased air-filled
porosity, whilst Douglas et al. (1992) found that
reduced ground pressure gave decreased bulk
density in the topsoil compared to conventional
ground pressure.

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Vol. 11 (2002): 219–231.

The pasture field showed less response to the
highest fertilizer rate than the fields with leys
on mineral soils. This may be explained by the
low pH and the botanical composition of the
pasture field, which consisted of common bent
and smooth meadow grass. Grassland dominat-
ed by common bent usually has a lower yield
than leys dominated by timothy (Nesheim 1986).

The small yield response of compaction on
the sandy soils correspond with Wopereis et al.
(1990), who found no significant effect of com-
paction when nitrogen application was N 200 kg
ha–1 or more. Without nitrogen input on sandy
soils, Wopereis et al. (1990) found that both nor-
mal and heavy compaction caused significant
yield loss. The variable response to increased
fertilizer application on peat soils trafficked by
light and medium weight tractors suggests that
different degrees of compaction may affect the
plants above and below soil surface differently.
Heavy traffic may have decreased the ability of
the plants to utilize applied plant nutrients on
the peat soils due to direct damage of the plants.
Rasmussen and Møller (1981) found that yield
loss due to compaction was much greater than
could be explained by changes in soil physical
conditions. Crushing of plant material under the
tractor wheels may be an important reason for
yield loss in grassland. The decreased yields and
low effect of fertilizer in the wheel tracks both
on sandy soils and peat soils in the present in-
vestigation, showed that tractor wheels passing
affected nutrient uptake and plant growth. Al-
though some farmers compensate with extra fer-
tilizer for compaction by heavy agricultural ma-
chinery, Tveitnes and Njøs (1974) and Douglas
and Crawford (1993) have shown that addition-
al nitrogen cannot fully compensate for the yield
loss caused by compaction. Using the recom-
mended fertilization level N 180 kg ha–1 or N
120 kg ha–1 on peat soils the present investiga-
tion shows that additional nitrogen supply of N
60 kg ha–1 is needed if the same yield should be
obtained using medium heavy machinery instead
of light tractors. In the systems with medium
heavy machinery, this additional N is not utilized
by the plants, but may be lost as leaching or due

to denitrification because of poor aeration
(Douglas and Crawford 1998). The results there-
fore indicates that use of lighter machines on peat
soils will give lower fertilizer costs for the farm-
ers, and decrease the risk of increased leaching
and other losses of plant nutrients from the soil.

The results do not show any significant dif-
ferences in yield between the forage harvesting
system and the two-step harvest system with
round baling on the peat soils, while the two-
step harvest system and medium tractor load
gave smallest yield on sandy soils. Due to re-
peated passes in the same tracks in the two-step
harvest system a smaller percentage of the sur-
face area was covered by tracks, while in the
forage harvesting system the tracks were more
evenly distributed (see Figs. 1a and 1b). The re-
sults indicate that the load of the machinery was
more important than the harvesting system on
peat soils. The poor yields in the two-step har-
vest system with medium load on the sandy soils
was a result of poor growth and response to fer-
tilizer N in the tracks, and small negative effects
of the more evenly distributed tracks in the for-
age harvesting system. Accumulation of soil
compaction effects year by year was found in
peat soils in the study of Haraldsen et al. (1995),
who carried out investigations during a five-year
period. It is therefore possible that the experi-
mental period of three years in the present in-
vestigation is too short for development of sig-
nificant differences in yield between the differ-
ent harvesting systems on peat soils.

Conclusions

In field experiments in northern Norway, the ef-
fects of agricultural machinery varied between
sandy soils and peat soils. On sandy soils there
was no significant difference in yields or in soil
physical properties in the plough layer between
traffic-free and forage harvesting systems. On
the peat soils, all traffic treatments gave lower
yields than no traffic. Use of the heaviest equip-

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Volden, B. et al. Effects of traffic and fertilization on grassland yields

ment (tractor 4.3 Mg) gave the greatest yield
reduction. When light machines (tractor 2.8 Mg)
were used, there was a significant yield increase
from N 120 to 180 kg ha–1, but no significant
yield increase at higher nitrogen levels. When
the heaviest machines were used, N 60 kg ha–1,
additional nitrogen was necessary in order to
obtain the same yield as with the use of light
machines. The results showed that the weight of
the agricultural machines was more important for
decrease in yield level than the harvesting sys-
tem on peat soils.

These results have important implications for
practical farming. Farmers on sandy soils with
low organic matter content will experience lim-

ited benefit by the use of light tractors (< 3.0
Mg) compared to moderately heavy tractors
(4.0–4.5 Mg). Farmers on peat soils, on the oth-
er hand, will have better utilization of applied
fertilizer if they use light machines compared to
moderately heavy machines. Use of light ma-
chines on peat soils may therefore have a posi-
tive influence on the farmers’ economy and be
the best practice in relation to the environment.

Acknowledgements. The paper is an outcome of the project
“Yield stability of grassland in northern Norway” which
has been supported by the Research Council of Norway
(Grant no. 119043/110). We thank Dr. Hugh Riley for cor-
recting the language and giving comments on the manu-
script.

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Title
Introduction
Material and methods
Results
Discussion
Conclusions
References