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Soil & Tillage Research 94 (2007) 376–385

Subsoil loosening in a crop rotation for organic farming

eliminated plough pan with mixed effects on crop yield

Jørgen E. Olesen *, Lars J. Munkholm

Danish Institute of Agricultural Sciences, Department of Agroecology, P.O. Box 50, 8830 Tjele, Denmark

Received 15 February 2006; received in revised form 23 August 2006; accepted 28 August 2006

Abstract

Compacted subsoil may reduce plant root growth with resulting effects on plant uptake of water and nutrients. In organic farming

systems subsoil loosening may therefore be considered an option to increase nutrient use. We investigated the effect of subsoil

loosening with a paraplow to ca. 35 cm depth within a four-crop rotation in an organic farming experiment at Foulum (loamy sand)

and Flakkebjerg (sandy loam) in Denmark. In each of the years 2000–2003, half of four plots per site were loosened in the autumn

bearing a young grass-clover crop (mixture of Lolium perenne L., Trifolium repens L. and Trifolium pratense L.) established by

undersowing in spring barley (Hordeum vulgare L.). The grass-clover was grown for another year as a green manure crop and was

followed by winter wheat (Triticum aestivum L.), lupin (Lupinus angustifolius L.):barley and spring barley in the following 3 years.

On-land ploughing was used for all cereal and pulse crops. Penetration resistance was recorded in all crops, and the results clearly

showed that subsoil loosening had effectively reduced the plough pan and that the effect lasted at least for 3.5 years. Measurements

of wheat root growth using minirhizotrons at Foulum in 2002/2003 did not show marked effects of subsoil loosening on root

frequency in the subsoil. Subsoil loosening resulted in reduced growth and less N uptake of the grass-clover crop in which the

subsoil loosening was carried out, probably due to a reduced biological nitrogen (N) fixation resulting from a smaller clover

proportion. This had a marked effect on the growth of the succeeding winter wheat. Negative effect of subsoil loosening on yield of

winter wheat and spring barley was observed without manure application, whereas small positive yield effect of subsoil loosening

was observed in crops with a higher N supply from manure. Yield decrease in winter wheat was observed in years with high winter

rainfall. There was no significant effect of subsoiling on grain yield of the lupin:barley crops, although subsoiling had a tendency to

increase crop growth and yield during dry summers. Our results suggest that subsoil loosening should not be recommended in

general under Danish conditions as a measure to ameliorate subsoil compaction.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Subsoil loosening; Subsoiling; Root growth; Yield; Nitrogen uptake; Organic farming

1. Introduction

Compacted subsoil is a widespread problem on

Danish arable soils (e.g. Schjønning and Rasmussen,

1989). It may even overshadow the effects on soil

quality of differences between conventional and organic

* Corresponding author. Tel.: +45 89991659; fax: +45 89991619.

E-mail address: JorgenE.Olesen@agrsci.dk (J.E. Olesen).

0167-1987/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2006.08.015

farming practices (Schjønning et al., 2002). Compac-

tion is often most severe immediately below the plough

layer in the form of a plough pan.

Mechanical subsoil loosening (subsoiling) can be

used to reduce soil strength beneath the plough layer,

and this has been shown in several investigations to

increase root penetration (Marks and Soane, 1987;

Hipps and Hodgson, 1988). However, the physical

effects of subsoiling often do not last more than 1–2

years after which the plough pan is reestablished (Hipps

J.E. Olesen, L.J. Munkholm / Soil & Tillage Research 94 (2007) 376–385 377

and Hodgson, 1988; Tessier et al., 1997). Recompaction

can be mitigated by using on-land ploughing and light

traffic (Munkholm et al., 2005a). Avoiding recompac-

tion by light traffic and on-land ploughing may be more

beneficial for root growth than regular application of

subsoiling (Munkholm et al., 2005b).

Subsoil loosening may be expected to increase crop

yield in cases where restricted root growth in the

untreated soil leads to shortages of water or nutrients

(Barraclough and Weir, 1988). Subsoiling has thus been

commonly found to increase grain yield of spring sown

crops in years of moderate to severe droughts (Braim

et al., 1984; Marks and Soane, 1987). Less consistent

results have been found for winter wheat (Triticum

aestivum L.), probably because winter wheat plants

often have sufficiently deep roots in spring even when

root growth is restricted by a plough pan (Marks and

Soane, 1987; Anken et al., 2004).

Nitrogen (N) is the major yield limiting factor in

organic arable crop production (Berry et al., 2002;

Olesen et al., 2002). If root growth could be improved to

enhance N uptake from the subsoil, then yield could

improve. Such an improvement in early root growth and

N uptake in winter wheat due to improved root growth

with subsoil loosening was demonstrated by Barra-

clough and Weir (1988). However, in organic farming

rotations this effect would depend on the availability of

mineral N in the subsoil and on possible secondary

effects of subsoiling on biological N fixation (BNF),

which is a major source of N in organic cropping

systems.

The objective of this study was to investigate the

effect of subsoil loosening on crop growth and yield in

an organic crop rotation experiment, where a root

hampering plough pan had previously been identified

(Djurhuus and Olesen, 2000).

2. Materials and methods

An experiment on subsoil loosening was carried out

during 2000–2004 in a crop rotation experiment

established in 1997 (Olesen et al., 2000). The experi-

ment was carried out at two locations representing

different soil types and climate regions in Denmark.

Foulum (568300N, 098340E) is situated on a loamy sand

(8.8% clay, pH(CaCl2) 6.0) with an annual rainfall of

704 mm, and Flakkebjerg (558190N, 118230E) is on a

sandy loam (15.5% clay, pH(CaCl2) 7.5) with an annual

rainfall of 626 mm.

Daily measurements of air temperature and rainfall

were taken at official meteorological stations within

1 km of the experimental locations.

2.1. Crop rotation experiment

Full details on the design of the crop rotation

experiment were given by Olesen et al. (2000). This

paper focuses on subsoil loosening in a 4-course

rotation with spring barley (Hordeum vulgare L.), grass-

clover, winter wheat and an intercrop of lupin (Lupinus

angustifolius L.)/barley. All cereal and pulse crops were

grown to maturity. The grass-clover was a mixture of

ryegrass (Lolium perenne L.), white clover (Trifolium

repens L.) and red clover (Trifolium pratense L.), and

the grass-clover was established by undersowing in

spring barley. Weed harrowing was used, where

possible, to control weeds in cereals and legumes

(Rasmussen et al., 2006). All straw and grass-clover

production was incorporated or left on the soil in all

treatments, and the grass-clover was thus managed as a

green manure crop.

The experimental factors were (i) manure (with (M+)

and without (M�) animal manure applied as slurry),

and (ii) subsoil loosening (with (L+) and without (L�)

subsoiling in the young grass-clover crop).

Manure in the form of anaerobically stored slurry

was solely applied to cereal crops at rates corresponding

to 40% of the N demand for cereals. The N demand was

based on a Danish national standard (Plantedirektoratet,

1997). Spring barley and winter wheat each received

slurry corresponding to a target rate of 50 kg NH4-N

ha�1.

All four crops in the rotation were represented every

year with the two manure treatments in a randomized

design with two replicates resulting in a total of 16 main

plots at each location. Subsoil loosening was carried

out by subdividing these main plots, resulting in a split-

plot experiment. Subsoil loosening was applied in the

young grass-clover sequentially over 4 years from 2000

to 2003, and there were therefore over time an increa-

sing number of plots at each site where subsoil

loosening had been applied. Since winter wheat follo-

wed the grass-clover, this crop provided the largest

number of years for determining difference in grain

yield from subsoil loosening. The gross size of the main

plots was 216 and 169 m2 at Foulum and Flakkebjerg,

respectively.

2.2. Subsoil loosening

Subsoil loosening was carried out to 35 cm depth

using a paraplow (Howard Rotavator Company). In

each of the years 2000–2003 half of four main plots per

location was loosened in the autumn in the young grass-

clover (established in the spring by undersowing in

J.E. Olesen, L.J. Munkholm / Soil & Tillage Research 94 (2007) 376–385378

spring barley). The grass-clover crop was cut three

times during the following growing season by the use of

light machinery. The grass-clover crop was ploughed

under in autumn, and winter wheat was established.

After subsoil loosening there was thus a whole year with

continuous crop growth and light traffic, and without

tillage. On-land ploughing was applied as primary

tillage to counter plough pan reformation.

2.3. Penetration resistance and bulk density in situ

Soil penetration resistance was measured to a depth of

60 cm with an automated cone penetrometer (Olsen,

1988), using an ASAE R313.1-recommended 20.27 mm

diameter/308 semi-angle cone. Measurements were

performed in the spring in the years 2001–2004 at

approximately field capacity. Measurements were

performed at 15 points in both the loosened and reference

treated part of plots. The number of measured plots at

each location accumulated over the years from 4 in 2001

to 16 in 2004, and the number of years with measure-

ments therefore also varied between crops from 4 in the

grass-clover to 1 in spring barley.

Wet bulk density (WBD) was measured in situ at

approximately field capacity in spring 2003 at Foulum

with a high-resolution dual probe gamma-ray transmis-

sion system (Nucletronics ApS, Magleby, Denmark) as

described by Munkholm et al. (2003). Volumetric water

content was determined at the same time and depths as

for the WBD measurements employing a neutron

method with the neutron source and detector located on

the same probe rod as the gamma-ray detector for the

dual probe gamma-ray system. Dry bulk density was

derived from WBD and volumetric water content data.

The WBD was determined in the Foulum plots loosened

in 2001 and grown with winter wheat in 2003.

Measurements were performed at 5–50 cm depth for

each 5 cm increment at two points in both the loosened

and reference treated part of plots.

2.4. Porosity and air permeability

Minimally disturbed soil cores (100 cm3, 6.1 cm

diameter, 3.4 cm height) were collected at Foulum in

March 2003 from loosened and non-loosened plough

pan layer (25–30 cm). Soil cores were collected in metal

cylinders held in position by a special flange ensuring

vertical sampling and driven into the soil by means of a

hammer. After careful removal of the soil-filled

cylinder, the end surfaces were trimmed with a knife.

All soil cores were sealed with plastic caps and stored at

2 8C until analysis. Cores were sampled in the four

Foulum plots loosened in 2001 and grown with winter

wheat in 2003. Three cores were taken at pre-selected 1

m2 areas within both the loosened and non-loosened

part of plots. In total 72 cores were sampled.

Soil cores were capillary wetted to saturation on

tension tables and then drained to a matric potential of

�10 kPa. Sample weight was recorded at �10 kPa and

after oven drying (105 8C for 24 h). Soil porosity was

estimated from bulk soil density and particle density.

Volume of pores >30 mm (ea,100) was calculated as the

difference between total porosity and volume of water

retained at �10 kPa.

Air permeability was measured by the steady-state

method described by Iversen et al. (2001) on cores

that had been equilibrated at �10 kPa matric potential

(Ka,10). Flow of air through the sample was controlled

by a pressure regulator and measured by a precision

flow meter. Flow was recorded at a pressure difference

of 0.5 kPa as measured by a water manometer. Prior

to permeability measurements, soil was gently pressed

at the very edge of the metal ring to minimize the

risk of air leaking along the ring (Ball and Schjønning,

2002).

2.5. Root growth

In winter wheat at Foulum, root observations were

carried out several times during the 2002/2003

growing season using a minirhizotron technique

described by Thorup-Kristensen (2001). Glass tubes

(outer diameter 70 mm, length 1.5 m) were inserted

into the soil with recently emerged wheat in October

2002. Minirhizotrons were installed at an angle of 308from vertical, reaching a depth of approximately 1.0 m

in the soil. Root growth was registered using two

replicate counting grids painted along the minirhizo-

trons, one on the left hand and one on the right hand

side on the upper and outer surface of the minirhizo-

trons. Each grid consisted of 4 columns and 60 rows

(4 � 60 fields). Field size was 2.0 cm � 2.0 cm and

each field represents a vertical soil horizon of 1.73 cm

depth. Measurements were made with a mini-video

camera moved along the two central columns for each

grid. Root frequency was determined from videotape

as the fraction of fields within each 10.4 cm soil layer,

where roots were visible (i.e. 4 horizontal � 6

vertical = 24 fields within each 10.4 cm layer). Five

tubes were installed in both the loosened and non-

loosened part of each winter wheat plot, i.e. 40 tubes in

total. Data from the 0–10 cm depth were rejected for

the 2002 spring and summer observations due to poor

quality of recordings.

J.E. Olesen, L.J. Munkholm / Soil & Tillage Research 94 (2007) 376–385 379

Table 1

Mean temperature and sum of precipitation during the winter (Novem-

ber–March) and summer (April–July) of the growing seasons 2001/

2002 to 2003/2004 at Foulum and Flakkebjerg

Year Temperature (8C) Precipitation (mm)

Winter Summer Winter Summer

Foulum

Normal 1.2 11.4 264 219

2001/2002 2.9 12.6 295 241

2002/2003 0.9 12.5 176 276

2003/2004 2.6 11.4 266 236

Flakkebjerg

Normal 1.5 11.8 239 204

2001/2002 3.5 12.6 259 230

2002/2003 1.4 13.1 157 249

2003/2004 2.8 11.7 237 251

The climatic normal refers to the period 1961–1990 (Olesen et al.,

2000).

2.6. Crop biomass and yield

Grain yield was measured at harvest maturity using a

combine harvester. Harvest area was 48 and 32 m2 at

Foulum and Flakkebjerg, respectively. Grain weight

was determined by weighing 600 dried grains per plot.

Samples of total above-ground biomass in the cereal

and pulse crops were taken in 1 m2 sample areas in each

plot at growth stage 85, 1–2 weeks before yellow

maturity.

Samples of above-ground biomass in the grass-

clover were taken in 1 m2 sample areas in each plot at

time of cutting and mulching (two to three times per

year). Samples were separated into clover and grass.

Sampling was performed in 2003 and 2004 at Foulum

and in 2004 at Flakkebjerg.

Dry matter content of grain and plant samples were

determined after oven drying at 80 8C for 24 h. Total N

in grain and plant samples was determined on finely

milled samples from each plot by the Dumas method

(Hansen, 1989). Total potassium (K) in grain was

determined by flame emission after dry ashing followed

by dissolution in nitric acid. Total phosphorus (P) in

grain was determined colormetrically after dry ashing

followed by dissolution in acid and addition of

ammonium molybdovanadate.

2.7. Spectral reflectance

Spectral reflectance in each plot was measured at

weekly or bi-weekly intervals during the growing

seasons of 2002–2004 to estimate seasonal develop-

ment in light interception by the crop canopy.

Measurements in the grass-clover crop were only

taken in 2003 and 2004. Four measurements were

taken above the crop, each measurement covering

approximately 0.25 m2 of the net area of each plot.

The hand held equipment used consisted of two sensor

units (Christensen, 1992). One unit measured red

(650 � 10 nm) and near infrared (800 � 10 nm)

reflection from the canopy. The second unit measured

incoming radiation. Sensor units used at Foulum

in all years and at Flakkebjerg in 2002 were of

type SKR1800 with a 158 view and connected to an

A/D converter of type SDL2500, both from Skye

Instruments Ltd., UK. The instrument used at Flak-

kebjerg in 2003 and 2004 was of type MSR87 from

CROPSCAN Inc., MN, USA. Ratio vegetation index

(RVI) was calculated as the ratio of near infrared

to red reflectance, taken as a measure of the green

leaf area index of the crop canopy (Christensen and

Goudriaan, 1993).

2.8. Statistical analyses

Averages of measured values were calculated for

each subplot (i.e. loosened and non-loosened parts of

main plots) and used as input in general linear models

(GLM) for treatment effects, taking manure as the main

plot factor and subsoil loosening as the split-plot factor

in a split-plot design. Data on air permeability and pore

organization were log-transformed for normal distribu-

tion. Data for penetration resistance were analysed for

variance due to year, manure and loosening. Crop

response to subsoiling was calculated for each main plot

as the difference between measurements of the L+ and

L� treatments. This difference was then subjected to

analysis of variance, which included effects of site, year

and manure application. We used the GLM procedure of

the statistical software SAS for the analysis of variance

(SAS Institute, 1998). Relationships between dry matter

yield response to subsoiling and various climatic and

crop variables were calculated by Pearson correlation

coefficients using the CORR procedure of SAS.

3. Results

3.1. Climatic conditions

Mean temperature during winter and summer

periods of the three experimental years was close to

or above the long-term climatic normal at both locations

(Table 1). Winter rainfall was considerably lower than

normal at both sites during the winter of 2002/2003, but

close to normal in the other two winter seasons. Summer

rainfall was slightly above the climatic normal in all 3

experimental years.

J.E. Olesen, L.J. Munkholm / Soil & Tillage Research 94 (2007) 376–385380

Fig. 2. Penetration resistance for the four treatment combinations of

subsoil loosening (L�without, L+ with) and manure application (M�without, M+ with) at Flakkebjerg. Measurements were taken in spring

(a) 0.5 years (grass-clover), (b) 1.5 years (winter wheat) and (c) 2.5

years (lupin:barley) after subsoiling. Data are averages across 1–4

years of observation (ny). Depth intervals where significant treatment

effects (P < 0.05) were observed are indicated by vertical bars.

3.2. Soil physical properties

Subsoiling effectively loosened the plough pan at

both sites (Figs. 1a and 2a). Subsoil loosening decreased

penetration resistance in the plough pan layer (25–

35 cm) from ca. 2 to 0.5–0.7 MPa, when measured 6

months after subsoil loosening. During subsequent

years, penetration resistance increased gradually in the

loosened plough pan layer at both sites. There was a

clear and significant positive effect 1.5 years after

subsoil loosening when winter wheat was grown. In

subsequent years (i.e. 2.5 and 3.5 years after subsoil

loosening), there was still a tendency for a loosening

effect (Figs. 1c and d and 2c). However, the effect was in

general not significant at the P = 0.05 level. At Foulum,

penetration resistance was greater in the M+ than in the

M� treatment at 0.5, 2.5 and 3.5 years after subsoil

loosening in part of the 30–50 cm layer.

In situ bulk density at Foulum in 2003 for plots

treated in 2001 confirmed penetration resistance results

(data not shown). Bulk density was significantly lower

in the loosened plough pan layer. At 30 cm depth, bulk

densities was 1.39 and 1.21 g cm�3 for L� and L+,

respectively.

At Foulumin2003 higher proportionofpores>30 mm

occurred after subsoiling (L+: 20.7 m3 100 m�3 and L�:

Fig. 1. Penetration resistance for the four treatment combinations of

subsoil loosening (L�without, L+ with) and manure application (M�without, M+ with) at Foulum. Measurements were taken in spring (a)

0.5 years (grass-clover), (b) 1.5 years (winter wheat), (c) 2.5 years

(lupin:barley) and (d) 3.5 years (spring barley) after subsoiling. Data

are averages across 1–4 years of observation (ny). Depth intervals

where significant treatment effects (P < 0.05) were observed are

indicated by vertical bars.

16.8 m3 100 m�3) at 25–29 cm depth. There was also a

tendency for increased air permeability (P = 0.09) (L+:

14.6 mm2 and L�: 9.2 mm2) and decreased bulk density

(P = 0.11) (L+: 1.35 g cm�3 and L�: 1.44 g cm�3).

There was no significant effect of subsoiling on pore

organization estimated as Ka/ea (Groenevelt et al., 1984).

Manure application resulted in significantly lower

bulk density (M+: 1.36 g cm�3 and M�: 1.42 g cm�3),

but had no significant effect on air permeability,

pores >30 mm and pore organization. There were no

significant interactions between manure application

and subsoiling.

3.3. Root growth

At Foulum root frequency for L+ was lower than for

L� in the 30–40 cm layer in the spring (March–May

2003) (data not shown). This contradicted our

expectations based on soil physical measurements.

The difference was only significant at P = 0.05 level at

one measurement date. In the lower part of the root zone

(i.e. 62–104 cm depth) there was a tendency for higher

root frequency in early April in L+ than in L�(P = 0.09) (Fig. 3). In addition, lower root frequency

occurred for manured than unmanured soil (P = 0.01) in

early April. In late May and early June there was an

interaction between manure application and subsoil

J.E. Olesen, L.J. Munkholm / Soil & Tillage Research 94 (2007) 376–385 381

Fig. 3. Root intensity at 62–104 cm depth during the 2002/2003

growing season of winter wheat at Foulum in treatments without

(L�) and with (L+) subsoil loosening and without (M�) and with

(M+) manure application.

loosening, i.e. manure resulted in greater root frequency

for L� but lower for L+.

3.4. Crop production

Highest grain yield of winter wheat was obtained in

2003 at both locations (Table 2). This was also the year,

where the most positive (Flakkebjerg) or the least

negative (Foulum) yield effect of subsoil loosening was

obtained. However, there were no significant effects of

subsoiling on grain yield in any of the crops, when the

results were analysed for each location and year

separately. There was a greater tendency for yield

reduction from subsoiling in treatments without manure

application than with manure application (Table 3).

Total above-ground dry matter was not affected by

subsoiling without manure, but was possibly affected by

subsoiling with manure.

Table 2

Mean dry matter grain yield (Mg ha�1) at Foulum and Flakkebjerg for

treatments without (L�) and with (L+) subsoil loosening (average of

two manure treatments)

Crop Year Foulum Flakkebjerg

L� L+ L� L+

Winter wheat 2002 4.25 4.01 3.98 3.68

2003 5.11 5.08 4.60 4.81

2004 4.99 4.33 4.50 4.57

Lupin:barley 2003 2.64 2.61 2.98 3.08

2004 3.44 3.37 2.66 2.68

Spring barley 2004 4.53 4.56 3.45 3.14

None of the treatment differences were significant in individual year

and location combinations.

The increase in grain yield from subsoiling was

positively correlated with grain N uptake in the main

plots at both locations (Table 4). There was also a

tendency at both locations for negative effects of winter

temperature and winter precipitation on the yield benefit

from subsoiling.

Subsoiling reduced the grain weight of both species

in the lupin:barley mixture (Table 3).

3.5. Nutrient uptake

There were tendencies for reduced N concentration

and uptake of N in winter wheat after subsoil loose-

ning, although some of the effects were significant

(Table 3). Average reductions were 3 and 8 kg N ha�1

in N uptake of grain and total above-ground biomass,

respectively. The results for the spring sown crops were

less consistent, but with a tendency for higher N uptake

in total biomass in the manured treatments after

subsoiling.

There was a tendency for increased P concentrations

in grains of all crops after subsoil loosening, whereas

there was a significant decrease in K concentration in

spring barley from subsoil loosening (Table 3).

3.6. Spectral reflectance

The RVI in Table 5 is a measure of the green leaf area

index and thus of the light interception of the crop

canopy (Christensen and Goudriaan, 1993). There was

a tendency for reduced RVI in the grass-clover after

subsoil loosening (P = 0.16), and thus a smaller green

leaf area index. This effect was most pronounced

during the first half of the growing season. Mean RVI of

winter wheat was little affected by subsoil loosening,

and this was also the case for the spring sown crops at

Foulum. However, there was a tendency for higher RVI

after subsoiling at Flakkebjerg during the second part

of the growing season in both lupin:barley and spring

barley.

3.7. Grass-clover

Cumulative N uptake in above-ground biomass of

grass-clover was reduced by about 70 kg N ha�1 year�1

from subsoil loosening (Table 6), although this was not

significant at any of the locations (P = 0.10 at Foulum,

P = 0.27 at Flakkebjerg). The reduction was probably a

result of differences in BNF, since the difference in N

uptake between subsoil loosening treatments was

considerably larger in the clover compared with the

grass component.

J.E. Olesen, L.J. Munkholm / Soil & Tillage Research 94 (2007) 376–385382

Table 3

Mean effect of subsoil loosening on various crop parameters for treatments without (M�) and with (M+) manure application using data from all

years at both locations

Parameter Crop M� M+

Dry matter grain yield (Mg ha�1) Winter wheat �0.17 0.02

Lupin:barley �0.01 0.03

Spring barley �0.25+ �0.04

Above-ground DM (Mg ha�1) Winter wheat �0.25 0.19

Lupin:barley �0.04 0.09

Spring barley �0.32 0.59+

Grain weight (mg) Winter wheat 0.5 0.6

Lupin:barley (lupin) �2.8+ �1.0

Lupin:barley (barley) �1.2* �1.1+

Spring barley �0.1 0.4

N in grain (g N kg�1) Winter wheat �0.2 �0.5

Lupin:barley 0.5 0.0

Spring barley �0.1 0.2

N grain yield (kg N ha�1) Winter wheat �4 �2

Lupin:barley 2 0

Spring barley �4 0

Above-ground N (kg N ha�1) Winter wheat �6 �10

Lupin:barley 2 6

Spring barley �3 11+

P in grain (g P kg�1) Winter wheat 0.1 0.0

Lupin:barley 0.2+ 0.1

Spring barley 0.1 0.1

K in grain (g K kg�1) Winter wheat �0.1 �0.2

Lupin:barley 0.1 �0.1

Spring barley �0.4* �0.3+

* Significance levels: 0.01 < P < 0.05.+ Significance levels: 0.05 < P < 0.10.

Table 4

Correlation coefficients between dry matter yield increase in winter

wheat from subsoil loosening and mean grain N uptake, and tem-

perature and precipitation during winter (November–March) or sum-

mer (April–July)

Variable Foulum Flakkebjerg Both locations

Grain N uptake 0.71** 0.54+ 0.51*

Winter temperature �0.98 �0.89 �0.67

Summer temperature 0.10 �0.46 �0.12

Winter precipitation �0.99+ �0.84 �0.80*

Summer precipitation 0.88 0.94 0.52

Data for individual plots were used in correlation analyses for

correlation with grain N uptake (12 observations per location).

Average yield data per location and year were used for correlation

with weather data (3 observations per location).* Significance levels: 0.01 < P < 0.05.

** Significance levels: 0.001 < P < 0.01.+ Significance levels: 0.05 < P < 0.10.

Table 5

Mean ratio vegetation index (RVI) of the individual crops before and

after 15 June at Foulum and Flakkebjerg for treatments without (L�)

and with (L+) subsoil loosening

Crop Location Before 15

June

After 15

June

L� L+ L� L+

Grass-clover Foulum 12.6 11.3 13.5 12.6

Flakkebjerg 12.8 11.0 12.5 11.5

Winter wheat Foulum 7.5 6.9 4.4 4.4

Flakkebjerg 6.8 7.1 6.8 6.7

Lupin:barley Foulum 5.3 5.4 9.1 9.0

Flakkebjerg 4.8 4.3 13.7 15.5

Spring barley Foulum 12.0 12.5 9.9 9.7

Flakkebjerg 7.1 7.0 17.6 19.1

None of the treatment differences were significant in individual year

and location combinations.

J.E. Olesen, L.J. Munkholm / Soil & Tillage Research 94 (2007) 376–385 383

Table 6

Mean cumulative N uptake (kg N ha�1 year�1) in above-ground

biomass of grass-clover cuttings at Foulum and Flakkebjerg for

treatments without (L�) and with (L+) subsoil loosening (average

of two manure treatments)

Component Foulum Flakkebjerg

L� L+ L� L+

Grass 97 83 62 54

Clover 246 192 204 137

Total 343 275 266 192

Data for Foulum were average of 2003–2004, whereas for Flakkebjerg

were 2004 only. None of the treatment differences were significant.

4. Discussion

4.1. Soil physical effects

Plough pan reformation was effectively mitigated by

on-land ploughing during the first 2 years after subsoil

loosening, and there was even after 3.5 years a

significantly lower penetration resistance in the

previous plough pan of the loosened compared with

the untreated soil (Figs. 1 and 2). A similar study in an

organically farmed sandy loam showed that on-land

ploughing was necessary to mitigate plough pan

reformation (Munkholm et al., 2005a).

4.2. Nitrogen cycling

Average dry matter grain yield of winter wheat was

about 4.5 Mg ha�1 without manure application and

5.5 Mg ha�1 with manure application. Typical yield in

conventional farming on similar soil types in Denmark

is about 7.0 Mg ha�1 (Plantedirektoratet, 1997). It is

therefore very likely that crop yield was limited by low

N supply.

Much of the N supply for winter wheat in this

specific crop rotation comes from the previous grass-

clover crop, which was managed as a green manure crop

to increase soil N fertility (Olesen et al., 2002).

Subsoiling was carried out during autumn in the young

grass-clover crop. There was no major visible damage

to the grass-clover at the time of subsoiling. However,

subsoiling apparently reduced the growth of clover and

thereby BNF during the following growing season

(Table 6). This was probably the main reason for the

lower winter wheat yield when subsoiled without

manure application (Table 3).

There was greater yield benefit from subsoiling when

the N supply was increased with manure or through

BNF in the grass-clover (Table 4). Low N supply was

expected without manure application, especially in

cases of higher winter rainfall, which increases N and

results in low residual N. Under conditions of high N

leaching, improved root penetration with subsoil

loosening may not have increased N uptake, since

the N would have already leached from the soil profile.

The winters 2001/2002 and 2003/2004 were wet (i.e.

higher risk of N leaching), which may have contributed

to reduced grain yield from subsoiling in these years

(Table 2). A generally lower level of N leaching at

Flakkebjerg compared with Foulum (Askegaard et al.,

2005) may also explain why more positive yield effects

of subsoiling were obtained at Flakkebjerg than at

Foulum.

A low N supply would not only have reduced above-

ground biomass and grain yield, but also root frequency,

as indicated by the minirhizotron measurements at

Foulum (Fig. 3). Subsoiling generally improves root

penetration in succeeding crops, especially in the

previously compacted zone and below (Braim et al.,

1984; Marks and Soane, 1987; Hipps and Hodgson,

1988). Minirhizotron measurements in Fig. 3 supported

this generalisation. It is possible that the effects of N

supply and subsoiling may interact under certain

circumstances to provide a particular large improve-

ment of root growth and density under conditions of

both high N supply and low soil strength. This would

lead to larger benefits of subsoil loosening under high N

supply as found in the correlation analyses in Table 4.

Subsoil loosening should also reduce nitrate leaching.

However, minirhizotron measurements at Foulum did

not show a more dense root growth of wheat with both

subsoil loosening and manure application.

4.3. Water use

There is an alternative explanation for the effect of

subsoiling in improving winter wheat grain yield at

higher N supply (Table 3). Crops with a higher N supply

will have a larger leaf area index, and therefore, higher

evapotranspiration, which means that they may be more

susceptible to summer droughts. Subsoiling has been

found to increase yield in years of moderate to severe

drought (Marks and Soane, 1987; Barraclough and

Weir, 1988). There were indications that this played a

role for the spring sown crops at Flakkebjerg (2003 and

2004), since RVI after 15 June was higher with than

without subsoiling (Table 5). It was not possible to

detect this effect in winter wheat, possibly because

enhanced N supply following the grass-clover without

subsoiling may have overshadowed better root growth

and less drought stress with subsoiling.

J.E. Olesen, L.J. Munkholm / Soil & Tillage Research 94 (2007) 376–385384

4.4. Perspectives

Our results call into question when to perform

subsoiling within a crop rotation. We chose to subsoil in

the young grass-clover, since this gave a long period

afterwards with light traffic and extensive root growth to

stabilise the subsoil loosening, which we thought was

critical for avoiding recompaction (Munkholm et al.,

2005a,b). However, we did not anticipate the negative

effects of subsoiling on the growth of clover, which

subsequently reduced BNF and restricted N supply and

yield of subsequent winter wheat. In hindsight it may

have been better to perform the subsoil loosening prior

to spring barley. With on-land ploughing this should

have still yielded a lasting reduction in soil strength

beneath the plough layer.

The effects of subsoil loosening were in general

very small, despite yield benefits may have been

slightly larger had there not been a negative effect on N

uptake in the grass-clover. Our results suggest that

subsoil loosening should not be recommended in

general under Danish conditions with organic farming

as a measure to ameliorate subsoil compaction.

Biological amelioration may be a suitable alternative

to mechanical subsoil loosening (Munkholm et al.,

2005b). Root and earthworm channels constitute

effective pathways through compacted layers and will

significantly improve root penetration. Such channels

will be favoured by on-land ploughing and light traffic

in combination with crop rotations that increase inputs

of organic matter to soil.

5. Conclusions

Subsoil loosening in a young grass-clover crop

reduced growth of clover and thus possibly biological

nitrogen fixation. This negatively impacted the yield of

a subsequent winter wheat crop, especially without

manure application. Small positive effect of subsoiling

on yield of winter wheat was obtained under conditions

of higher N supply with manure application. There

were indications that subsoiling increased crop growth

and yield of spring cereals and pulses during dry

summers.

Subsoiling reduced soil strength in the former plough

pan, but the effect on grain yield was modest under the

cool and generally moist Danish climate. So, results

suggest that subsoil loosening should not be recom-

mended in general under Danish conditions as a

measure to ameliorate subsoil compaction. Any positive

subsoiling effect would require a subsequent reduction

in weight load of the soil to mitigate recompaction.

Acknowledgements

We thank Per Schjønning for valuable inspiration

and advice. The technical assistance of B.B. Chris-

tensen, Stig T. Rasmussen, Michael Koppelgaard, Jens

B. Kjeldsen, Erling Nielsen and Eugene Driessen is

gratefully acknowledged. The ROMAPAC project was

funded by the Danish Ministry of Food, Agriculture and

Fisheries. The project was an integral part of the

activities under Danish Research Centre for Organic

Farming.

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