Susceptibility of a sandy loam soil topreferential ¯ow as affected by tillage

C.T. Petersena,*, H.E. Jensena, S. Hansena, C. Bender Kochb

aDepartment of Agricultural Sciences, Laboratory for Agrohydrology and Bioclimatology,

The Royal Veterinary and Agricultural University, Agrovej 10, DK-2630 Taastrup, DenmarkbChemical Department, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark

Received 30 May 2000; received in revised form 17 October 2000; accepted 10 November 2000

Abstract

Flow patterns for water and solutes in structured soils are often heterogeneous. Understanding the spatial variability of ¯ow

is essential in solute transport studies and for management of chemical movement. We hypothesized the occurrence of effects

of alternative tillage operations for winter wheat on ¯ow patterns and that such effects could be revealed by dye tracing.

Tracer studies using the anionic dye Brilliant Blue FCF were conducted on a sandy loam soil (Agrudalf) subjected to four

different tillage treatments: (T1) Ð harrowing two times with a springtine harrow, drilling; (T2) Ð direct drilling; (T3) Ð

ploughing, light subsurface compaction, one harrowing with a PTO-driven rotary harrow, drilling; (T4) Ð ploughing, one

harrowing with a springtine harrow, drilling. Studies were conducted (i) in the autumn after plant emergence 4 weeks after

tillage and planting, and (ii) in the spring 7 months after tillage and planting. Twenty-®ve millimetres of water containing

4.0 g lÿ1 of the dye was applied uniformly to the soil surface within 1 h. Plots were excavated 1 day after dye application.

Stained ¯ow patterns on 22 vertical 1.00 m2 soil pro®les from each treatment were photographed and subjected to image

analysis.

Deeply penetrating ¯ow paths were found for all treatments both in the autumn and in the spring. The number of

individually bounded, stained ¯ow pathways per metre pro®le length averaged over depth in the 30±100 cm soil layer (NP30±

100) and the degree of coverage of pro®le faces with dye in the 0±20 cm layer (DC0±20) were signi®cantly affected by tillage

treatment, both in the spring and in the autumn �P < 0:001�. Averaged for the 22 pro®les per treatment, NP30±100 equalled to

2.0, 3.5, 0.7, and 1.8 mÿ1 for T1, T2, T3 and T4, respectively, while DC0±20 equalled to 57, 44, 64 and 65%, respectively.

Horizon boundaries and other observable structural features related to soil tillage and structural development appeared to be

very important for the initiation of preferential ¯ow. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Tillage; Preferential ¯ow; Brilliant Blue; Dye tracing; Field experiments

1. Introduction

Preferential ¯ow phenomena, particularly macro-

pore ¯ow, can lead to rapid transport of water and

surface applied chemicals to deep subsoil layers, and

may thereby increase the leaching losses of a broad

variety of chemicals used in agriculture (e.g. Flury,

Soil & Tillage Research 58 (2001) 81±89

* Corresponding author. Present address: Department of Agri-

cultural Sciences, Laboratory of Agrohydrology and Bioclimato-

logy, The Royal Veterinary and Agricultural University, Agrovej

10, DK-2630 Taastrup, Denmark. Tel.: �45-3528-3389;

fax: �45-3528-3384.

E-mail address: cpe@kvl.dk (C.T. Petersen).

0167-1987/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 8 6 - 0

1996; Hall and Mumma, 1994; Kladivko et al., 1991).

Tillage processes affect soil structure and hydraulic

properties in the topsoil which are expected to be

important for the generation of preferential ¯ow (e.g.

Ritsema et al., 1998; Staricka et al., 1991; Horton et al.,

1989; Beven and Germann, 1982). Thus, systematic

effects of tillage on ¯ow patterns and chemical trans-

port are expected. Most studies on the effect of tillage

on preferential ¯ow and chemical transport have been

focussed on the two extremes of primary tillage, viz.

mouldboard plough and no-till management systems.

The results of this research generally suggest that no-

till enhances the proportion of water transported pre-

ferentially (e.g. Harris et al., 1993; Shipitalo et al.,

2000). There are very limited data on the effect of

secondary tillage on preferential ¯ow.

Direct quanti®cation of preferential ¯ow under ®eld

conditions, particularly macropore ¯ow, is very dif®-

cult, not only because of spatial variability but also

because of the temporal variability which is often

involved in the ¯ow processes (Demuth and Hiltpold,

1993). Staining techniques as discussed by Flury and

FluÈhler (1995) and applied, e.g. by Ehlers (1975),

Flury et al. (1994), and Petersen et al. (1997a) do

not, in a strict sense, quantify ¯ow. These techniques

can, however, be used to visualize the primary ¯ow

pathways of water and solutes with high spatial reso-

lution. Dye tracing can, therefore, contribute signi®-

cantly to an understanding of preferential ¯ow and

transport mechanisms in soil and to the design of

ef®cient schemes for soil sampling in solute transport

studies. For tracing water, the applied dye should

preferably be relatively mobile and distinctly visible.

The present work is based on the use of dye tracing

in ®eld experiments. The purpose was to investigate

effects of four different seedbed preparation methods

for winter wheat on preferential ¯ow patterns appear-

ing in the upper 1 m layer of a sandy loam soil.

2. Materials and methods

2.1. Soil

The soil considered in this study is situated 20 km

west of Copenhagen on the experimental farm Rùr-

rendegaard belonging to the Royal Veterinary and

Agricultural University. The soil is developed on

moraine deposits from the Weichselian Glaciation,

and is classi®ed as an Agrudalf. It bears the impress

of long-term cultivation, with an agric horizon located

directly below the plow layer at 25±35 cm. Aggregates

with clay skins have been developed in the argillic Bt

horizon. The depth to the C horizon (containing

calcite) is 110 cm on an average. The Bt horizon

exhibits a well-developed structure with angular to

columnar aggregates. The upper horizons are charac-

terized by weak, subangular aggregates, tending

towards a weak platy structure in the agric horizon.

Biopores are dominated by root and vertically oriented

earthworm channels transecting the aggregates. The

root channels are generally smaller than 2 mm in

diameter whereas the worm channels are between 3

and 8 mm in diameter, extending to 20 mm in the

worm cavities. Rough estimates of the number of

earthworm channels have been made on large lumps

of soil taken from the pro®le. Common numbers found

in the Bt horizon at 50 cm depth are 2±6 channels per

100 cm2 horizontal cross section. There are coatings

of clay- and humus-rich particles on the pore walls.

Even some coarse sand particles have been transported

via the pores as evidenced by the presence of such

particles within the pores laid bare above the pre-

viously mentioned coatings on the pore walls. The

number of open earthworm channels is relatively low

right below the Ap horizon (0±25 cm). The number of

channels decreases rapidly from about 90 cm depth

even though some channels penetrated below 130 cm

depth. The soil is easily wettable at the moisture

contents investigated in the present study. Some pro-

®le characteristics are given in Table 1.

2.2. Tillage treatments

Seedbed preparation and drilling for winter wheat

were performed under near-optimal soil moisture

conditions. There were four treatments: (T1) Ð two

harrowings with a springtine harrow (4±6 cm depth),

drilling; (T2) Ð direct drilling (drill with roller

shares); (T3) Ð ploughing (25±28 cm depth), light

subsurface compaction, one intense harrowing with a

PTO-driven rotary harrow (5 cm depth), drilling; (T4)

Ð ploughing (25±28 cm depth), one harrowing with a

springtine harrow (4±6 cm depth), drilling. The treat-

ments were applied to four texturally uniform, level

®eld plots. Straw, except stubbles, from a previous

82 C.T. Petersen et al. / Soil & Tillage Research 58 (2001) 81±89

winter wheat crop was removed. T3 resulted in a more

homogeneous seedbed structure and even soil surface

as compared with T4. Traf®c was avoided after sow-

ing. The four treatments represent the range of meth-

ods used in agriculture in Denmark fairly well.

2.3. Dye tracer and dye application

The dye tracer used to stain ¯ow paths of water was

Brilliant Blue FCF. Tracer characteristics of Brilliant

Blue FCF are given by Flury and FluÈhler (1994, 1995).

Depending on pH, the dye tracer is either neutral

or it dissociates to a mono- or bivalent anion

�pKa � 5:83 and 6:58�. Thus, the tracer should gen-

erally be anionic, i.e. adsorb weakly on the soil in this

study. Recent ®ndings (Ketelsen and Meyer-Windel,

1999) suggest that Brilliant Blue FCF may adsorb

somewhat more strongly than expected from its

charge, in particular to the clay fraction of the soil.

The dye was applied in solution at a concentration

of 4.0 g lÿ1 to 1:6� 1:6 m2 test plots randomly

located within each ®eld plot using an automated

sprinkling apparatus similar in principle to the ones

described by Ghodrati et al. (1990) and Flury et al.

(1994). The device consists of a motor-driven spray

bar with nozzles (type 4110-10 from Hardi Interna-

tional A/S) aligned for one-dimensional application of

the solution direct under the bar, a constant pressure

regulator, and a suction pump connected to tanks

containing the dye solution. Twenty-®ve millimetres

of dye solution was applied within 1 h in all experi-

ments. This was achieved by letting a timer switch the

device on and off at equal time intervals of 2 min. The

device was designed to ensure a controlled and spa-

tially uniform distribution of dye tracer at the soil

surface. The uniformity of application depends on the

nozzle distance above the soil surface and on the

nozzle pressure. The best combination of these para-

meters was found in the laboratory. For an inner area

of 110� 110 cm2, it was possible to achieve a coef®-

cient of variation of the amount of applied solution of

5±6% in the direction perpendicular to the travel

direction, and 2±3% in the travel direction when

measuring the amount of irrigation water in

10� 10 cm2 trajectories placed side by side. In the

®eld, wind drift was reduced by a wooden frame

surrounding the treated area, and all vertical soil

pro®les (see below) were made parallel to the travel

direction of the bar. All test plots were covered with

tarpaulins in a wet situation 4 days before the ®rst dye

application in the autumn and in the spring in order to

establish and preserve an initial water content within

the pro®les close to the ®eld capacity.

2.4. Sampling

Sampling was performed in the autumn (October) 4

weeks after tillage and planting, and again in the

spring (late in April) 7 months after tillage and

planting. The rainfall received between planting and

Table 1

Dry bulk density, rb, initial moisture content just before dye application, yi, and textural composition (average values, each based on 10±15

single observations)

Property Sample depth and treatmenta

10±15 cmb 35±40 cm (all) 60±65 cm (all) 85±90 cm (all)

T1 T2 T3 T4

rb in autumn (103 kg mÿ3) 1.60 a 1.57 a 1.47 b 1.41 b ± ± ±

rb in spring (103 kg mÿ3) 1.58 a 1.58 a 1.45 b 1.49 b 1.66 1.65 1.69

yi in autumn (m3 mÿ3) 0.298 a,b 0.312 a 0.285 a,b 0.279 b 0.280 0.293 ±

yi in spring (m3 mÿ3) 0.325 0.317 0.304 0.308 0.275 0.283 ±

Clay, <0.2 mm (kg3 kgÿ3) 0.107 0.148 0.222 0.207

Silt, 0.2±50 mm (kg3 kgÿ3) 0.222 0.214 0.195 0.235

Sand, 50±2000 mm (kg3 kgÿ3) 0.671 0.638 0.582 0.558

a Treatments T1, T2, T3, and T4. Values for depths below 10±15 cm are based on 3±4 samples from each treatment. They represent all

treatments (all) since no differences were observed between treatments.b Values in the same line followed by different letters are signi®cantly different �P < 0:05�.

C.T. Petersen et al. / Soil & Tillage Research 58 (2001) 81±89 83

the autumn sampling was 52 mm whereas a total of

380 mm was received between planting and the spring

sampling. The plots were excavated to 130 cm depth

perpendicular to the direction of the last plowing 1 day

after dye application. An excavator was used to dig a

trench in front of the treated area and to remove all

loosened soil from the bottom of the excavation. Soil

below the treated area was loosened with hand spades

and vertical cross sections were prepared for detailed

description. The cross sections were trimmed with a

trowel and a knife. Special care had to be taken to

remove all deposits of dye coming from the excavation

process. The plots were excavated systematically in 11

parallel vertical cross sections separated 10 cm from

each other. Flow patterns and relevant structural fea-

tures appearing on the cross sections were described

and photographed. A 50 mm camera and Fujicolor

Superia 200 ®lm was used for all photos designed for

image analysis. A 100� 100 cm2 metal frame was

placed on the cross sections before taking these

photos. The frame was aligned with all previous frame

positions and with the soil surface. 100 cm3 soil

samples were taken during the excavation process

to determine dry bulk density. Initial moisture content

was measured using the TDR technique (Topp et al.,

1980).

2.5. Image analysis

All stained dye patterns on the photos appearing

within the metal frame as well as the corners of the

frame (approximate size on prints: 15� 15 cm2) were

transferred manually with a ®ne black pen to trans-

parent plastic sheets. The only distinction made in this

process was whether blue dye was visible or not at the

different parts of the photo. All transfers were made or

controlled by one person.

The new black or transparent representations of the

¯ow patterns were scanned with a resolution of

300 dpi (dots per inch). The resulting binary repre-

sentations were displayed on a computer screen. The

four corners of the metal frame with known coordi-

nates were marked on the screen after which the

program made a projective geometric transformation

of that part of the image being inside the frame to a

quadratic representation with 464 true horizontal rows

and 464 true vertical columns. For each row (i.e.

depth), the computer calculated (1) the fraction of

pixels, which were turned on, i.e. representing a

stained area, DC, and (2) the number of horizontal

transitions from pixels which were turned off to pixels

which were turned on, NP. DC and NP were averaged

for different depth increments within each soil pro®le.

The average width of ¯ow paths in the 0±20 cm layer

(PW0±20) was calculated from averaged DC and NP

values for that layer as DC0±20/NP0±20.

3. Results

Based on a visual inspection, the digitized and

geometrically corrected representations of ¯ow pat-

terns shown in Fig. 1 are fairly representative for the

four treatments, even though the within plot variability

was considerable. It appears that preferential ¯ow

paths penetrating deeply into the subsoil were found

for all treatments, both in the autumn and in the spring.

3.1. Flow pathways in the subsoil

Almost all the visualized, vertically oriented ¯ow

pathways in the subsoil below 30 cm depth were

channels with diameters between 0.5 and 8 mm.

Dye below 50 cm was only seen in close vicinity to

stained earthworm channels implying that earthworm

channels were the dominating, primary ¯ow pathways

in the deep subsoil. It was possible to follow these

continuous, gently bent channels over long distances

in the subsoil by careful excavation with a knife. Most

of the stained earthworm channels terminated 35±

40 cm below the soil surface, probably as a conse-

quence of tillage and traf®c, i.e. earthworm channels

were not the dominating pathways above 35 cm. Only

sections of the stained channels appeared on the true

vertical cross sections. Larger stained areas were

found at the lower ends of some of the channels, often

on stones or ped surfaces, indicating the appearance of

internal catchment. In general, the dye had moved less

than 1 mm into the walls of the stained earthworm

channels. However, during the excavation process,

traces of dye up to a few centimetres long were found

in ®ne root channels branching off on ped surfaces

from dyed earthworm channels. It was not possible to

see these minor traces on the true vertically cut pro®le

faces, generally, due to some inevitable smearing.

Traces of dye were found below 100 cm depth on

84 C.T. Petersen et al. / Soil & Tillage Research 58 (2001) 81±89

19, 20, 2, and 10 vertical pro®le faces, respectively,

out of the 22 pro®les excavated for each of the

treatments T1, T2, T3, and T4.

The number of stained vertical ¯ow paths, NP,

tended to be larger for T2 and smaller for T3 than

for all other treatments at all depths below 30±35 cm,

both in the autumn and in the spring (Fig. 2). Treat-

ment effects were most obvious in the upper part of the

subsoil above 60 cm depth but signi®cant differences

between treatments were found both at 30±40, 40±60,

60±80 and 80±100 cm depth (Table 2). NP was sig-

ni®cantly smaller for T3 than for the other treatments

Fig. 1. Digitized ¯ow patterns representing the four tillage treatments in the autumn (T1_a±T4_a) and in the spring (T1_s±T4_s).

Fig. 2. Depth pro®les of the number of ¯ow pathways in the 25±100 cm layer for the four treatments in the spring (T1_s±T4_s) and in the

autumn (T1_a±T4_a). Average� standard error for each 1.7 cm depth increment.

C.T. Petersen et al. / Soil & Tillage Research 58 (2001) 81±89 85

at 60±80 and at 80±100 cm depth, except for treatment

T4 at 80±100 cm depth in the autumn �P < 0:05�. NP

was larger for T2 than for all other treatments at all

depth intervals, except the 60±80 cm interval in the

autumn and the 80±100 cm interval in the spring

�P < 0:05�.NP decreased with depth below 40 cm depth, most

rapidly for T2 (Fig. 2). However, NP was a little

smaller below the topsoil±subsoil interface at 30±

35 cm depth than at 40 cm depth, except for T3 in

the autumn. On excavation in the ®eld of this soil layer

with relatively few visualized vertical pathways it was

dif®cult to ®nd but very few inclined, stained chan-

nels. However, a number of ®ne (<1.0 mm), stained

root channels became visible in this layer when break-

ing dried soil samples in the laboratory.

The number of stained ¯ow paths averaged for the

whole 30±100 cm layer, NP30±100, did not differ sig-

ni®cantly for any treatment between autumn and

spring �P > 0:05�. Furthermore, the Shapiro±Wilk

statistic (Shapiro and Wilk, 1965) calculated for all

the 22 NP30±100 values per treatment did not for any

treatment lead to a rejection of the hypothesis of

normality �P < 0:05�. Consequently, data from iden-

tical treatments from the autumn and the spring situa-

tion were considered as representing the same normal

distribution. Pairwise t-tests revealed that the within

treatment means of NP30±100 differed in the order

T2 > T1;T4 > T3 �P < 0:001� (Table 3).

3.2. Flow pathways in the topsoil and at the topsoil/

subsoil interface

In the uppermost part of the pro®les there were a

0.1±3 cm thick, completely dyed-in soil layer. Least

thickness of this layer was found after direct drilling

(0.1±0.5 cm), whether in the autumn or in the spring.

Below the completely dyed-in layer, the stained areas

split into more complicated patterns. The appearance

of these patterns differed considerably between treat-

ments, the number of independently stained ¯ow paths

being obviously largest in the direct drilled plot

(Fig. 1). The fractional area dyed in the 0±20 cm

layer, DC0±20, was smaller for the treatment T2 than

for all other treatments, except from T4 in the spring

situation �P < 0:001�. The average width of ¯ow

paths in the 0±20 cm topsoil layer, PW0±20, was sig-

ni®cantly smaller for T2 than for all other treatments,

both in the autumn and in the spring �P < 0:001�(Table 4). DC0±20 was larger for T1 and T2 in the

spring situation than in the autumn �P < 0:001�.Most patterns stopped or changed abruptly in shape

25±30 cm below the soil surface indicating a structural

interface created by the plow. Such an interface was

evident even for the not recently ploughed treatments

T1 and T2 (Fig. 1). Some more or less continuous

narrow horizontal pathways appearing at 3±6 cm

depth in the direct drilled plot in the spring (Fig. 1)

were probably generated by frost. Loose soil volumes

between structural elements left unbroken by the plow

and by secondary tillage appeared to be preferred ¯ow

pathways in ploughed plots in the autumn situation,

particularly soil volumes with stubble residues. In

general, however, preferential ¯ow took place in a

broad variety of pathways in the topsoil, including

®ssures and channels.

Table 2

Number of independently stained ¯ow paths per metre pro®le

length (NP) at 30±40, 40±60, 60±80, and 80±100 cm deptha

NP30±40 NP40±60 NP60±80 NP80±100

T1_a 3.4 a,b 2.4 a,b 1.8 a 0.9 a

T2_a 6.8 c 6.0 c 2.1 a 1.3 b

T3_a 2.2 b 1.4 a 0.6 b 0.1 c

T4_a 4.1 a 3.7 b 1.4 a 0.3 c

LSD0.95 1.3 1.4 0.8 0.3

T1_s 4.2 a 3.4 a 1.1 a 0.5 a

T2_s 7.5 b 5.6 b 2.2 b 0.7 a

T3_s 1.3 c 1.3 c 0.2 c 0.1 b

T4_s 2.6 d 2.8 a 1.1 a 0.4 a

LSD0.95 1.1 1.2 0.6 0.3

a Average values for the 11 pro®les per treatment in the autumn

(T1_a±T4_a) and in the spring (T1_s±T4_s). 95% level least

signi®cant difference, LSD0.95. Means with the same letter are not

signi®cantly different.

Table 3

Number of stained ¯ow paths per metre pro®le length averaged

over depth in the 30±100 cm layer, NP30±100

Treatment NP30±100a

Mean Std

T1 1.96 0.49

T2 3.51 0.99

T3 0.74 0.41

T4 1.83 0.85

a Mean and standard deviation (Std) for N � 22 pro®les per

treatment.

86 C.T. Petersen et al. / Soil & Tillage Research 58 (2001) 81±89

4. Discussion

Observed differences between autumn and spring in

DC0±20, DC0±20 being larger in the spring situation,

generally (Table 4) should be related to temporal

differences in topsoil structure brought about by

cycles of frost and thaw. However, NP found in the

subsoil for the different tillage treatments were similar

in the autumn and in the spring, suggesting that

temporal changes of soil structure were less important

for the activation of deeply penetrating preferential

¯ow paths than tillage itself. The rainfall (52 mm)

received between planting and sampling in the autumn

may have accelerated the temporal changes in soil

structure.

The existence of local minima for NP at 30±35 cm

depth corresponds with the observation that the num-

ber of open earthworm channels was low in this layer.

Since many deeply penetrating earthworm channels

below 35 cm are fed with dye solution there should

either be many preferential ¯ow pathways also in the

30±35 cm layer which do not appear in the ®eld, or

ef®cient pathways must be subdivided below 35 cm

depth. It is suggested that the relatively small stained

root channels observed in the laboratory at 30±35 cm

serve as distributors to the larger earthworm channels

in the deep subsoil. Upwards the root channels may be

connected with ®ssures created by the plow at the

topsoil/subsoil interface (Petersen et al., 1997b) or

they may be connected more directly with capillary or

non-capillary preferential ¯ow paths in the plow layer.

The signi®cance of preferential ¯ow in ®ne (capillary)

root channels as well as in capillary inter-aggregate

pores have previously been recognized (Othmer et al.,

1991).

The number of stained root channels at 30±35 cm

were not quanti®ed. It is likely that the number would

be larger after ploughless tillage, particularly with

direct drilling, due to better pore continuity to near

the soil surface. At 25±30 cm, Comia et al. (1994)

found larger volume of pores with equivalent pore

diameter >100 mm, larger saturated hydraulic con-

ductivity, and larger air permeability with ploughless

tillage than with conventional tillage including

ploughing to 25 cm. Our dif®culties in ®nding the

root channels in the ®eld may be ascribed to the weak

platy, partly destroyed soil structure right below the

plow layer.

Local minima for NP at about 30 cm depth were

also reported by Gjettermann et al. (1997) from dye

tracing experiments conducted one and a half year

after plowing at a similar soil type but only for high

application rates of the dye solution (50, 25, and

12.5 mm hÿ1), not for low rates (6.3 and 3.1 mm hÿ1).

They reported that for the application intensity of

3.1 mm hÿ1 all stained earthworm channels penetrat-

ing to more than 90 cm depth could be traced all the

way to the soil surface.

The most ef®cient reduction of NP was found with

the most intensive tillage treatment (T3). This is in

agreement with Petersen et al. (1997b) who in a tracer

study with Brilliant Blue FCF found that rotovation

given as supplement to traditional seedbed preparation

(plowing and harrowing with springtine harrow)

reduced the penetration of stained ¯ow paths into

the subsoil of a sandy loam soil.

Ghodrati and Jury (1990) investigated the effects of

disturbing the soil structure in the upper 30±40 cm

with a trencher on ¯ow patterns in the root zone. They

investigated a sandy soil without visible macropores

Table 4

Fractional area dyed, DC0±20 (%), and width of ¯ow paths, PW0±20 (%), averaged over depth in the 0±20 cm layera

Treatment DC0±20, autumn (%) DC0±20, spring (%) PW0±20, autumn (%) PW0±20, spring (%)

Mean Std Mean Std Mean Std Mean Std

T1 44 a 4 70 a 6 5.6 a 1 8.4 a 2.6

T2 32 b 4 55 b 12 2.6 b 0.5 5.0 b 1.5

T3 58 c 8 70 a 5 7.4 a,c 1.7 9.3 a 1.2

T4 69 d 7 60 a,b 7 9.2 c 1.9 9.2 a 2.4

a Mean and standard deviation (Std) for N � 11 pro®les per treatment. Means with the same letter are not signi®cantly different at the

0.1% level.

C.T. Petersen et al. / Soil & Tillage Research 58 (2001) 81±89 87

using the anionic dye tracer Acid-Red 1. The distur-

bance dramatically altered the ¯ow patterns. Both the

tendency of ®nger formation just below the treated soil

layer and the percentage of dye coverage in the subsoil

below 70±80 cm were increased. In our study, how-

ever, soil disturbance, particularly by T3, reduced the

deep penetration of dye patterns. The different results

may be explained by differences in soil type and soil

treatment, a homogenizing effect of soil disturbance

on soil structure being more likely in the present study.

Finger formation in the subsoil caused by the devel-

opment of unstable wetting fronts were not observed

in the present study. Furthermore, Ghodrati and Jury

(1990) applied 100 mm of irrigation water to their

plots compared with 25 mm in the present study. In a

study by Schwartz et al. (1999), tillage did not affect

the deep penetration and spreading in the root zone of

¯ow paths stained with Brilliant Blue FCF. However,

as in the present study, they found that soil disturbance

by tillage increased the fraction of stained soil in the

Ap horizon.

Any close relationship between the amount of dye

deposited in the subsoil at the surface of earthworm

channels, and the area and intensity of the blue ¯ow

patterns can hardly be expected because the dye was

concentrated within very small soil volumes and

because of geometrical and optical factors (e.g. sha-

dows cast over parts of the stained channel surfaces).

The dye application caused no noticeable ponding

at the soil surface. The crop establishment as esti-

mated in the autumn and in the spring was fully

acceptable for all treatments with 303±354 and

245±285 plants per m2, respectively.

5. Conclusions

Based on dye tracer experiments conducted at the

1 m scale on a structured sandy loam soil subjected to

different tillage operations, the following conclusions

can be drawn:

1. Preferential ¯ow was a prevailing phenomenon.

Earthworm channels were the dominating prefer-

ential ¯ow paths (PFPs) in the subsoil below

50 cm.

2. Tillage had a marked effect on ¯ow patterns. The

number of PFPs at 30±100 cm depth was larger

after direct drilling than after all other treatments,

including ploughless tillage with harrowing to 4±

6 cm depth �P < 0:001�. After ploughing, mode-

rate topsoil compaction followed by harrowing to

5 cm depth with a PTO-driven rotary harrow

reduced the number of PFPs at 30±100 cm depth

as compared with traditional harrowing with a

springtine harrow �P < 0:001�.3. A signi®cant number of deeply penetrating

preferential ¯ow pathways in terms of vertically

oriented channels were initiated at 30±40 cm

depth.

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