effects of uniaxial compaction on aeration and structure of ploughed or direct drilled soils

ELSEVIER Soil & Tillage Research 31 (1994) 135-148

509 &

Effects of uniaxial compaction on aeration and structure of ploughed or direct drilled soils

B.C. Bal l *'a, E . A . G . R o b e r t s o n b

aSoil Science Department, SAC, West Mains Road, Edinburgh EH9 3JG, UK bScottish Centre of Agricultural Engineering, SAC, Bush Estate, Penicuik EH25 OPH, UK

(Accepted 26 October 1993 )

Abstract

Cores of intact soil and of soil aggregates in the field condition were compacted uniaxially in the laboratory in a series of progressive increments of stress. Before compaction and after each increment of stress, air-filled porosity, relative diffusivity, diffusion time delay and air permeability were measured. Pore continuity and pore organisation indices were calculated from these measurements. Diffusion time delay is a measure of the time taken for gas to diffuse through a core from the start of a diffusion measurement.

During compaction relatively small changes in air-filled porosity were accompanied by large changes in relative diffusivity and air permeability. After the application of a given stress, these properties decreased less in direct drilled than in ploughed soil, particularly when wet. This was associated with the lower compactibility of the long-term direct drilled treatment as a result of improvement in soil structure associated with organic matter ac- cumulation. Stresses of only 77 kPa were sufficient to reduce permeability and relative diffusivity values to zero in wet, intact ploughed cores. Pore continuity was lower under direct drilling than under ploughing, but was less affected by compaction. The direct drilled soil probably contained more dead-end pores than the ploughed soil. This dead-end porosity was less affected by applied stress than pore continuity.

Keywords: Laboratory compaction; Field cores; Air permeability; Gas diffusivity; Pore continuity

1. Introduction

The influence o f compac t ive loads on soil bulk densi ty has been extensively invest igated in the labora tory using the uniaxial compress ion test. Such tests are

*Corresponding author.

0167-1987/94/$07.00 © 1994 Elsevier Science B,V. All rights reserved SSDI 0167-1987 ( 93 )00377-D

136 B.C. Ball, E.A.G. Robertson ~Soil& Tillage Research 31 (1994) 135-148

useful in the assessment of soil compactibility (I.arson et al., 1980). Although these tests are usually made on disturbed soil, their results have been extended to field compaction by modelling (Soane et al., 1980; Gupta and Allmaras, 1987 ). The results are usually given in terms of dry bulk density. However, Soane (1990) concluded that the results of compactibility tests would be better expressed in terms of porosity, hydraulic conductivity and gas diffusivity since these properties have a greater influence on soil productivity.

The influence of compaction on soil aeration properties (e.g. gas diffusivity and air permeability) has not been extensively studied because of the require- ment to test soils with their field structure intact. Usually, in such studies, laboratory measurements have been made on cores from compaction treatments applied in the field (e.g. Campbell et al., 1986; McAfee et al., 1989; Roseberg and McCoy, 1992). Roseberg and McCoy (1992) measured air permeability in samples equilibrated to matric potentials close to saturation and were thereby able to distinguish macropore and matrix air permeability and derive separate indices of pore geometry. In order to measure the effects of successive compaction treatments on gas movement in one sample, these treatments must be applied in the laboratory. Blackwell et al. (1990) applied uniaxial compaction to short, intact samples in the laboratory and recorded a decline in air permeability and in the volume of macropores. Kirby (1991) studied the effects of compression and shearing on reconstituted soil samples and further showed that changes of air permeability in soil with strong preferential orientation ofmacropores were dominated by the continuity of these macropores.

The increase in soil compaction resulting from a given stress is highly dependent on soil water content (Soane et al., 1980). In disturbed samples from long- term direct drilled soil, Ball et al. ( 1989 ) found that the influence of water content on compaction was moderated by enhanced organic matter contents. There has been little attempt so far to link differences in compactibility associated with organic matter and soil water content with influences on gas movement properties and soil structure. Dawidowski and Koolen (1987) deformed undisturbed samples in undrained triaxial tests and showed that air permeability dramatically decreased with deformation along with a decrease in soil matric potential. An- other study using disturbed samples indicated that air permeability after uniaxial compaction related more to differences in sample preparation than to tillage treatments, although the measurements provided some insight into the mechan- ics of compaction (O'Sullivan, 1992).

Here we tested the influence of compaction on gas movement and soil structure in long-term ploughed and direct drilled treatments on two soil types sampled on several occasions corresponding to times when field traffic could occur. The organic matter content was considerably greater in the direct drilled than in the ploughed soil and the water content differed markedly between sampling times. Samples were either intact with structure and water content in the field condition or were made of aggregates prepared by breaking up intact structures in order to simulate tillage. We applied uniaxial compaction as progressive increments and measured air permeability and gas diffusivity between each increment. Indices of

B.C. Ball, E.A. G. R obertson ~Soil & Tillage Research 31 (1994) 135-148 137

soil structure were also derived from gas diffusion and gas permeability measurements.

2. Materials and methods

Tests were made on cores of soil aggregates and on intact cores removed from the field with minimal disturbance. All samples were taken from a depth of 10- 60 mm and were retained within sampling cylinders (depth 50 mm, diameter 73 mm) during measurements of gas movement. The direct drilled and ploughed treatments, each on two soil types, were sampled within the South Road long- term field experiment located at Bush Estate near Penicuik. The experiment was described by Ball et al. (1989). The soil types are a eutric Cambisol of Macmerry series and a Gleysol which is Winton-Macmerry complex (FAO-UNESCO, 1974). The soils were originally described by Ragg and Futty ( 1967 ). Ploughing and drilling operations occurred in late August and early September each year. The direct drilling treatment had been applied each year since 1968. Particle size distributions and organic matter contents for the top 60 mm of soil are given in Table 1. Organic matter was estimated from organic carbon determined by wet oxidation on samples taken in December 1989.

Intact cores were taken in late September 1989, in early July 1990 and in late March 1991. On the first and third occasions, two plots only from each of the four tillage treatment × soil combinations were sampled but in 1990 four plots were sampled per combination. Intact cores were tested at field water content in order to represent the field situation. Thus, the water contents before compaction for the intact cores given in Table 2 represent the field condition at sampling. In 1989 and 1990 the soils were near field capacity (approximately 0.25 g g- 1 ), but in 1991 the soils were wetter than field capacity. Where necessary, soils were stored at 4 °C before testing.

Aggregates were sampled once only in late September 1989 at the same time as the first batch of intact cores. The aggregates were gently broken by hand at field water content and passed through a 10 mm and then a 5 mm sieve. This operation simulated tillage for seedbed preparation. Only the aggregates less than 5 mm in

Table 1 Particle size distribution and organic matter contents (g per 100 g) at a depth of 0-60 mm

Cambisol Gleysol

Coarse sand 2.0-0.2 mm 19 18 Fine sand 0.2-0.06 mm 31 29 Silt 0.06-0.002 mm 35 36 Clay < 0.002 mm 15 17 Organic matter Ploughed 6.4 5.4 Direct drilled 6.8 6.4

138 B.C. Ball, E.A.G. Robertson /Soil & Tillage Research 31 (1994) 135-148

Table 2 Gravimetric water contents and dry bulk densities of samples before compaction and after the greatest compaction. Least significant difference (LSD) values are given only for the second sampling when means were of four replicates. P and D refer to the ploughed and direct drilled treatments

Gravimetric water content Dry bulk density (gg-~) (Mgm -3

Before After Before After compaction greatest compaction greatest

stress stress

Aggregates 9/89 (First sampling) Cambisol P 0.229 0.203 0,97 1.56

D 0,303 0.272 0,90 1.40 Gleysol P 0.207 0.186 1.00 1.54

D 0.293 0.270 0.91 1.39 Intact 9/89 (First sampling) Cambisol P 0.261 0.233 1.13 1.38

D 0.250 0.243 1.26 1.41 Gleysol P 0.291 0.279 1.09 1.39

D 0.273 0.240 1.27 1.40 Intact 7/90 (Second sampling) Cambisol P 0.220 0.215 1.22 1.41

D 0.272 0.267 1.23 1.33 Gleysol P 0.230 0.226 1.32 1.48

D 0.264 0.259 1.28 1.36 LSD for stress 0.018 0.030 LSD for tillage X soil 0,029 0.072 Intact 3/91 (Third sampling) Cambisol P 0.279 0.269 1.15 1.44

D 0.339 0.328 1.18 1.31 Gleysol P 0.300 0.289 1.26 1.42

D 0.344 0.322 1.23 1.37

diameter were used because these were considered to be small enough to give a reasonably homogeneous structure. This aggregate fraction was about half the total weight of soil and was spread over a sand suction table and equilibrated at - 6 kPa for 1 week. The aggregates were then packed loosely by hand in a sam- piing cylinder to a dry bulk density of about 1 Mg m - a. One core per tillage treatment × soil combination was prepared.

Gas diffusion and permeability were first measured in the uncompacted cores. Cores were then compacted uniaxially by applying a range of static loads in an oedometer with a piston which was a close fit inside the sampling cylinders. The piston face consisted of porous ceramic which was permeable to water. The loads were applied progressively over about 2 s with the piston contacting the upper surface of the core. For the aggregates, loads were chosen to give nominal stresses of 25, 77, 103, 155,258, 413 and 826 kPa which were applied sequentially. For the intact cores, loads of 77 and 155 kPa were applied. An additional initial load of 39 kPa was applied to most of these cores sampled on the third occasion be-

B.C. Ball, E.A. G. Robertson / Soil & Tillage Research 31 (1994) 135-148 139

cause they were considered to be highly compactible. Gas diffusion and permeability measurements were repeated after the application of each stress. Each stress was applied for about 5 min, until the rate of change of sample length was small (typical strain rate less than 10 -4 mm s - l ) . After each stress application, the load was removed and the cores were allowed to relax. A relaxation period of at least 10 min was allowed before starting to measure gas diffusion.

Relative gas diffusivity and air permeability were measured using the methods of Ball et al. ( 1981 ). From these measurements we also calculated indices of pore continuity and pore organisation. Pore continuity is the quotient of relative diffusivity divided by air-filled porosity (Ball et al., 1988 ). Pore organisation is the quotient of air permeability divided by macroporosity (Blackwell et al., 1990). This index is a measure of permeability per unit volume and, in the calculation, air permeability and air-filled porosity represent intrinsic permeability and macroporosity. Blackwell et al. (1990) proposed that these two correspond to air permeability and air-filled porosity at - 6 kPa. The diffusion time delay was also measured. This is the time taken for gas initially to diffuse across a sample and is estimated by making frequent measurements of gas concentration at the start of a diffusion measurement.

A functional model of porosity has been proposed which assumes that pores are divided between arterial, peripheral and isolated pores (Arah and Ball, 1994). Arterial pores are continuous, whereas peripheral pores are mainly dead-ends. Relative diffusivity is assumed to be influenced by the arterial pores, whereas diffusion time delay is influenced by both arterial and peripheral porosity. Iso- lated pores have no influence on diffusion. The pore continuity index allows the influence of air-filled porosity on relative diffusivity to be accounted for so that within this model the pore continuity index relates to the arterial porosity. When the diffusion time delay is plotted against the pore continuity index, changes in diffusion time delay independent of those in pore continuity indicate changes in the content of peripheral or dead-end porosity.

3. Results

Data for aggregates are for individual cores, data for intact cores are means over replicate plots. The gravimetric water contents and dry bulk densities of the cores before compaction and after application of the greatest stress are given in Table 2. Least significant differences are attached to data for the second sampling of intact cores only, since these were replicated fourfold. On the other dates, sample replication was twofold which was considered to be inadequate for satisfac- tory analysis of variance. At the second sampling, differences in dry bulk densities between tillage treatments and applied stresses were significant (P< 0.05 and P < 0.00 l, respectively). The interaction between tillage and applied stress on dry bulk densities was highly significant (P<0.001) . This interaction shows that the direct drilled soil compacted less than the ploughed soil over the same range of applied stresses, even though the direct drilled soil was wetter. The samples dried

140 B.C. Ball, E.A.G. Robertson I Soil & Tillage Research 31 (1994) 135-148

0.5

0.4-

E 0 . 3

8 0.2.

0.1,

0

0.2"

0.15-

~ 0.1

0.05

0-

I000- I

100 ~

~ to-

t

0.1

0.01

--D--CP CD

-.~--GP .,-.z'-/'/-- . . . . . . . . . . . . . . . ~ GD

• . t k ..... 1() 100 I000

Applied stress (kPa)

~.~.,'" ";,, '%,..,

10 100 1000 Applied stress (kPa)

~--/~ . . . . . . . . ..~

1'0 to00 Applied stress (kPa)

Fig. 1. Effect of applied stress on air-filled porosity, relative diffusivity and air permeability in individual aggregate cores. In this and in subsequent figures, C and G refer to Cambisol and Gleysol and P and D refer to the ploughed and direct drilled treatments.

by between 0.01 and 0.03 g g- 1 during the tests as a result of evaporation and surface losses during handling between testing.

Air-filled porosities, relative gas diffusivities and air permeabilities versus applied stress are given in Fig. l for the aggregates and in Figs. 2-4 for the intact cores. In the aggregates, relative diffusivities and air permeabilities decreased with increasing stress beyond the first increment, such that the rank ordering of the treatment and soil combinations was generally preserved. Most relative diffusiv-

B.C. Ball, E.A. G. Robertson ~Soil& Tillage Research 31 (1994) 135-148 141

0.3

E % 0.2

£

8 0.1 <

]" Intact cores : first sampling "'~ ~.--.. . . --[3--CP

-*z&--Gp GD

I I

go t;o t~o 2;o Applied stress (kPa)

second sampling

~E 0.2 ~

, I , ,I ]

0 50 1 O0 150 200


0.3 l third sampling

~E~E 0 2 1 " \ , \

~- 0.1

O I - , ~ , , 0 50 100 150 200


Fig. 2. Effect o f applied stress on air-filled porosity in intact cores sampled on three occasions (mea n s o f two or, for the second sampling, four replicates) . Error bars represent the typical values o f the range for two replicates or o f two standard errors for four replicates at each applied stress.

ities and air permeabilities were greater under ploughing than under direct drilling (Fig. 1 ). Further, the difference between ploughing and direct drilling was greater for the Gleysol than for the Cambisol.

In the intact cores, there was a reduction in air porosity (Fig. 2 ), relative diffusivity (Fig. 3) and air permeability (Fig. 4) with increase in applied stress, although the decreases were considerably smaller than those for the aggregates at corresponding stress increments. In the intact cores, most relative diffusivities

142 B.C. Ball, E.A.G. Robertson ~Soil& Tillage Research 31 (1994) 135-148

0.09

0.06

"~ 0 .03 tw

I I

• . Intact cores : first sampling

0 s'0

0.04

0.03 "'" " - .

4o 4o Applied stress (kPa)

second sampling

0.02

0.01

0.04

003 t " ,

0.02 \ ,

0.01

0

s; t;0 40 Applied stress (kPa)

third sampling

, I

, 1 - - t ~ t

50 t00 150 Applied stress (kPa)

--I3--CP -II- CD "'~"GP -'=k- GD

2oo

200

Fig. 3. Effect of applied stress on relative diffusivity in intact cores sampled on three occasions (means of two or, for the second sampling, four replicates). Error bars represent the typical values of the range for two replicates or of two standard errors for four replicates at each applied stress.

were greater in ploughed than in direct drilled soil. However, ploughed soil tended to compact more than direct drilled soil (Fig. 2 ), particularly where the core was initially loose (e.g. at the first sampling) or wet (e.g. the Gleysol at the third sampling). At the third sampling, application of 77 kPa stress caused water to be expelled from most of the intact Gleysol ploughed cores. Such consolidation oc- curs when air-filled porosity is at or near zero, resulting in very low or zero diffusivities and permeabilities (Figs. 3 and 4). Consolidation prevented further


8.

E

1000

100

I000-

100

tO

Intact cores : first sampl ing

" ' - . . . . . . .

s; t;o ,;o Applied stress (kPa)

second sampl ing

t 000

I I I

0 100 150 Applied stress (kPa)

third sampl ing

1 0 0

t | "'lJ_ ~-

l 0 . '~" , ~,~

0 5O 100 150 Applied stress (kPa)

--E]--CP - I I - CD --A-" GP

GD

200

200

200

Fig. 4. Effect of applied stress on air permeability in intact cores sampled on three occasions (means of two or, for the second sampling, four replicates). Error bars represent the typical values of the range for two replicates or of two standard errors for four replicates at each applied stress.

compaction of these samples. The decreases in air-filled porosity, relative diffusivity and air permeability during applications of stress are likely to involve a combination of water redistribution due to decreased matric potential and physical rearrangement of the soil structure.

Diffusion time delays and pore continuity indices are shown in Fig. 5. For the aggregates, since differences between soil types were small in comparison with those between tillage treatments, treatment means are shown in Fig. 5 (A). For

144 B.C. Ball, E.A. G. Robertson / Soil & Tillage Research 31 (1994) 135-148

1000.

500.

250.

._8 100-

=_ D

50.

(A) Aggregate cores

0'1 0'2 0:3 014 Pore continuity

"O-D "'O"P

015

==

500 -

2 5 0 -

100

(B) Intact cores

--[3--Cp • -m- CD --A-.GP

155 "dr GD

0 155 "- I = [ 3 - " ' - 7 7 155 ~ ' , . . " 77 T~|

" - , 00 77 - "j" - . -_ *

"0 0

065 0'1 0~5 Pore continuity

Fig. 5. Diffusion time delay and pore continuity index in (A) aggregate cores (means of both soil types) and (B) intact cores for the second sampling (means of four replicates). The arrows indicate the direction of increasing stress. The stresses applied to the aggregates are given in the text. The stresses (kPa) applied to the intact cores are shown near the points in (B). Error bars (2 SE) represent typical values.

the intact cores, means are given for the second sampling occasion only, when replication was fourfold and where diffusion t ime delays were recorded for all samples at each applied stress. As compact ion increased, the pore cont inuity index generally decreased and the diffusion t ime delay increased. Pore organisa- t ion, plotted against air-filled porosity in Fig. 6, decreased with increasing stress. Dif fus ion t ime delays and pore cont inuity and organisation indices for the intact cores averaged over the three sampling dates and load are given in Table 3.


[3-

1000-

100 -

10-

--O--CP - - I - CD -'A-- GP

GD --<>--Aggregates

~ . ~ /1x0 - ' ~ . . - Intact Aggregates " ~ .... <>o

/ 25.. . . "

7 7 ~ ~ " " 7 / . . ~ / / ~ r 7 ." " lOa . . . . . . . . . . o

"~,,.. r7 ../1 . ~

/ .-<>

K = 10~ tm 2 ls~t . .<> 413 .-- -"

." K a = 11.tm 2

~ 8 2 6

011 012 0'.3 014 Air porosity (m 3 m 3)

Fig. 6. Effect of applied stress on pore structure as revealed by the pore organisation vs. air-filled porosity characteristic. The numbers correspond to the applied stresses (kPa). Air permeability (Ka) isolines are shown.

Table 3 Diffusion time delays, pore continuity and pore organisation indices in intact soil cores, averaged over applied stresses and sampling dates

Time delay Pore coninuity Pore organisation (s) (/~m 2)

Cambisol Ploughed 153 0.121 66 Direct drilled 275 0.066 165

Gleysol Ploughed 130 0.185 205 Direct drilled 389 0.051 111

4. Discussion

4.1. Sampling time and water content

The time since tillage at the first sampling was 1 month and at the second sampling was 7 months. On the first occasion the ploughed soil was less dense (Table 2 ) than the direct drilled soil before applying stress, but on the second occasion,

146 B. C. Ball, E.A, G. Robertson / Soil & Tillage Research 31 (1994) 135-148

the ploughed soil had become more dense, such that there was little difference between tillage treatments. Since the soil received no vehicle traffic between samplings, the increase in bulk density would have resulted mainly from weathering. Differences in relative diffusivity between these samplings (Fig. 3 ) roughly match the differences in bulk density (Table 2). However, air permeability seemed to be less well related to bulk density. The apparent increase in Cambisol direct drilled permeability between samplings is probably a result both of the death and decomposition of roots and of the large variability between samples on any one occasion. At the third sampling the high soil water content dominated the response of the samples to compaction. This shows the importance of soil water content or potential at stress application in determining soil responses. In order to detect a time trend through the season, samples would need to be equilibrated to the same matric potential before applying stress.

4.2. Aggregates and intact cores

The cores of aggregates compacted more than the intact cores. The maximum density reached was greater under ploughing than under direct drilling where it was about the same as in the intact cores. However, much greater stresses had to be applied to the aggregates to achieve these densities. O'Sullivan (1992) also reported that very high stresses had to be applied to aggregates obtained from direct drilled soils to reproduce field bulk densities.

At corresponding stresses, relative diffusivities and most air-filled porosities were greater in the cores of aggregates than in the intact cores. The cores of aggregates still conducted gases after application of the greatest stress, whereas some intact cores were blocked to gas movement after the application of relatively small stresses. At corresponding stresses, diffusion time delays were smaller and pore continuities were greater in the aggregates than in the intact cores. This indicates the marked difference in the structure of the air-filled pores between sample types. On the pore organisation versus air porosity characteristic (Fig. 6 ), values for the aggregates were to the right of those for the intact cores. Values for the Cambisol ploughed intact cores were closest to those for aggregates. The likely pore structural properties can be interpreted from the relative positions of these points (BlackweU et al., 1990). Thus, the aggregates probably contain fewer channels and fissures and more vughs and packing pores than most of the intact cores. In the cores of aggregates, the sharp increase in diffusion time delay (Fig. 5 (A)) in response to application of the highest stresses may indicate a major structural change or that the cores were close to pore water saturation.

4.3. Tillage and soil structure

The difference in compactibility between tillage treatments shown by bulk densities (Table 2 ) is also revealed by gas diffusivities. The greater compactibility of the ploughed soil was attributed by Ball et al. ( 1989 ) to low initial bulk density and o~anic matter content. Using aggregates from similar soils, O'Sullivan (1992)

B. C Ball, E.A. G. Robertson ~Soil& Tillage Research 31 (1994) 135-148 147

reported smaller increases in specific volume under direct drilling than under ploughing which he attributed to the greater structural stability in the direct drilled soil as a result of greater organic matter content. In addition, the strength of undisturbed soils increases with time (Davies, 1985 ), which also increases soil re- sistance to compaction. Roseberg and McCoy (1992) found that wheel traffic under conventional tillage affected porosity and air-flow features more than under direct drilling, indicating that the pore geometry was better preserved after wheeling the direct drilled soil. Thus, the small pore continuity indices of the direct drilled soil may indicate structure resistant to compaction.

Under direct drilling, the Gleysol was the more compactible of the two soils, yielding, in general, smaller permeabilities and diffusivities than the Cambisol. In addition, the Gleysol was more poorly structured, having longer diffusion time delays and smaller pore continuity and organisation indices than the Cambisol (Table 3). The difference between soils is likely to be associated with organic matter content (Table 1 ). The high diffusion time delays (Fig. 5) in the direct drilled Gleysol indicate a large content of dead-end porosity (Arah and Ball, 1994). Earlier independent observations revealed that this direct drilled soil had a laminar structure where vertical root and worm channels ran through horizontal laminae (Ball and Robertson, 1994 ). The samples were taken vertically. Thus the vertical channels aligned with the core main axis and functioned as arterial pores. The pores between the laminae were mostly horizontal (i.e. normal to the direction of flow of gas within the core) and were thus likely to act as peripheral or dead-end porosity.

5. Conclusions

( 1 ) Relatively small changes in air-filled porosity after stress application were accompanied by large changes in relative diffusivity and air permeability. The extent of these changes was highly dependent on the water content at stress application. Cores of aggregates produced by simulated tillage compacted more than intact cores but continued to conduct gases after the application of very high stresses.

(2) Although air permeability and relative diffusivity were smaller in long- term direct drilled than in ploughed soil, the reduced compactibility of the direct drilled soil was shown by the smaller decreases in air permeability and relative diffusivity in response to applied stress than in ploughed soil, particularly under wet conditions.

( 3 ) At a given stress, functional arterial porosity (pore continuity ) was greater in the ploughed than in the direct drilled soil. Peripheral or dead-end porosity, as indicated by diffusion time delay, was generally greater in direct drilled soil than in ploughed soil and was relatively less affected by applied stress than pore continuity.

148 B. C Ball, E.A. G. Robertson ~Soil& Tillage Research 31 (1994) 135-148

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effects of uniaxial compaction on aeration and structure of ploughed or direct drilled soils

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