Modification of porespace by tillage in two stagnogley soils with contrasting management histories

Soil & Tillage Research, 10 (1987) 303-317

303

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Modification of Porespace by Tillage in Two Stagnogley Soils with Contrasting Management Histories J.T. DOUGLAS":' and M.J. GOSS 2

Agricultural and Food Research Council Letcombe Laboratory, Wantage, Oxfordshire, 0X12 9JT (Gt. Britain) (Accepted for publication 5 June 1987)

ABSTRACT Douglas, J.T. and Goss, M.J., 1987. Modification of porespace by tillage in two stagnogley soils with contrasting management histories. Soil Tillage Res., 10:303-317. The soil porespace was studied in two long-term tillage experiments on two clayey stagnogleys in Southern England. The soils differed in respect of mineral and organic composition and previous management history. In both soils the total volume of pores and the volume fraction of macropores in the topsoil horizon declined with direct drilling compared with annual ploughing. This difference between tillage treatments appeared to develop more slowly in the soil that was formerly under continuous arable cultivation than in the soil that was previously in long-term grassland. Fluid transport coefficients were greater in ploughed topsoil in both soils; however, at the boundaries between topsoil and subsoil, and in the upper subsoil, permeability and gaseous diffusivity were greater after direct drilling. At a long-term arable site, soil was more consolidated below the depth of ploughing or shallow tillage, whereas in a former grassland soil ploughing disrupted the continuity of channel-type macropores.

INTRODUCTION

Simplified forms of tillage result in some stratification of topsoil structure dependent on the depth of implement action (Douglas et al., 1986). However, the type of soil structure produced by any cultivation system will be influenced by previous management ( Low, 1972). When arable soils cease to be tilled, the pore volume and the volume of 'Present address: Scottish Centre of Agricultural Engineering, Bush Estate, Penicuik, Midlothian, EH26 0PH, Gt. Britain. '-'Present address: Agricultural and Food Research Council Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Hefts, AL5 2JQ, Gt. Britain. :~Author to whom correspondence should be addressed.

0167-1987/87/$03.50

© 1987 Elsevier Science Publishers B.V.

304 TABLE I Details of the two clay soils

Site location Previous agricultural use Soil group Particle-size distribution ~, (%, w/w) Clay Silt Sand Organic carbon content ~'2 (%, w/w) Wetness class:~

Lawford series

Denchworth series

Northfield, by Baulking, Oxfordshire Arable (ploughed) Stagnogley

Compton Beaucharnp, Oxfordshire Grassland for more than 15 years Stagnogley

39 (37) 40 ( 40 ) 21 (23) 2.6 III

49 (49) 47 (47) 4 (4) 4.4 III

~Topsoil layer. Values in parentheses are for the upper subsoil layer (Bgl). ~From Stengel et al. (1984). :~Hodgson (1976) macropores in the topsoil layer generally diminish (Douglas et al., 1980; Ball, 1981; O'Sullivan and Ball, 1982; de Jong et al., 1983; Douglas et al., 1986). Heavy clay soils generally have large total porosities, b u t only a small volume of macropores. In clay subsoils macroporosity volume may be less than 5% and under-drainage is required for maximising crop production (Ellis et al., 1984). We report here on the nature and rate of soil changes that result from simplified tillage ( direct drilling or shallow-tine cultivation) and from traditional mouldboard ploughing on two stagnogley soils where previous land use had differed. MATERIALS AND METHODS

Sites The experiments were conducted at 2 sites in the Vale of White Horse, Oxfordshire: one at Northfield farm near Baulking on Lawford series, the other at Compton Beauchamp on Denchworth series. Both are non-calcareous clay soils. The Agricultural and Food Research Council's Letcombe Laboratory started the experiments in 1974, and a full description of the soils, their management and cropping sequence over the first 4 seasons were given by Cannell et al. (1980). Details of the sites and soils relevant to this study are summarised in Table I. The site at Northfield had a long history of arable cultivation, but that at Compton Beauchamp was old grassland.

305

Treatments Crop residues were first burned. At both sites the following treatments were applied: ( i ) Conventional tillage (P) by a mouldboard plough to about 20-cm depth followed by appropriate secondary tillage to produce a seedbed. A zig-zag harrow was used to level the soil after drilling. (ii) Direct drilling (DD) with a triple-disc drill without prior soil disturbance. The drill was followed by a zig-zag harrow (1 or 2 passes ). Together these moved much of the upper 3 cm of soil each year. At North field a third t r e a t m e n t was also applied: (iii) Shallow-tine cultivation (ST) comprising 2 passes of a coil-tine cultivator to a final depth of 5-8 cm, followed by appropriate secondary tillage to produce a seedbed. A zig-zag harrow was also used after drilling. Pore space, bulk density and water and gas transport properties The volumes of solid, liquid and gas phases were measured on intact core samples taken into the laboratory; various fluid transport coefficients were measured in the cores, as well as from measurements in the field, with tensiometers inserted in two microplot areas (2.5 × 2.5 m), on 2 plots of each treatment. The core sizes were either 7.6-cm diameter by 5-cm thick (henceforth referred to as "small" ) or 15-cm diameter by 10-cm thick ( "large" ). Collecting and retaining rings, constructed of steel (small) or PVC (large), were inserted by impact and by jack pressure (Fig. 1), respectively. Details of the number and size of cores taken in each experimental year are summarised in Table II. TABLE II Details of sampling Site

Experimentalyear 74-75 (1)

Northfield Occasion Depth range studied (cm) Core size (sample replicates) Compton Beauchamp Occasion Depth studied (cm) Core size (sample replicates)

75-76 (2)

76-77 (3)

77-78 (4)

78-79 (5)

79-80 (6)

autumn autumn a u t u m n

winter

winter; summer

winter

0-15

0-15

0-5,0-15,

0-40,0-30

15-30,0-40

5 25

S(20)

S(20)

8(20),S(20)

S(4),L(2)

S(8),L(8),S(4)

S(4)

autumn a u t u m n autumn 0-15 0-15 0-15

spring 0-40,15 30

S(20)

S(4),L(16)

8(20)

8(20)

Core size: S = small; L = large; see text for dimensions.

80 81 (7)

306

/ h clear I

watertape

PVC

constant head device coars~

suppc

Fig. 1. Techniques for collection of large soil cores (above) and measurement of saturated hydraulic conductivity (below) ; p o r t s 'A' are for removal of entrapped bubbles and h = head of water.

Prior to the collection of cores we identified particular discontinuities in the soil profile: between superficially-tilled and untilled topsoil, and between tilled layers and "undisturbed" subsoil. Cores were collected specifically either to include or to avoid such boundaries. In 1980 cores were collected also from low-lying areas on direct-drilled plots where crop growth was relatively poor; these areas appeared to be the furrows

307 of a former "ridge and furrow" system, presumably established during an earlier period of arable cultivation. The pore-size distribution was estimated from the cores by measuring the volume of water drained from saturation to selected water potentials, using silica-flour tension tables. The dry bulk density was obtained on oven-dry samples. Relative diffusivity of krypton-85 and air permeability were measured by the techniques of Ball et al., (1981) in apparatus adapted to accommodate both large and small cores. In this technique, a small volume of SSKr in air was injected into one sealed gas chamber, and diffused through the core sample to a receiving chamber. The count rate of emitted beta radiation is proportional to the concentration of 85Kr in each chamber. From measurements of differences in count rates between chambers the diffusion coefficient of SSKr was calculated by Fick's first law. Relative diffusivity is the ratio of diffusion coefficients of SSKr in the soil sample (D) to that in free air ( Do ). The permeability of the cores was measured by piping air through the diffusion apparatus at a known pressure difference across the sample. Pressure was measured by an electronic differential manometer, and the rate of flow was measured using soap-film tube meters; the air permeability (k) was calculated from Darcy's Law. The saturated hydraulic conductivity of the large cores was measured by the method illustrated in Fig. 1. Within each horizon features of soil structure, such as earthworm channels, increased variability (Douglas, 1986 ), but cores containing these features were tested and data included in analyses and interpretation.

RESULTS

Old arable site at Northfield (Lawford series)

Years I and 2 (experimental years 1 and 2, 1974-75 and 1975-76) At the time of sowing in Years 1 and 2 there were no significant differences between treatments in bulk density and water content in the upper 15 cm ( Table III). The soil was drier, less swollen, and therefore more dense in the second year (autumn 1975).

Year 3 (1976-77) At the time of sowing, the 0-5 cm layer of the direct-drilled soil was denser at the same depth than after shallow cultivation or ploughing. One month after sowing the top 5 cm was still denser than the ploughed soil. In the 5-15-cm layer differences were not significant (Table III).

308 T A B L E III Bulk d e n s i t y a n d w a t e r c o n t e n t at, or n e a r to, t h e t i m e of sowing in t h e L a w f o r d series, 1974-77 ( Years 1-4 ) Year

Depth (cm)

Bulk d e n s i t y (g c m - 3 ) DD

ST

P

se

Significance o f difference {P )

W a t e r c o n t e n t (g g - ' ) DD

ST

P

se

1974

0- 5 5-10 10-15

0.87 1.10 1.15

0.86 1.10 1.14

0.87 1.01 1.10

0.029 0.029 0.019

-

0.30 0.38 0.40

0.30 0.39 0.39

0.28 0.38 0.39

0.013 0.012 0.013

1975

0- 5 5-10 10-15

1.07 1.21 1.30

1.07 1.20 1.26

1.06 1.19 1.23

0.024 0.038 0.034

-

0.23 0.28 0.28

0.21 0.26 0.27

0.21 0.26 0.27

0.012 0.011 0.011

1976 (i) (ii) 1

0- 5 0- 5 5-10 10-15

1.03 1.08 1.19 1.23

0.92 -

0.92 1.03 1.15 1.19

0.019

<0.01 <0.05 -

0.38 0.39 0.36 0.36

0.39 -

0.39 0.39 0.38 0.38

0.010

0- 5

1.02

0.92

0.87

0.022

< 0.001

0.33

0.34

0.28

0.008

1977

'4 weeks after sowing.

Year 4 (1977-78) At the time of sowing the upper 5 cm of soil on the direct-drilled plots was denser than either of the tilled soils (Table III). Saturated hydraulic conductivity was greater, and bulk density less, in the upper 30 cm of ploughed soil than in direct-drilled soils (Table IV). In the winter, total porosity and the volumes of macropores of diameter > 50/~m were greater in the tilled layer of the ploughed soil than in the equivalent topsoil horizon of the direct-drilled land (Fig. 2 ). Below approximately 20-25 cm the pore volumes and bulk densities were similar in the 2 treatments. T A B L E IV Hydraulic c o n d u c t i v i t y (Ksat) a n d bulk d e n s i t y ( B D ) o f large s a t u r a t e d cores from L a w f o r d series, N o v e m b e r 1977 (Year 4) Depth (cm)

0-10 10-20 20-30

BD ( g c m -:~)

Ks~t (10-4 m S- t )

DD

P

DD

P

0.80 __0.227 0.38 +__0.050 0.06 __+0.054

1.13 __ 0.013 1.04 ____0.042 _ 1.06 __ 0.096

1.22 4- 0.066 1.25 4- 0.015 1.36 4- 0.006

0.95 +__0.01 1.01 ____0.001 _ 1.05 __ 0.039

309 0

0"05

A~r-filled porespace (v/v) 0.10

0-15

0.20

I-

'i

lO

I,

.c2o

3o

-iJa

Direct -drilled

Hi {-

P,o d ~

standard error

4O

Fig. 2. Volume of air-filled pores at - 6 kPa water potential in Lawford series, 1978.

Year 5 (1978-79) In January 1979 (Table V) the volume of pores > 50/lm diameter in small cores, collected from immediately above the boundary between topsoil and subsoil, was greatest in the ploughed soil and least in the shallow tine-cultivated soil, with direct-drilled soil intermediate; there was a similar trend in air permeability at - 6 kPa water potential. Values of pore volume and permeability (k) were less below the boundary than above it, with smallest values in the shallow-tilled soil. In large cores, which contained the topsoil/subsoil interface centrally, saturated hydraulic conductivity was least in the shallowtilled soil. TABLE V Bulk density, volume of pores > 50 pm diameter, air permeability (small cores) at - 6 kPa water potential, and saturated hydraulic conductivity (large cores) in the zone of the topsoil/subsoil boundary, Lawford series, January 1979 (Year 5) Parameters measured and sample position relative to boundary

Direct-drilled

Shallow-tine cultivated

Bulk density ( g c m -~) Above 1.22 ± 0.007 1.25 ± 0.017 Below 1.31 _+0.011 1.31 ± 0.026 Pore volume > 50 Bm (v/v) Above 0.07 ± 0.005 0.04 ± 0.004 Below 0.04 ± 0.004 0.03 ± 0.003 Permeability ~ (10 s cm 2) ± se log mean Above 35 ± 0.20 3 ± 0.25 Below 4 ±0.41 1 ±0.33 Saturated hydraulic conductivity ~ (107 m s - ~) ± se log mean Through 327 _+0.17 4 ±0.36 ~Geometric means (other values are arithmetic means).

Ploughed

1.10 ± 0.021 1.28 ± 0.019 0.11 ± 0.013 0.04 ± 0.005 230 3

± 0.07 ±0.23

172

±0.14

310 T A B L E VI

Air-filled porespace ( e, ), air permeability ( k ) and relative diffusivity ( D/Do) at water potential ( - 6 k P a ) in the upper 40 cm of Lawford series, June 1979 (Year 5 ) Depth

Direct-drilled

Shallow-tine cultivated

Ploughed

(cm)

O- 5 5-10 10-15 15-20 20-25 25-30 30-35 35-40

D/Do

~. (v/v)

k(cm2) l

0.19 0.07 0.06 0.04 0.04 0.04 0.05 0.05

2.4×10 -v 1.1 x I O - s 5 . 3 × 10 - s 1.2X 10 -8 8.4×10 -s 4.4X 10 - s 2 . 0 × 1 0 -8 1.4X10 -s

ea

k(cm2) l

D/Do

~ (v/v)

k(cm2) 1

D/Do

3 . 1 × 1 0 -6 1.Ox 10 - s 5.0 X 10 -9 3.0X 10 -9 1.5×10 -s 1.2X 10 -7 3 . 0 X l O -9 4 . 0 × 1 0 -9

1.9X10 -2 4.6X 10 -4 4.1X 10 -4 2.8X 10 -4 1 . 5 × 1 0 -3 1.7X 10 - a 6 . 2 X 1 0 -4 5 . 3 × 1 0 -4

0.22 0.12 0.10 0.09 0.03 0.03 0.04 0.04

1 . 4 × 1 0 -6 6.7X 10 -8 1.9X 10 -7 2.8X 10 - s 1 . 0 X 1 0 -9 3.0X 10 -9 2.0X10 -s 3.3×10 -s

3 . 5 X 1 0 -2 7 . 8 × 1 0 -3 9.5X 10 -a 2 . 0 × 1 0 -3 2 . 0 × 1 0 -4 5.0X 10 -4 1.6X10 -3 2 . 5 X 1 0 -3

(v/v) 1 . 2 X l O -2 1.5X 10 -~ 4 . 3 × 1 0 -3 2.5X 10 -3 2 . 0 X 1 0 -3 1.3X 10 -3 1 . 8 X 1 0 -'~ 1 . 6 X 1 0 -3

0.19 0.05 0.03 0.02 0.05 0.04 0.04 0.05

~Geometric means.

These differences between cultivation treatments were confirmed in a micro-plot irrigation experiment. Water applied to the soil surface in May penetrated the subsoil less rapidly in the ploughed and shallow-tilled soil than in the direct-drilled soil. In small cores collected from the micro-plots, air-filled porespace (ca) and transport coefficients were least in those from the boundary zone of the shallow tillage treatment (Table VI ). Year 7 (1980-81) In the 5-cm-thick layer immediately below that disturbed by drilling or superficial tillage, at 5-10 and 7-12 cm in direct-drilled and shallow-tine-cultivated soils, respectively, air permeability was greater than in 1979; air-filled porespace was similar in direct-drilled soil and greater in shallow-tilled soil than occurred at - 6 kPa in Year 5 (Table VII) The boundary between topsoil and subsoil was again least permeable after shallow tine-cultivation, concomitant with least air-filled porosity. In the annually ploughed soil, permeability T A B L E VII

Air-filled porespace ( e . ) , and air permeability ( k ) at field water content (wf) Lawford series, February 1981 (Year 7 ) Direct-drilled

Shallow-tine cultivated

Depth

wf

ea

kI

Depth

wf

(cm)

(gg-I)

(v/v)

( c m 2)

(cm)

(gg-1)

0.07 0.06

3 . 9 X 1 0 7 7-12 0.38 1 . 5 X 1 0 -7 14-19 ~ 0.36

5-10 0.37 13-182 0.37

Geometric means. ~Topsoil/subsoil boundary included.

Ploughed

~. (v/v)

k1 ( c m ~)

0.06 0.03

1 . 1 X 1 0 -7 1.8>(10 -8 20-252 0.36

Depth

wf

(cm)

(gg-1)

~, (v/v}

h~ ( c m 2)

0.06

5.9×10 -s

311 TABLE VIII Bulk density and water content at about the time of sowing in the Denchworth series, 1974-76 ( Years 1-3 ) Depth (cm)

Bulk density (gcm -3) Directdrilled

Ploughed

Water content (g g- ~) se

Significance of difference

Directdrilled

Ploughed

se

(P) 1974 O- 5 5-10 10-15 1975 O- 5 5-10 10-15 1976 O- 5 5-10 10-15

0.84 0.99 0.94

0.74 0.81 0.83

0.016 0.030 0.033

<0.01 <0.01 <0.05

0.55 0.47 0.48

0.43 0.54 0.56

0.029 0.010 0.014

0.97 1.10 1.11

0.89 0.92 0.96

0.015 0.024 0.012

<0.05 <0.01 <0.001

0.26 0.27 0.27

0.24 0.32 0.32

0.010 0.007 0.007

0.95 1.07 1.10

0.79 0.95 0.97

0.021 0.025 0.013

<0.05 < 0.05 <0.01

0.54 0.45 0.44

0.55 0.50 0.50

0.006 0.003 0.007

in the 5-cm layer encompassing the boundary was less than at the equivalent depth after direct drilling, although air-filled pore volume was similar. Taken over all seasons the soil that was direct-drilled became significantly denser only 3 years after the start of the experiment, and this was confined to the top 5 cm. By the winter of Year 4 the plough layer was more porous than the equivalent layer of the direct-drilled soil. Ploughing created a significant discontinuity at the boundary of the topsoil (Apg) and upper subsoil (Bgl) horizons, clearly evident by Year 5. Surprisingly, by this time air permeability was reduced in the Bgl in the shallow tine treatment and, more importantly, also in the 10-cm layer below the depth of tillage in that treatment.

Old grassland site at Compton Beauchamp (Denchworth series) Years 1, 2 and 3 (1974-75, 1975-76, 1976-77) Throughout the experiment the 0-15-cm layer was less dense, and total porosity greater, after mouldboard ploughing than after direct drilling (Table VIII). Year 6 (1980-81) The boundary between topsoil and subsoil was observed at about 19-cm depth in the direct-drilled soil and 24-cm depth where the soil had been ploughed annually; generally, the boundary was more distinct after ploughing.

312 Relative diffusivity, DIDO

O lxlO-S

1K10-"

1~10-3

1"10-2

1o

J: 20 ~'e~

~ ,i

30

"'"--.~ ~ "-..~.

riUed -4-Direct-drilled (furrow) .-o. Ploughed

•

40

Fig. 3. Relative diffusivity at - 1 kPa water potential in Denehworth series, 1980.

0

lx10 -s

lx10-4

Relative diffusivity, D/Do lxlO-3

lxlO-2

lxlO-1

~2c G) Q

3C

O/

a

Fig. 4. Relative diffusivity at - 1 and - 6 kPa water potential in Denchworth series, 1980 in cores yielding relative diffusivity values closest to the mean at each depth shown in Fig. 3. P e r m e a b i l i t y a n d relative d i f f u s i v i t y ( D / D o ) w e r e v e r y s m a l l in m a n y cores a t a w a t e r p o t e n t i a l of - 1 k P a (Fig. 3 ) . E x p e r i e n c e w i t h t h e p e r m e a b i l i t y t e c h n i q u e h a d s h o w n t h a t p r e c i s i o n w a s low t h r o u g h v i r t u a l l y i m p e r m e a b l e cores. A n y s a m p l e t h a t did n o t e x h i b i t t h r o u g h - f l o w of air at a p r e s s u r e differe n t i a l of 0.4 k P a w a s d e e m e d to h a v e zero p e r m e a b i l i t y . I n t h e p l o u g h e d t r e a t -

313

ment only the upper 3 sample depths (encompassing 0-15 cm) and the 35-40cm layer were permeable; in contrast, all depths in the typical direct-drilled soil provided samples with measurable permeability in at least half of the replicates. Relative diffusivity was measurable on all cores by allowing long periods for diffusive flow. In the upper 15 cm, D/Do at - 1 kPa water potential was greatest in the ploughed soil; below this depth, diffusivity was generally least in the ploughed treatment. There were similar variations in values of k between treatments and depths. From 0- to 25-cm depth in the direct-drilled soil, both coefficients were smaller in the furrow areas. A broadly similar difference between tillage treatments was observed at - 6 kPa, though unaccountably D/Do (Fig. 4) and k increased more in the ploughed soil with decreasing water potential than in the direct-drilled soil. There was most air-filled porosity in the whole topsoil layer of the ploughed soil; in the subsoil, porosities were similar under both cultivation treatments (Fig. 5). There were no differences between the typical direct-drilled areas and the furrow, nor did air-filled pore volume increase more in one t r e a t m e n t between - 1 and - 6 kPa than in any other. Measurements on large cores containing the topsoil/subsoil boundary indicated that DID,, and k were greater in the soil at both water potentials after direct drilling than after ploughing {Table IX), though air-filled pore volume was 0.05 and 0.07 (v/v) at - 1 and - 6 kPa, respectively, in both treatments. The profiles of an index of pore continuity, c, calculated from D/Do/ea (Ball, 1981 ), are presented in Fig. 6., for ploughed and direct-drilled soil from each site, at - 6 kPa water potential. T h e y indicate that there were greater differences between cultivation treatments t h a n between soils. Index c tended to be greater in the ploughed topsoil, but in the interface zone and the subsoil c was greater after direct drilling; in the latter t r e a t m e n t significant vertical discontinuities in soil structure were absent. DISCUSSION AND CONCLUSIONS

The previous m a n a g e m e n t histories of the two soils influenced their responses to the different tillage systems. Simplified ( zero or shallow) tillage on the old arable soil (Lawford series) led to a progressive change in structure, whilst on the Denchworth series direct drilling maintained features of the existing structure of the former grassland, and ploughing created an instantaneous and pronounced alteration. After several years of the treatments, however, the structural attributes were qualitatively similar for both soils. On the Lawford series measurements of air-filled porespace and permeability made in Year 7 ( Table VII) confirmed that soil structure had deteriorated under shallow tillage, manifest as compaction beneath the working depth of the tined cultivator ( ~ 7 c m ) . A 'plough-pan' had also begun to develop in the

314 Air-fined porespace (v/v) 0.10 0.15 T i ' . . . . . . . . J___~ F~

0-05 C

0.20 /

J

_____TA.... 1(]

T I

~2C

0.25 ' sed

F

T I

---¢i--"

___.1__n 3G

¥-T-I

g 1 i Direct-drilled T:typical, F:furrow ( Ploughed

--F-r-T"~

40

Fig. 5. Volume ofair-filledpores at - 6 kPawater potential from typical (T) and furrow (F) areas in Denchworth series, 1980. Index c O

1=10-z

l x 1 0 -~

lx10-'

1

,,,,-;"

e-2.

,c 2 0 oj Q

""c~'b 30

/j/f/'~

""

DD -~" P -o- DD o~- P

Denchworth Denchworth Lawford Lawford

40 Fig. 6. Pore continuity index 'c' at - 6 kPa water potential in the Lawford and Denchworth series, in 1979 and 1980 respectively.

soil that was ploughed every year. At no time was there any pan detected on the Denchworth soil. The greater permeability of the direct-drilled soil, at the depths equivalent to interfaces between tilled and untilled soil in cultivated land, indicates greater pore continuity. This could be attributed either to maintenance of structure under direct-drilling, or to processes of natural development, or to a combination of both. Nevertheless, in later years of the experiment on the old arable soil, there were more macropores after sequential direct drilling than after ploughing, at particular depths in the profile. Both soils are classified as stagnogleys. Stengel et al. (1984) showed, however, that the Law-

315 T A B L E IX Air-filled porespace ( Q ) , relative diffusivity (D/D,,) and air permeability (k) at different water potentials of large cores containing the topsoil/subsoil boundary, Denchworth series, 1980 (Year

6) Treatment

Direct-drilled Ploughed

D/D,,

e~(v/v)

k(cm') ~

-12

--6

-1

-6

-1

-6

0.05 0.05

0.07 0.07

2.0X10 -':~ 1.4X 10 -':~

3.2X10 :~ 2.6X 10 -:~

6.7X10 : 8.2X 10 ~

9.4X10 : 2.0X 10 ;

tGeometric means. '-'kPa.

ford-series soil was more compactible than the Denchworth-series soil because of its particle-size distribution and organic matter content. Susceptibility to compaction would be greater in the Lawford series than in the Denchworth series soil because of the smaller ratio of organic carbon to clay content (0.07 and 0.09, respectively), and to the broadly-graded particle-size distribution of the Lawford series. Consequently implements could have consolidated the soil layer directly below the depth of working on the Lawford series. In the old grassland soil treatment differences at the interfaces were due largely to the presence of earthworm channels, which connected the topsoil to the subsoil in the direct-drilled soil, but which were absent in the ploughed soil. Differences in permeability and diffusivity means could be attributed to the random presence of continuous cylindrical macropores (Douglas et al., 1980; Douglas, 1986), e.g. the coefficients of variation of D/D,, in the 15-35-cm layer of the direct-drilled soil were double those in the same depths in the ploughed soil. Such results suggest that annual ploughing destroyed the continuity of' such channels between topsoil and subsoil, and a diminished population density of deep-burrowing earthworms (Barnes and Ellis, 1979) reduced the rate of renewal or replacement. Under direct-drilling, soil disturbance would be limited to displacement in the surface 3-4 cm by the seed-drill and harrows, and to that due to the mainly vertical stresses from wheel traffic. It is likely, therefore, that there would be less disruption of the soil pore system, inherited from the period under grass, after direct drilling than after mouldboard ploughing. Physical conditions in these soils cannot be predicted easily from measurements of pore volume alone. For example, air-filled porespace at - 6 kPa water potential did not differ between "typical" and furrow areas, although there was good crop growth in the "typical" areas and poorer shoot and root development in the furrow areas (Ellis et al., 1981). The assessment of soil properties between areas of good and poor growth failed to establish any differences. However, they observed slightly fewer earthworm channels on the poorer areas in

316 Year 6, and this would result in poorer transmission characteristics which would have led to impaired drainage and aeration. The results for the profiles of continuity (index c) are similar to those obtained by Ball (1981) in the topsoil of the Lawford series. He did not give any values for the subsoil of this land under different tillage treatments. Soil structure has many attributes. Changes in structure can be detected by measuring bulk density and porosity, b u t for crop growth the size and continuity of pores are more important, and these must be inferred from fluid-transport properties. Contrasts in soil porosity arising from differing tillage regimes largely account for differences in water retention and distribution (Goss et al., 1978), root proliferation (Ellis and Barnes, 1980), crop water extraction (Goss et al., 1984 ) and secondary drainage requirement ( Harris, 1984 ) previously reported on these, and similar, clayey soils. As yields of autumn-sown cereals are rarely diminished on heavy soils with direct drilling compared with ploughing (Cannell et al., 1980; Christian et al., 1984) it must be concluded that the geometry of the porespace (particularly its continuity) in the latter system compensates for its smaller volume in the topsoil. Alternatively, the volume of macropores created by topsoil tillage may be in excess of the requirements of cereal crops under the climatic conditions of these experiments. ACKNOWLEDGEMENTS The authors are grateful to staff of the former AFRC Letcombe Laboratory for their assistance and co-operation, particularly Dr. R.Q. Cannell for advice, D.G. Christian for management of the field experiments and J.V. Armstrong for technical support.

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317 Douglas, J.T., 1986. Macroporosity and permeability of some soil cores from England and France. Geoderma, 37: 221-231. Douglas, J.T., Goss, M.J. and Hill, D., 1980. Measurements of pore characteristics in a clay soil under ploughing and direct drilling, including use of a radioactive tracer (144Ce) technique. Soil Tillage ires., 1: 11-18. Douglas, J.T., Jarvis, M.G., Howse, K.R. and Goss, M.J., 1986. Structure of a silty soil in relation to management. J. Soil Sci., 37: 137-151. Ellis, F.B. and Barnes, B.T., 1980. Growth and development of root systems of winter cereals grown after different tillage methods including direct drilling. Plant and Soil, 55: 283-295. Ellis, F.B., Barnes, B.T. and Douglas, J.T., 1981. Agricultural Research Council Letcombe Laboratory Annual Report, 1980, pp. 12-13. Ellis, F.B., Christian, D.G., Bragg, P.L., Henderson, F.K.G., Prew, R.D. and Cannell, R.Q., 1984. A study of mole drainage with simplified cultivation for autumn-sown crops on a clay soil. 3. Agronomy, root and shoot growth of winter wheat, 1978-80. J. Agric. Sci., 102: 583-594. Goss, M.J., Howse, K.R. and Harris, W., 1978. Effects of cultivation on soil water retention and water use by cereals in clay soils. J. Soil Sci., 29: 475-488. Goss, M.J., Howse, K.R., Vaughan-Williams, J.M., Ward, M.A. and Jenkins, W., 1984. Water use by winter wheat as affected by soil management. J. Agric. Sci., 103: 189-199. Harris, G.L., 1984. Agricultural and Food Research Council Letcombe Laboratory Annual Report 1983, p. 22. Hodgson, J.M., 1976. Soil Survey Field Handbook. Soil Survey Technical Monograph No. 5, Harpenden, p. 88. Low, A.J., 1972. The effect of cultivation on the structure and other physical properties of grassland and arable soils (1945-1970). J. Soil Sci., 23: 363-380. O'Sullivan, M.F. and Ball, B.C., 1982. Spring barley growth, grain quality and soil physical conditions in a cultivations experiment on a sandy loam in Scotland. Soil Tillage Res., 2: 359-378. Stengel, P., Douglas, J.T., Gudrif, J., Goss, M.J., Monnier, G. and Cannell, R.Q., 1984. Factors influencing the variation of some properties of soils in relation to their suitability for direct drilling. Soil Tillage ires., 4: 35-53.