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Measurements of pore characteristics in a clay soil under ploughing and direct drilling, including use of a radioactive tracer (144Ce) technique
Soil & Tillage Research, 1 (1980/1981) 11--18
11
Elsevier Scientific Publishing Company, Amsterdam -- Printed in Belgium
MEASUREMENTS OF PORE CHARACTERISTICS IN A CLAY SOIL UNDER PLOUGHING AND DIRECT DRILLING, INCLUDING USE OF A RADIOACTIVE TRACER (144Ce) TECHNIQUE
J.T. DOUGLAS, M.J. GOSS and D. HILL
Agricultural Research Council Letcombe Laboratory, Wantage, Oxon, 0X12 9JT (Great Britain) (Accepted 10 July 1980)
ABSTRACT 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 (~44Ce) technique. Soil Tillage Res., 1: 11--18. In a comparison of soil pore space after direct drilling and mouldboard ploughing of a clay soil total porosity and volume of transmission pores (> 50 urn) were greater in the cultivated layer of the ploughed (0--25 cm) soil than in the equivalent layer of the directdrilled soil. Below this depth values converged. At the interface of topsoil and subsoil (about 25 cm depth) saturated hydraulic conductivity was significantly lower in ploughed soil than in direct-drilled soil. Infiltration of a radioactive tracer solution (~44Ce) confirmed that the continuity of transmission pores was less after ploughing compared with direct drilling. The results are discussed in relation to the physical conditions observed in this and similar fine-textured soils.
INTRODUCTION
Several papers have reported more rapid infiltration of water (Ehlers, 1976; Goss et al., 1978), greater concentrations of oxygen in winter (Dowdell et al., 1979) and, in some soils, more rapid root growth to the deeper parts of the profile after direct drilling than after ploughing (Ellis and Barnes, 1980). Enhanced pore continuity in uncultivated soil has been considered the most likely explanation, perhaps associated with cracks between peds (Ellis et al., 1979) and greater earthworm activity (Barnes and Ellis, 1979). However, in direct-drilled soil the volume of larger pores (greater than 50 ~m equivalent diameter), sometimes called "transmission" pores (Greenland, 1977), in the surface layers is less and the bulk density greater than in soil which has been ploughed (Boone et al., 1976; Pidgeon and Soane, 1977; Gantzer and Blake, 1978). Therefore, the effects listed above seem unlikely to depend solely on differences in total pore volume or pore size distribution. Here we report results of a study of porosity and pore function which suggest that the transmis0167--1987/80/0000--0000/$02.50 © 1980 Elsevier Scientific Publishing Company
12 sion properties o f soil pore space can be impaired by sequential ploughing compared to direct drilling. EXPERIMENTAL Measurements were on " u n d i s t u r b e d " samples f r o m a soil with a clay content o f 50% (Denchworth series) where a long-term comparison of direct drilling and m o u l d b o a r d ploughing that com m enced in 1974 was in progress. Details o f the soil, experimental treatments and yields for crops including winter wheat, winter oats and winter oil-seed rape have been published elsewhere (Cannell et al., 1980). In summary, the soil has been direct-drilled with a triple-disc drill or ploughed to 25 cm depth each a u t u m n after burning of residues from the previous crop. Ploughing was then followed by secondary cultivation as appropriate to produce a seed-bed. The cultivation treatments were on plots 42 m X 26 m in fivefold replication. A drainage system imposed on the site consisted of tiles at 90 cm depth, 40 m apart with porous backfill, and mole drains drawn transversely over the pipes at a depth of 50 cm, and a b o u t 3 m apart. The volume of pores was measured on all five replicates and pore continuity assessed on only f our replicates. Soil samples were collected to a depth of 40 cm (below which direct differential effects of cultivation were unlikely to occur). Total porosity was assessed from measurements of bulk density on cores 7.6 cm diameter by 5 cm thickness and others 15 cm diameter and either 10 cm or 35 cm thickness. These larger cores contained the interface between topsoil and subsoil (at approximately 20--25 cm depth) and were sufficiently large to a c c o m m o d a t e the structural features of the soil and any vertical deviations in the level of the interface. The volume of transmission pores (> 50 pm) was obtained from the difference in volumetric water c o n t e n t at saturation and after equilibration at --6 kPa potential on a sand tension table. The continuity of pores was assessed on samples 15 cm X 10 cm by measuring saturated hydraulic conductivity (K) (Klute, 1965). Saturation was achieved in the laboratory by wetting the samples from the b o t t o m at a small water potential (approximately 0.5 kPa) and, after flooding, application of a mild suction at the top of the sample to remove any entrapped air. The measurement of flow rate was made using a constant hydraulic head o f 2 cm. After the measurement of K and the volume of transmission pores, the dry bulk density of the samples was determined to give an indication of total pore volume. We also used a new m e t h o d based on t he distribution of cerium-144 30 min after infiltration in solution to assess pore continuity. Cerium-144, a rare earth, was chosen as the labelling element because the tripositive cation is highly basic and strongly adsorbed by the soil (Nishita et al., 1956), the level of H-emission energy and isotope half-life (t~/2= 284 d) make it suitable for auto-radiography and it is readily available. " U n d i s t u r b e d " soil cores (15 cm diameter and 35 cm deep) were collected when the soil water potential was close to - 6 kPa. A 0.1 pM solution of cerium chloride containing 5 ttCi
13
cerium-144, was added to a vertical tube, 7.5 cm in diameter, placed centrally on top of the soil core. The volume infiltrated was equivalent to 10 mm solution over the cross-sectional area of the whole sample. Thirty min after the solution had been applied the cores were laid horizontally and the soil divided lengthwise in two using a guillotine. The procedure prevented any ceriumlabelled soil being distributed vertically in the column by the cutting action. The soil was then frozen and auto-radiographs obtained of the cut surfaces. R E S U L T S AND D I S C U S S I O N
Table I indicates that in 1977 bulk density was lower in the cultivated layer of the ploughed soil. Consequently, total pore volume was greater. Concomitant with the greater total porosity was a greater volume of transmission pores (> 50 pm) remaining after cultivation (Fig.l). Total and transmission pore volume were significantly correlated (r = 0.87***) over both treatments. Below the depth of cultivation porosity values converged, suggesting that implements or traffic had not caused more compaction in one treatment than in the other. These results agree with those found by other workers (eg. Pidgeon and Soane, 1977; Gantzer and Blake, 1978) but do not explain the general improvement in infiltration, oxygen concentrations and root growth seen in other soils (see Introduction). The mean value of K at the interface of topsoil and subsoil measured in 1978 for direct-drilled soil was double that after ploughing (Table II). The difference was significant (P = 0.01) by a t-test and could not be attributed to differences in bulk density at this depth (1.08 g cm -3 and 1.04 g cm -3 for direct-drilled and ploughed cores respectively) or in the measured volume of transmission pores (approximately 5% and 7% for direct-drilled and ploughed soil respectively). A limited number of smaller cores (7.6 cm diameter and 5 cm thick) taken immediately adjacent did not show any additional compaction below plough depth since the previous sampling in 1977. TABLE I Bulk d e n s i t y profile to 40 cm a f t e r direct drilling and after ploughing t o 25 c m d e p t h o f a clay soil ( D e n c h w o r t h series), 1977 D e p t h (cm)
0--5 5--10 10--15 15--20 20--25 25--30 30--35 35--40
Bulk d e n s i t y ( g c m 3) Direct-drilled
Ploughed
Significance level o f difference
0.91 1.00 1.03 1.12 1.19 1.26 1.30 1.34
0.82 0.91 0.94 0.95 1.08 1.22 1.29 1.35
* * ** * * NS NS NS
14 Volume of transmission pores (%,V/v) 5 10
01
10
Direct
- d r ~ /
.~ 20 ~
~pthof
15
20
......................................... '.............. o /Ploughed
~_~..
°'--cultivation 30
"O
~
Standard error
40
Fig.1. Volume of transmission pores (> 50 ~m) at different depths in direct-drilled and ploughed soil, 1977. TABLE II Saturated hydraulic conductivity at the interface of topsoil and subsoil in direct-drilled and ploughed soil, 1978 Treatment means ( 1 0 - ' ms -~)
Standard samples With earthworm channels plugged
Ploughed
Direct-drilled
0.42
1.01
--
0.22
The range of K values with direct-drilled soil was wider than with ploughed soil (Fig.2). Approximately cylindrical earthworm channels with diameters ranging from 1--8 mm were visible at the two ends of m a n y of the direct~ drilled soil sample cores and were apparently continuous through them. When these channels were artificially blocked at each end with plugs of remoulded soil K decreased by 80% (Table II) and was clearly lower than the mean value after ploughing, so earthworm activity was apparently a prime cause of the difference between direct-drilled and ploughed treatments. Auto-radiographs (Fig.3) show that in the upper layer of direct-drilled soil the distribution o f tracer was confined to a smaller area than in the ploughed soft: consistent with the greater volume and complex geometry o f large pores after cultivation. The infiltration was carried out when only pores > 50 ~m were air-filled (water potential being close to - 6 kPa) and solution movement was arrested 30 min after infiltration. Therefore it was assumed that labelling was confined to transmission pores. From the absence of darkening it appears
15 80
•!Ploughed / \ i /J !
60
/
\
/
Q.
¢n "5 4O
\
'
#
20
i
i / //~lrect - drilled
0
0'-0.1
0.1:1.0
1.0'-2.0
2.0~-3"0
K intervals (units: lO"ms-')
Fig. 2. The distribution of saturated hydraulic conductivity (K)values in direct-drilled and ploughed soil at the interface of topsoil and subsoil, 1978. that cerium-144 was not transported below the cultivated layer in the ploughed soil column but clearly labelled pores at the equivalent depth in the directdrilled soil. Labelling also continued through the length of the direct
16
I', '
A
,, ,,I
PLOUGHED
I',
A
~-I
DIRECT-DRILLED
Fig.3. A u t o - r a d i o g r a p h s f r o m p l o u g h e d a n d direct-drilled soil c o l u m n s a f t e r a p p l i c a t i o n o f 144Ce. A u t o - r a d i o g r a p h d i m e n s i o n s b e f o r e r e p r o d u c t i o n : 15 c m × 35 cm. A = z o n e of a p p l i c a t i o n o f tracer s o l u t i o n ; B = dark s p o t at base o f cultivated layer caused by flow o f tracer s o l u t i o n a r o u n d an e m b e d d e d stone.
17
applied to water movement in the soil used in this study. When the subsoil is wetter and shrinkage cracks are closed aeration can be impaired after direct drilling (Burford et al., 1980) and we have also noted that the water-table can be located closer to the soil surface. One or more factors and their interaction could produce this latter situation. For example the mole drains may have degenerated to a greater extent on the direct~drilled areas than on the ploughed land because of deeper cracking (Ellis et al., 1979). Alternatively the contrasting pore systems in the two treatments may interact differently with the mole drain channels; thus Trafford and R y c r o f t (1973) have suggested that lateral movement of water in highly porous cultivated layers is an important c o m p o n e n t of drainage regimes in clay soils. The greater density of direct~drilled topsoil, combined with the lower impedance at the topsoilsubsoil boundary, may result in less lateral flow and a different distribution of tension-free water (or water at high potentials) from that in ploughed land. However, this must remain as speculation because of the paucity of published information on this aspect. Generally, however, we surmise that the presence of vertical, approximately tubular pores in direct~drilled clay soil made by earthworms will be beneficial in terms of movement of water, gas and roots by compensating for the greater overall density in the surface layers and by lessening impedance at the interface of topsoil and subsoil. Little is known of the contribution of root channels and planar inter-ped voids to soil conditions under different cultivation regimes. This is an important area for further study, particularly in soils with few deep-burrowing earthworms. CONCLUSIONS
Total porosity and volume of transmission pores (> 50 pm) was greater in the upper layer of cultivated soil than in direct-drilled soil. In the subsoil values converged. Measurements of saturated hydraulic conductivity at the topsoil--subsoil interface and infiltration of radioactive tracer indicated that transmission pores were more continuous in the direct-drilled soil than in the ploughed soil. The presence of earthworm channels was the prime cause of this difference in this soil. The enhanced link between soil horizons formed by these pores will possibly improve aeration status and encourage more rapid root growth. Nevertheless it may be that subsurface drainage requirements for direct-drilled and ploughed land may differ and more research is necessary. ACKNOWLEDGEMENTS
The authors wish to thank Dr F.B. Ellis and his staff for the high standard of soil management in the long-term field experiment and Dr R.Q. Cannell for helpful comment. The technical assistance provided by J.V. Armstrong is gratefully acknowledged.
18 REFERENCES Barnes, B.T. and Ellis, F.B., 1979. Effects of different methods of cultivation and direct drilling, and disposal of straw residues on populations of earthworms. J. Soil Sci., 30 : 669--679. Boone, F.R., Slager, S., Miedema, R. and Eleveld, R., 1976. Some influences of zerotillage on the structure and stability of a fine-textured levee soil. Neth. J. Agric. Sci., 24: 105--119. Burford, J.R., Dowdell, R.J., Crees, R. and Hall, K.C., 1980. Soil aeration and denitrification. Agric. Res. Counc. Letcombe Lab. Ann. Rep., 1978, p. 26. Cannell, R.Q., Ellis, F.B., Christian, D.G., Graham, J.P. and Douglas, J.T., 1980. The growth and yield of winter cereals after direct drilling, shallow cultivation and ploughing on non-calcareous clay soils, 1974--78. J. Agric. Sci. (Cambridge), 94: 345--359. Dowdell, R.J., Crees, R., Burford, J.R. and Cannell, R.Q., 1979. Oxygen concentrations in a clay soil after ploughing or direct drilling. J. Soil Sci., 30: 2 3 9 - 2 4 5 . Ehlers, W., 1976. Water infiltration and redistribution in tilled and untilled loess soil. GSttinger Bodenkdl. Ber., 44: 137--156. 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., Elliott, J.G., Pollard, F., Cannell, R.Q. and Barnes, B.T., 1979. Comparison of direct drilling, reduced cultivation and ploughing on the growth of cereals. 3. Winter wheat and spring barley on a calcareous clay. J. Agric. Sci. (Cambridge), 93: 391--401. Gantzer, C.J. and Blake, G.R., 1978. Physical characteristics of Le Sueur clay loam soil following no-till and conventional tillage. Agron. J., 70: 853--857. 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. Greenland, D.J., 1977. Soil damage by intensive arable cultivation: temporary or permanent? Philos. Trans. R. Soc. Lond. (B), 281: 193--208. Klute, A., 1965. Laboratory measurement of hydraulic conductivity of saturated soil. In: C.A. Black (Editor), Methods of Soil Analysis, Part 1. Agronomy, 9: 210--221. Nishita, H., Kowalesky, B.W., Steen, A.J. and Larson, K.H., 1956. Fixation and extractability of fission products contaminating various soils and clays. I. Sr90, Y91, R u l 0 6 , Cs137 and Ce144. Soil Sci., 81: 317--326. Pidgeon, J.D. and Soane, B.D., 1977. Effects of tillage and direct drilling on soil properties during the growing season in a long-term barley monoculture system. J. Agric. Sci. (Cambridge), 88: 431--442. Quisenberry, V.L. and Phillips, R.E., 1976. Percolation of surface applied water in the field. Soil Sci. Soc. Am. J., 40: 484--489. Trafford, B.D. and Rycroft, D.W., 1973. Observations on the soil-water regimes in a drained clay soil. J. Soil Sci., 24: 380--391.
11
Elsevier Scientific Publishing Company, Amsterdam -- Printed in Belgium
MEASUREMENTS OF PORE CHARACTERISTICS IN A CLAY SOIL UNDER PLOUGHING AND DIRECT DRILLING, INCLUDING USE OF A RADIOACTIVE TRACER (144Ce) TECHNIQUE
J.T. DOUGLAS, M.J. GOSS and D. HILL
Agricultural Research Council Letcombe Laboratory, Wantage, Oxon, 0X12 9JT (Great Britain) (Accepted 10 July 1980)
ABSTRACT 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 (~44Ce) technique. Soil Tillage Res., 1: 11--18. In a comparison of soil pore space after direct drilling and mouldboard ploughing of a clay soil total porosity and volume of transmission pores (> 50 urn) were greater in the cultivated layer of the ploughed (0--25 cm) soil than in the equivalent layer of the directdrilled soil. Below this depth values converged. At the interface of topsoil and subsoil (about 25 cm depth) saturated hydraulic conductivity was significantly lower in ploughed soil than in direct-drilled soil. Infiltration of a radioactive tracer solution (~44Ce) confirmed that the continuity of transmission pores was less after ploughing compared with direct drilling. The results are discussed in relation to the physical conditions observed in this and similar fine-textured soils.
INTRODUCTION
Several papers have reported more rapid infiltration of water (Ehlers, 1976; Goss et al., 1978), greater concentrations of oxygen in winter (Dowdell et al., 1979) and, in some soils, more rapid root growth to the deeper parts of the profile after direct drilling than after ploughing (Ellis and Barnes, 1980). Enhanced pore continuity in uncultivated soil has been considered the most likely explanation, perhaps associated with cracks between peds (Ellis et al., 1979) and greater earthworm activity (Barnes and Ellis, 1979). However, in direct-drilled soil the volume of larger pores (greater than 50 ~m equivalent diameter), sometimes called "transmission" pores (Greenland, 1977), in the surface layers is less and the bulk density greater than in soil which has been ploughed (Boone et al., 1976; Pidgeon and Soane, 1977; Gantzer and Blake, 1978). Therefore, the effects listed above seem unlikely to depend solely on differences in total pore volume or pore size distribution. Here we report results of a study of porosity and pore function which suggest that the transmis0167--1987/80/0000--0000/$02.50 © 1980 Elsevier Scientific Publishing Company
12 sion properties o f soil pore space can be impaired by sequential ploughing compared to direct drilling. EXPERIMENTAL Measurements were on " u n d i s t u r b e d " samples f r o m a soil with a clay content o f 50% (Denchworth series) where a long-term comparison of direct drilling and m o u l d b o a r d ploughing that com m enced in 1974 was in progress. Details o f the soil, experimental treatments and yields for crops including winter wheat, winter oats and winter oil-seed rape have been published elsewhere (Cannell et al., 1980). In summary, the soil has been direct-drilled with a triple-disc drill or ploughed to 25 cm depth each a u t u m n after burning of residues from the previous crop. Ploughing was then followed by secondary cultivation as appropriate to produce a seed-bed. The cultivation treatments were on plots 42 m X 26 m in fivefold replication. A drainage system imposed on the site consisted of tiles at 90 cm depth, 40 m apart with porous backfill, and mole drains drawn transversely over the pipes at a depth of 50 cm, and a b o u t 3 m apart. The volume of pores was measured on all five replicates and pore continuity assessed on only f our replicates. Soil samples were collected to a depth of 40 cm (below which direct differential effects of cultivation were unlikely to occur). Total porosity was assessed from measurements of bulk density on cores 7.6 cm diameter by 5 cm thickness and others 15 cm diameter and either 10 cm or 35 cm thickness. These larger cores contained the interface between topsoil and subsoil (at approximately 20--25 cm depth) and were sufficiently large to a c c o m m o d a t e the structural features of the soil and any vertical deviations in the level of the interface. The volume of transmission pores (> 50 pm) was obtained from the difference in volumetric water c o n t e n t at saturation and after equilibration at --6 kPa potential on a sand tension table. The continuity of pores was assessed on samples 15 cm X 10 cm by measuring saturated hydraulic conductivity (K) (Klute, 1965). Saturation was achieved in the laboratory by wetting the samples from the b o t t o m at a small water potential (approximately 0.5 kPa) and, after flooding, application of a mild suction at the top of the sample to remove any entrapped air. The measurement of flow rate was made using a constant hydraulic head o f 2 cm. After the measurement of K and the volume of transmission pores, the dry bulk density of the samples was determined to give an indication of total pore volume. We also used a new m e t h o d based on t he distribution of cerium-144 30 min after infiltration in solution to assess pore continuity. Cerium-144, a rare earth, was chosen as the labelling element because the tripositive cation is highly basic and strongly adsorbed by the soil (Nishita et al., 1956), the level of H-emission energy and isotope half-life (t~/2= 284 d) make it suitable for auto-radiography and it is readily available. " U n d i s t u r b e d " soil cores (15 cm diameter and 35 cm deep) were collected when the soil water potential was close to - 6 kPa. A 0.1 pM solution of cerium chloride containing 5 ttCi
13
cerium-144, was added to a vertical tube, 7.5 cm in diameter, placed centrally on top of the soil core. The volume infiltrated was equivalent to 10 mm solution over the cross-sectional area of the whole sample. Thirty min after the solution had been applied the cores were laid horizontally and the soil divided lengthwise in two using a guillotine. The procedure prevented any ceriumlabelled soil being distributed vertically in the column by the cutting action. The soil was then frozen and auto-radiographs obtained of the cut surfaces. R E S U L T S AND D I S C U S S I O N
Table I indicates that in 1977 bulk density was lower in the cultivated layer of the ploughed soil. Consequently, total pore volume was greater. Concomitant with the greater total porosity was a greater volume of transmission pores (> 50 pm) remaining after cultivation (Fig.l). Total and transmission pore volume were significantly correlated (r = 0.87***) over both treatments. Below the depth of cultivation porosity values converged, suggesting that implements or traffic had not caused more compaction in one treatment than in the other. These results agree with those found by other workers (eg. Pidgeon and Soane, 1977; Gantzer and Blake, 1978) but do not explain the general improvement in infiltration, oxygen concentrations and root growth seen in other soils (see Introduction). The mean value of K at the interface of topsoil and subsoil measured in 1978 for direct-drilled soil was double that after ploughing (Table II). The difference was significant (P = 0.01) by a t-test and could not be attributed to differences in bulk density at this depth (1.08 g cm -3 and 1.04 g cm -3 for direct-drilled and ploughed cores respectively) or in the measured volume of transmission pores (approximately 5% and 7% for direct-drilled and ploughed soil respectively). A limited number of smaller cores (7.6 cm diameter and 5 cm thick) taken immediately adjacent did not show any additional compaction below plough depth since the previous sampling in 1977. TABLE I Bulk d e n s i t y profile to 40 cm a f t e r direct drilling and after ploughing t o 25 c m d e p t h o f a clay soil ( D e n c h w o r t h series), 1977 D e p t h (cm)
0--5 5--10 10--15 15--20 20--25 25--30 30--35 35--40
Bulk d e n s i t y ( g c m 3) Direct-drilled
Ploughed
Significance level o f difference
0.91 1.00 1.03 1.12 1.19 1.26 1.30 1.34
0.82 0.91 0.94 0.95 1.08 1.22 1.29 1.35
* * ** * * NS NS NS
14 Volume of transmission pores (%,V/v) 5 10
01
10
Direct
- d r ~ /
.~ 20 ~
~pthof
15
20
......................................... '.............. o /Ploughed
~_~..
°'--cultivation 30
"O
~
Standard error
40
Fig.1. Volume of transmission pores (> 50 ~m) at different depths in direct-drilled and ploughed soil, 1977. TABLE II Saturated hydraulic conductivity at the interface of topsoil and subsoil in direct-drilled and ploughed soil, 1978 Treatment means ( 1 0 - ' ms -~)
Standard samples With earthworm channels plugged
Ploughed
Direct-drilled
0.42
1.01
--
0.22
The range of K values with direct-drilled soil was wider than with ploughed soil (Fig.2). Approximately cylindrical earthworm channels with diameters ranging from 1--8 mm were visible at the two ends of m a n y of the direct~ drilled soil sample cores and were apparently continuous through them. When these channels were artificially blocked at each end with plugs of remoulded soil K decreased by 80% (Table II) and was clearly lower than the mean value after ploughing, so earthworm activity was apparently a prime cause of the difference between direct-drilled and ploughed treatments. Auto-radiographs (Fig.3) show that in the upper layer of direct-drilled soil the distribution o f tracer was confined to a smaller area than in the ploughed soft: consistent with the greater volume and complex geometry o f large pores after cultivation. The infiltration was carried out when only pores > 50 ~m were air-filled (water potential being close to - 6 kPa) and solution movement was arrested 30 min after infiltration. Therefore it was assumed that labelling was confined to transmission pores. From the absence of darkening it appears
15 80
•!Ploughed / \ i /J !
60
/
\
/
Q.
¢n "5 4O
\
'
#
20
i
i / //~lrect - drilled
0
0'-0.1
0.1:1.0
1.0'-2.0
2.0~-3"0
K intervals (units: lO"ms-')
Fig. 2. The distribution of saturated hydraulic conductivity (K)values in direct-drilled and ploughed soil at the interface of topsoil and subsoil, 1978. that cerium-144 was not transported below the cultivated layer in the ploughed soil column but clearly labelled pores at the equivalent depth in the directdrilled soil. Labelling also continued through the length of the direct
16
I', '
A
,, ,,I
PLOUGHED
I',
A
~-I
DIRECT-DRILLED
Fig.3. A u t o - r a d i o g r a p h s f r o m p l o u g h e d a n d direct-drilled soil c o l u m n s a f t e r a p p l i c a t i o n o f 144Ce. A u t o - r a d i o g r a p h d i m e n s i o n s b e f o r e r e p r o d u c t i o n : 15 c m × 35 cm. A = z o n e of a p p l i c a t i o n o f tracer s o l u t i o n ; B = dark s p o t at base o f cultivated layer caused by flow o f tracer s o l u t i o n a r o u n d an e m b e d d e d stone.
17
applied to water movement in the soil used in this study. When the subsoil is wetter and shrinkage cracks are closed aeration can be impaired after direct drilling (Burford et al., 1980) and we have also noted that the water-table can be located closer to the soil surface. One or more factors and their interaction could produce this latter situation. For example the mole drains may have degenerated to a greater extent on the direct~drilled areas than on the ploughed land because of deeper cracking (Ellis et al., 1979). Alternatively the contrasting pore systems in the two treatments may interact differently with the mole drain channels; thus Trafford and R y c r o f t (1973) have suggested that lateral movement of water in highly porous cultivated layers is an important c o m p o n e n t of drainage regimes in clay soils. The greater density of direct~drilled topsoil, combined with the lower impedance at the topsoilsubsoil boundary, may result in less lateral flow and a different distribution of tension-free water (or water at high potentials) from that in ploughed land. However, this must remain as speculation because of the paucity of published information on this aspect. Generally, however, we surmise that the presence of vertical, approximately tubular pores in direct~drilled clay soil made by earthworms will be beneficial in terms of movement of water, gas and roots by compensating for the greater overall density in the surface layers and by lessening impedance at the interface of topsoil and subsoil. Little is known of the contribution of root channels and planar inter-ped voids to soil conditions under different cultivation regimes. This is an important area for further study, particularly in soils with few deep-burrowing earthworms. CONCLUSIONS
Total porosity and volume of transmission pores (> 50 pm) was greater in the upper layer of cultivated soil than in direct-drilled soil. In the subsoil values converged. Measurements of saturated hydraulic conductivity at the topsoil--subsoil interface and infiltration of radioactive tracer indicated that transmission pores were more continuous in the direct-drilled soil than in the ploughed soil. The presence of earthworm channels was the prime cause of this difference in this soil. The enhanced link between soil horizons formed by these pores will possibly improve aeration status and encourage more rapid root growth. Nevertheless it may be that subsurface drainage requirements for direct-drilled and ploughed land may differ and more research is necessary. ACKNOWLEDGEMENTS
The authors wish to thank Dr F.B. Ellis and his staff for the high standard of soil management in the long-term field experiment and Dr R.Q. Cannell for helpful comment. The technical assistance provided by J.V. Armstrong is gratefully acknowledged.
18 REFERENCES Barnes, B.T. and Ellis, F.B., 1979. Effects of different methods of cultivation and direct drilling, and disposal of straw residues on populations of earthworms. J. Soil Sci., 30 : 669--679. Boone, F.R., Slager, S., Miedema, R. and Eleveld, R., 1976. Some influences of zerotillage on the structure and stability of a fine-textured levee soil. Neth. J. Agric. Sci., 24: 105--119. Burford, J.R., Dowdell, R.J., Crees, R. and Hall, K.C., 1980. Soil aeration and denitrification. Agric. Res. Counc. Letcombe Lab. Ann. Rep., 1978, p. 26. Cannell, R.Q., Ellis, F.B., Christian, D.G., Graham, J.P. and Douglas, J.T., 1980. The growth and yield of winter cereals after direct drilling, shallow cultivation and ploughing on non-calcareous clay soils, 1974--78. J. Agric. Sci. (Cambridge), 94: 345--359. Dowdell, R.J., Crees, R., Burford, J.R. and Cannell, R.Q., 1979. Oxygen concentrations in a clay soil after ploughing or direct drilling. J. Soil Sci., 30: 2 3 9 - 2 4 5 . Ehlers, W., 1976. Water infiltration and redistribution in tilled and untilled loess soil. GSttinger Bodenkdl. Ber., 44: 137--156. 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., Elliott, J.G., Pollard, F., Cannell, R.Q. and Barnes, B.T., 1979. Comparison of direct drilling, reduced cultivation and ploughing on the growth of cereals. 3. Winter wheat and spring barley on a calcareous clay. J. Agric. Sci. (Cambridge), 93: 391--401. Gantzer, C.J. and Blake, G.R., 1978. Physical characteristics of Le Sueur clay loam soil following no-till and conventional tillage. Agron. J., 70: 853--857. 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. Greenland, D.J., 1977. Soil damage by intensive arable cultivation: temporary or permanent? Philos. Trans. R. Soc. Lond. (B), 281: 193--208. Klute, A., 1965. Laboratory measurement of hydraulic conductivity of saturated soil. In: C.A. Black (Editor), Methods of Soil Analysis, Part 1. Agronomy, 9: 210--221. Nishita, H., Kowalesky, B.W., Steen, A.J. and Larson, K.H., 1956. Fixation and extractability of fission products contaminating various soils and clays. I. Sr90, Y91, R u l 0 6 , Cs137 and Ce144. Soil Sci., 81: 317--326. Pidgeon, J.D. and Soane, B.D., 1977. Effects of tillage and direct drilling on soil properties during the growing season in a long-term barley monoculture system. J. Agric. Sci. (Cambridge), 88: 431--442. Quisenberry, V.L. and Phillips, R.E., 1976. Percolation of surface applied water in the field. Soil Sci. Soc. Am. J., 40: 484--489. Trafford, B.D. and Rycroft, D.W., 1973. Observations on the soil-water regimes in a drained clay soil. J. Soil Sci., 24: 380--391.