Soil strength and soil pore characteristics for
direct drilled and ploughed soils
Per Schjùnning
a,*, Karl J. Rasmussen
baDanish Institute of Agricultural Sciences, Department of Crop Physiology and Soil Science,
Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark
bDanish Institute of Agricultural Sciences, Department of Agricultural Engineering, Research Centre Bygholm,
PO Box 536, DK-8700 Horsens, Denmark
Received 13 December 1999; received in revised form 17 July 2000; accepted 28 July 2000
Abstract
Direct drilling has often been reported to increase density and strength and to affect pore continuity and tortuosity of the upper soil layers. In this study these aspects were studied for three texturally differing soils 4±6 years after initiation of continuous trials with direct drilling and mouldboard ploughing. The soils studied were a coarse sandy soil (Korntved, 5% clay), a sandy loam (Ballum, 8% clay) and a silty loam (Hùjer, 19% clay). The crop rotation at Korntved was spring barley and winter rye while at Ballum and Hùjer it was spring barley and winter wheat. Both crops were grown every year. All ®elds had been mouldboard ploughed for decades prior to the trial period. The ploughed treatment (PL) was imposed in the autumn and the seedbed preparation and drilling were performed with an S-tined seedbed harrow and a traditional drill. The direct drilled (DD) treatment received no tillage other than the drilling which was performed by a triple-disc drill. Straw and stubble were burned. In the 4th, 5th and 6th years of the trial period, minimally disturbed soil cores were taken from the 4 to 8, 14 to 18 and 24 to 28 cm depths, i.e. two horizons above the ploughing depth of 20 cm, and one horizon below this depth. Longer cores were sampled in the 18±27 cm depth in order to include this transition layer. Furthermore, in the 4th year of the trial period shear strength was measured in the ®eld at 2-week intervals in the spring with a vane shear tester in the two upper layers mentioned. All samplings and measurements took place in the ®eld grown with spring barley. In the laboratory air diffusivity and air permeability were measured at ®eld-sampled water content and again when the soil cores were drained to a matric potential ofÿ100 hPa. Cone penetration resistance was measured with a 2 mm diameter penetrometer. Separate core samples from the 14 to 18 cm depth of the Korntved and Hùjer soils were used for estimating soil cohesion and soil internal friction by a shear annulus method at ®eld-sampled water content.
For all soils, DD increased soil bulk density in the two upper soil layers. The shear vane tester also generally estimated higher shear strength for the DD compared to the PL treatment. The shear annulus measurements in the laboratory revealed no differences between tillage treatments for the Korntved soil, while a tendency of higher cohesion and internal soil friction was found for the DD treatment on the Hùjer soil. The cone penetration measurements indicated a stronger top-soil and fewer high-strength soil elements in the 24±28 cm horizon for the DD than for the PL treatment. Generally the DD treatment had lower volume of macropores (i.e. pores>30mm) in the 4±8 and 14±18 cm depths than the PL treatment. This was re¯ected in reduced air diffusivities and air permeabilities for these horizons. An exception was the 14±18 cm depth of the Ballum soil, where increased air diffusivity and air permeability was measured at ®eld-sampled water content. Continuity indices
*Corresponding author. Tel.:45-8999-1766; fax:45-8999-1719.
E-mail address: [email protected] (P. Schjùnning).
calculated from air diffusivity and air permeability measurements showed that the DD soil from the 4 to 8 and 14 to 18 cm depths had less continuous and more tortuous macropores than the ploughed soil.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Direct drilling; Ploughing; Soil strength; Soil pore characteristics; Pore continuity; Plough pan
1. Introduction
Tillage has been performed since ancient times in order to ful®l three major requirements: (i) to improve soil tilth, (ii) to combat weeds and (iii) to incorporate plant residues and organic manures. For most soil management systems and crop rotations, all three requirements are still valid. Mouldboard and disc ploughing have been the most obvious way of meeting these demands and are still the most widely used primary tillage methods in many countries including Denmark (Riley et al., 1994; Rasmussen, 1999). How-ever, the mechanical turnover of the top 20±25 cm layer of soil is an energy-demanding procedure, it brings new weed seeds to the top layer ready for germination, and it kills a lot of soil fauna active in the turnover of organic matter (AndreÂn and LagerloÈf, 1983). Furthermore, several investigations have reported a signi®cant densi®cation of the soil layer just below the ploughing depth (e.g. Ehlers, 1973; Rydberg, 1987; Francis et al., 1987). The continued ploughing of many soils with tractors of ever-increas-ing weight and power has created critical conditions for soil processes such as air exchange (Teiwes and Ehlers, 1987; Schjùnning, 1989) and water movement (Comia et al., 1994; Ball et al., 1998). Direct drilling or shallow tillage have been shown to affect the soil characteristics of importance to these soil functions for the horizon just below the equivalent depth of ploughing, compared to continuously ploughed soil. For this critical soil layer conversion to direct drilling or shallow tillage has been shown to increase the volume of macropores (e.g. Ehlers, 1973; Comia et al., 1994) and to improve air conductivity by diffusion as well as by convection (Douglas and Goss, 1987; Schjùnning, 1989; Comia et al., 1994).
On the other hand, repeated direct drilling or shal-low tillage will increase the density of the non-culti-vated soil layers compared to ploughed soil (Rydberg, 1987; Douglas and Goss, 1987) and decrease the
volume of macropores (Douglas et al., 1980; Schjùn-ning, 1989). Schjùnning (1989) found a signi®cant reduction in air diffusivity for shallow tilled topsoil layers compared to ploughed soil and modelled this effect as being of even higher importance to the air exchange of the soil pro®le than the low diffusivity found in the dense pan below the ploughing depth in a ploughed soil. Water in®ltration has been found to increase for reduced tilled and direct drilled (DD) soils as compared to ploughing (Ehlers, 1997) which probably is due to increased numbers of persistent surface-connected macrochannels created by earthworms (Ehlers, 1975; Ehlers and Claupein, 1994) or by roots. In contrast, for some soils the saturated hydraulic conductivity has been found to be higher in ploughed soil than in DD soil (Ball et al., 1994, 1998).
Also soil strength has been shown to be signi®cantly different in ploughed and DD soil. Graham et al. (1986) reported restricted root growth due to a high mechanical strength at 15 cm depth of a DD silt loam soil. Braim et al. (1992) found considerably higher penetration resistances for DD soil than for ploughed soil. Schjùnning and Rasmussen (1989) considered the spatial variation of soil strength below tillage depth (5±10 cm) for a shallow-tilled soil and found a high frequency of areas with a high penetration resistance of >2 MPa. At the 25±30 cm depth, on the contrary, the shallow tillage system had facilitated a reduction in the mechanical strength compared to the continu-ously ploughed soil.
2. Materials and methods
2.1. Soils and trial treatments
Field trials with repeated mouldboard ploughing and direct drilling were performed at three locations in Denmark for 6 years. The soils ranged from a coarse sandy soil at Korntved, a sandy loam at Ballum and a silt loam at Hùjer (Table 1). The Korntved and Ballum soils originated from morainic material, while the Hùjer soil derived from marine sediments. Prior to the experiment all soils had been in a conventional tillage system with annual ploughing to a depth of 20 cm. The crop rotation at Korntved was spring barley and winter rye, while at Ballum and Hùjer it was spring barley and winter wheat. Straw residues were burned. Both crops were grown every year but all investigations were performed in the ®eld with the spring barley crop. Conventional tillage included ploughing in the autumn to 20 cm depth and seedbed preparation by shallow harrowing in the spring prior to drilling. The DD plots were subject to no other tillage operation but drilling by a triple-disc drill. The two tillage treatments were compared in a randomised, complete block design with three blocks at the Hùjer location and four blocks at the two other locations. The tillage plots measured 15:00 m10:25 m at the Korntved and Ballum locations and 20:00 m10:50 m at the Hùjer location.
2.2. Soil sampling and ®eld measurements
In years 4±6 following the start of the continuous trial treatments, soil was sampled for physical mea-surements in the laboratory. In each year, minimally disturbed soil cores were collected at the time of plant emergence from the soil depths 4±8, 14±18
and 24±28 cm. In year 4, an extra sampling took place approximately 4 weeks after the ®rst one. The soil cores were collected randomly from a cross-section of the tilled soil; i.e. no attempt was made to identify the location of each core relative to the drilled spots. The soil cores were retrieved in metal cylinders (diameter6:10 cm, height3:42 cm, volume100:0 cm3) forced into the soil by means of a hammer. The cylinders were held in position by a special ¯ange ensuring a vertical downward move-ment into the soil. After careful removal of the soil-®lled cylinder, the end surfaces were trimmed with a knife. At each sampling, four replicate cores were taken from each combination of trial plot and sam-pling depth, which means that a total of 1056 cores were collected for the Measuring Procedure I.
In year 4, additional 250 cm3soil cores (diameter 6:10 cm, height8:55 cm) were sampled at all loca-tions, 18±27 cm depth, with the technique described above. Three replicate cores were taken at plant emergence and again 4 weeks later for each location and trial plot (except at Hùjer where sampling was only at plant emergence). This gave a total of 114 cores, which were used in the Measuring Procedure II. Furthermore, at the Hùjer and Korntved locations, six replicate 100 cm3 cores were collected at each plot from the 14 to 18 cm layer at those two sampling dates, yielding a total of 192 minimally disturbed soil cores for the Measuring Procedure III.
In year 4, at plant emergence and again approxi-mately 2, 4 and 6 weeks later, 100 cm3soil cores were collected for determination of dry soil bulk density at the 4±8 and 14±18 cm soil depths. On the same dates, vane shear strength was measured in the ®eld at the 4± 6.5 and 14±16.5 cm depths using the method of Schaf-fer (1960). Eight replicate measurements were made for each combination of plot and measuring depth.
Table 1
Soil texture of the 0±20 cm layer of the soils investigated
Location Textural
Korntved Sand 1.5 4.6 3.4 12.4 78.1
Ballum Sandy loam 2.4 8.1 8.2 41.8 39.5
Hùjer Silt loam 2.9 19.0 51.3 25.2 1.6
At all sampling events, soil cores were collected from each plot at two separate subplots of approxi-mately 1 m2in order to minimise the effect of micro-scale variability. The replicate vane shear measure-ments were also performed in these subplots. The sampling and measurement programme is summarised in Table 2.
2.3. Laboratory measurements
Measuring procedure I: Soil air diffusivity and air permeability were measured at the water contents at the time of sampling and again following adjustment of the matric potential toÿ100 hPa on a tension table. A non-steady state method was used for the diffusivity determination, in principle as suggested by Taylor (1949) and with the technique described by Schjùn-ning (1985a). Air permeability was measured by the steady-state method of Grover (1955), where air at a constant pressure difference, displaced by a descend-ing ¯oat chamber, ¯ows through the soil column at a rate that is proportional to the air permeability. Prior to these measurements, careful kneading of the soil sur-face at the boundary to the metal ring minimised the risk of air diffusing or leaking along the metal ring. For the cores sampled in the trial years 4 and 5, the measuring procedure was concluded with determina-tion of the penetradetermina-tion resistance of the soil. This measurement was carried out in accordance with the suggestions of Whiteley et al. (1981). The samples were placed in the apparatus, in which ®ve steel penetrometers (2 mm diameter, 308-apex semi-angle) concurrently and automatically were pushed into the
soil sample at a rate of 3 mm minÿ1. At a soil `depth' of 8 mm (4 times the probe diameter) the penetration resistance was recorded automatically.
Measuring procedure II: Air permeability was mea-sured at the ®eld water content and again following adjustment of the samples toÿ100 hPa matric poten-tial. The technique was as described above.
Measuring procedure III: Shear annulus strength (Schjùnning, 1986) was determined at normal loads of 30, 90 and 150 kPa, with soil cores at ®eld-sampled water contents. The principle in the shear test is torque measurement during a rotational shearing action of a grousered shear annulus. The maximum shear strength was calculated from the shear-strength±strain relation-ship and the recorded torque, taking the mean shear radius as representative of the shearing soil annulus as detailed by Schjùnning (1986).
Particle density (pycnometer method; Blake and Hartge, 1986) was determined on air-dried soil bulked from replicate plots for each of the 4±8, 14±18 and 24± 28 cm soil depths. Porosity was calculated from bulk and particle density, and the air-®lled pore fraction further estimated by subtraction of the volumetric water fraction.
2.4. Climatic conditions
Mean annual precipitation and temperature for Denmark is 712 mm and 7.78C, respectively (1961± 1990 average). Precipitation excess of 200±400 mm during the winter ensures that soil is at ®eld capacity every spring. For trial year 4, when sampling and ®eld measurements took place on several occasions during
Table 2
Summary of sampling events for laboratory measuring procedures (I±III, see text) and of ®eld vane shear strength measurements (F)
Depth (cm) Years from start of trial
4 5 6
aVane shear measurements: 4±6.5 and 14±16.5 cm. bOnly at Korntved and Hùjer.
a 6-week period following the germination of the crop, precipitation approximately balanced the evapotran-spiration (data not shown), leaving the soil at a water content close to that of ®eld capacity.
2.5. Statistical analysis
Data from each year were analysed using a general linear model assuming that the random effects (errors) were independent and followed a normal distribution with mean zero and constant variance. Some variables were log transformed before the analysis in order to improve the distributional assumptions. The hypoth-esis of no treatment effects were tested by F-tests using the treatment by block term as error (both treatments were present in all blocks). Data from all years were analysed together in another general linear model. In order to take into account that in 2 of the 3 years, observations came from the same ®eld (and thus must be expected to be correlated) this model included
both systematic and random effects (see, e.g., Searle et al., 1992). The effects of treatment, year and the interaction treatment by year were assumed to be systematic effects. The effects of blocks within ®eld, treatment by block within ®eld and sampling dates were assumed to be random effects. Each random effect (each error) was assumed to be independent and to follow a normal distribution with mean zero and constant variance. The calculations were performed using the procedures glm and mixed of SAS (SAS, 1996).
3. Results and discussion
3.1. Soil strength
Except for the 4±8 cm depth of Ballum soil, there was a clear trend of higher bulk density in DD soil in the 4th year of the trial period (Table 3). The water
Table 3
Bulk density, volumetric water content at plant emergence (sampling No. 1), and vane shear strength at 0, 14, 28 and 42 days after emergence for the 4th year of continuous tillage treatment
Measuring time No.
Tillage system
Location
Korntved Ballum Hùjer
4±8 cm 14±18 cm 4±8 cm 14±18 cm 4±8 cm 14±18 cm
Bulk density (g cmÿ3) Average PL 1.44 1.46 1.53 1.50 1.36 1.38
DD 1.48 1.51 1.53 1.54 1.47 1.44
LSD0.05 NS 0.02 NS NS 0.05 NS
Water content (m3/(100 m3)) 1 PL 11.3 12.9 30.8 30.6 32.9 34.3
DD 11.3 12.0 27.4 25.3 34.1 35.6
LSD0.05 NS NS 1.5 2.1 NS NS
Vane shear strength (kPa) 1 PL 31.6 28.5 30.5 28.9 31.0 39.5
DD 26.3 34.3 39.3 41.8 61.7 64.2
LSD0.05 NS NS NS 4.3 16.5 13.8
2 PL 17.1 16.6 36.8 32.4 37.7 47.6
DD 19.4 28.7 37.8 48.2 61.0 67.0
LSD0.05 NS 3.8 NS 4.5 21.2 NS
3 PL 25.6 22.0 46.9 37.3 35.6 41.4
DD 23.5 27.5 48.6 47.1 59.3 56.9
LSD0.05 NS 2.1 NS 8.8 17.8 13.9
4 PL 28.2 28.0 71.2 55.7 89.1 94.6
DD 31.5 42.6 69.7 73.7 107.5 76.3
LSD0.05 NS 9.2 NS 5.3 NS NS
Average PL 25.6 23.8 46.3 38.6 48.4 55.8
DD 25.2 33.3 48.8 52.7 72.3 66.1
content at plant emergence (®rst sampling date) might be regarded as an estimate of ®eld capacity and was found to be equal for the two treatments at the Korntved and Hùjer locations, while it was signi®-cantly lower for the DD treatment than the ploughed treatment (PL) at Ballum. Vane shear strength was highest for the ®ner textured soils, which is in accor-dance with Ball and O'Sullivan (1982). Only small increases in the vane shear strength were registered during the period studied. This was due to the climatic conditions, since the soil had about the same water content at all measuring times (data not shown). For the 4±8 cm depth in the coarse sandy soil at Korntved and the sandy loam at Ballum, the tillage system had only minor effects on the vane shear strength (Table 3). For the other combinations of location and measuring depth, averaged over the four measuring dates, direct drilling increased the vane shear strength by about 10± 24 kPa compared to ploughed soil, with the highest value for the silty loam at 4±8 cm depth. These increases corresponded to 18±49%. Also, Ball and O'Sullivan (1982) found increased vane shear strength in DD soil compared to mouldboard ploughing. Graham et al. (1986) and Ball and O'Sullivan (1982) found the limiting soil strength Ð as measured by a vane shear tester Ð for unrestricted shoot and root growth of spring barley to be approximately 50
and 65 kPa, respectively. Based on these ®ndings, a restriction in plant growth might be expected for the sandy loam at Ballum and especially for the silt loam soil at Hùjer. This is in accordance with the plant population data reported for the present trials by Rasmussen (1988).
Similar to the ®eld vane measurements, the labora-tory shear annulus estimates of strength remained unchanged from the ®rst sampling date (at plant emergence) to the second sampling date 4 weeks later (data not shown). Therefore, measurements from both samplings were bulked into one analysis of the effects of soil tillage system on the soil strength (Fig. 1). For the coarse sandy soil at Korntved, the level of cohesion calculated from the shear annulus measurements was found to be approximately 70±75% of the vane shear strength measured in the ®eld (Fig. 1). This is in accordance with the most often quoted ®ndings (e.g. Fountaine and Brown, 1959), that the vane will over-estimate soil cohesion. Noticeable therefore, is the data obtained for the silty loam at Hùjer, display-ing a higher cohesion than the vane estimate of strength (Fig. 1). This may be due to anisotropy of the soil, the vane shearing the soil primarily in a vertical plane of failure, while the loaded annulus in the laboratory shears the soil in a horizontal plane. For the coarse sandy soil at Korntved at all three levels
of normal load, nearly identical strength values were obtained for the two tillage systems. This results in nearly identical estimates for the cohesional and fric-tional components of the strength for this soil. For the silty loam at Hùjer, higher values of strength were registered for the DD compared to the ploughed soil, especially at higher levels of normal load (Fig. 1). Although this was re¯ected in higher values of cohe-sion and internal friction for this soil, the effects were not statistically signi®cant.
The sinkage of the shear annulus when applying the normal load is shown in Fig. 2, full lines indicating the relative height of the soil core before shearing the annulus (`static' sinkage), and broken lines the situa-tion after shearing (`dynamic' sinkage). Naturally, for both soils higher levels of sinkage (compaction) were obtained at higher levels of normal load. For the two highest normal loads, the `static' as well as the `dynamic' sinkage were higher for the coarse sandy soil at Korntved than for the silty loam at Hùjer. For the 30 kPa normal load, on the other hand, `static' sinkage was about identical for the two soils, and for the aggregated silty loam soil the `dynamic' sinkage indicated an increase in soil volume due to aggregates `rolling' on each other. Generally, the largest sinkage was registered for ploughed soil (Fig. 2), probably re¯ecting the differences in bulk density as well as the soil strength, as discussed above.
The cone index (CI) obtained by the micro-penet-rometer measurements on soil cores drained to ÿ100 hPa water potential in the laboratory (cores sampled in years 4 and 5 after changing to direct drilling) appeared to be considerably lower for the coarse sandy soil at Korntved than for the loamy soils at Ballum and Hùjer (Fig. 3). For all three locations at the 4±8 and 14±18 cm depths, direct drilling increased the CI compared to annually ploughed soil. There were no signi®cant differences in CI for the 24±28 cm depth. However, it should be noted that for the Hùjer soil, aloweraverage value of CI was measured for the soil that had been DD for 4±5 years compared to the annually ploughed soil (Fig. 3). The frequency distribution clearly reveals that this is due to a considerable lower number of soil cores having pene-tration resistance above 2 MPa. Braim et al. (1992) also reported cone penetration resistance considerably higher for the upper layers of DD soil compared to ploughed soil. However, the maximum values registered were moderate compared to this investiga-tion and compared to the limits for restricinvestiga-tion of root growth reported in the literature (e.g. Taylor, 1971; Dexter, 1987). Blackwell et al. (1986) and Graham et al. (1986) demonstrated that mechanical strength in DD soil exceeding approximately 1.4 MPa slowed the growth of seminal roots of winter wheat. When comparing these ®ndings to the strength
data from the present investigation (Fig. 3), it is obvious that a restriction in root growth would be expected especially for the DD silty loam soil at Hùjer which may be the reason for the lower plant population registered for that location (Rasmussen, 1988).
3.2. Soil pore characteristics
Air- and water-®lled pore space at the time of sampling, the total soil porosity, the relative air diffu-sivity, and the air permeability at these water contents are tabulated as averages of the samplings during the
trial years 4±6 (Table 4). As sampling took place at a water content of about ®eld capacity (and the extra sampling in trial year 4 also took place at an equivalent water content), these data can be taken as an indication of the situation at the start of the growing season. For the coarse sandy soil at Korntved, no effects of the tillage treatments were generally observed for the 24± 28 cm soil depth, while at the two upper sampling depths and especially at the 4±8 cm depth, direct drilling decreased soil porosity and air-®lled pore space (Table 4). For both these soil layers the decreased air-®lled pore space resulted in signi®cantly lower (although still comfortably high) values of
relative diffusivity and air permeability. This is in accordance with most reports for DD soil compared to ploughed soil (Ball, 1981b; Douglas and Goss, 1987; Comia et al., 1994; Ball and Robertson, 1994). On the sandy loam soil at Ballum, DD seemed to reduce the soil water content compared to the ploug-hed soil. Although also a trend of lower porosity for the DD compared to the PL treatment was observed, signi®cantly higher volumes of soil were air-®lled in the DD soil. This further yielded signi®cant increases in diffusivity and permeability (Table 4). It is notice-able, that the trend of reduced water content at ®eld capacity was observed also for the 24±28 cm depth.
Table 4
Air- and water-®lled soil pore volumes, relative air diffusivity and air permeability for soil cores at sampling as averaged for all sampling dates in years 4±6 of continuous tillage treatmenta
Location Soil depth
(cm)
Tillage system
Soil volume (m3/(100 m3)) Relative diffusivity
( DS=D0 1000)
Air permeability, log(KS,mm2)
Air-filledb Water-filledb Porosityb
Korntved 4±8 PL 32.9 12.2 45.1 100.0 1.54
DD 28.4 12.9 41.2 76.9 1.25
LSD0.05 1.6 (2) 0.7 (2) 1.4 (2) 8.9 (2) 0.07 (2)
14±18 PL 28.5 13.7 42.5 81.5 1.40
DD 28.3 13.0 41.6 72.0 1.33
LSD0.05 NS (1) NS (2) NS (1) 9.3 (1) NS (1)
24±28 PL 28.3 13.4 41.4 76.6 1.36
DD 27.7 13.8 41.3 67.1 1.35
LSD0.05 NS (2) NS (1) NS (0) NS (1) NS (0)
Ballum 4±8 PL 13.1 27.6 40.7 13.6 0.77
DD 15.0 24.9 39.8 16.8 0.76
LSD0.05 1.6 (3) 0.9 (3) NS (0) NS (2) NS (0)
14±18 PL 13.1 28.2 41.2 13.1 0.71
DD 16.5 23.8 40.2 26.4 0.98
LSD0.05 1.7 (2) 1.2 (3) NS (0) 4.7 (2) 0.21 (2)
24±28 PL 12.9 25.1 38.0 11.3 0.65
DD 15.2 23.4 38.6 16.7 0.85
LSD0.05 1.9 (2) 1.0 (3) NS (0) 3.6 (1) 0.18 (0)
Hùjer 4±8 PL 10.9 34.9 45.3 11.7 0.32
DD 8.7 35.4 43.7 6.4 0.19
LSD0.05 NS (0) NS (0) NS (2) NS (0) NS (0)
14±18 PL 9.0 36.8 45.3 9.2 0.71
DD 6.7 36.8 43.1 6.0 0.63
LSD0.05 NS (0) NS (0) 1.8 (0) NS (0) NS (0)
24±28 PL 6.1 38.3 44.1 3.5 0.12
DD 7.4 38.9 46.1 4.8 0.52
LSD0.05 NS (1) NS (0) NS (0) NS (1) NS (1)
aThe numbers in parentheses indicate the number of individual sampling events (out of the maximum possible of four) with signi®cant
trend (P0:05) similar to the one found for the average of all four samplings.
bAir- and water-®lled pore volumes do not always exactly sum up to yield total porosity volume. This is due to the statistical procedure
For the silty loam soil at Hùjer, the only statistically signi®cant effect was a reduced porosity in DD com-pared to PL at the 14±18 cm depth. It should be noted, however, that this was due to a reduction in the volume of air-®lled pore space from 9.0 to 6.7% v/v. Also for the 24±28 cm depth, the air-®lled pore space for both tillage treatments was below the 10% v/v limit often quoted as the minimum for optimal plant growth. Generally, direct drilling tended to reduce air-®lled porosity for the two upper soil depths, leading to reduced diffusivity and permeability. However, the effect is much smaller than was found when perform-ing shallow tillage on the same soil (Schjùnnperform-ing, 1989). Noticeable is also an increase in porosity at the 24±28 cm depth for soil that was not ploughed for 4±6 years. Probably the increased porosity was due to a higher volume of macropores as re¯ected in a higher air-®lled porosity for DD than for ploughed soil. Although statistically insigni®cant, this led to rather dramatic increases in air diffusivity and especially air permeability (Table 4). Neither these effects were statistically signi®cant for the average results from 3 years (four samplings). However, the trend was found signi®cant for one of the samplings which was the case also for the increased volume of air-®lled pore space at ®eld capacity for the DD compared to ploughed soil (Table 4). The results from the 24 to 28 cm depth of the Hùjer soil are in accordance with the results from a long-term tillage trial at the same location, reported by Schjùnning (1989). In that trial continuous non-ploughing tillage also led to an ame-lioration of the dense plough pan that had built up in a continuously ploughed system. Also Douglas and Goss (1987) and Comia et al. (1994) reported higher ¯uid transport coef®cients for ploughless tillage com-pared to ploughed soil for the horizon below normal ploughing depth.
Table 5 and Fig. 4 report the results from the measurements on the soil cores when drained to a matric potential ofÿ100 hPa. For the Korntved and Hùjer soils, essentially all the trends discussed for the soils at their ®eld-sampled water contents (Table 4) were also found when the water content was controlled by a hanging water column of 100 cm in the labora-tory. This means the same trends in the volume of macropores (Table 5) and signi®cantly higher esti-mates for relative diffusivity and air permeability in ploughed soil at the upper soil layer for the coarse
sandy soil at Korntved (Fig. 4). Although not signi®-cant, both these parameters were higher for DD than for ploughed soil at the 24±28 cm depth at the silty loam at Hùjer (Fig. 4). Also for the sandy loam at Ballum, the relative diffusivity displayed the same trend as for the ®eld-moist soils, i.e. signi®cantly higher diffusivities were found for DD soil compared to ploughed soil for the 14±18 and 24±28 cm depths. Air permeability, on the other hand, had lower values in DD soil.
For the coarse sandy soil at Korntved and for the sandy loam at Ballum, the air permeability measured in the longer soil cores sampled at the 18±27 cm depth was found to equal the level measured in the 3.4 cm cores (Fig. 4). However, for the silty loam at Hùjer, this estimate of permeability was only about one tenth of the values measured for the 3.4 cm cores at the 14±18 and the 24±28 cm depths. Further, it should be noted that the deviation from these estimates is most pronounced for the ploughed soil (note the logarithmic scale). This indication of a low perme-ability for the transition layer between ploughed top-soil and undisturbed subtop-soil is in accordance with results of Douglas et al. (1980) and Douglas and Goss (1987).
According to the model of Ball (1981a), the effec-tive diameter,DB, of soil pores active in the transport
of air through the porous soil system can be calculated asDB28K= DS=D01=2, whereKis the air perme-ability,DSthe diffusion coef®cient for gas in soil, and
D0the diffusion coef®cient for gas in the atmosphere.
For the 4±8 and 14±18 cm depths of the Korntved soil and especially the Ballum soil, direct drilling decreased this pore size estimate (Table 5). Note also the signi®cant decrease in pore size estimate for the 24±28 cm depth of the Ballum soil. This is remarkable as a highervolumeof pores>30mm was found in this horizon. No signi®cant effects were found for the silt loam at Hùjer.
The parameter,CG DS=D0=e, relating the rela-tive diffusivityDS/D0and the air-®lled pore space,e, is
an expression of the effectiveness of the air-®lled pore space in conducting air by diffusion, i.e. an index of continuity of the soil pore system (Gradwell, 1961). For the coarse sandy soil at Korntved, a signi®cant reduction in theCGcontinuity index was found for the
at Ballum, while no signi®cant effects were observed for the Hùjer soil.
The index, POK=e (Groenevelt et al., 1984), relating air permeability to the air-®lled pore space can also be taken as an expression of continuity of the soil pore system and is often quoted as pore organisa-tion (PO) (Blackwell et al., 1990). In accordance with theCG-index, a reduced continuity of the coarse sandy
soil at Korntved was found when converting from ploughing to direct drilling (Table 5). A lower PO-index was also found for DD soil in the 4±8 cm layer of the loamy sand at Ballum. The silty loam at Hùjer displayed no signi®cant effects for this parameter. However, although not statistically signi®cant, it should be noted that for the 24±28 cm horizon there
is a corresponding increase in both effective pore diameter and the two continuity indices (Table 5).
Generally, the effects of tillage on the soil pore characteristics are similar to the results obtained by Ball (1981b), Schjùnning (1985b, 1989), Ball et al. (1989) and Ball and Robertson (1994). In other words, when draining soil pores with necks in the range 30±50mm, the pores in DD or shallow tilled soil appear to be less continuous and more tortuous than in ploughed soil. However, as found by Ball (1981b) and Schjùnning (1985b, 1989), the situation may well be the opposite when pores of sizes 150± 200mm only are drained of water. From that evidence, DD soils appear to exhibit larger, channel-like pores than ploughed soil.
Table 5
Volume of pores with tube-equivalent diameter >30mm and selected model estimates of soil pore characteristics as averaged for all sampling dates in years 4±6 of continuous tillage treatmenta
Location Soil depth (cm) Tillage system Pores>30mm
(m3/(100 m3))
Effective pore diameter,DB(mm)
Continuity indices
CG(mÿ310ÿ2) log(PO,mm2)
Korntved 4±8 PL 32.3 118 26.3 2.05
DD 27.5 99 21.2 1.81
LSD0.05 1.6 (2) 7 (1) 1.5 (2) 0.06 (2)
14±18 PL 28.1 112 23.4 1.95
DD 27.8 106 21.5 1.87
LSD0.05 NS (1) 6 (0) 1.4 (1) 0.06 (0)
24±28 PL 28.4 105 24.2 1.92
DD 28.2 105 23.0 1.90
LSD0.05 NS (0) NS (0) NS (0) NS (0)
Ballum 4±8 PL 12.6 181 10.3 2.01
DD 13.4 161 8.7 1.80
LSD0.05 NS (2) 16 (2) 1.3 (0) 0.11 (2)
14±18 PL 12.6 188 10.1 2.00
DD 14.6 141 11.2 1.80
LSD0.05 1.5 (2) 19 (3) NS (0) 0.14 (1)
24±28 PL 12.2 156 8.6 1.79
DD 14.1 138 9.1 1.71
LSD0.05 1.7 (2) 14 (1) NS (0) NS (2)
Hùjer 4±8 PL 7.5 130 5.7 1.32
DD 5.2 140 5.4 1.28
LSD0.05 NS (0) NS (0) NS (0) NS (0)
14±18 PL 7.0 203 10.7 1.93
DD 4.9 201 9.2 1.92
LSD0.05 NS (0) NS (0) NS (0) NS (0)
24±28 PL 4.6 170 7.9 1.70
DD 6.2 181 8.1 1.83
LSD0.05 NS (1) NS (0) NS (0) NS (0)
aThe number in parentheses indicates the number of individual sampling events (out of the maximum possible of four) with signi®cant
4. Conclusions
When compared with continuously ploughed soil, 4±6 years with direct drilling caused the upper soil layer (0±20 cm) to:
be more dense and display higher strength, and for the silt loam also a tendency of higher cohesion and soil internal friction;
exhibit a reduced volume of macropores (>30mm) for the sandy soil and the silt loam, whereas the opposite was found for the sandy loam;
display a reduced continuity of pores when drained to a matric potential ofÿ100 hPa.
For the soil below 20 cm the DD soil appeared to:
exhibit a reduced frequency of high-strength spots in the silt loam,
display increased volumes of macropores (>30mm) in the sandy loam and the silt loam,
display a higher continuity of macropores for the silt loam.
The results also showed that the pore system in the two soil layers in the sandy soil and the sandy loam was well connected whether the soils were ploughed or directly drilled. On the contrary, the ploughing had on the silt loam introduced a limiting permeability, which was only partly eliminated by 4 years of direct drilling. No general recommendation concerning til-lage system can be given from the studies. However,
Fig. 4. Relative air diffusivity,DS/D0, and log air permeability (mm2) measured in 100 cm3soil cores at a matric water potential ofÿ100 hPa.
The air permeability values indicated at 20 cm depth derive from longer, 250 cm3soil cores sampled in the 18±27 cm depth. (*) Ploughed,
the results highlight the bene®ts and drawbacks for both tillage systems in terms of soil physical properties.
Acknowledgements
We thank Dr. K. Kristensen and Dr. B. Hansen of the Biometry Research Unit of the Danish Institute of Agricultural Sciences for invaluable statistical advice.
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