Effects of soil water hysteresis and the direction of sampling on aeration and pore function in relation to soil compaction and tillage
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E LS EV I E R Soil & Tillage Research 32 (1994) 51-60
Effects of soil water hysteresis and the direction of sampling on aeration and pore function in relation to
soil compaction and tillage
B.C. BalP'*, E.A.G. Robertson b
aSoil Science Department, SAC, West Mains Road, Edinburgh, EH9 3JG, UK bScottish Centre of Agricultural Engineering, SAC, Bush Estate, Penicuik, Midlothian, EH26 OPH,
Accepted 23 March 1994
Abst rac t
Cores of intact soil were taken in vertical and horizontal directions in two tillage exper- iments from horizons subjected to compaction. Cores from below the plough layer were equilibrated at a range of matric potentials by drainage. Cores from near the soil surface were subjected to hysteresis by drainage to -2 kPa from saturation, further drainage to - 10 kPa and finally re-wetting to - 2 kPa. Air-filled porosity, relative diffusivity, diffu- sion time delay and air permeability were measured at each potential, and pore continuity and pore organisation indices were calculated. Diffusion time delay is a measure of the time taken for gas to diffuse across a core at the beginning of a measurement of diffusion.
Hysteresis influenced both air-filled porosity and the relationships between air-filled porosity and gas diffusivity and air permeability. Soil aeration was generally more favour- able after wetting to - 2 kPa from - 10 kPa than after drainage to - 2 kPa from saturation. Hysteresis effects were greater in ploughed than in direct drilled soils. Direction of sam- pling influenced diffusion and flow properties, mainly in compact direct drilled soil from near the surface. These effects were partly attributed to the sampling method but gave evidence of greater vertical than horizontal orientation of macropores in ploughed soil and greater horizontal than vertical orientation of macropores in direct drilled soil.
Keywords: Macropore orientation; Field cores; Gas diffusivity; Air permeability; Pore continuity
* Corresponding author.
0167-1987/94/$07.00 1994 Elsevier Science B.V. All rights reserved SSDI 0167-1987 ( 94 ) 00405-4
52 B.C. Ball, E.A.G. Robertson / Soil & Tillage Research 32 (1994) 51- 60
In moist climates in the United Kingdom, many arable crops are sown in the autumn and spend the winter in soil close to field capacity. Soil aeration status may influence crop survival in such wet soil, particularly if the soil has also been compacted (Campbell et al., 1986; Ball and Robertson, 1990). In such soils, the presence of a sufficient volume of continuous macropores is particularly impor- tant for soil aeration (Blackwell et al., 1986; Ball and Robertson, 1994). These macropores may show some preferential orientation. For example, in compact direct drilled soils with lenticular or laminar structure, horizontally oriented planar voids may occur between structural units (Bullock et al., 1985) whereas worm holes penetrating the structural units may be vertically oriented (Ehlers, 1975 ). Thus pore type or continuity may be anisotropic. In order to detect such direc- tional effects, measurements need to be made both horizontally and vertically at the same depth, which can be achieved by taking core samples in different orientations.
The matric potential of the soil water at field capacity is approximately -6 kPa in south-east Scotland (Duncan, 1979). During autumn and winter, the soil is subjected to wetting and drying cycles in a limited range around this potential. At any given matric potential within this range, the soil water content is likely to be influenced by hysteresis (Mualem, 1984). Since soil water hysteresis involves the differential blockage of pores by air during wetting and drying (Mualem, 1984),air-filled macropore continuity is also likely to be affected by hysteresis. Soil water hysteresis is thus likely to influence soil aeration. Core measurements of gas diffusivity and permeability provide useful indicators of soil aeration and, provided that air-filled porosity is known, macropore continuity indices can be assessed from measurements of diffusion (Ball et al., 1988 ) and air permeability (Blackwell et al., 1990).
The objective of this study was to discover whether relative diffusivity, air permeability and macropore continuity are influenced by the direction of sam- piing and by soil water hysteresis in wet soils where soil structure has been influ- enced by tillage and compaction.
Relative gas diffusivity and air permeability were measured in soil cores using the methods of Ball et al. ( 1981 ). Diffusivity was determined by measuring the diffusion of the tracer gas 85Kr between gas chambers attached to both ends of a core sample. In this procedure, 85Kr is injected into one chamber and the concen- tration change in both chambers is determined by regular counting offl radiation at a plastic scintillator-photomultiplier tube assembly. The counts are used in a non-linear regression technique to estimate diffusivity. Diffusivities are ex- pressed relative to those in free air adjusted to the temperature of measurement using a formula cited by Currie (1960). Diffusion time delay, the time before
B.C. Ball, E.A.G. Robertson / Soil & Tillage Research 32 (1994) 51-60 53
first appearance of the gas in the second chamber, is estimated by extrapolating the concentrations in that chamber to zero. Air permeability was measured in the same cores using the same apparatus.
From the relative diffusivity and air permeability measurements, we calculated indices of pore continuity and pore organisation. These indices relate porosity to gas conductivity. Pore continuity is determined by the overall length and degree of interconnection of the conducting paths (Ball et al., 1988 ). The pore continu- ity index is the relative diffusivity divided by the air-filled porosity (Ball et al., 1988). Pore organisation is the air permeability divided by macro-porosity (Blackwell et al., 1990). Pore organisation is effectively a measure of the conti- nuity of the macroporosity. To ensure that only the macroporosity was conduct- ing, Blackwell et al. (1990) recommended measurement of air permeability and air-filled porosity at - 6 kPa matric potential, when macropores of effective di- ameter > 0.05 mm are drained. Here we measured air-filled porosity at both - 2 and - 10 kPa when macropores of effective diameter > 0.15 mm and > 0.03 mm respectively were drained. The relationship between pore organisation and air- filled porosity is a characteristic which can be used to describe changes to the macroporosity of soils caused by management practices or biological activity (Blackwell et al., 1990). Although air permeability cannot be shown in the data points of the characteristic, isolines for air permeability can be shown, thereby allowing the value for each data point to be estimated.
In the functional model of porosity proposed by Arah and Ball (1994) pores are assumed to be either arterial, peripheral or isolated. Diffusion through soil occurs primarily through arterial pores. Peripheral pores consist of a series of dead-ends and loops lying off the arterial pores. Arah and Ball (1994) suggest that relative diffusivity is influenced by the arterial pores whereas diffusion time delay is influenced by both arterial and peripheral porosity, and that isolated po- res have no influence on diffusion. The pore continuity index takes account of the influence of air-filled porosity; thus, in the model, the pore continuity index is determined by the arterial porosity. Changes in diffusion time delay indepen- dent of those in pore continuity index indicate changes in the content of periph- eral porosity.
3. Materials and methods
Direct drilled and ploughed treatments were sampled in long-term arable field experiments of the former Letcombe Laboratory and of the Scottish Agricultural College (SAC). The Letcombe experiment was located in Berkshire, England on an Orthic Luvisol (FAO-UNESCO) of the Hamble series, a silt loam containing 16% clay (Jarvis et al., 1984). The SAC experiment was located in Midlothian, Scotland on a Gleysol (FAO-UNESCO) which is a Winton-Macmerry complex, a loam containing 18% clay (Ragg and Futty, 1967 ). The Letcombe experiment was described by Ellis et al. (1982) and the SAC experiment by Ball et al. ( 1989 ).
Tests were made on intact cores removed from the field with minimal distur-
54 B.C Ball, E.A.G. Robertson ~Soil & Tillage Research 32 (1994) 51-60
bance. Cores were retained within sampling cylinders 50 mm long and 73 mm diameter during subsequent treatments and measurements. The cores were taken 2 to 3 months after drilling of winter cereals and stored, where necessary, at 4 C before testing.
On the Luvisol, the increase in bulk density attributable to cultivation was greatest at just below the plough depth of 200-250 mm (Douglas et al., 1986). Five samples per tillage treatment were taken from just below this depth, hori- zontally from 240-310 mm and vertically from 250-300 mm. Horizontal sam- ples were taken parallel to the crop rows. Samples were equilibrated to - 2, - 10, and -20 kPa matric potential on sand suction tables and to - 100 kPa on pres- sure plates. The equilibration times for the successively decreasing potentials were approximately 6,10, 14 and 21 days. No hysteresis treatment was applied to these samples. Two samples per tillage treatment were also taken vertically from 30- 80 mm depth. These cores were tested for soil water hysteresis as described below for the Gleysol.
On the Gleysol, a direct drilled plot was chosen where crop growth appeared to suffer from adverse soil physical conditions, particularly in the season