SOUL Tillage Research

ELSEVIER Soil & Tillage Research 38 (19%) 203-216

Effect of soil macroporosity and aggregate size on seed-soil contact

A.D. Brown a, * , A.R. Dexter a, W.C.T. Chamen a, G. Spoor b a Silsoe Research Instituie, Wrest Park. Silsoe MK45 4HS, UK b Silsoe College, Cranfield Vniuersity, Silsoe MK4S 4DT, UK

Accepted 29 March 1996

Abstract

The influence of soil structure on the degree of seed-soil contact within a seedbed is poorly understood. This paper presents a simple analogue of seed-soil contact which allows the examination of the influence of macroporosity and relative aggregate size on the degree of contact within a bed of deformable spheres. A method is described in which a rigid disc or sphere representing a seed is placed within a bed of deformable spheres of uniform size representing soil aggregates. The structure is then compressed uniaxially to a given macroporosity. Contact areas were measured by a technique involving the use of paint, dismantling of the sample, and image analysis. Results show that degree of contact increases as macroporosity decreases. Greatest levels of contact are achieved where rigid and deformable spheres are of similar size. This result appears to be a consequence of maximum stress concentration occurring at this size ratio. Contact points were unevenly distributed over the surface of the rigid sphere. The applicability of these findings is considered.

Keywords: Seed-soil contact; Soil compaction; Macroporosity; Aggregate deformation

1. Introduction

Adequate seed-soil contact is a prerequisite for rapid emergence and good crop stand establishment (Hadas, 1975; Singh et al., 1985; Forbes and Watson, 1992) as it provides a route through which soil water can enter a seed. Indeed, tillage operations may be carried out specifically to ensure that reasonable contact exists (Allen, 1988; Colvin et al., 1988; Rainbow, 1994). Words such as ‘adequate’ and ‘reasonable’ are frequently

* Corresponding author.

0167.1987/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO I67- 1987(96)0 1030-6

204 A.D. Brown et al./Soil & Tillage Research 38 (1996) 203-216

used in describing seed-soil contact, as are the terms ‘poor’ and ‘insufficient’. There is a considerable degree of subjectivity in such descriptions of seed-soil contact which are based on the prevalent soil conditions: texture, aggregate size distribution and level of compaction. There appears to have been no attempt to quantify the amount of contact with these soil factors, thus little is known as to what actually constitutes ‘good’ or ‘poor’ contact. Authors thus ascribe poor germination to inadequate contact (Karlen and Gooden, 1987; Marshall and Naylor, 1984; Voorhees et al., 1985), without establishing what degree of contact the prevalent soil conditions impose or the contact required in order that water transfer from soil to seed does not restrict imbibition rate. This contact requirement may also be expected to vary with soil water availability and soil hydraulic conductivity. The lack of understanding of how soil structure translates into seed-soil contact has practical implications. Tillage operations undertaken to increase the degree of contact may have other adverse effects; for example, aggregate size reduction may increase a soil’s vulnerability to erosion while compaction can cause impedance of seedling shoots and roots. If seedbed preparation techniques are to be carried out to improve contact, it is important to know how much contact is required to benefit emergence or avoid poor establishment, what soil conditions will provide such contact and whether or not these conditions are likely to impose restrictions on later crop development. This paper presents a model which attempts to identify effects of soil structure, (soil aggregate size and macroporosity) on the area of contact between soil crumbs and seeds.

2. Theory

Models of seed-soil contact have previously been made. Bruckler (1983) proposed a model of imbibition which included consideration of seed area available for the uptake of water in both liquid and gaseous phases. By comparing imbibition rates of maize seeds partially coated with varnish in free water with the rates of seed imbibition in soil, Bruckler constructed an empirical model of seed-soil contact where water potential was non-limiting so that:

4 -=a(1 -n) +b 4

(‘1

where IZ is air-filled structural porosity, S,/S, is the proportion of surface area of seed available for liquid imbibition, and a and b are soil parameters.

Under limiting water conditions a decrease in matric potential of the soil water caused an apparent decrease in area available to liquid imbibition such that:

(2)

where + is matric potential, and c and d are fitting parameters for soil type. If a reference bulk density is used, (e.g. 0.9 mg mA3), Eq. (2) can be used for other

bulk densities by multiplication of the calculated value of S,/S, by the ratio of actual bulk density to the reference bulk density. While this model has been used with some

A.D. Brown et d/Soil & Tillqe Reswrch 38 (1996) 203-216 205

success in germination studies of maize (Bruckler, 1983) and wheat (Bouaziz and Bnrckler, 1989), it suffers from the disadvantage that model coefficients require calibration for soil type and aggregate size range.

Collis-George and Hector (1966) adopted a geometrical approach to calculate the fractional wetted area of a seed within a bed of spherical granules. It is this wetted area which determines the surface over which water transfer can occur into the seed rather than seed-soil contact per se. The wetted area of a seed is determined by the formation of air-water menisci between soil aggregates and the seed coat and is thus influenced by the proximity of aggregates (i.e. soil structure), and the soil manic potential which affects the radii of curvature of the menisci. The authors applied their model to both open and closely packed beds of spheres over a matric potential range of - 1 kPa to - 1 MPa using seed sphere size ratios of 4 and 1. The model may allow adequate estimation of the degree of contact that exists in non-aggregated soil where compaction is mainly achieved by particle rearrangement, but cannot be expected to describe the situation within a moist clay soil in which some degree of volume change is attributable to aggregate deformation (Harris, 1971). Day and Holmgren (1952) demonstrated that plastic deformation is a dominant mechanism in the compression of loose assemblages of moist soil. To a first approximation aggregates themselves may be considered as incompressible and do not undergo appreciable volume change (Kezdi, 1974); conse- quently, compaction causes an increase in bulk density but not in aggregate density. The present investigation attempts to examine the influence of aggregate deformation and uses a simplified geometry for the purposes of reproducibihty. The use of spheres in the study of soil structure is common in the literature (Collis-George and Hector, 1966; Davis et al., 1973; Dexter and Hewitt, 1978). Uniform sized spheres of modelling clay are used to represent soil aggregates while seeds are modelled as rigid spheres or discs. Compression of beds of clay spheres results in bulk volume change through particle deformation and reorientation allowing investigation of contact over a range of macrop- orosities.

Crop seeds have equivalent spherical diameters ranging from less than 1 mm (e.g. celery and carrot) to greater than 10 mm (e.g. broad bean and chickpea), while aggregate sizes between 1 and 5 mm in diameter are considered to provide a suitable seedbed (Braunack and Dexter, 1989). Thus a wide range of seed:aggregate size ratios, from 1: > 5 to > 5: 1, can occur in a well-prepared seedbed. Altering the relative sizes of plastic and rigid spheres enables the incorporation of sphere diameter ratio effects (rigid sphere diameter:plastic sphere diameter). A flat disc, having zero curvature, simulates a sphere:aggregate diameter ratio of 1:O. To obtain a representative proportional area of contact for an infinitely large seed, consideration of disc orientation relative to the direction of compressive load must be made.

The opposite extreme, with a sphere:aggregate diameter ratio of 0: 1, can be consid- ered theoretically. A seed can be situated in one of two positions in a seedbed consisting of infinitely large aggregates: (1) within the inter-facial area of neighbouring aggregates (proportion of seed surface in contact with soil = 1); (2) elsewhere on the aggregate surface (proportion of seed surface in contact with soil = 0).

Gn the assumption that seeds are randomly distributed on the surfaces of the aggregates, the mean proportional contact for a population of seeds in a compacted bed

206 A.D. Brown et d/Soil & Tillage Reseurch 38 (1996) 203-216

Fig. 1. Position of hypothetical seeds within a bed of infinitely large, plastic aggregates. 0, effective proportion of seed surface in contact with surrounding aggregates = 0. l , effective proportion of seed surface in contact with surrounding aggregates = 1.

will be equal to the probability of a seed being positioned within the interfacial areas of two aggregates (see Fig. 1). In an investigation of the effects of compression on beds of deformable spheres, Davis et al. (1973) introduced the terms R and D, where:

total area of interfaces between a crumb and its neighbours R=

surface area of a cell (3)

and volume of crumb

D= volume of cell

A cell is a hypothetical cube formed by tangent planes between a crumb and its neighbour and enclosing the crumb. It can be seen that D is equal to (1 - macroporosity) and R is equivalent to the mean proportion of contact for a population of infinitely small seeds. In the model presented by Davis et al. (1973), compression of spherical crumbs is achieved through the flattening of the spheres at interfaces producing truncated spheres of greater radii but of unaltered volume. The authors calculated values of D and R for closely packed spheres, (face-centred cubic lattice) and open packed spheres (simple cubic lattice), using the parameters r (sphere radius) and 9 (the semi-angle formed by tangents at the edge of interfacial areas between spheres), respectively (see Fig. 2). Davis et al. (1973) showed that, for a simple cubic lattice:

4R = r(cosec28 - 1) (5) and

120 = r[cosec28(9 - 4cosec8) - 3] (6)

For a face-centred cubic lattice, they showed that:

fiR = 7~( r2 - 1) (7) and

d?D=rr 3r2-q-1) i

A.D. Brown et d/Soil & Titfuge Reseurch 38 (19%) 203-216 207

Fig. 2. Deformation of spheres (after Davis et al., 1973). NP, interface between neighbouring spheres; r. truncated sphere radius; NQ and PQ, tangents at edge of interface edge; 0, semi-angle formed by tangents; - - -, cell boundary

Eqs. (5)~(8) can be rearranged to express D in terms of R for both packing arrangements, such that:

D=n

-(;+1)[9-4(f+li’-3)- 12

and for face -centred (close) packing:

D= I -

(91

(10)

208 A.D. Brown et ol./Soil & Tilluge Research 38 (1996) 203-216

Therefore the macroporosity at which a population of infinitely small seeds has a mean proportion R of its surface in contact with soil aggregates can be found as:

n=l-D (11)

Graphs of R against macroporosity are presented in Fig. 4(b) for comparison with experimental results.

3. Method

In an attempt to model the seed-soil contact areas existing between seeds and surrounding soil aggregates, rigid spheres were used to represent seeds while spheres of modelling clay were used to form artificial aggregates. The clay spheres were incom- pressible: measurement of volume under applied load revealed that no compression of the material was occurring. The sizes of both artificial seeds and aggregates were varied in order to investigate the effects of sphere:aggregate diameter ratio. Three sphere:ag- gregate diameter ratios were initially investigated, 2.5: 1, 1: 1 and 1:2. A fourth series in which the model seed was effectively infinitely larger than the aggregates will be described later. Dimensions of model seeds and aggregates are shown in Table 1. In each trial, uniform clay spheres were arranged randomly to half fill a cylindrical chamber which had a detachable bottom. Uniformity of size of the clay spheres was attained by careful weighing of a standard amount of clay. The clay was then rolled into a ball. Variability of diameter of resultant spheres was statistically insignificant (P < 0.05). A rigid sphere was placed within the cylinder and remaining spheres added to fill the container. The spheres representing seeds had a line etched onto their surfaces dividing the sphere into two hemispheres. Care was taken to place the sphere such that this line was perpendicular to the plane of applied load allowing the distribution of contacts on upper/lower surfaces and lateral surfaces to be compared. The container was then placed on a loading frame and the clay structure compacted to a calculated thickness conferring a preselected macroporosity on the structure such that:

(12)

where h is height to which clay column is compacted, A is section of cylinder, M, is mass of clay aggregates, p, is density of clay, and V, is volume of model seed.

The PTFE lining of the cylinder minimised friction between the aggregates and the inner surface of the cylinder. After loading, the cylinder was removed from the loading frame and the bottom detached to allow removal of the structure from within the cylinder. The clay structure was placed in a basin. Matt black paint was poured over the structure six to ten times at various orientations to ensure the paint penetrated to the centre of the structure. When the structure was dismantled it was apparent that the paint had indeed coated all exposed surfaces of the rigid sphere. For higher macroporosity trials (n > 0.4) the structure was unable to withstand disturbances caused by re-orienta- tion. It was thus necessary in these cases to retain a cylindrical support provided by wide

A.D. Bronm et d/Soil & Tilluge Reseurch 38 (1996) 203-216 209

Table I Dimensions and initial packing macroporosities of model seeds and aggregates

Sphere:aggregate Model seed Model aggregate Initial packing diameter ratio diameter (mm) diameter (mm) macroporosity

2.5: I 14.0 35 .o 0.424 ( + 0.009) I:I 25.0 25.0 0.424 ( f 0.005) I:2 35.0 17.5 0.438 (* 0.020) I:0 (disc) 30.0 30.0 0.430 (+ 0.020)

Standard errors in parentheses. Cylinder chamber dimensions: radius 50 mm, height 80 mm.

bore plastic tubing. Plastic mesh was placed over the tube ends to allow paint to flow freely in and out of the structure while maintaining the structure’s integrity.

After the paint had dried, the clay structure was dismantled and the rigid sphere retrieved. Residual clay on the sphere was removed to expose areas of the sphere untouched by the black paint. These areas were painted white to clearly distinguish them as points of contact. Typical appearance of spheres can be seen in Fig. 3. The number and distribution of contacts relative to the equator were recorded. The proportion of the model seed in contact with clay spheres was found using image analysis. The investiga- tion included three seed:aggregate diameter ratios at four macroporosities with three replicates of each trial.

For a sphere:aggregate diameter ratio of l:O, rigid spheres were replaced with flat discs of PTFJZ. Discs were placed amongst the spheres at O”, 45” and 90” to the plane of applied stress. After loading and dismantling, imprints of the clay spheres where they were in contact with the disc were clearly discernible on the disc surface. The circumference of the disc and the areas of the contact points were traced in black ink onto transparencies. Areas of contact were measured using image analysis. Results were taken from both upper and lower surfaces of the disc, and three replicates were performed each at four macroporosities.

3.1. Image analysis

Image analysis was used to find the proportion of black areas (non-contact) to the proportion of white areas (contact) on the surface of the rigid spheres from two-dimen- sional monochrome images. A simple program was constructed in which a computer used an eroded mask image of a completely white sphere as a template in which the number of black pixels and white pixels were counted. The template discounted an outer ring of a width of 4% of the image diameter in order to avoid including areas of the sphere which were considerably distorted due to its curvature. Results from all replicates were pooled to find the mean proportional surface area of the artificial seed in contact with surrounding spheres. This method was considered to negate any bias caused by reducing a three-dimensional image into two dimensions. The proportion of black to white pixels measured should tend towards the actual value as the number of randomly orientated views of the sphere is increased. This was verified by using the technique to estimate known areas on spheres. No significant differences were observed between

210 A.D. Brown et al./Soil & Tillage Research 38 (1996) 203-216

Fig. 3. Spheres representing artificial seeds showing areas of contact, (white areas) at two macroporosities, n = 0.08 (a) and n = 0.41 (b), at sphere:aggregate diameter ratio 2.5: I.

estimated and actual areas. In order to assess accurately the proportion of contact, 16 sets of orientations were analysed for each sphere. Each set of observations included one randomly orientated image and its obverse. Taking further views of the spheres did not alter the mean estimated contact area significantly. To obtain a pattern of contact distribution on the model seed a further six images were analysed: four lateral views

A.D. Brown et d/Soil & Tillage Reseurch 38 (1996) 203-216 211

attained by quarter turns around the equator, and upper and lower views in the plane of the equator. Proportional areas of contact on discs could be calculated from single images as areas were recorded in only two dimensions.

4. Results

Initial packing macroporosities did not vary significantly (P < 0.05) for different sphere:aggregate size ratios (see Table 1); consequently, the degree of deformation to achieve a given macroporosity should be similar for all conditions.

Fig. 4(a) shows the change in the proportion P of the surface area of the rigid sphere in contact with surrounding plastic spheres over a macroporosity range of n = 0.1-0.5.

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0.7

0.6

0.5

d 0.4

0.3

0.2

0.1

0.0

macroporosity

00 01 0.2 0.3 0.4 0.5 0.6

macroporosity

Fig. 4. (a) Contact between rigid spheres and aggregates. P, proportion of rigid sphere surface in contact with aggregates. Sphere:aggregate diameter ratios: +, 1:O; n , 2.5: I; A, 1: 1; v , 1:2. (b) Change in R for open and closely packed deformable spheres, where R is interfacial areas between a crumb and its neighbours as a proportion of the surface area of a cell (equivalent to P for a population of infmitely small seeds).

, simple cubic packing; -- --, face-centted cubic packing.

212 A.D. Brown et al./ Soil & Tillage Research 38 (1996) 203-216

Table 2 Linear regression coefficients for P against macroporosity for four sphere:aggregate diameter ratios

Coefficients Sphere:aggregate diameter ratio

2.5: 1 1:l 1:2 l:o

a 0.72 (+O.OOl) 0.76 ( f 0.002) 0.61 (+ 0.002) 0.57 ( * 0.005) h - 1.33 (*0.004) - 1.33 ( * 0.065) - 0.98 ( f 0.004) - 1.24(&0.018) r2 0.999 0.982 0.997 0.992

Standard errors in parentheses.

Results from the four sphere:aggregate diameter ratios form a series of curves on which regression analysis was performed. The data were fitted to the equation:

P=a+bn

0.2 -

0.0 0.1 0.2 0.3 0.4

macroporosity

tj 0.8 - 2 3

; 0.6 - z 8 $

2 0.4 - e

‘5

0.2 -

I I I I I

0.0 0.1 02 0.3 0.4

macroporosity

Fig. 5. (a) Distribution of contacts on the surface of spheres. Sphenzaggregate diameter ratio 2.5:l. l , upper/lower views; n , lateral views; A, mean proportional contact. (b) Distribution of contacts on the surface of spheres. Sphenxaggregate diameter ratio 1:l. 0, upper/lower views; l , lateral views; A, mean proportional contact.

A.D. Brown et d/Soil & Tillage Reseurch 38 (1996) 203-216 213

Values of the coefficients a and b are presented in Table 2. Although proportional contact would not be expected to vary linearly over the complete macroporosity range and cannot be legitimately extrapolated to the Y-axis, for the experimental conditions a linear fit provided an adequate description of the data over the range of macroporosities investigated. Below the macroporosity range investigated, curves must converge to a proportional contact value of 1 as macroporosity approaches zero. A logit transformation was used to perform analysis of parallelism on the regression lines. It was found that regression slopes did not vary significantly between sphere:aggregate diameter ratios. Projected Y-axis intercept values for sphere:aggregate diameter ratios of 2.5:1 and I:2 were not significantly different, while that of diameter ratio 0:l was significantly lower and that of diameter ratio 1: 1 was significantly greater (P < 0.05). Thus greatest proportional contact was achieved over this macroporosity range at a sphere:aggregate diameter ratio of 1: 1 whereas the lowest occurred at a diameter ratio of l:O. Ratios of 1:2 and 2.5: 1 gave intermediate values. Standard error bars are notably longer for results obtained using discs (sphere:aggregate diameter ratio l:O). This is a consequence of the results having been calculated from the mean of three orientations, (0”, 45” and 90% to the direction of loading. Comparison of 90” and 0” orientations with upper/lower and lateral views of spheres also indicate the lower contact areas attained at the 1:0 diameter ratio (graphs not presented). Fig. 4(b) shows the curves calculated for sphere size ratio 0: 1 for open and close packing; random packing of spheres would produce intermediate proportional contact values. These values are generally lower than those found experi- mentally except at very low macroporosities (n < 0.2).

The distribution of contacts for sphere:aggregate diameter ratios of 2.5:1, and 1:l are shown in Fig. 5(a) Fig. 5(b). Results from sphere:aggregate diameter ratio 1:2 could not be included as during compression the relatively smaller rigid spheres were prone to change their orientation and thus lateral and upper/lower views could not be distin- guished. Contact areas are unevenly distributed over the surface of the model seed, mainly being concentrated on upper and lower surfaces relative to the direction of the applied stress. This pattern of contact was also noted on the disc at a diameter ratio of l:o.

5. Discussion

The proportion of contact between model seed surface and aggregates is shown in Fig. 4(a) Fig. 4(b). As compression reduced macroporosity, the area of contact on all faces of the artificial seeds increased, most notably on upper and lower surfaces (see Fig. 5(a) Fig. 5(b)). A small but significant increase in the number of contact points was noted as macroporosity decreased except for results from sphere:aggregate diameter ratio 1:l (see Table 3). The mean number of contact points for this sphere size ratio ranged from 7.33 to 8.67, which falls between the number of contacts expected in open and close packed assemblages of uniform spheres (six and 12 contact points, respec- tively) (Deresiewicz, 1958).

The sphere:aggregate diameter ratio, as indicated by Fig. 4(a) Fig. 4(b), clearly influences the degree of contact over the macroporosity range investigated. Contact area

2i4 A.D. Brown et d/&St & Tillage Research 38 (19961203-216

Table 3 Number of contact points between spheres and aggregates for three sphenxaggregate diameter ratios over a range of macrooorosities

Macroporosity

0.08 0.21 0.34 0.41

Diameter ratio 2.5: 1

26.3 ( f 2.0) 23.0 ( f 2.0) 24.3 ( f 0.9) 19.0 f 1.0

Macroporosity

0.17 0.28 0.38 0.44

Diameter ratio 1: 1

7.3 ( + 0.3) 7.7 ( * 0.9) 8.0 ( + 0.6) 8.7 ( + 0.7)

Macroporosity

0.18 0.28 0.38 0.49

Diameter ratio I:2

5.0 ( * 0.0) 4.7 ( f 0.3) 4.0 ( 2 0.0) 3.7 ( + 0.3)

Standard errors in parentheses.

is greatest at a given macroporosity where spheres are of a similar size. Where the disparity in sizes is large, contact areas are reduced. These results are supported by a model of fracture probability of brittle spheres presented by Sammis et al. (1987), who were investigating the mechanical processes of fault gouge formation within rock. The authors argued that the fracture probability of a sphere was determined by the relative size of its nearest neighbours. Fracture probability was shown to be greatest when neighbouring spheres were of equal size as this geometry maximised stress concentra- tions at points of contact leading to fracture propagation. Stress transmission between spheres of unequal size occurs over a greater area than between spheres of equal size, and thus values of stress required to cause failure are less likely to be attained. (See Sammis et al. (1987) for a fuller discussion.)

Concentration of stresses within the clay spheres used in this investigation caused plastic failure at contact points, resulting in increasing interfacial areas as the spheres deformed. A difference in contact areas would thus be expected if each of the assemblages in the present model had been subjected to the same stress. The fact that differences in contact areas were observable between sphere size ratios when com- pressed to a given macroporosity is a consequence of the distribution of deformation within the structures. Where rigid spheres are of different dimensions to plastic spheres, deformation round the model seed will be lower than that occurring between uniformly sized plastic spheres. Where sphere size ratio is equal to unity, the stress concentration will initially be similar to that between the uniformly sized plastic spheres; thus greater interfacial areas between sphere types would be expected in the latter case for a given macroporosity.

Excessive compression or consolidation of seedbeds to increase seed-soil contact can reduce soil macroporosity below a critical level, which can lead to aeration and impedance problems. This investigation suggests that minimal reduction in macroporos- ity to produce a given degree of seed-soil contact can be achieved when seed and soil aggregates are of closely similar size. Where seed and soil aggregate sizes are of considerably different magnitudes, reduction in macroporosity by, for example, rolling of the seedbed, may not greatly increase seed-soil contact as macroporosity reduction will occur more readily between the similarly sized aggregates than the seed and aggregates. There may be disadvantages associated with altering aggregate size ranges in

A.D. Brown et al./Soil & Tilluge Research 38 (1996) 203-216 215

order to improve seed-soil contact; for example, reduction in aggregate size in order to improve contact between small seeds and aggregates during rolling could lead to aeration problems at low matric potentials. Extrapolation of the findings of this investigation to the field situation is, however, hampered by the many dissimilarities between modelled and actual seedbed conditions; for example, the plastic behaviour of the spheres and uniform size ranges used in the model are important simplifications. Seedbeds contain a range of aggregate sizes while the degree of aggregate plasticity is controlled by water content. Under dry conditions brittle fracture will result from sufficient loading of aggregates and the proposed contact model is not applicable under such circumstances. Further refinements to the model, such as using more than one size of plastic aggregate or changing seed shape, could be added to increase its similarity to seedbeds.

Models of seed-soil contact give single estimations of proportional contact between a seed and soil. This proportional area may, in actuality, alter during the imbibition period due to seed swelling which brings the seed coat into contact with soil aggregates. Similarly, clay aggregates which lose water to the seed will shrink, possibly reducing contact with the seed. Seeds appear to swell normally over a substantial part of the imbibition process (Collins et al., 1984; Vertucci, 1989); therefore if the soil is subject to normal shrinkage proportional seed-soil contact may not alter greatly, whilst in non- shrinking soils contact may increase appreciably with time.

Whilst the model is designed to give an estimate of seed-soil contact, it is in reality seed-soil water contact which determines the wetted area of the seed over which water transfer can occur. There is thus a need to relate seed-soil contact and matric potential of the soil water to seed-soil water contact. A combination of the Collis-George and Hector model and the approach described here may be able to give a more accurate estimation of conditions at the seed-soil interface. Finally, it should be remembered that the significance of the influence of seed-soil contact and seed-soil water contact on germination and emergence rates is insufficiently understood and requires further study before tillage operations to increase such contact can be recommended.

6. Conclusions

The study shows the effects of varying the degree of compaction and relative sphere sizes on the contact that exists between a rigid sphere and surrounding deformable spheres in relation to seed-soil contact. Compaction (reduction of macroporosity) increased the proportion of the surface area of the rigid sphere in contact with neighbouring spheres through deformation of the artificial aggregates. Contact points were not evenly distributed over the sphere, being mainly concentrated on faces perpendicular to that of the applied load. At any macroporosity over the range investigated, the greatest proportion of contact was achieved where rigid and deformable spheres were of closely similar size. The results suggest that under experimental conditions a predetermined proportion of contact is achievable with minimal macrop- orosity reduction when rigid and deformable spheres are of equivalent size.

216 A.D. Brown et d/Soil & Tillage Reseurch 38 12996) 203-216

Acknowledgements

The image analysis programme used in this study was written by Professor J. Marchant. The principal author is grateful for the guidance of R. White in the statistical analysis of the data. This investigation forms part of a Ph.D. studentship funded by Silsoe Research Institute.

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