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Particle-size distribution of soil and the perception of texture. (Short Communication).

Introduction

In a comparison of the soil texture classification systems in use in Australia and the United States, Minasny and McBratney (2001) have suggested that most countries including Australia should consider adopting the particle-size limits and texture classes of the United States Department of Agriculture (USDA). This suggestion is examined critically in relation to principles established by Marshall (1947) for the design of the Australian diagram.

Particle-size limits of the silt fraction

In Australia the particle-size fractions recommended by the International Society of Soil Science (ISSS) have been adopted with upper limits for the clay, silt, and sand fractions set at effective diameters of 2, 20, and 2000 [micro]m, respectively (Prescott et al. 1934). In the USDA system they are 2, 50, and 2000 [micro]m. In both systems the relation between particle-size distribution and texture classes is represented on equilateral triangles. The ISSS system was chosen partly because its clay limit of 2 [micro]m was preferred to that of the USDA, which was 5 [micro]m at that time. However, the description of texture in the field followed as closely as possible the well-established USDA practices. The history of particle-size systems and texture diagrams is discussed by Marshall (1947) and Minasny and McBratney (2001).

It is desirable to eliminate differences between the two systems and Minasny and McBratney (2001) suggest that the USDA upper limit of 50 [micro]m for the silt fraction should be adopted in Australia. They point out also that this would have the advantage of giving a more uniform spread of data points over the diagram. This would happen because the average silt content would increase to twice its present relatively low values (Marshall 1947). On the other hand in support of the present 20 [micro]m limit it may be said that the restricted spread has had no effect on texture standards. Also, tests by 8 experienced soil scientists on a separated fraction of soil sized between 20 and 50 [micro]m favoured the 20 [micro]m limit (Marshall 1947); six described its texture as sand, one as silt, and one was undecided.

If the 50 [micro]m limit were to be adopted, the Australian texture diagram (Fig. 1) could be adjusted to the new size limits as shown approximately in Fig. 2, where the more uniform spread of the texture classes can be seen. This diagram was based by Marshall (1947) on a direct comparison of the percentage of soil in the 2-50 [micro]m and 2-20 [micro]m size fractions for silt, using representative soils on which both fractions had been measured. The average ratio between them of 2 to 1 was used, where applicable, to determine the percentage of silt at various points in Fig. 2 from that at corresponding points on Fig. 1. A complementary decrease was made in the percentage of sand. The calculated position of the line separating the silty classes from the rest is the 50% silt line. But since the ratio is not applicable when the clay content exceeds 50%, the line was altered arbitrarily to extend it into that area.

[FIGURES 1-2 OMITTED]

This conversion was presented as an approximate one. But the method could be made sufficiently accurate if enough samples were used to provide ratios appropriate to different parts of the diagram. It would be useful for this and other purposes to have measurements made routinely for both 20 [micro]m and 50 [micro]m limits.

Minasny and McBratney (2001) have demonstrated a similar spread of texture classes when the silt limit is changed from 20 [micro]m to 50 [micro]m using an empirical equation for the conversion.

Class limits and soil behaviour

The transverse limits of most of the texture classes in the Australian diagram (Fig. 1) follow an upward trend from the left (sandy classes) to the right (silty classes). This trend was found from the scatter of points representing the particle-size distribution of 800 samples allotted to texture classes by field description (Marshall 1947). The general use of horizontal lines for limits of clay content in texture diagrams does not accord with this trend.

The texture of a soil is determined in the field from its behaviour under manipulation in the hand. It is evident that the behaviour of the clay fraction (as perceived from the plastic, cohesive, and adhesive properties of the soil) is affected in some way by the size distribution of the non-clay fraction (2-2000 [micro]m). For a given grade of behaviour to be perceived the clay content has to be greater at the silty side than the sandy side of the diagram.

Apparently, the effectiveness of the clay fraction is reduced with decrease in the size of particles in the non-clay fraction. Two reasons can be given for this:

1. Under manipulation a soil will probably feel less clayey if its non-clay particles have a large specific surface area.

2. The presence of a large amount of silt with particles near 2 [micro]m in size implies that there will also be much of the same type of material <2 [micro]m included as a relatively inactive component of the clay fraction that contributes little to cohesion and adhesion. In general, the finer the material in the 2-2000 [micro]m fraction the greater will be the content of inactive particles in the clay fraction, <2 [micro]m, thus making the clay less effective.

Comparison of the Australian and USDA diagrams

When this principle was built into the texture classes of the Australian diagram (Fig. 1) it gave a lower average clay content to a sandy class than to the corresponding silty class. This created a serious departure from the USDA diagram of that time in which 2 horizontal lines at 27% and 40% clay represented the lower and upper limits for the clay content of the sandy clay loam, clay loam, and silty clay loam classes. Some years later the USDA diagram shown in Fig. 3 was introduced (Soil Survey Staff 1975). Amongst the changes was the lowering of the clay content limits for sandy clay loam and adjoining sandy classes. This and other changes brought the USDA diagram much closer to agreement with the Australian diagram.

[FIGURE 3 OMITTED]

But differences remain. If Figs 1,2, and 3 are compared, it can be seen, for example, that the USDA diagram has retained horizontal lines for its class limits for clay content. This does not accord with the sloping trend discussed in the previous section. Also when the USDA diagram lost part of its continuous limit lines at 27% and 40% clay content it lost its system of associated classes. There is no longer a smooth transition, for example, between sandy clay loam and clay loam. The abrupt change in these and other limits misrepresents the relation between adjoining classes.

There are thus 2 reasons for retaining the Australian diagram. It has sloping limits for clay content that accord with field texture descriptions based on soil behaviour, and it provides a smoother transition in particle-size distribution between neighbouring classes.

Field description of texture

A further reason for retaining a national system is concerned with usage. Properties such as exchangeable sodium can cause a field description to differ widely from the texture defined by the size distribution. In Australian practice the texture of a soil as described in the field stands unchanged in such circumstances (McDonald et al. 1990). It has a value of its own in representing perceived texture as distinct from texture based on size distribution alone. If the two differ greatly, the reason for this can be sought analytically. The diagram sets a standard for texture in an average way and is not used to adjust an individual description.

In contrast, the USDA system (Soil Survey Staff 1975) defines the texture of a soil from the particle-size distribution. The preference for measurement over description is understandable but in this case it would affect Australian usage for field texture as given by McDonald et al. (1990). Of course it would be possible to adopt the USDA diagram without its associated usage, but this could cause confusion.

Conclusions

A suggestion that Australia should adopt the USDA texture diagram because of its international status is opposed on the following grounds:

1. The Australian diagram has a more systematic set of limits for the various texture classes. These avoid the abrupt changes in particle-size distribution that occur between some neighbouring classes in the USDA diagram.

2. The USDA diagram has a number of horizontal lines of uniform clay content as class limits. These do not accord with the relation between particle-size distribution and texture as perceived from the behaviour of the soil under manipulation in the field.

Also there are differences in the usage of the two systems on how to assess the texture when field description does not agree with particle-size distribution.

The upper limit for the effective diameter of particles in the silt fraction is 20 [micro]m in Australia, following a recommendation of the ISSS. Reasons for and against increasing this are given. If it were changed to 50 [micro]m as has been suggested, the spread of points on the diagram would be greatly improved. This would also give uniformity with the USDA fraction sizes. Methods are available for converting the diagram to these sizes without changing the texture standards or losing the present system with smooth transitions between classes and slopes in its transverse limit lines.

Acknowledgments

The author thanks Prof. J. M. Graves and Dr T. J. Richards for preparing the manuscript and diagrams, and the referees for their helpful suggestions.

References

Marshall TJ (1947) Mechanical composition of soil in relation to field descriptions of texture. Council for Scientific and Industrial Research, Australia, Bulletin No. 224.

McDonald RC, Isbell RF, Speight JG, Walker J, Hopkins MS (1990) 'Australian soil and land survey field handbook.' 2nd edn (Inkata Press: Melbourne)

Minasny B, McBratney AB (2001) The Australian soil texture boomerang: a comparison of the Australian and USA/FAO soil particle-size classification systems. Australian Journal of Soil Research 39, 1443-1451.

Prescott JA, Taylor JK, Marshall TJ (1934) The relationship between the mechanical composition of the soil and the estimate of texture in the field. Transactions 1st Commission, International Society of Soil Science, pp. 143-153.

Soil Survey Staff (1975) 'Soil taxonomy.' Agriculture Handbook No. 436, Soil Conservation Service. (USDA: Washington, DC)

Manuscript received 17 June 2002, accepted 28 October 2002

T.J. Marshall

Formerly: Division of Soils, Commonwealth Scientific and Industrial Research Organisation, Australia. Present address: 4 Diosma Road, Eltham, Vic 3095, Australia.
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Author:Marshall, T.J.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Mar 1, 2003
Words:1762
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