Soil survey applications

Applied Geography (1982), 2. 69-l 1

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63 1982 Butterworths

Essay review Jarvis, M. G. and Mackney, D. (eds) Soil survey applications (Soil Survey Technical Monograph No. 13). Harpenden: Soil Survey, 1979. 195 pp. E4.00 limp. Avery, B. W. Soil class@ation for England and Wales (Soil Survey Technical Monograph No. 14). Harpenden: Soil Survey, 1980. 67 pp, f 1.OOlimp. Thomas, M. F. and Coppock, J. T. (eds) Land assessment in Scotland. Proceedings of the Royal Scottish Geographical Society Symposium, University of Edinburgh. 25 May 1979. Aberdeen: Aberdeen University Press. 1980. 143 pp. In recent years, concern with land assessment has been increasing. not only in relation to the traditional functions of agriculture and forestry, but far more broadly in the context of recreation. ecological conservation and construction engineering. The pioneers in these approaches have almost certainly been the Dutch, who have developed highly integrated systems of land assessment, linked closely to soil survey and classification. As the three publications reviewed here illustrate, similar procedures are now being introduced in Britain. One of the basic inputs into any land assessment is soil survey data. In most cases this information takes the form of generalized data derived from soil maps. The quality of this information, and hence of the assessments made from it, therefore depends upon the quality of the initial soil survey and the rigour, flexibility and precision of the soil classification system used to categorize and present this information. It is now widely accepted that soil classification involves more than reducing a large bulk of information on soil conditions to the most aesthetically pleasing set of soil classes; a successful classification must also be user-oriented, and must satisfy the particular needs of its ultimate clients. From this awareness has arisen the principle, supposedly adopted in the USDA Comprehensive System of Soil Classification, that soils be classified not on the basis of interpretations of genesis, or assumed correlations with extrinsic conditions such as climate or vegetation, but according to the measurable characteristics of the soil profiles. This, too, is the aim of the soil classification system which has been devised over recent years by the Soil Survey of England and Wales; Avery states (p. 2): As the classification is intended for use in general purpose surveys of both cultivated and uncultivated land, the classes are in principle defined by properties that can be evaluated in the field or inferred from field examination . . . and that are relatively permanent.

The classification can therefore be judged on the basis of how well it achieves its aim of provihing a foundation for meaningful, general-purpose surveys of soil conditions. The general structure of the Soil Survey classification is similar to that of the USDA scheme. It is a hierarchical classification consisting of 10 major groups. 43 groups and 118 subgroups. Classification is based on a combination of different features at each level. including soil composition. geology and the presence of diagnostic horizons; as such it is a polythetic classification. Particular stress is laid on soil properties assumed to have specific practical significance. This, undoubtedly, is a strong point of the classification. It is possible to quibble at times about the detailed limits of certain diagnostic criteria, and several of these criteria cannot be determined without recourse to laboratory analysis, making the application of the classification difficult in the field, but in general the principle is to be applauded. The emphasis placed on drainage characteristics is especially welcome in British conditions, for without doubt these represent one of the main limitations upon land use in many areas. Welcome, too, is the avoidance of the unmemorable and often

horrendous portmanteau words used in the USDA scheme, although as a result subgroup names become somewhat unwieldy, e.g. ‘humic gleyic rendzina-like alluvial soils’. Conceptually. the main weakness in the classification is its occasional inconsistency. Thus, despite the avowal to use only ‘specific soil properties to define class limits’, it is still necessary to infer genetic processes in order to distinguish some soils (for example. man-made soils at major group level and alluvial soils at group level). In practice, however. the value of the classification will be determined only when it is seen whether the soil classes recognized by the system reflect real and significant variations in soil performance and utility. To some extent it is already possible to make this analysis, for the classification has been in use since 1975 and hence information is already accruing on the performance of the system. Unfortunately. little of this information was available when the earlier technical monograph (Soil sun-~)’ qdicutions) was developed. for although published in I979 it is based on papers initially presented in December IY76. None the less. the monograph brings together a number of useful studies. It considers the role of soil surveys not in relation to general land uses. but in terms of specific applications and management techniques: direct drilling, conventional cultivation. grassland farming. slurry application, pipeline corrosion. various recreational uses and wildlife conservation. Perhaps the most laudable feature of the whole report is its recognition that different land uses require different methods of land capability assessment. and. implicitly, that general-purpose soil surveys have major limitations. Traditional methods of land capability assessment in Britain. for example. are directed towards arable farming, and grassland soils are commonly downgraded because the land is generally unsuitable for cereal cropping. This is despite the fact that grassland farming may be as profitable on these soils as arable cropping on supposedly higher quality land. and that yields of grass are often much closer to their physiological optimum than are yields of grain crops. Harrod’s attempt to devise a method of evaluating land specifically for grassland is therefore welcome. the more so because within it he separates the assessment of yield potential from that of trafficability and poaching risk. The map of soil suitability for grassland which results from combining these measures reflects the importance of summer moisture availability and drainage status in controlling the capability of the land for grazing. The survey of soil suitability for direct drilling (Cannell. Davies and Pidgeon) is similarly interesting. Yields from direct drilling of wheat and barley on a range of soil groups are presented. and these show that direct drilling is most successful (relative to conventional cultil,ation) on brown earths. rendzinas and argillic brown earths, while yields are low on tnost gley soils and on brown sands. In general wheat performs relatively better under direct drilling than does barley. Unfortunately, many of the soils which present difficulties for direct drilling are also those that cause problems for conventional cultivation. as the paper by Jones illustrates. Assessing soils of the English west Midlands in terms of their ease of cultivation. he shows that cultivation is most difticult on soils of high clay content. low air capacity and high retained water capacity. Thus. most of the gley soils present major problems for ploughing. especially in spring. Several of the other papers are equally valuable: Hartnup and Jarvis’ discussion of soils in relation to civil engineering and planning. and Lea’s paper on slurry acceptance. in particular. But in one or two cases the efforts do not seem to do justice to the topic. most notably the over-brief summary of soil surveys in relation to playing fields by Escritt and the outline of methods to assess soil droughtiness (surely one of the most significant yet underrated soil properties) by Thomasson. More generally. it is difficult to avoid a certain unease at the approach embodied in many of these assessment procedures. With few exceptions. the principles of evaluation remain highly simplified and mechanistic: all too often there is no explicit appreciation of the dynamics and interactions involved or the

structure of the system one is dealing with. As a consequence, assessment at times becomes no more than a categorization on the basis of seemingly intuitively defined and arbitrarily applied criteria. This is not to criticize the authors; rather it serves to highlight the lack of fundamental research into the role of soils in these land-use systems. To a great extent a similar criticism can be levelled at many of the papers in the third item reviewed here (Land assessment in Scotland). Here, too, there is a tendency to apply a range of rather static procedures. Nowhere is this more apparent than in Locke’s summary of land assessment for forestry, which even relied on Dudley Stamp’s 1930s maps of land utilization for basic information. On the other hand, both the evaluation of the country’s scenic heritage, by Turner, and McCarthy’s assessment of land for nature conservation, employ highly subjective methods which, in the former case at least, are so obscure as to be incomprehensible. In almost all these cases one yearns to see evidence of a more dynamic understanding. This even applies to Francis’ otherwise interesting paper on the role of climate in land assessment. The author argues, rightly, that the balance between rainfall and evaporation is agriculturally crucial, but selects to represent this in terms of maximum summer potential soil-water deficit. He is left then to lament that ‘once again this parameter emphasizes a climatic difference that hides the true agricultural significance’ because in relatively moist environments a large moisture deficit is advantageous, whereas in drier areas it is a disadvantage. Again, the problem lies not so much with the individual authors, but with the direction which research into land assessment has followed in Britain over the last decade. What can be queried is whether a reiteration of results from these sorts of approach serves any purpose for the geographer or planner. Not all the papers should be tarred with the same brush, however. Johnstone and Tivy present an informative and thoughtful review of methods to assess the physical capability of land for rural recreation, stressing the problems of incorporating subjective data into land-use inventories, and the need to relate the method of assessment to the specific needs of the user. Jeffers-belatedly, perhaps, in view of the papers which precede him-argues strongly for the use of dynamic models in land-use assessment, and states that, ‘many proposed schemes for land-use survey and assessment, in the United Kingdom, in Europe and elsewhere, have been compromised by a subjective choice of sample units, a scheme of classification based on a priori judgements, and methods of data collection and evaluation which do not permit a valid analysis and re-evaluation of the assessment . . .‘. He says also, ‘we still need an ability to examine the consequences of the policies that are adopted by those agencies having an impact upon land use’ and goes on to illustrate the value of linear programming methods for this purpose. Significantly. the one paper which adopts a modelling approach-the assessment of the effects of afforestation on water resources by Calder and Newson-is also the paper which faced the most severe criticism by speakers at the symposium. Using a general model of evaporation losses for forest crops, taking account of precipitation, interception and humidity, they produce conservative estimates which indicate serious potential increases in evaporation losses in up to 30 per cent of the afforestable land in Scotland. Whether these predictions are valid is uncertain-the authors themselves are at pains to stress the inbuilt assumptions-but in the context of the symposium proceedings as a whole it is to be welcomed for its vitality and its originality. Sadly, many of the other papers lacked both these attributes. David J. Briggs Department of Geographv, University of SheJield