WORKING GROUP 3 
CURTIS 
155 
Complexes, Series, Types and Phases. The Soil Series has been used exten 
sively as a mapping unit on scale of about 1 : 50,000 and survey organisations 
have shown considerable interest in aerial photographs as aids for the delinea 
tion of Series boundaries. It is therefore appropriate to examine the problems 
which arise due to the definitions of the Soil Series. 
According to the current definition [Soil Survey Staff, 1951] Soil Series are 
to be regarded both as mapping units and as fundamental taxonomic units 
linked to Major Soil Groups in the classification. The Soil Series consists of a 
group of soils possessing similar profile characteristics and developed from a 
particular type of parent material. In practice the uniformity required in a 
Soil Series is looked for in that part of the profile below the ploughed layer or 
its equivalent. 
The definition of the Soil Series, therefore, raises three points that are signi 
ficant to the photo interpreter. First, the Series is defined in terms of profile 
characteristics which are not directly visible on aerial photographs. Thus 
photo interpretation of Series boundaries must rest upon the somewhat un 
certain basis of correlation of profile characteristics with surface features. 
Secondly, the definition places emphasis upon the subsoil characteristics of the 
profile and largely disregards the surface soil horizons. Therefore, the surface 
conditions, which are observed and recorded by the photo interpreter, are 
largely excluded from consideration except where they reflect subsoil conditions. 
Thirdly, the Series represents a grouping of soil individuals on the basis of 
selected criteria. Thus the Series is a concept and does not represent a clearly 
defined natural phenomenon. Where the Series is used to mark an area on a 
map it means that 85% or more of the area contains soils which have selected 
criteria conforming to the concept of the Series. [7th Approximation, 1960]. 
There is no certainty that the boundary placed around these selected criteria 
will bear a close relationship to surface features. Indeed the criteria used for 
its definition may be unsuited for photo interpretation. It is hardly surprising, 
therefore, that estimates of the accuracy of photo interpretation of Series units 
[Pomerening & Cline, 1953] gave best performances of 72% correct in a 
simple area and 33% in a complex area. 
The Series unit as defined above is also somewhat unsatisfactory for ground 
survey purposes. The boundaries of soil units which are defined solely on the 
basis of profile characteristics are difficult and time consuming to map. 
Therefore, the surveyor seeks mappable surface features which mark the 
boundaries of the typical profile characteristics of the Series e.g. geomorphic 
inflexions, vegetation patterns. However, the surveyor is often unable to make 
full use of the elements of the landscape because surface features do not nec 
essarily coincide with the conceptual boundaries of the Series and Major Soil 
Groups. It has been suggested, therefore, that Soil Series as mapping units 
should not be related directly to taxonomic Soil Groups. Instead the Series 
should be regarded primarily as a geographic concept, defined as a landscape 
unit with a limited range of profiles and parent materials [Avery, 1956]. 
Series units defined in this manner would be suitable for photo interpretation