A Method of Presenting Soil Survey Data on Maps

MANY modern detailed soil survey maps are characterized by intricate patterns of a large number of mapping units. The complexity of these maps, and difficulties that many individuals have in interpreting them, may result in their exclusion from uses to which they can be readily adapted. This suggests that some very useful purposes may be served by supplementary interpretive maps or guides to the interpretation of the detailed maps. The objectives of this paper are to review briefly some characteristics and limitations of two kinds of supplementary maps that are commonly prepared fr.om the data on detailed soil survey maps and to present a method of preparing an additional kind of map. One important kind of supplementary map that is commonly prepared from the data on detailed soil survey maps shows the distribution of groups of mapping units defined in terms of their relative suitability to a particular use or their responsiveness to a particular kind of management. The preparation of a map of this kind demands an appraisal of the individual mapping units in terms of their suitability to a specified use or their responsiveness to a specified kind of management. A large number of maps of this kind can be prepared from the data on the detailed soil survey maps. Some of these maps may be useful for a number of purposes, but the use of any one map is usually limited to the specific purpose for which it was prepared. The limitations of use of maps of this kind should be understood by those who employ them. One of their important limitations relates to the fact that mapping units of greatly different characteristics may be placed in the same group for one purpose and in greatly different groups for another purpose. These maps cannot be used as a source of information regarding specific characteristics of the soils, they merely supplement the detailed soil survey map. Another kind of map, the so-called "single factor" map, can be compiled from soil survey data for the purpose of showing the distribution of soil groups defined by a single characteristic. The number of maps of this kind that can be prepared from the data on a detailed soil survey map is great; it is limited only by the number of definable soil characteristics. The limitations of use of any one "single factor" map are due largely to variations in factors other than the one considered in the preparation of the map. The significance of any one factor is not uniform in all of the combinations in which it may occur. "Single factor" maps may serve certain very useful purposes. They have definite educational possibilities because the units may be defined in a manner that will cause the user of the map to become "soil characteristic conscious", but the use of only a small number of these maps in this manner may be dangerous ; the concept of a soil as an individual of complexly inter-related characteristics may not result. A set of these maps could be superimposed upon one another to illustrate the reason for the detail and complexity of the detailed soil survey map. "Single factor" maps may also have other uses. Certain single soil characteristics or a combination of a small number of characteristics often are of primary significance in the evaluation of the mapping units for a particular purpose. For example, the lime content of the soils, or a combination of lime content, natural drainage, and texture may be of primary significance in determining the suitability of the soils of some areas for alfalfa production. The additional method of presentation of soil survey data that is discussed in this paper is an outgrowth of the "single factor" concept. It is based upon the following assumptions: ( I ) That within a given landscape a relatively small number of selected "single factors" can be used to distinguish between the soils, and (2) that a systematized scheme of cartographic expression can be related to the selected factors. The objective of the method is to develop a cartographic scheme that will simplify the understanding and interpretation of the map. The fact that a small number of "single factors" can be used to distinguish between the soils does not mean that differences between the soils are restricted to these factors. A large number of characteristics are accessory to the distinguishing factors. . The first requirements in the application of this method are a thorough knowledge of the mapping