Practical Problems in Soil Taxonomy and Soil Mapping in Great Plains States

A s in other parts of the country, the soil surveyor r\. on the Great Plains has to learn to distinguish clearly between the recognition of soil types in the taxonomic sense and the drawing of boundaries between areas of soils which carry soil-type names. It is far easier to prepare an accurate description of a soil series or of a soil type than it is to draw lines on a map that will show the exact distribution of each. Many soil types — perhaps most of them — do not occur everywhere in easily mapped geographic units, even very local ones, but are more or less interspersed with other types. Veatch and others have brought out this fact on many different occasions. The purpose of this paper is to call attention to a few representative problems in soil taxonomy and soil mapping in the Great Plains States, and to show how established procedures can be applied to improve the quality and practical usefulness of soil surveys. The region under discussion includes the area from Texas north through the Coastal Plain, Great Plains, and Central Lowland to Canada. Westward, it includes the Rocky Mountains and basins of Montana, Wyoming, and eastern Colorado. This group of states is arbitrarily set up for technical reasons and is not strictly a physiographic area, although the Great Plains Province, in a physiographic sense, comprises the greater part of'it. Traditionally, the soil type is. the basic mapping unit, and the phase is the secondary unit used most commonly in soil survey work throughout the United States. Because of this fact, there has been some tendency to define soil types on the basis of what can be mapped easily rather than on the basis of what may be included properly in the soil type from a taxonomic standpoint. Probably this is because soil-survey organizations in the United States have adhered rather rigidly to the soil type as the basic unit of mapping. Within limits imposed by map scale, the surveyor must draw lines on the map around areas of like landscapes, which may or may not comprise areas of pure soil types. In the past, there has been a tendency for soil scientists to become enslaved more or less to the soil type as a mapping unit and so to ignore or underestimate soil differences which are difficult to delineate on maps. Thus, we may overlook differences in soils which have an-important bearing on agricultural problems. To justify this deficiency it has been said that if the chief soil within a given mapping unit is Barnes loam, for example, one should not say too much about other soil types included in the mapping unit called Barnes loam, because of the danger that readers of the soil survey reports will become con=fused. Thus, we have failed, at times, to describe adequately important inclusions in our mapping units. The inclusions in mapping units commonly called "soil types," are of two general sorts, viz., ( i ) catenary and (2) noncatenary associates of the type shown on the map. In other words, many of our socalled soil types, as they appear on some maps, are actually either catenary or noncatenary soil complexes. Let us consider a few examples. The Barnes catena of the Chernozem zone in eastern North and South Dakota, and the "Darlingford association" in Manitoba, the approximate equivalent of the Barnes catena in the United States, are representative examples of the problem under discussion. When a soil surveyor first enters an area of soils of the Barnes catena, he recognizes at once two absolutely essential map units in the catena. He notes ( i) the presence of large areas of well-drained to somewhat imperfectly drained soils on the undulating to rolling high ground, and (2) .areas of marshes and intermittent lakes with Wiesenboden soils in kettle-like morainic depressions throughout the area. He recognizes at once that his map must have lines around the depressions to separate the generally nonarable marshy soils from the arable higher ground. In reconnaissance work this separation may be sufficient. If, however, he calls all of the soils outside of the depressions by the name of "Barnes loam," he soon comes to realize that the Barnes series includes quite a wide range of soil profiles and a considerable range of usesuitability and productivity. Perhaps his next step would be to separate stony phases or types from nonstony ones on the high ground. If he is making a detailed soil map, he begins to attend to differences between two kinds of soil on the more steeply rolling areas. On ridge tops and steep slopes, he finds ( i) a soil in which the A horizon is thin and in many places calcareous to the surface, and with many very light-colored spots exposed in plowed fields; and (2) another unit where the A horizon is considerably thicker, and grades into a slightly developed B horizon between it and the calcareous gracial till. Thus, he decides to separate the Buse loam, a thin Rendzina-like soil, from the Barnes loam, a typical Chernozem. Looking a little further, he notes that the soils in the gently sloping swales between the ridges have much thicker black A horizons, duller colored B horizons, and a dull, mottled coloring in the deeper subsoil. He learns from the farmers and from his own observations that in most years this member of the Barnes catena is the best soil. He finds many areas of nearly level or slightly depressed surface where this soil is mappable; and so he decides to set up the tentative Aastad silty clay loam as a separate mapping unit.