The Selenium Content of Vegetation and the Mapping of Seleniferous Soils 1

N June, ~939, the South Dakota Agricultural Experiment Station started grazing experiments with cattle on a ranch located in a seleniferous area of the state. To select the most desirable location for the experiment, both the opinions of the ranchers in that vicinity and the selenium analyses of soil and vegetation samples were considered. The ranch which was finally selected was reputed to be one of the most seleniferous in the area, and all ranchers who had operated it had experienced considerable trouble with selenium poisoning or "alkali disease" among their livestock. It was necessary that as much information as possible be accumulated concerning the selenium content of the vegetation on this land in order to set up the pastures and to evaluate the results of the grazing studies; therefore, in ~938 studies were started on the selenium content of the vegetation. Secondary purposes of this study were (a) to determine the factors which influence the selenium content of plants and (b) to develop a practical method for mapping seleniferous land in sufficient detail to be of value on small ranches. To date, most of the work which has been done in locating seleniferous soils has been of a reconnaissance nature. In addition to actual analysis of plants and soils, the occurrence of cases of selenium poisoning, of "indicator" plants, or of certain geological formations known to be seleniferous have been used in studying the occurrence of selenium. Whereas these means of locating seleniferous areas are of great value in reconnaissance work, their use in detailed mapping is quite limited. Studies made earlier have shown that the analysis of plants is the most practicable and reliable means of determining, for the purpose of detailed mapping, the location of areas which are capable "of producing vegetation of selenium content high enough to be toxic to livestock? There are, however, several factors which cause variations in the selenium content of plants and these must be considered when plant analyses are used as an index of the amount of "available" selenium in soils. These factors have been discussed in an earlier publication and may be listed as follows: The amount of selenium in the soil; the chemical form of the selenium in the soil; the accumulation of selenium in the subsoil; the geological formation from which the soil was formed; the sulfur content of the soil; climatic conditions; the kind of plant; plant associations; the stage of growth of the plant; thrift and condition of the plant; the method of drying the plant for analysis; and the part of the plant analyzed. An effort has been made