During the excavation of root systems of native and crop plants throughout the Great Plains region during the past five years (Weaver, 19I9, I920, 1922) contact has been made in more than thirty fields with the so-called hardpan. Such a layer of soil underlies much of this area of low rainfall, at depths varying from I5 inches to 3 feet. It varies from 8 inches to over i.5 feet in thickness. In connection with studies on the physical and chemical composition of the soils and their seasonal water-content under natural and cropped conditions, a more detailed study has been made of the soil in this hardened layer. The major portion of this work was done at Burlington, Colorado, a station selected as being typically representative of High Plains conditions, but analyses of hardpan from Flagler, Colorado, and Colby, Kansas, were also made.
THE HARDPANMechanical and chemical analyses of the soils at Burlington are given in Tables i and 2. These represent the several foot sections to a depth of six feet, except in the first foot, where samples were taken to depths of six and twelve inches, respectively. The samples from Flagler and Colby were obtained from the hardpan layer at depths of approximately 1.3 to 2.1 and 1.9 to 2.6 feet, respectively. At Flagler the field had been broken only one year, at Colby for a number of years. The cropped area at Burlington was on land broken only 3 years. Here a well-formed hardpan occurred at a depth of about 2 feet. The change in soil structure was quite distinct, the harder layer continuing to a depth of 4 feet, where it gave way to a loess-like soil very loose and powdery in structure. Hence in Tables i and 2 the first three samples are of soil above the hardpan, the next two in the hardpan, while the last two are from the soil below it.An examination of Tables i and 2 shows that the hardpan at Flagler and also that at Colby appears to be due solely to the calcareous nature of the soil. The former contains a relatively low percentage of clay and at the same time more inorganic carbonate along with calcium oxide than any one of the Burlington samples. The Colby sample is higher in clay content, but holds nearly twice the amount of carbonate compounds. At Burlington the case is different. Here calcareousness alone does not account for the condition. Although the carbonates increase with depth, they do not reach a concentration sufficient to bring about the hardening of the soil, unless their effect is supple- 237
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