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Relation of Minimum Moisture Content of Subsoil of Prairies to Hygroscopic Coefficient

F. J. Alway, G. R. McDole and R. S. Trumbull
Botanical Gazette
Vol. 67, No. 3 (Mar., 1919), pp. 185-207
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Page Count: 23
Subjects: Botany & Plant Sciences Environmental Science Biological Sciences
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Relation of Minimum Moisture Content of Subsoil of Prairies to Hygroscopic Coefficient
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1. During a 6-year period, in which the weather was exceptionally favorable for a study of the minimum moisture content of the subsoil, moisture studies were carried out on Nebraska prairies, both in the buffalo-grass formation in the southwestern part of that state, where the climate is typically semi-arid, and in the prairie-grass formation near Lincoln, which lies within the limits of the humid region. The fields were sampled to a depth of 6 ft. or more, and in the case of every sample the hygroscopic coefficient as well as the moisture content was determined, and the moisture condition is expressed as the ratio of moisture content to hygroscopic coefficient, this having the advantage of expressing the relative moistness while at the same time indicating whether either free water (1.1 or above) or growth water (1.6 or above) is present, and if so the amount of each. 2. The subsoils of the semi-arid prairies were characterized by their persistent dryness. Usually throughout more or less of the first 6 ft. a ratio of 1.5 or lower was found, and commonly in one or more of the foot sections a ratio as low as 1.1 was encountered. After droughts of unusual severity the whole of the subsoil to a depth of 6 ft., and in some cases of 12 ft., showed a ratio of approximately 1.0. 3. There was no appreciable further reduction of the moisture content when, after the subsoil had been reduced to this very dry condition, there followed a 4 or 5-month period of practically rainless autumn and winter weather. After such droughts the surface foot was found but little drier than the subsoil. 4. The subsoils of the humid prairies, on the contrary, showed no distinct reduction of the moisture content through a greater depth than 5 ft., and even in this a ratio as low as 1.2 or 1.3 appeared only under the severest drought conditions. The normal moisture condition in the deeper subsoil (6-20 ft.) appears to correspond to a ratio lying between 2.0 and 2.4. 5. The dry condition of the deeper subsoil so common in the semi-arid prairies is to be attributed to the presence of perennials with a vertical root range of 15 ft. or more, while the moist condition characteristic of that of the humid prairies is regarded as evidence that the roots of the native vegetation are but little developed below the fifth foot. The occurrence of areas in the semi-arid prairies, even after a severe drought, in which the subsoil below the sixth foot is quite moist, is to be attributed to the absence or fewness of deep rooted perennials in such places. 6. After the subsoil at any level has been exhausted of the water in excess of the hygroscopic coefficient it remains in this dry condition until the precipitation conditions are sufficiently favorable to raise the ratio to 2.0 or upward throughout the whole distance from the surface down to the level in question. Accordingly during many wet periods following droughts the upper moistened portion of the subsoil will be isolated from any deeper lying moist layer by a zone in which the subsoil is too dry to permit of the penetration of plant roots. 7. While in the semi-arid prairies after protracted droughts the moisture conditions in the first 6 ft. are quite uniform, under more normal weather conditions they vary much from place to place, thus rendering the results obtained in single borings unreliable as an index of the general moisture conditions.