Measuring Soil Temperature by Standard Thermometer Suspended in Iron Pipe

Abstract

Soil temperature affects not only the functional activity of tree roots, but also the rate and duration of vegetative growth. The differences in the vegetation of north and south slopes which are particularly evident in the more extreme climatic conditions of many parts of the West are largely the result of variations in soil temperature. The rapid increase in the development of reproduction which usually occurs after the removal of an overwood may be to a considerable degree the result of increased soil temperature. This is true particularly of species near the northern limit of their range. The temperature of the upper layers of the soil fluctuates more or less in harmony with atmospheric changes; with increasing depth the peaks and depressions disappear and a progressive smoothing of the curve takes place. The daily range at a depth of 2 feet rarely exceeds 1? F. and is inappreciable below three feet. Seasonal variations penetrate to much greater depth. The measurement of these changes is a necessary part of the study of the habitat, and is a field of investigation which has been overshadowed in the past by more obvious lines of research. Soil temperatures at varying depths are usually measured by means of the so-called standard soil thermometers, which are expensive and subject to breakage. In recent years many investigators have measured soil temperatures by means of the ordinary standard thermometer lowered to the required depth within an iron pipe driven into the soil, and projecting six inches or more above the surface. In some instances where the temperature is taken at less than two feet in depth a wooden pipe is used, as it affords better insulation. In investigations during recent years at the Yale School of Forestry iron pipes have been inserted at varying depths in the soil, projecting six inches above the surface. Two years ago (extending over a period of two months) comparative readings were taken, from standard soil thermometers and from standard thermometers in iron pipes at depths of six inches, one foot, two feet, and three feet. These investigations showed that with the pipes extending six inches above the soil the variations in readings between the two sets of thermometers at two feet or more in depth rarely exceeded Y20 F. and usually were much less. At less depths, however, the difference in the read-