Linear Changes in the Shelby Loam Profile as a Function of Soil Moisture

With.the establishment of the erosion stations, studies and experiments were directed toward measurements of the movement and loss of topsoil on cultivated slopes. Prior studies of the soil had been limited primarily to measurements of the inherent changes in the soil itself. Measurements of erosion are made by two methods, both of which are usually used in each study. The first method considers the loss of soil from an area, field, or plot by individual rains, and is determined by catching the entire quantity or a known aliquot of the soil eroded in large basins of concrete, wood, or metal. The second method considers .actual loss of material from the plot itself as determined by the changes in elevation of the surface. This latter method does not give a measure of the loss by individual rains, but over a long period of time shows from what position on a slope the losses are occurring. These elevation readings are taken with an engineer's level at a sufficient number of points over the plot to give an accurate measure of the soil movement. Usually readings at 5 or 10-foot intervals with the slope and 1 to S-foot intervals across the .slope are considered satisfactory. Readings from a typical plot in continuous corn on an 8 per cent slope are shown in figure 1-b. The first set of readings, taken in April, 1932, shows the original elevation of the surface while the second set of readings shows the surface elevation in October, 1934. The difference supposedly represents the erosion that has occurred from various portions of the slope. As plotted, these profiles represent the variation in feet from an average slope on the series of 8.07 per cent. During the summer of 1934, one of the most severe droughts in recent years was experienced. Large cracks appeared in the soil. In places these extended to depths of three to four feet. Upon excavating below the surface horizon of silt loam into the plastic clay subsoil, it was found that the cracks were larger and more numerous than appeared from the surface. Sometimes vertical cracks in the lower horizon would not extend completely through the surface horizon. Considerable damage occurred in grassed waterways and in gully bottoms healed by vegetation due to the breaking of the sod where cracks occurred. Such breaks formed lines of weakness which cut through when rains causing runoff were again experienced. At this same time, numerous reports were received of the failure of the foundations of large buildings. Shrinkage of the soil from drying caused irregular settling of the foundations which resulted in distortion and cracking of the superstructure. Damage was most apparent in buildings of masonry or brick. Reports of such damage were most common from those towns and cities located on a glacial till, of which the Shelby loam is the common soil type under consideration. Following the recent period of drought(June to August, 1936) the first rain of torrential character carried appreciable amounts of surface soil into the deeper subsoil cracks. Such a process, if repeated a sufficient.number of times, would undoubtedly change both the chemical and physical nature of the soil profile. The appearance of the cracks described in the above paragraphs raised the question of whether or not' contraction of the soil in a vertical plane might not also be occurring. To check this, a complete series of elevation readings were taken on one of the erosion test plots in alfalfa for which elevation readings taken