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Field measurements of capillary tension

Richards, S.J., Lamb, J. Jr.
Agronomy journal 1937 v.29 no.9 pp. 772-780
rain, runoff, soil water, measurement, weather, capillarity, New York
Field measurements of capillary tension over parts of two growing seasons (1935 and 1936) are presented. Tensions were much higher during the comparatively dry summer of 1936. Relative values of capillary tension at various layers of the soil profile were consistent. Changes occurred where water was being lost or added before corresponding changes reached the other layers. The changes follow closely the precipitation data. Records of the capillary tension for two soils and for three crop relations on the same soil showed decided differences. These differences were maintained over the two summers. Application is made of the same apparatus used in measuring capillary tension for observing variations in the level of the free water table. Experimental curves relating moisture percentage and capillary tensions for samples of the Lordstown surface soil were found to differ, depending on whether the soil was drying or wetting. The drying curve was also changed when the soil structure was disturbed. The use of capillary tension for expressing soil moisture conditions eliminates the uncertainties which are introduced by the hysteresis and structure effects when moisture percentage is used. Capillary tension has the additional advantage of applying equally well for stony soils. Tensions cannot be measured with porous clay apparatus when they exceed 1 atmosphere, but within the range of 1 atmosphere they are readily obtainable.