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Some Field Observations with Tensiometers (1)

ABSTRACT

Eighty-three indicating tensiometers and two duplex recording tensiometers were installed in the field at the Soil Conservation Service Experiment Station at Clarinda, Iowa, during the summer of 1936. Because of the extreme drouth, the study was restricted to fallow plots where the moisture conditions remained well within the operating range of the instruments. The comparatively rapid tension changes which were found indicate the moisture was present in the soil in a connected liquid phase up to within a few inches of the surface.

In a uniform fallow plot of Marshall silt loam soil, quadruplet sets of tensiometers with cups at the 6, 12, 24, and 36-inch depths were installed at three locations down the slope. The readings over the 48-day period indicate that at any given time the tension at the 6 and 12-inch depths was fairly uniform over the plot even when the tension at these depths was changing rapidly from one day to the next in response to rainfall or evaporation. Consistent differences occurred in the readings at the 24 and 36-inch depths which indicated that at these depths the water was held less securely by the soil toward the bottom of the slope. This is what one would expect in case of a seepage flow down the slope caused by a lower lying less permeable soil layer.

Quadruplet tensiometer sets of similar design were also installed in a check plot and four adjacent fallow plots which for five years had been given organic matter treatments. Two of these plots received annually eight and sixteen tons per acre of manure, and the other two received light and heavy, green sweet clover applications equivalent in dry matter content to the manure applications. The tensiometer data from these plots, taken as a whole, are fairly consistent in indicating that adding organic matter to the surface seven inches of soil decreased the capillary tension at all depths where measurements were taken, the heavier organic matter additions causing the larger tension reductions. The tensiometer data indicate the moisture contents of the various plots ranged exactly in the order which would be expected from rainfall runoff data which have been available on these plots for several years.

Charts from duplex recording tensiometers installed in fallow soil indicate that in dry weather large diurnal fluctuations occur in the capillary tension at the 6 and 12-inch depths. Between these depths, the recorders showed that the average water moving field, the net force per unit mass of liquid water in the soil, was upward except for short periods after rain, and values as high as 16.5 times gravity in the upward direction were recorded. Charts are given which illustrate how multiple pen recording tensiometers give an automatic record of the time required for rain water to reach definite depths in a soil. It appears that recording instruments will be of considerable usefulness in studying the effects of various soil treatments on evaporation loss, infiltration, and water conservation.

¹ Research Assistant Professor of Soils, Iowa Agricultural Experiment Station and Associate Soil Conservationist, Soil Conservation Service, respectively.