Tensiometers measure soil moisture tension (SMT) or the degree of difficulty experienced by plant roots in extracting moisture from the soil. SMT is related to soil moisture content. The tensiometer acts much as a gas gauge does - it indicates a hign soil moisture content (low SMT) or low moisture content (high SMT).
Tensiometers are useful for keeping the farmer or irrigator knowledgeable about the moisture status in soils on a day to day basis. These instruments are especially well adapted for use in scheduling irrigation on sandy soils such as those at Oakes.
Tensiometers were used at Oakes during both the 1970 and 1971 growing seasons. During 1971 they were installed in second year alfalfa, soybeans, corn, and sugarbeets. They functioned well in all cases.
Tensiometers were relied upon to schedule irrigations for the sugar beet fertilizer nitrogen-water level trial. Figures showing tensiometer readings for the W1 (dryland) and W3 (high water level) at the 1, 2 and 3 foot depths from late June to mid-September are shown in Figure 4 and Figure 5, respectively. SMT ranges from 0 to 800 on the graph and is shown on a daily basis. When SMT is zero the soil is saturated. When a tensiometer reads 500 the sandy soil holds about 1/3 to 1/2 the maximum amount of available water it will hold.
In figure 4, the SMT at the 1-foot depth decreases after each rain of about 1/2 inch (SMT tension decreases as available water increases). The amount and date of precipitation are shown in the upper part of the figure.On July 20 roots began utilizing water from the 2-foot depth and on August they began using water from the 3-foot depth. There was definite period of water shortage experienced by this treatment from July 25 to August 30.
Compare the above data for treatment W1 with that for treatment W3 shown in Figure 5. Precipitation and the amount of irrigation water applied to treatment W3 are shown at the top of the graph. The W3 plot was first irrigated when the SMT at the 1-foot depth reached 500 (the beet root system did not yet extend to 2 feet). The sudden decrease in SMT on July 16 corresponds to an irrigation of 2 inches of water. As the water content in the surface foot increased, SMT decreased. SMT at the 1-foot depth increased again until the beets were irrigated on August 2. The irrigation water for this and the remaining 3 irrigations was applied when the average SMT at the 1 and 2-foot depths was 500. The SMT at the 3-foot depth for treatment W3 remained lower throughout the summer when compared with the 3-foot depth for treatment W1.
When tensiometers are used to schedule irrigation on sandy soils in this manner it is unlikely that crope will be subjected to unneccesary soil moisture stress periods. In addition, when tensiometers are used in conjunction with soil moisture holding capacity data, which is currently being collected, a farmer can prevent over-irrigation (adding too much irrigation water), which is an undesirable management practice which may give rise to leaching losses of mobile plant nutrients.
1971 Sugarbeet Research and Extension Reports. Volume 2, pages 60 - 61.