Soil Moisture Content Sensor Setup Guide
Use these as starting points for estimating the 'full' (field capacity) and 'refill' (management allowable depletion) points for managing irrigation using soil moisture sensors. These are rough guesses since the actual value can depend on the particular soil structure, compaction, % gravel, organic matter content, and especially on sensor installation. Therefore modify these over time using the soil moisture dynamics and field and crop observations. However, once you have these points established they should remain constant over time for that field and sensor installation.
How To Use The Numbers
How much to irrigate?: Use the difference between field capacity (in/in) and the current soil moisture content (in/in) multiplied by the soil depth that the sensor represents (inches) will give you the soil water deficit in inches. That is the amount of water you would apply today to refill the soil without overfilling and losing water to deep percolation (exceeding field capacity). If your measurement is in percent, just divide by 100 to get in/in.
Example: I have 3 sensors at 6, 18, and 30 inches deep. Because of their depths, each of these represents 12 inches of soil depth. The 6 inch deep sensor represents soil depths of 0 to 12 inches, the 18 inch deep sensor represents 12 to 24 inches, and the 30 inch deep sensor represents the 24 to 36 inches soil depths. In this example, if the field capacity for each of these sensors is 0.28 in/in, and the current measured soil water content is 0.23, 0.25, and 0.27 (inches of water/inch of soil) for each of the three depths respectively, then the soil water deficit (space for additional water in the soil) is 0.28-0.23 = 0.05, 0.28-0.25 = 0.03, and 0.28-0.27 = 0.01 inches/inch, respectively. Then: 0.05 (inches of water/inch of soil) x 12 (inches of soil) = 0.6 (inches of water), 0.03 (in/in) x 12 (in) = 0.36 (in of water), and 0.01 (in/in) x 12(in) = 0.12 (in of water). Totalized for the entire root zone, these represent the total soil water deficit: 0.6 + 0.36 + 0.12 = 1.08 inches of water that can/should be applied without fear of over-irrigating and losing water to deep percolation. i.e. you should plan to apply 1.08 inches or less.
Avoid Water Stress!: Be careful to irrigate before the measured soil water content goes below the maximum allowable depletion (refill point).