1. Accuracy of Soil Moisture Devices
Sanjay Shukla
Agricultural and Biological Engineering
UF-IFAS

2. Accuracy of Devices Factors Soil type (water holding capacity)
Texture, organic matter, salinity
Sandy soil
Low plant available water (e.g. 5-6%)
Example: 25% error – sufficient to close to wilting point
Soil specific calibration
Silt and Clay
Relatively less accuracy is acceptable
Crop type
Cost
Type of device
Not a big limiting factor with multi-depth devices

3. Examples of Types of Devices

4. Sensor Evaluation – Preliminary Results Example Purposes only – Default Calibration - Not For Distribution

5. Sensor Accuracy and Irrigation Scheduling
Example
Sandy soil with low water holding capacity
Plant available water = 6 %, FC = 9%, PWP = 3%
25 % error in measuring 6% PAW
actual moisture = 6%; moisture read from sensor = 7.5 or 4.5% (under irrigation, adverse impact on crop)
If errors are systematic
can be calibrated for specific soil
important for sandy soils
errors can be substantially reduced
calibration is time consuming
calibrate with the gravimetric method data

6. Which Device Should I Use?
Factors to consider
Area of the farm and crop type
Soil type and variability (texture, water holding capacity, types of soils within same farm)
Accuracy - applicability of default calibration to your soil
For coarse textured soils, better accuracy is needed
User’s comfort level and ease of use
Maintenance requirement
Cost (sensor, installation, and maintenance cost)
Installation cost is high for short duration crops
e.g. tomato, every 4-5 months

7. Which Device Should I Use?
Using a soil moisture device, even inaccurate, is better than not using any
If cost is a consideration, using handheld devices such as Hydrosense is a better option
Soil specific calibration is important, especially for the coarse textured soils