Capacitive soil moistures sensors are inexpensive and fairly easy to program for determining "wet" and "dry" soil conditions. However, arbitrary sensor thresholds are not adequate for determining irrigation start times or permanent wilting points in response to soils losing water to evaporation over time.

This video summarizes an experiment to see if capacitive soil moisture sensor response may be fine tuned by carefully measuring volumetric water content of a silty clay over time and comparing results to published recommendations.

The spreadsheet referenced in this video is posted here : https://docs.google.com/spreadsheets/... . This contains additional references and worksheets that might of interest to tinkerers feel free to double check my calculations or use it to run your soil experiment ;)

Here's the paper that I studied in detail see Table 1 sourced to Ratliff et al. (1983); Hanson et al. (2000) used for deriving that multicolored chart for predicting VWC thresholds:
1. "Understanding Soil Water Content and Thresholds for Irrigation Management" https://extension.okstate.edu/factsh...

These are the papers for Table 1 referenced in my primary source:
2. "Field Limits of Soil Water Availability as Related to Laboratory Measured Properties"
https://www.ars.usda.gov/ARSUserFiles...
(My sincere apologies to ARS and Ratliff for the mispronunciation in the video.)

3. "Monitoring soil moisture helps refine irrigation management"
https://calag.ucanr.edu/Archive/?arti...

Here is another excellent reference from the UA Cooperative Extension
4. "Methods of Measuring for Irrigation Scheduling"
https://itc.tamu.edu/files/2018/05/az...

5. The multicolored chart in the video is sourced to a commercial irrigation sensor website, but I lost the link for proper attribution :( It sources its data to Ratliff which matches the numbers in Table 1 in my primary reference, so the attribution should really go to Ratliff and Hanson.

Note that it's been about 25 years since I took a soil science class. To be clear, this video is meant to share my exploration and lab notes on a topic of interest to me personally. Any corrections or suggestions for improvement are most welcome via the comments section.