There's a bunch of threads on this, but none of quite solve the issue of adding cheap, accurate, and commercially-available soil moisture sensing to Rachio.
This is the ideal way to "close the loop" of the Rachio soil-moisture guesstimation.
Edyn seems like the only available option (via IFTTT), but it's relatively expensive and the reviews suck (e.g. it reports the same moisture value no matter what). And it doesn't actually use the moisture value to feedback into the Rachio algorithm.
My plan is to use the $14 Xiaomi plant sensor (sometimes called "Flora" or "Miflora") to push the data to Rachio (using a $10 Pi Zero W as a Bluetooth to WiFi bridge). Previously, people talked about an API that allows moisture percent to be pushed to Rachio, but I can't find the details of that API. (I plan on having multiple moisture sensors to measure different zones, so hopefully the API supports that.)
Or, if IFTTT is the only option for Rachio Integration, is there a way for the Rachio to accept the moisture percent in the JSON body of the Webhooks? Currently, the only relevant Rachio slots would be "Start watering time" or "Start a zone" but neither of those accept a moisture level.
(Side note: The "GreenThumb" soil sensor seems like a good alternative sensor if it was more widely available.)
Edit: Found the API here, but I don't see anything that would allows input of soil moisture percent: https://rachio.readme.io