This one is a quite nice one as well: https://www.tindie.com/products/ProtoArt/greenthumb-wifi-soil-moisture-sensor/
It is developed by a member of the Dutch tweakers.net community. From him I know that a few people who met through tindie are developing an even better sensor.
On the other hand Xiaomi has a nice bluetooth based sensor, the people at home-assistant have already reverse engineered the protocol. It also measures light, temperature and nutrient levels.
https://community.home-assistant.io/t/xiaomi-mi-plants-monitor-flower/3388/11