Nuclear soil moisture and density gauge.
https://diqn32j8nouaz.cloudfront.net...r_fullsize.jpg
How they work:
https://www.youtube.com/watch?v=JOKVZbp7Tzc
Previously:
https://www.homemadetools.net/forum/...023#post207242
https://www.homemadetools.net/forum/...271#post184969
https://www.homemadetools.net/forum/...561#post212418
https://www.homemadetools.net/forum/...499#post215643
https://www.homemadetools.net/forum/...276#post176982
