Interesting question Wim Stenfert Kroese.
I immediately thought of a sensor restrained within butterfly valve-
separated tubular chambers, and a coffee grinder assembly. Grain
enter a fore-chamber, is ground to the proper textture, sampled, then
exhausted. Of course, there would be considerable calibration
required, but I believe suitably-ground grain kernels, with good
chamber density management, would create sufficient contact with a
good, absolute soil moisture sensor to get the job done.
I have a rough schematic, for your consideration. Not shown are the
requisite dislodging/backflow mechanisms.
Tom Reynolds, CID, NRCS-TSP
Water Balance, LLC
Tempe, AZ 602-463-5072