The real problem here relates to what sort of electrodes you are going to use.
As best I know, Arduino does not make a soil moisture sensor, so you are presumably referring to some or other module advertised as being "suitable to use with" an Arduino (or a Raspberry Pi, PIC, etc.).
So perhaps you cite - Web link - which so-called "soil moisture sensor" you have in mind, and we can discuss it.
Basically, all of these except the capacitive variety, have major problems with their electrode material which is also a concern with skin electrodes, depending on for how long you expect to use them or how repeatedly. 