"system is effectively linear"
This is vague. What's the "system"? Reread Maturana and Varela, or Arakawa and Gins, or Heisenberg, Planck, Bohr. Where does the organism begin and phenomenon end? Maturana would remind us that "system" in the sense that you might mean it, meaning the electronic sensor + wires + microprocessor + code + wire + LED + battery + wire is only an artificial "god's eye view" linguistic fiction. (Read also the (in)famous essay about the artifactuality of physical measurement in gyro guidance system for ICBMs, reprinted in the Science Studies Reader, ed. Mario Biagioli.)
The mapping from soil humidity to human felt experience of the humidity is a composition of mappings, only some of which are computable, most of which are non-linear, most of which are in fact unknown. Call these mappings f1, f2, ..., fn.
If f is linear, and g is non-linear, then f º g is non-linear.
If f and g are both non-linear, you have to be pretty damn lucky to find f º g linear.
Psycho-perception, alone, aside from anthropology and phenomenology, tells us that linearity is bogus.
In fact, the situation is far worse. Since at least one of the factor mappings fi is indeterminate, even if all the other factor mappings were determined, the composition f1 º f2 º ... º fn would still be indeterminate.
In fact the point of asking for monotonicity and reproducibility is precisely because that they together yield an "unambiguous" measure upon which individual humans can train. "Reading" and "unambiguous" are vague and paper over vast areas of uncertainty. (Cf. the psych-perception literature; read Husserl.) I suggested monotonicity and reproducibility as two practical qualities sometimes attainable by experimentalists with modest technical means. At least if y'all achieve these two qualities in your sensor system, then we'll be able to muddle along.
To measure soil humidity, I bet there are ceramic wicks that'll do this totally analog, with no macro-current needed. A chief value for doing digital is not so much to display humidity to the human, but its side effect: to get the humidity data into our Ozone system. Speaking of which, I think we should run photo-cells into Ozone. Maybe Claire can make some up from the Memory+Place leftovers -- asking Zohar, Patricia ?
Regards, Xin Wei