Arduino capacitive sensing at the simplest level is seeing how long it takes to charge a pin and the thing you have plugged into it after
OUTPUT LOW
INPUT_PULLUP
With a bent paperclip and foil square button, if I got > 5 1 micro count loops from LOW to HIGH, a finger field was coupled with the button field and the count could be 100's without touching.
Try a larger surface area and see if damp grain doesn't have an electric field to couple with and charge.
Need to calibrate that scale? Got known weights?
How about you check the field from process start to end and
Watch
For
Change.