To measure the soil moisture of a house plant, I've put two knives in the soil (covered with shrink sleeve), and attached a capacitance meter. The dielectric constant of water is really high, and as a result, when I add water to the dry soil of this plant, the capacitance increases!
However, the measurement is extremely noisy. The noise seems to depend on the size of the plant, measuring a glass of water is almost noise free, but when measuring a 1 meter tall plant, there's more noise than data and even averaging 1000 measurements isn't enough.
I thought it could be EMI (radio waves) related, so I attached the ground of the capacitance meter to the soil to short the interference to ground, but now I'm measuring a capacitance of zero. Does anyone know what's going on here? I've spent the whole day reading about the theoretical underpinnings of capacitance but still have no clue.