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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.

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    \$\begingroup\$ 2 knives with heatshrink? Is it 100% watertight? If not (which I highly suspect) the noise and all other effects will be because in soil there's a lot of free ions that there are very few of in a glass of water, to conduct between electrodes. \$\endgroup\$ – Asmyldof Aug 2 '15 at 14:15
  • \$\begingroup\$ Earth itself has a large capacitor,hence you cant keep your electrodes grounded,you gotta take samples of soil,use parallel electrodes with reasonable space usually depends on area of your electrodes,electrodes should be water-tight. \$\endgroup\$ – MaMba Aug 2 '15 at 18:26
  • \$\begingroup\$ It is likely that your capacitance meter doesn't like its "ground" connected to the circuit under test. Capacitance measurements always require some sort of stimulus and measurement, the test end of the circuit is probably designed to measure an isolated capacitor. You might be right about the reason for the noise, but your solution is wrong. What kind of capacitance meter is it? A handheld DMM? An Arduino? An LCR meter? \$\endgroup\$ – tomnexus Aug 2 '15 at 21:14
  • \$\begingroup\$ @Asmyldof the heatshrink is indeed 100% watertight, as confirmed by the resistance function of my DMM. \$\endgroup\$ – Plumpie Oct 20 '15 at 14:19
  • \$\begingroup\$ @MaMba there seem to be capactive soil moisture probes on the market though that do in fact use capactive measurement, designed for in situ use and not soil samples \$\endgroup\$ – Plumpie Oct 20 '15 at 14:20
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Soil, and ground is a hotbed of electrical signals. Metals, oxides and other conducting materials react with fluids and generate currents by galvanic processes. Currents that can be large enough to 'eat away' railroad hardware (which is why, in some areas, large zinc or other metals are buried to main the 'hardware' intact...

I doubt you will be able to measure capacitance reliably in soil, unless in a protected environment, isolated from the rest of the ground.

I suspect that your capacitance meter gave up, because of the 'bad quality' of the ground capacitor. After all, the ground also acts as a resistor, in parallel with the capacitor you're trying to measure.

I doubt shrink sleeve will protect your electrodes from conducting. Maybe covering a bottle on the inside with foil could work. But still, the capacitance will still effectively be a C in parallel with an R, and will be very difficult to measure.

Also, imagine that the capacitance between your electrodes, will be just a couple of pF, compared to all the capacitance to the rest of the 'earth'.

The capacitance in the glass will work, because the resistance is very high. Put in some salt, and you'll see how C varies! And then you'll probably be measuring the capacitance of the shrink sleeve, not the water.

Try measuring R directly - at least then there are no oscillators involved, as used in measuring capacitors.

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  • \$\begingroup\$ Measuring soil moisture using the change in dielectric constant is a standard industrial way - more so than resistive measurements. I agree that proper isolation between the electrodes is essential. \$\endgroup\$ – corecode Aug 2 '15 at 15:14
  • \$\begingroup\$ I doubt that a simple instrument, as used to measure components, will be able to measure capacitance in soil. I've read quite a bit about these techniques, and TDR (Time domain reflectometry) seems promising. Hardly implementable for one's garden work though. \$\endgroup\$ – jcoppens Aug 2 '15 at 19:43
  • \$\begingroup\$ @jcoppens TDR is about using the capacitance of the soil as well - after all, a transmission line in TDR measurements consists of the inductance of the wire you're passing through the ground and the capactive coupling to the other wire. \$\endgroup\$ – Plumpie Oct 20 '15 at 14:24

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