# Why is soil more conductive than water?

Compare the following two sources:

High quality deionized water has a conductivity of about $5.5~\mathrm{\mu S/m},$ typical drinking water in the range of $5$-$50~\mathrm{ m S/m,}$ while sea water about $5 ~\mathrm{S/m}$[2] (i.e., sea water's conductivity is one million times higher than that of deionized water).

Source

It seems highly unintuitive to me that sand can be more conductive than drinking water. I think I am missing something here.

The comparison certainly depends on the soil, and the dissolved salts in the water, and the interaction between the two. Recall from early chemistry that pure water has very few charge carriers. Add any amount of dissolved salts and it quickly becomes conductive. In the natural world, 1mS/m conductivity in water is pretty rare. It's more often 10mS/m to 100mS/m - and that's still called "fresh(ish)".
"Soil" has a wide range of conductivity, but it is strongly influenced by the water content and the clay content. Dry, pure sand is very, very resistive. The resistivity of natural sand is defined by the water content and conductivity (look up Archie's law). Clay soil resistivity also depends on water content and salinity, but dry clay still has some conductivity because of the free ions of the clay minerals. A very readable technical note on the subject is Technical Note 5 Electrical Conductivity of Soils and Rocks by Duncan McNeill, and available on-line from the website of Geonics, in their library (www.geonics.com).

• What about sand soaked with rain-water? Or PVC soaked in rain-water? – user40300 Oct 1 '16 at 5:13
• I think you're going to have to distinguish between lab (pure) and real-world (dirty) conditions. In this case, it will make quite a lot of difference. Real sand always has a significant amount of clays and other crap that will make the rain water more conductive as soon as it hits. – Greg Hodges Oct 2 '16 at 18:40
• ...to continue: The conductivity of most any damp soil can be estimated by the percentage pore space and the conductivity of the water. The mineral content is generally more resistive than any groundwater, so the porosity and water conductivity controls the formation conductivity. (That's Archie's law that I mentioned.) As for the PVC - what sort of PVC? PVC pellets would be very resistive, so again it would be pore space and water conductivity as per Archie's law. – Greg Hodges Oct 2 '16 at 18:43

Pure water is actually a really good electrical insulator.

The conductive properties of water depends on the ions dispersed in it. For pure water, the only ions present are $\text{OH}^-$ and $\text{H}^+$ ions produced in the water dissociation equilibrium. This is why sea water, which contains a large quantity of $\text{Na}^+$, $\text{Cl}^-$ and other ions has such a large conductivity.

Conductivity of water also depends on the quantity of gas contained in it: the more gas it contains, the less conductive it is, because gases are terrible electrical conductors. Quoting from Wikipedia:

The electrical conductivity of ultra-pure water is $5.5 × 10^{−8}$ S/cm (18 MΩ·cm in the reciprocal terms of electrical resistivity) and is due only to H$^+$ and OH$^−$ ions produced in the water dissociation equilibrium. This low conductivity is only achieved, however, in the presence of dissolved monatomic gases. Completely de-gassed ultrapure water has a conductivity of $1.2 × 10^{−4}$ S/m, whereas on equilibration to the atmosphere it is $7.5 × 10^{−5}$ S/m due to dissolved CO$_2$ in it.

(Notice that $5.5 × 10^{−8}$ S/cm = $5.5 × 10^{−6}$ S/m.)

We therefore have, in order:

1. Ultra-pure water + monatomic gases: $5.5 × 10^{−6}$ S/m
2. Ultra-pure water + CO$_2$: $7.5 × 10^{−5}$ S/m
3. De-gassed ultra-pure water: $1.2 × 10^{−4}$ S/m
4. Drinking water: $5 × 10^{−3} - 5 × 10^{−2}$ S/m
5. Seawater: $5$ S/m

You can therefore see why soil is generally more conductive than water: it is because it contains such a large number of ions. Of course, you also have to take into consideration the quantity of gas contained in the soil, because as we have seen a large quantity of gas will usually decrease the conductivity. For example it is clear that dry sand will have a relatively low conductivity because of the large quantity of air trapped between the grains.

• What category is rain-water? – user40300 Oct 2 '16 at 4:11
• Rainwater in the air is highly pure. Probably close to the ultra-pure mentioned above. But once it hits the ground, even "clean" sand, it starts to pick up ions. The highest resistivity (i.e. lowest conductivity) I think my colleagues and I have ever seen in natural water is a several thousands ohm-m, =conductivity of 0.2 mS/m, in freshwater lakes in crystalline geology in northern Canada. Even there, in lakes only recently de-glaciated ("recent" geologically) there is still sediment to contribute ions to the water. – Greg Hodges Oct 2 '16 at 18:48