Planet Earth Online - Water Repelling Soils

If you've ever neglected a houseplant, you'll know this problem - when you finally come to water it, the water doesn’t soak into the soil, but it forms beads all over...
13 March 2011

Interview with 

Stefan Doerr, Swansea University


The surface chemistry of the lotus leaf repels water, providing the basis of its self-cleaning mechanism.


Sarah -   Now, if you've ever neglected a houseplant (I know I have,  I'm not very good at keeping houseplants alive!) you'll know this problem; when you finally come to water it, the water doesn't soak into the soil, but it forms beads all over the surface.  On a larger scale, these hydrophobic soils can cause major problems.  To discover how, Planet Earth podcast presenter, Richard Hollingam has been to meet a scientist who loves the rain...

Richard -   Well I really couldn't have picked a better day to investigate the effects of moisture of soil.  I'm in the grounds of Swansea University with Stefan Doerr and you've got a water bottle here.  Why do we need a water bottle when it's raining quite so much?

Lotus Leaf Repelling WaterStefan -   Yeah, that's a good point.  Well, I'm just going to add a little bit more water to two different soils here on campus.  One is under a conifer tree and the other one is under a deciduous tree and I'm just going to show you the effect.

Richard -   Sweep away the leaves...

Stefan -   Yup, so we've got a nice smooth soil surface here under the conifer tree, and look at that, it's actually not going in.  It's beading up.

Richard -   That's really weird.  It's almost like mercury.  As if you poured mercury on the surface, not that you're allowed to do that anymore.  As if you poured mercury on the sort of the beads of water - on a soaking wet day?

Stefan -   Well exactly.  Everywhere else, it's soaking wet.  Let's just move to somewhere else and let's put some water onto what I would say a normal soil.  If we pour some water on it, we get what we normally expect to get, the water infiltrates fairly rapidly although the soil is already fairly wet from the rain we had now for probably 24 hours.

Richard -   Which raises the question, why is it not soaking in, under that conifer tree?

Stefan -   Well, exactly.  I mean, that's what we call soil hydrophobicity or soil water repellency.

Richard -   Now you've been investigating this, not outside, but actually in the lab.

Stefan -   To really understand the phenomenon, we basically have to go into the lab.  We have to investigate it in quite some detail, and at a range of scales as well.


Richard -   Even inside, you can't get away from the rain.  This is the rainfall simulation laboratory which consists a large steel frame, a bottle - a bucket if you like - on top, and a box which is simulating the rainfall.  So it's dropping in drops through a wire mesh onto two different types of soil.

Stefan -   On the right hand side, we have the wetable soil and the rain is causing little craters in the loose soil surface.  It's loose material and it's soaking up very nicely. And on the left hand side, it's not forming craters, and as we've seen outside, the water is just sitting on the surface, just like mercury.  We want to be able to predict when soil becomes hydrophobic, under which conditions, when does it actually switch to a hydrophobic condition?  So for example when we have drought periods in the UK or elsewhere, and the soil become relatively dry, hydrophobicity tends to switch on with all these implications.  As you can see now here on the rainfall simulation plot, the water is just running off the surface and that for example may be something that could've contributed to the devastating floods in 2007.  There's evidence that the soils weren't particularly wet.  In 2007, we had a very dry spring in April, a very, very dry period, followed by an obviously very, very wet period, but the soils didn't actually soak up the water everywhere as they should've done.  So we think hydrophobicity may have well played a role.

Richard -   Now you've been studying this, haven't you - at the microscopic level, well actually beyond the microscopic level.

A drop of water on a leaf.Stefan -   Yes and what we've done in recent years, we've looked at the chemistry of those compounds that actually cause water repellency.  What we do is we use a technique called atomic force microscopy.  That's basically a microscopy that's not using light or x-ray.  It's in a sense using a microscopic, or rather a nanoscale, little tapping device that scans across the surface of a soil particle, and that's basically at a scale you couldn't actually see with a normal microscope.

Richard -   So you can see what type, well "see". Perhaps we shouldn't use the word 'see.'

Stefan -   No, that's correct, yes.

Richard -   But you can detect the individual molecules and what effects they're having.

Stefan -   Yes.  In a sense, you can't actually visualise the molecules themselves- that would be pushing it, but what we can see is, at the nanoscale basically, how the organic coating on the soil's surface varies.  If we have a wetable soil surface, it looks quite different than on a hydrophobic surface and the physical properties at that particular level.  So for example, are these compounds - do they form a smooth layer or do they actually look more like a landscape with craters and little globules of organic material, and the latter is actually the case.

Richard -   Do they come from something living then?

Stefan -   Yes, absolutely.  I mean, it's absolutely clear that these compounds are organic, they are derived from living organisms in a sense.  It could be just upgraded from plant leaves.  For example, plant leaves often repel water, they're waxy. But it could also be from microorganisms like fungi or bacteria that actually produce hydrophobic compounds.

Sarah -   Stefan Doerr at Swansea University, investigating the effect of hydrophobic soils. 


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