Finding common ground

20 August 2012 by Karen Vancampenhout, posted in Soil Science

Two years ago, I started this blog after attending the World Congress of Soil Science in Brisbane. At this conference, a major concern was that soil scientists aren't particularly succesful at interesting the general public in their science. This summer, at EUROSOIL 2012, the issue was raised again, concluding that soils are probably too boring or too difficult to appeal to a broader public.

Well, I might be a little biased here, but I could not disagree more. To me, it's just a matter of 'finding common ground' (pun intended): introducing soils that tell a story people can relate to. And if the new rover can even find 'familiar' soil on Mars, we should be able to do so on our 'home ground' (pun also intended, I'm trying to make a point here). So, I want to take on the challenge to prove it to you.

I will tell you more about these soils-with-a-story in my next post. But first, I want to show that soil science doesn't have to be "difficult". On an academic level it certainly is terribly complicated. Da Vinci once said that we know more about the movement of celestial bodies than we do about the soil underfoot. Five hundred years later, he is still right: soil is an immensely complex ecosystem, where billions of unknown creatures live in a matrix of anorganic and organic material that changes from oxic and eutrophic to waterlogged or nutrient limited in a matter of millimeters and where more chemical reactions take place simultaneously than in the biggest chemical plants. Biology, geology, chemistry and physics all clash to form a mindboggling but fascinating environment we know very little about.

But you do not have to study deep-space dark matter to be able to enjoy gazing at the stars or 'root' for the new Mars rover (sorry, couldn't resist). Likewise, many aspects of soils can be understood by absolutely everybody.

For instance, the Mars rover is investigating particle-size distribution, also called soil texture, one of the most important soil properties. The soil texture reflects the size of the little particles of rock that make up the soil. Some of them are quite big: 63 µm up to 2 mm (or 50 µm - 2 mm in the USDA system). These are called 'sand'. You can see these grains with the naked eye or a hand lens, and feel them between your fingers. Also, its the "crunch" between your teeth when you accidentally get some soil into your mouth. Some particles are really small (<2µm), we call them 'clay'. Clay is really sticky and quite firm, like the stuff you used in kindergarten. Everything in between is called 'silt'. Silt feels like flour, is quite sticky and very plastic.

Not every soil has the same quantities of sand, silt and clay. Depending on these relative quantities, we give the soil's texture a name. A sand soil (e.g. a beach) contains roughly more than 85% sand (didn't guess that one, now did you?), a clay soil roughly over 40% clay (no, really?) and a silt roughly over 80% silt (told you it was easy). Finally, a Loam is a soil with about 20% clay, 40% sand and 40% silt. Everything in between is given names like loamy sand, sandy loam, clay loam, sandy clay, silt loam, ... you get the idea. This is typically summarized in a graph called a texture triangle (FAO, 2006):

Knowing the soil's texture is important, because it tells you a lot about its properties and usability:

E.g. most loams are excellent agricultural soils. Too much sand makes a soil dry and unfertile, while too much clay makes it waterlogged and heavy. And by now you understand why parents decided long ago that it would be a lot more practical to let their children play in a sand box, rather than use silt or clay.


Upper left: A poor and dry sandy soil under heathland (Photo J. Deckers); Upper right: Loamy soils are excellent for agriculture but are vulnerable to erosion and compaction; bottom: Clayey soils have a high fertility but are difficult to cultivate

There. Got it? And I didn't even have to dumb that down.


Now, I am going to let you in on a secret. When soil scientists enter a field, they take a little soil in between their fingers, move it about a little, add a little water, fiddle a bit more and than magically tell you the soil texture:

Ideally, if a profile pit is dug, they mess around with a knife, a meter and a bottle of acid for a while first.

It all looks terribly complicated (knitted brow, tilted head and serious face are absolutely obligatory) but hey, a child can do it:

So, want to impress your friends next time you go on a walk? Watch this and learn: soil texture by feel (just a handful of soil and any type of water will do (as long as it isn't salty). The spatula's only a prop; a big (blunt) knife looks much more impressive). It does require some practice to get really accurate, but as most countries have their soil maps online (or even accessible as an app) you can easily cheat by checking beforehand.

There you go. Now honestly, was that difficult?

7 Responses to “Finding common ground”

  1. Katinka Reply | Permalink

    Now you've gone spoiling my soil texture expertise!
    A very nice post though. And I adore the picture of Sam :-)

  2. Michael Blume Reply | Permalink

    No, this was not difficult - it was great! Can't wait for more blog posts about an important science I admittedly barely acknowledged! Thanks for the sound start!

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