Tracing fine sediment in soil

Hi, I’m Robert Hardy and I work at Lancaster University. I’m developing ways of tracing soil in natural environment. Soil erosion is a problem in the UK as well as around the world good. Quality soil takes thousands of years to form, so it’s important that we understand how to preserve the soil that we’ve got. Soil is made up of a collection of different sized particles which come over a range of sizes. Some particles are very small, like this clay that you can see here, very fine, whereas at the other extreme, you have some much larger particles, such as these pebbles here. Here you can see a demonstration of particles that are present in soil moving.

We have a collection of sand and gravel, and when I pour the water on it, you’ll see that the sand moves much more quickly than the gravel does.

Currently, I’m doing some work tracing soil movement in a farmer’s field in the Lake District. Tracing of soil is where you try to understand where the soil moves in a natural environment. This farmer’s field has a collection of different sized particles in it, but most of them are quite small particles called silts. Silts have roughly the same size as the icing sugar you’ll find in your kitchen, and because they’re quite small, they tend to move rather rapidly. In order to study the movement of silts in this field, we need something which is a similar size to the silts but is easily identified. For this we use a green tracer like this one you can see here.

It’s basically made of sand, which has been coated with a special green paint. The paint on the tracer means that the tracer glows green when you shine blue light on it. Currently, you can see the tracer under normal light, but if you turn the lights off, you’ll notice that the tracer becomes much more visible and glows bright green. The tracer you’ve just seen has iron filings in it which gives it a magnetic susceptibility. This means that the tracer responds to a magnetic field a bit like iron filings do. This can be detected using this machine. It has a loop sensor on the bottom which is used to detect the magnetic susceptibility in the field.

In the field, the tracer is mixed with soil from the field and then placed into 1 by 1 square metre plots.

In the field, there are three sets of three plots which are on different steepnesses. We hope to find out the effect the steepness of the slope has on how the soil is eroded. This will help the farmer understand if he can reduce the amount of erosion on his field by making his field flatter. The plots of tracer that we put into the field are monitored in a number of ways. One way is by removing samples from the plots. This is a slow process but allows us to build up an accurate picture of how much material moves from the plot to further down the field.

Another technique that is used to sample the field is using a magnetic susceptibility metre as you saw earlier. This is a much quicker device taking only a few minutes to sample each plot but doesn’t give as accurate results. Sometimes you want to investigate how the soil will move under conditions which haven’t occurred this year in nature. Well, we could just wait around and wait for these things to occur. This isn’t very practical. So we go into the lab and use what’s known as a rainfall simulator. This allows us to rain on the soil box and simulate conditions outside on a much smaller scale. This is a simulation of a summer thunderstorm.

The soil here is quite dry and the rainfall is very intense. This means that the particles move rapidly down the soil box.

What you have just seen is just a few techniques that soil scientists are using to help us understand and preserve the soil that we have around the world. In the words of Roosevelt, “A nation that destroys its soil destroys itself.”