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  • Rebecca Armstrong

The jar test – analysing the soil at the TransMission Community Garden

This simple test helps you understand the type of soil you’ve got in your plot and what sort of plants you can grow there.

A glass jar with a layer of soil and then water with a ruler leaning against it. Next to is a note book with a pen on it.
Soil composition test

Our first work party at the TransMission Community Garden involved testing the soil to find out what it’s comprised of. Soil is made up of a mix of particles, organic matter, air, water and living organisms. The exact quantities decide the quality of the soil.

There are three main soil particles: sand, silt and clay. The percentages of these components make up the soil’s texture – and determine what will grow there. Finding out your soil’s texture is fun, easy and a great way to help you plan what to plant or if you need to rebalance the soil’s composition.

How it’s done

This simple test requires a glass jar, some water, a soil sample, a ruler, a calculator and time. Take soil from different parts of the site to be tested and mix it up, removing any stones or bits of vegetation. You can do this by passing it through a colander, but we just took out the most obvious lumps. This worked just as well.

Fill one third of your glass jar with the soil sample and top the rest up with water. Then – with the lid firmly screwed on – shake it vigorously. Then you just wait as the different particles settle at different times. First, after 60 seconds, you’ll see a layer of sand. The silt forms next – after about 2 hours. Then, 48 hours later, the clay layer will settle.

You then measure each layer as well as the total height of all three layers. From this you work out the percentage that each layer makes up of the whole and from this you can tell what soil type you’ve got.

The results

The total height of my soil sample was 3.5cm. Of this, 1.8cm was sand, 1.2cm was silt and 0.5cm was clay. This surprised me – in London I was expecting a lot more clay. To get the percentage for each layer, you divide the layer height by the total height and multiply by 100:

  • Sand: 1.8/3.5x100 = 51%

  • Silt: 1.2/3.5x100 = 34%

  • Clay: 0.5/3.5x100 = 14%

A triangle illustration showing different soil types
The blue circle shows where the three soil types meet, giving us loamy/sandy loam soil

According to the analysis, this means the TransMission garden soil is on the cusp of loam and sandy loam. It turns out that loam or sandy loam soils are best for growing most plants. So we’ve got pretty good soil. We could add more organic matter to make it more ‘loamy’ but that’s probably not necessary.

When working on the garden, we’ve often commented on the size and number of worms that call it home, as well as the diversity of beetle species. The site is lush with grasses, while our sunflowers did well and our bulbs are already showing well and our hawthorns are fighting fit. The soil test appears to confirm our thoughts that the soil there is of a high quality. We look forward to growing lots more plants!

This test was devised by Andrew Jeffers at the College of Agriculture, Forestry and Life Sciences, Clemson University. This free resource has full instructions, can be printed out and has a worksheet where you can fill in your own measurements.

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