The strength of an interlocking stabilised soil block (ISSB) depends first on the soil it is made from. Cement helps, but it cannot rescue a soil that is full of organic matter or far too clayey. The good news is that you can learn a lot about your soil in a few days, with a jar, a spade and a wooden box.

Collect a proper sample

Dig below the topsoil, usually 30 cm or deeper, because the dark top layer contains roots and decaying plant matter that weaken blocks. Take small amounts from several spots on your site and mix them. Remove stones, roots and lumps larger than a thumbnail.

Test 1: The jar test

This shows roughly how much sand, silt and clay you have.

1. Fill a clear, straight-sided jar about one third full with your sample.
2. Add water until the jar is three quarters full, then shake well for a minute.
3. Leave it on a flat surface. Sand settles within a minute or two, silt over the next hour or so, and clay can take a day or more.
4. Measure the thickness of each layer and compare it with the total depth.

Many soil-block guides suggest soils that are mainly sand and gravel, with enough silt and clay (often around a quarter to a third of the mix) to bind the grains. A jar that shows almost all clay or almost all sand is a warning sign.

Test 2: The ribbon test

Moisten a handful of soil until it is workable, roll it into a sausage, then squeeze it between thumb and finger to push out a flat ribbon. A ribbon that breaks quickly suggests a sandy soil. One that stretches long before breaking suggests a high clay content.

Test 3: The shrinkage box test

Build a narrow wooden box about 60 cm long and 4 cm square inside. Grease it, pack it with moist soil, smooth the top and leave it in the shade for several days. Measure how far the soil has pulled away from the ends and look for cracks. Heavy shrinkage and deep cracks point to too much clay, which will cause cracked blocks.

What to do with the results

• Too clayey: blend in clean sand and sieve the mix.
• Too sandy: blend in a more clayey soil, or plan for a slightly higher cement content.
• Organic or salty soil: find another source. These soils are poor candidates for blocks.

Field tests are a guide, not a guarantee. For a larger project or a multi-storey building, have the soil tested by a laboratory and involve an engineer.

Next steps

Once you know your soil, the next decision is how much cement to add. Read our guide to choosing the right cement ratio, or look at the Standard Straight Interlocking Block Press to see how the blocks are pressed.

• ISSB
• soil blocks
• soil testing