Shake a third of a jar of soil with water and let it settle for 24 hours: sand drops in minutes, silt in hours and clay stays suspended longest, so the layer thicknesses give the sand, silt and clay shares. Follow with the ribbon test, where a long smooth ribbon means high clay and a short crumbly one means more sand or silt.
Field tests that estimate the sand, silt and clay content of site soil, to judge its suitability for building, drainage and crops.
Settling Speed Sorts Soil Particles by Size
Sand grains are large and sink fast. Silt grains are smaller and sink in hours. Clay particles are so small that they stay suspended for a day or more. A jar of muddy water therefore separates itself into layers: sand at the bottom, silt above it, clay on top. Measure the layers and you have a rough texture analysis with no equipment. The carrier’s guidance for what the result means:
- Sandy soil — drains fast, poor nutrient retention, easy to dig, poor for adobe or cob.
- Clay soil — holds water, rich in nutrients, difficult to drain, excellent for building.
- Loam (a mix) — ideal for agriculture, moderate for building.
What You Need
- A clear straight-sided jar — with a lid.
- Soil from the depth that matters — foundation depth for building, topsoil for farming; take several samples at different points.
- A ruler — or a mark on the jar.
- A day of waiting.
Method: Jar Test
Step 1: Prepare Remove stones and roots. Break up lumps.
Step 2: Fill and shake Fill the jar about one third with soil, add water to near full, put the lid on and shake hard for a couple of minutes, until no clumps remain.
Step 3: Watch the layers form Sand settles within a minute or two. Mark its level. Silt settles over the next few hours; mark it again. Clay stays cloudy and settles last, over a day or more. Leave it for 24 hours, then mark the top of the final layer.
Step 4: Measure Measure each layer and divide by the total depth of the settled soil. Organic matter floats on top and is not counted.
Worked Example
A settled sample is 6 cm deep in total: 3 cm sand, 2 cm silt, 1 cm clay.
- Sand: 3 / 6 = 50%.
- Silt: 2 / 6 = 33%.
- Clay: 1 / 6 = 17%.
This is a loam-like mix with about half sand: usable for farming and quick to drain. For building, a 17% clay share is a modest binder, so test the soil separately before using it in earth mixes and look for a richer clay source. The cob article describes cob as mostly sand with clay as the binder, so the right proportion depends on the method. See Cob Building and Clay Testing.
This is an estimate: layers are not perfectly sharp, clay can clump and settle early, and measurements can be off by several percentage points.
Method: Ribbon Test
Wet a handful of soil until it holds together and squeeze it between thumb and forefinger, pushing it out in a ribbon. Long, smooth ribbons mean high clay content. Short, crumbly ribbons mean more sand or silt. See Ribbon Test and Jar Test for Soil Texture for more detail.
Common Mistakes
- Reading the layers too early — clay needs a full day.
- Sampling only the surface — topsoil can differ from soil at foundation depth.
- Treating the result as exact — it is a field estimate, good for deciding between sites.
Safety
Keep digging safe and avoid contaminated ground
Do not sample soil from near former industrial sites, dumps or burnt buildings for food growing, and wash hands after handling soil. Test holes should be kept shallow and covered after use.
What’s Next
- Learn next: Load-Bearing Capacity — whether the soil can carry weight
- Combine with: Soil Composition — the farming side of the same test