soil type testingloamy soil / jar test

Soil type refers to the texture of your soil. Soil is composed of mineral particles, organic matter and air spaces.

For earthbags & plaster, you'll want soil that's between about 18% and 30% clay.

Soil minerals come in three basic categories: sand, silt and clay. Sand is the largest particle. Silt particles are smaller than fine sand but can still be seen by the human eye. Clay particles are microscopic. The relative proportion of these particles determines the soil texture.

Loam soil—the ideal soil texture - is composed of all three categories (sand, silt and clay). The ideal loam soil contains 40% silt, 20% clay and 40% sand (and organic matter, if you're gardening). Loam is a separate category because none of its components account for more than 50% of volume.

Test Soil Type by Hand

You'll want to bring along a water bottle, and maybe some jars or bags for samples. (For the latter, also bring a magic marker & way to label your samples so you don't get confused.) These are very rough tests, but allow you to narrow soils down to those that are worth subjecting to the jar test (below).

TEST 1: Take a handful of soil, and pour some water on it. Feel it and rub it between your fingers. If it's rough and gritty and breaks up easily, it's predominately sandy.  If it feels sticky and like plastic, it's probably mostly clayey. If it feels slippery with particles that are smaller than sand, it's mostly silty. You want something that has elements of all three.

TEST 2:  Squeeze the wet soil in your fist, trying to form a lump with it. If it makes a solid lump that doesn't crumble, it's got a high clay content. If it crumbles at once, it's mostly silt or sand. Rub it back & forth between your hands (like you would with Play-Doh or, duh, clay). Does it hold up & make an elastic snake that holds together? Partially?

Test Soil Type by Jar

This test can take several hours to a day or two.

  1. If you have more than one sample, label or identify your jars on a piece of tape on a mason jar so you can keep things straight. What part of your land did they come from? Surface samples, or from depth?

  2. Place 5 inches(or 5 cm) of soil into the jar and add a tablespoon of powdered dishwashing detergent. The detergent is a surfactant, which keeps the soil particles separate, resulting in a more accurate test.  

  3. Fill the jar to the top with water, screw the lid on, and shake the jar for three minutes to thoroughly combine the soap, soil, and water, and to make sure no soil is stuck to the bottom or sides of the jar.

  4. Set the jar somewhere stable & out of the way to let the sediment settle.

  5. As the sedimentation progresses, check the sample periodically to watch the layers form and to note the size of the particles settling out.

Sand particles are the heaviest of the three and settle out of the solution after about a minute. The sand layer is coarser in texture than the silt and clay.

Silt is the next heaviest particle and will settle out after about an hour. The silt layer is darker than the sand.

Clay, the lightest particle in the mix, can take from one to two days to settle out of the solution. The clay layer that settles on top is fine textured and light in color.

If you like, you can calculate percentages and transfer the numbers to the soil texture triangle (below) to determine the composition of your soil. Click image to enlarge.
    1.    Measure each layer of sediment
    2.    Figure out the percentages of sand, silt, and clay in the sample. 
           For example, if you have 5 total inches of soil in your jar, then every 1/2 inch will be 10%. (1"=20%, 2"=40%, 3.5"=70%, etc).

Remember: your ideal soil for earthbags should fall into the "loam" category with approximately 20% clay, 40% silt and 40% sand.
Soil Types

Amending by Soil Type
If a soil tends toward an excess in clay, sand or silt, you can amend it by simply adding & mixing the required components. Both sand and silt are pretty commonly available in most areas (if necessary, you can buy them from an aggregate yard for a few dollars per cubic yard).

Clay is a little more difficult to find, if it's not common in your region. You might be able to find sources of clay-rich dirt in river bottoms, lakes and ponds. Construction sites may have cleared topsoil, leaving access to clay layers below that you might be able to get permission to harvest. You can even buy powdered clay from pottery suppliers, but this can get expensive.

Worst case is that you can use cement to stabilize clay-poor soil. It works, but cement-stabilized dirt will be subject to cracking & doesn't offer as good insulative properties as clay or lime. You can try try mixing a little lime with your cement. Start with about 8% per volume, make a few test batches with different percentages of clay, then set them out in the sun to cure.

A couple of good tests for earthbag-worthy fill are the drop test (seeing if a dried sample shatters when it hits the ground from chest height) and trying to gently drive a nail into your fully-cured sample. If you can, without it splintering or breaking apart, you've got gold.

The following is geared more towards gardening (credit:,
but might help with gaining a comprehensive sense of the three components & their qualities:

Type of Soil Test Jar
Sandy soils come from erosive sources and so are very common near the mountain foothills, along rivers and streams and certain coastal areas. Sandy soils are typically comprised of approximately 80 - 100% sand, 0 - 10% silt and 0 - 10% clay by volume.

Sandy soils are light and typically very free draining, usually holding water very poorly due to very low organic content.  
sandy soil / jar test
Loam soils are common in the valleys and flat areas (flood plains) surrounding rivers and streams.
Loam soils are typically comprised of approximately 25 - 50% sand, 30 - 50% silt and 10 - 30% clay by volume.  

Loam soils are somewhat heavier than sandy soils, but also tend to be fairly free draining, again, due to typically low organic content.
loamy soil / jar test
Clay soils are very common in certain areas, particularly around urban areas where fill soils have been used to establish grade in subdivisions and developments.  Clay soils are typically comprised of approximately 0 - 45% sand, 0 - 45% silt and 50 - 100% clay by volume. 

Clay soils are not typically free draining, and water tends to take a long time to infiltrate. When wet, such soils tend to allow virtually all water to run-off. Clay soils tend to be heavy and difficult to work when dry.    
clay soil / jar test

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