Two simple ways of testing for clay subsoil
Earthen building encompasses a variety of highly resilient construction techniques that include benefits ranging from affordability, incredible durability, and ecological sustainability, to increased health of the structure's occupants. Despite all these advantages, you won't get very far if you don't know how to find the correct type of subsoil to make good walls. Fortunately the right kind of clay rich subsoil is abundant in most places around the world and by learning what to look for and how to make a few simple tests of your soil samples, you can start building high quality structures out of earth.
Before getting started on any earthen building project you have to be able to tell if the soil you'll be using is suitable for construction. There are two main ways to test your subsoil for clay and other mineral content. The first and easiest way is to moisten a sample of the subsoil and test it's pliability with your hands. If the moist soil feels sticky and continues to stick to your hand when you turn it upside-down, that's a great indicator that the soil has a decent clay content. You can also try rolling the soil into a snake like form and let part of it hang of the side of your hand. If it curves and stays connected without splitting and falling off, this is another good indicator of clay content.
The second way to test for different mineral content takes a bit longer but can give you a better idea of the ratios in a given sample. Take an empty transparent glass jar and fill it half way with your subsoil sample. Fill the rest of the jar 80% of the way with water, then put the top on and shake the contents thoroughly. Once everything is mixed well, let the jar sit undisturbed for five minutes and take a look at the results after the time is up. The layers that have started to form indicate different types of mineral deposits as shown in the drawing. Rocks and sand will be the first to settle, followed by silt, and then clay. It may take up to a full day for all the clay to settle, but you'll know it's done when the water on top is clear. Anything floating on top of the water is organic matter or bits of trash. The biggest challenge is telling the difference between silt and clay. Both are very smooth, soft, almost silky, but only clay really sticks. While this test is a good way to get an idea of the proportions of each material in your subsoil, I prefer to use the touch and form tests when deciding whether or not a test soil is good for making cob or adobe, because not all types of clay are sticky enough to really hold the mix together.
It's also very important to test multiple soil sites on your land because the clay and other mineral content can differ significantly only a few feet away. Rather than simply digging systematically every few meters from each test site, there are many different signals you can look for that indicate where clay soil might be found before you ever start digging. Look for places on the land where water tends to pool. Water doesn't pass through clay very easily and standing water is a good indication that there is a clay vein underneath. In every bio-region there are also plants that thrive in clay soils and high water tables. Research what these plants are in your area and look to see if you can find them on your land. Indicators such as these could save you a lot of unnecessary time and digging when looking for subsoil suitable for earthen walls.
If you are still unable to find any clay soil on your land there are many places you could search. Often times when roads are installed or cut into the side of a hill, the process of construction will expose heavy clay soil that would otherwise be inconvenient to excavate yourself. Many times you can speak directly with your local municipality to ask if there are any land alteration projects going on such as road leveling, or site grading for another construction project, or the digging of a well or pond which might produce surplus clay soil. Get creative in where you look and who you talk to. Clay subsoil is incredibly common and is usually very easy to obtain.