How to test your soil?

No.046 — Read old posts on Tinyfarmlab.com
Reading Time 4 minutes

In the previous post, we talked about soil composition and the roles of clay, sand, straw, and water in making a good cob mix.

Today, we shall discuss how to test your soil.

If you are building in a rural area, you usually won't need to make many changes to the soil, and if you are lucky, you might even find the perfect soil already in the ground, ready to mix with straw.

In this case, you wouldn’t have to get more sand or clay.

These ideal building soils, known as "ready-mix," have between 10% and 25% clay and a high amount of coarse sand (50% or more).

Checking your soil’s suitability

Use a shovel to remove the top one foot of soil and set it aside. Then, check the suitability of the subsoil underneath.

You can start with the jar test.

Jar Test or Shake Test:

You will need many clear jars with a tight lid and straight sides, water, and a couple of ounces of salt.

This test is great for telling the amount of sand you have in your soil. It also helps you detect the amount of clay and silt.

Dig multiple holes where you need to check your soil.

Take a handful of soil. Crush the lumps fine.

Put different samples in separate jars and fill them up to one-third full.

Add water until the jar is 3/4th full.

Add a teaspoon of salt to accelerate the settling process.

Shake the jar like you mean it.

Shake it for 2-3 minutes and then suddenly stop and keep it on a flat surface.

The coarse sand will fall first. Make a mark where it settles after 3-4 seconds.

Fine sand and silt will settle in the next 15-20 minutes.

Clay will settle gradually. It would take days or even weeks for a complete settlement.

If you have ideal cob soil, you'll find a thick layer of coarse sand with a thinner layer of clay and little to no silt or fine sand.

Although this is a good test for identifying the amount of coarse sand,
Sometimes it isn't easy to differentiate fine sand, silt, and clay, especially silt and clay since they both have very fine particles.

This is why we need other tests to identify clay in the soil.

Crush Test

If the dry soil sample, when pressed with fingers, crumbles easily, it probably doesn't contain much clay. Dry soils with lumps that are difficult to break using fingers mostly contain clay.

S-ticky Test

Take the soil sample. Add a little water and make a paste of it.

Dab it between your fingers and see if it sticks.

Next, make a wet ball, the size of a golf ball.

Press it between your palms, flatten it, and make a ‘tiki’ or pancake.

Turn your hand and it should stick for at least 7-10 seconds before it falls.

The stickiness indicates the clay in the soil.

Shine Test

Take another wet ball and flatten it in your palm.

Tap it repeatedly.

In the sun, you would see a shine on the top.

Clay retains shine whereas if it wouldn’t been silt it would have gone dull.

Ring Test

Potters use this test to check clay soils for ceramics.

Roll some of the soil between your palms into a rod about the size of a pencil. Then, bend it around your finger.

The more clay it has, the more it will bend without breaking.

Squish Test

Make another golf ball lump, like wet dough.

Squeeze the soil tightly in your fist, leaving a small gap between your thumb and forefinger.

If the soil has a lot of clay, a ribbon of wet clay will come out through the gap.

If there's a lot of silt, water will drip from your fist.

The above tests help you identify whether your soil has enough clay and coarse sand. In case, one of them is missing you would need to haul either sand or clay from outside to make an ideal mix.

In the next mix, we shall talk about how to find clay, sand, and straw if you are not able to find them on your land and add it to your site soil to make an ideal mix.

Until then, you can practice the above test by digging a hole in your fam ror even in the local park.

Love,

Raghav and Ansh

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What you can watch - Testing Soil for Clay Content

This video shows how to test the soil to determine if there is enough clay content to use the soil for construction - cob, adobe, clay plasters, and more.

What you can listen to - TFF Podcast | Earth to Shelter: A Masterclass on the Science of Mud Buildings with Yask Kulshreshtha

Meet Yask Kulshreshtha: Founder of Muddled Studio.

A scientific researcher in sustainable construction materials, Yask's PhD thesis "Building affordable, durable, and desirable earthen houses" at TU Delft, Netherlands, tackles rural Indian housing challenges: affordability, durability, and desirability.

What You Can Read - Cement-based rammed earth "not much better than concrete"

"If you make a 0.1 per cent improvement to concrete mix across all the projects that are getting built from concrete in the country, it will make more of a difference than if rammed earth went from something that was comparable to other materials to this magical material that didn't cost any carbon."

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