Best Soil Types for an Earthship & How to Do a Soil Test

Before you build a million-pound Earthship home, make sure you do a soil test so your house doesn’t shift OR SINK!

earthship soil test
Above: Here’s what can happen if you build your home in the wrong place.

If the soil at your building site isn’t a good foundation for underneath an Earthship, your architect and/or structural engineer will probably recommend that you use a tractor or hire an excavator to scrape away what’s there and replace it with imported materials.

In this article, I’ll teach you everything you need to know about soil types for underneath your Earthship, and also how to do a soil test. Last but not least, I’ll share with you the results of my soil test underneath The Montague Earthship and how I’m going to prepare the foundation for the build.

Three Basic Soil Types

According to Rogue Valley Soil, “Soil is a mixture of sand, clay, silt, organic matter, bacteria and microorganisms that help decompose organic matter into nutrients that enrich the soil. A basic soil test measures the proportion of each of these to determine the kind of soil you have.”

It’s important to understand the 3 basic soil types:

  1. Sand (0.05-2mm in size)
  2. Silt (0.002-0.05mm in size)
  3. Clay (less than 0.002mm in size)

Sand vs Silt vs Clay

Your soil type can be one or a combination of the three basic soil types.

Sand is the same stuff you’ll find on a beach, and when superheated to temperatures above 3090°F / 1700°C it forms glass. Sand will not decompose over time because it contains no organic matter, which is of benefit. Other benefits of sand are that it’s not absorptive and it drains well – which is terrible for a garden but okay for supporting your Earthship home. The biggest issue with sandy soil underneath a house foundation is that over time particles can potentially wash away.

Silt retains water well, which means it’s terrible for drainage underneath your home. Additionally, the absorptive qualities of silt causes silt-heavy soil to expand when water is present. Expansion of the silty soil when it becomes moist can cause the foundation to move and weaken over time.

Clay has the smallest particle size of the three and is highly water-soluble, which means it will expand and contract significantly during wet and dry times, respectively. These extreme changes can put pressure on foundations, causing them to shift up and down, and eventually crack. Also beware that if you have heavy-clay soil underneath your Earthship home, and you’ve pounded a few rows of tires and winter comes, the ground underneath could expand and shift your tires as the water freezes. This could potentially cause you to have to start from scratch the following spring and re-pound all your tires again.

Three Additional Soil Types

There are a number of other soil types that you should know about including loam, peat and bedrock.

Loam is a balanced combination of sand, silt and clay and has been called an ideal soil type for underneath the foundation of a home. The benefit of crumbly and soft loam is that it holds water in a more balanced way that’s not so excessive like in soils heavy in clay or silt. Additionally, the balanced combination of sand, silt and clay means it will not wash away like soils heavy in sand, for example.

Peat is a type of soil that’s spongy and contains an accumulation of partially-decayed organic matter. Spoiler alert!!! The land at The Montague Earthship building site was high in peat, but more on that later. The fact that peat retains moisture well and will continue decomposing over time makes it an unsuitable foundation for an Earthship home.

Bedrock is a solid slab of rock with considerable weight-bearing capacity. If the bedrock is level, this is probably the ideal situation for underneath your Earthship home or other type of home.

How to Do a Soil Test?

So now you’ve got an understanding of the primary soil types and components of soil and you’re ready to test the soil underneath your chosen Earthship building site. How do you do a soil test?

The best way to do a soil test underneath your Earthship building site is to do The Jar Test. Here’s how it’s done:

  • Fill a 1 Liter glass mason jar about half way with a sample of soil
  • Go home and fill the jar with water, leaving about 1 inch of air at the top
  • Attach the lid and shake the jar vigorously until all clumps have been broken up
  • Set the jar on a table or counter so it can rest for 24 to 48 hours

Once the material in the jar has settled it will have separated into sand, silt and clay in layers. Your jar will look something like the jars in the image below.

As you can see, the clay will rest on the top of the mixture, the silt in the middle, and the sand at the bottom. These layers occur because the sand has the largest particle size, the silt medium particle size and the clay has the smallest particle size. The results will give you an indication of what type of soil you have, which you can then share with your architect and/or mechanical engineer who will let you know if they think it’s a safe foundation of your home.

Soil Test at The Montague Earthship

In preparation for building The Montague Earthship I did a soil test to see what was underneath this building site, and if it was suitable to support my home.

After collecting some soil at The Montague Earthship building site, adding some water, shaking the jar and waiting for 48 hours the results were in. I took a photo of the jar so you could see…

The results of my soil test were actually a bit confusing at first because the water in the jar near the top never seemed to clear up. Even after five days of sitting on the counter it remained cloudy and ‘dirty’ like in the image above. Also, you can’t really see any distinct layers of sand, silt and clay in the jar.

It turns out, the soil test you’re looking at above from Let’s Grow Farm is almost pure peat – the weakest, least structurally-supportive material possible for the foundation of a home. I knew at this point that the peat would have to be scraped away and that I would need to import some stronger materials to serve as the foundation.

The question that I needed to answer now was: How deep is the peat??

Excavating the Ground to See The Layers

A soil test is great for finding out what a soil sample is made of, but it cannot tell you the entire story of what’s underneath your building site. The only way to see the bigger picture is to dig down at the building site with a shovel or excavator to reveal the many layers that are there.

While the construction crew was building the driveway at Let’s Grow Farm, I had them use the excavator they were using to dig down at my Earthship building site to see what was there. Here’s what was revealed…

Soil-Excavation soil test

As you can see by the coffin-looking hole in the ground, there is about 1 foot of peat, and beneath that about 1 foot of pure sand, followed by solid bedrock.

I forwarded the soil test and the excavation image to my architect helping me design my earthship and he recommended we remove the peat, and replace it with 1 foot of graded rock to prep the site before the build. This will create a well-drained, solid foundation that will not decompose or shift over time with the changing of the seasons.

For an Earthship home (or any home), bedrock, sand and graded rock underneath is one of the most solid foundations you could possibly have.

Soon I’ll bring in a construction crew to prep the site for the building of The Montague Earthship. Oh, and I’ll be documenting all of it so you can watch it all unfold. Be sure to subscribe so you don’t miss it!

Conclusions

Soils are more stable when they contain more rock and compacted sand/gravel. This is the ideal situation that you should hope for (or create) at the building site of your Earthship (or any) home you plan to build.

It’s important to not build your home on a foundation of soil that’s too silt- or clay-rich as they can significantly expand and contract through wet and dry seasons, causing foundations to shift and crack.

Peat-heavy soil, like the soil at Let’s Grow Farm, is fluffy and high in organic matter, which cannot support significant weight, expands and contracts considerably during wet and dry times, and will actually decompose over time.

To remedy the problem of having peat-heavy soil, we’ve chosen to scrape out the foot of peat and replace it with graded rock. Building on bedrock with a layer of sand and graded rock is a maximally solid foundation for an Earthship or for any type of home.