The Nitty Gritty on Soil

Knowing your soil is essential for installing a Low Impact Development (LID) project like a rain garden, permeable paving, etc. Read this blog to learn what type of soil is in your yard.

Thinking about installing a Low Impact Development project like a rain garden, bioswale, or permeable pavers? There is a lot to consider. Use the Green Venture checklist to help you plan your project.

Site Conditions and Analysis Checklist

The Site Conditions and Analysis Checklist walks you through all of the details when it comes to looking at your site’s existing and future conditions.

One aspect of site analysis you need to understand the soil type on your project site. The type of soil you have in your yard can make a critical difference in the type of LID, its sizing, and your plant choices.

Soil types

Typically when we consider soil, we think of 4 major types: sand, silt, clay (and loam). These names describe not only the particle size/shape, the texture, but also the pore (or air) spaces they form between the particles.

soil pore space

For LIDs, the pore space is a key feature to understand, because pore space influences how much and how fast water can pass through the soil (infiltration). However, no soil is purely one type or another. Most are a mix, as seen in this soil triangle below.

Soil triangle

To build a small-scale LID project on your property, this is one way to check for soil type:

Ribbon test procedure:

Dig a small amount of soil from the area you are considering installing an LID project. Do multiple tests over the given area. The larger the proposed feature, the more samples are needed.
Take a clump of soil, big enough to form a small ball and add a few drops of water so it holds together.
Form the ball into a cylinder shape (like a wide baby carrot).
Between your thumb and forefinger, begin to flatten the soil, forming a “ribbon”
Measure how long the ribbon forms until it begins breaking.
If it breaks before reaching 2.5 cm long and it feels: gritty, the soil texture is sandy loam; smooth, the soil texture is silty loam and; if neither gritty nor smooth, the soil texture is loam.
If it breaks between 2.5-5 cm long and the soil feels: gritty, the soil texture is sandy clay loam; smooth, the soil texture is silty clay loam and if the ; soil does not have either a gritty or smooth feel to it, you have a clay loam.
If you can form a ribbon longer than 5cm, and the soil feels: gritty, the soil texture is sandy clay; smooth, the soil texture is silty clay and if the ; soil does not have either a gritty or smooth feel to it the texture is clay.

Make sure to write down your results.

When you start digging, you may be surprised at what you find and be confused if your soil doesn’t seem to fit into these neat categories. Soil science emerges from agriculture and geological sciences, usually looking at soils that are either more natural or managed (in terms of farming). Soils found in developed areas, especially dense, urban areas, defy easy categories. While digging you can find old building materials and trash. In recently constructed areas, sometimes topsoil is removed, leaving onto subsoils in its place. These are challenging things to encounter in your soil but not insurmountable. Luckily, there are many plant communities adapted to both the soil conditions and potential moisture conditions found in an LID. However, we do want to encourage caution if your analysis shows a very slow draining clay soil (especially if you notice consistent ponding in the area you are considering). This type of soil may not be the best location for in-ground LID projects like rain gardens. But other options may be possible!

Here are some more resources to help you understand soil type:

UBC hosts a wikipedia page on soil texturing that offers a good overview, including a handy flow chart.

Oregon State University has a thorough resource on infiltration testing involving digging a test hole (please note the specific directives on when and where to do this testing).