A Sum Greater Than its Parts: Soil Composition

Ask any gardener what kind of soil they have, and likely they will definitively say “clay,” “sand,” or in some cases, “dirt,” “junk,” or other words not fit to print here. The reality, though, is that soil is a combination of mineral material, organic matter, air, and water. The differences in soils lay in what percentage there is of each of those.

Enter the soil pyramid chart, also known as the soil texture triangle or, by some, as the tertiary soil diagram. Knowing it by any of those terms will not get you a date on a Friday night, but will help you to figure out what you are working with in your garden. The ubiquitous soil pyramid chart is a tool devised by agronomists and soil scientists and used around the world to classify soils by their constituent parts. Soil health, when viewed holistically, is at the intersection of its biological components, such as the living or organic matter in it; the chemical components, which refer to the nutrients within the soil; and the physical properties of the soil, which refer to the particles. The soil pyramid chart helps to identify soil composition by using the physical properties.

Sand particles are larger than silt particles, which are larger than clay particles. Colloidal particles are those suspended in water, and they are even smaller than clay particles. The relative percentages of clay, sand, and silt in tandem reveal a soil texture on the chart. Soils may be sand, loamy sand, sandy loam, sandy clay loam, loam, silty loam, silt, silty clay loam, clay loam, sandy clay, silty clay, or clay.

For the pyramid to be a useful tool for average gardeners, they would need to be able to sieve out and weigh the various particles in a soil sample. Most growers, even those with experience, are not likely to have the type of equipment needed for such an analysis. A lab is generally needed to perform this test. But in case you were wondering…

1. Particles greater than 2 millimeters (mm) are deemed to be gravel and not considered to be part of the actual soil.

2. Particles ranging in size from 0.05 mm to 2 mm, which is visible to the naked eye, are considered sand. Sand may further be divided into the categories of very coarse, coarse, medium, fine, and very fine. Sand particles are generally round.

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3. Particles from 0.05 mm down to 0.02 mm are considered silt (larger silt particles are only visible only with a hand lens). Silt particles are generally irregularly shaped.

4. Particles less than 0.02 mm, which are only visible with a microscope, are considered clay. Clay particles tend to be flat.

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Why This Matters

Okay, you’ve determined that your soil is silty clay loam or loamy sand or any of the other classifications—but why does this matter? Knowing your soil texture will help to inform your decisions relating to watering frequency, fertilizing frequency, and even what types of crops to grow. Soil texture influences things such as water movement, water storage, aeration, and nutrient holding capacity. It can also let you know how early in the season you can till, plow, or otherwise work the soil, since soils with less moisture warm up earlier in the spring than soils with more moisture. Here’s a breakdown of how soils generally behave:

1. Predominantly clay soils are good at holding water and nutrients, but they are not good at water infiltration, have poor aeration, drainage, and are difficult to work with.

2. Predominantly sandy soils are the opposite of clay soils. They have good aeration, good drainage, good water infiltration, are easy to work with, but are poor at holding nutrients and water.

3. Predominantly silty soils are fair at all the aforementioned criteria regarding water and nutrient holding capacities, infiltration, and workability.

4. Predominantly loamy soils are the same as silty soils in those same categories.

It should be noted that soil texture is not necessarily correlated to soil quality. The other components of soil such as the organic matter and nutrients affect the quality of the soil.

The Ribbon Test

If you are among the 99.9 per cent of gardeners that do not have the necessary equipment to sieve and weigh your soil sample, there is an easier method to determine your soil texture. It is not as scientific or precise as the soil pyramid chart, but it will reveal the basic characteristics of your soil, allowing you to determine the sample’s texture. The ribbon test is a measure of a soil’s plasticity, or ability to change shape when pressure is applied to it. Sandy soils are not “plastic,” while clay soils are highly plastic.

The first steps involve collecting and moistening a soil sample. Since you are not testing for nutrients or pH, the source of water is unimportant. Grab a handful of the wet soil and try to form a sphere about the size of a ping-pong ball. If the sample will not form a ball and the reason is not for lack of moisture, then you probably have a sandy soil. If you do still have a soil ball, place it between your thumb and forefinger and gently squeeze. If the sample falls apart when squeezed, then you likely have a loamy sand soil sample (say that 10 times fast). If the sample stays intact and you can flatten it into a ribbon-like shape, the fun has begun.

If you form a ribbon that breaks off less than 25 mm (one inch) after it has formed and:

1. It feels gritty, you probably have sandy loam.

2. It feels smooth, you probably have silty loam.

3. It is equally gritty and smooth, you probably have loam.

If you form a ribbon that extends over 25 mm, but less than 51 mm (two inches) before breaking off and:

1. It feels gritty, you probably have sandy clay loam.

2. It feels smooth, you probably have silty clay loam.

3. It feels equally gritty and smooth, you probably have clay loam.

Finally, if your ribbon is longer than 51 mm before it breaks off and:

1. It feels gritty, you probably have sandy clay.

2. It feels smooth, you probably have silty clay.

3. It feels equally gritty and smooth, you probably have straight up clay.

The Jar Test

If squeezing wet soil doesn’t sound very appealing, another do-it-yourself method for analyzing soil texture is the jar test. For this to work, you need a clean, clear jar. A one-liter (one-quart) quart works well. Fill the jar approximately two-thirds full with your soil sample and the remainder with water. Close it up, shake the jar, and allow to sit for a couple of hours so that the components can settle out.

Any organic matter will float to the top; disregard this as it’s not part of the measurement anyway. The larger sand particles, however, will settle to the bottom. The next strata will be silt particles, with clay or colloids on the top. Using a ruler, measure the height of each of the strata to determine the percentage of each in your sample. (Again, do not measure the organic matter or any water that may be above the clay/colloid level.) So, for example, let’s say a sample jar is about 20 centimeters (eight inches) high. Inside, the sand level is 7.6 cm (three inches) thick, the silt layer is another 2.5 cm (one inch) thick, and the clay is another 1.3 cm (0.5 inch), making for a total of 11.4 cm (4.5 inches) of sediment/strata height. When figuring out the percentage of the soil components, you would use the 11.4 cm (4.5 inch) height in your calculations, not the 20 cm (eight inches) of the sample jar. So, in this example, the soil is 67 per cent sand, 22 per cent silt, and 11 per cent clay.

When interpreted correctly, the soil pyramid chart will help you to understand the physical nature of your soil type. For a more detailed analysis of your soil, for a fee, your local co-operative extension office can either test your soil or direct you to a lab that will test your soil and can help you to identify your soil texture. The lab, extension service, or professionals at your local garden center can then help you to interpret your results, if needed, and offer guidance relating to watering and fertilizing frequencies for your particular site.