Let me preface this piece with the caveat that I am not trained as a soil scientist (though I did take a couple of university soils courses many years ago). Nevertheless, as a gardener I am always working with soil, and I’ve been curious enough to do some more research over the years.
In any good book on growing plants, you are likely to come across terms such as clay-loam, soil alkalinity, humus, topsoil, etc. Understanding what some of these terms actually mean, and how they relate to your gardening, will help you to be more successful at growing your native (or other) plants.
First off, some definitions.
Texture: refers to the size of the soil particles (this is not the clumps of soil you may see in your garden, but the elemental particles that make them up). Soil scientists have determined that there are 3 basic soil particle sizes – Sand, Silt and Clay – and these combine to form 12 soil types that you would typically see in gardens (more on these later).
Sand is made up of ground minerals and rocks that are rounded or irregular in shape. These range in size from 2 mm down to 0.05 mm in diameter and feel gritty when rubbed between your fingers.
Silt particles are between 0.05 and 0.002 mm in diameter and feel a bit like flour when dry.
Clay particles are extremely small – less than 0.002 mm in diameter. In fact, they are so small that you can really only see the individual particles through an electron microscope. We often refer to clay soils as “heavy”. (One suggestion for this term is that plowing clay soils takes 2 to 4 times the tractor power to pull the plow through it as in “light” sandy soils.)
The Implications of Texture
If your soil is mostly sand, you cannot form a ball of it when it is wet – it will just crumble and fall apart. This is because the large particle size of sand results in large air gaps between the particles and thus water doesn’t have a strong enough bond to hold the particles together. Although these gaps allow water to enter easily, it also lets the water flow right through. As a result, sandy soils dry out quickly.
Some plants have adapted to sandy soils – Big Bluestem (Andropogon gerardii) and Prairie Smoke (Geum triflorum) are just two examples – and thus will typically not do well in wetter areas with heavier soils. By the same token, plants that have evolved in wetter areas, especially in clay-rich soils, are unlikely to survive in sandy soils. This is, in part, because without moisture retaining mechanisms, their roots will simply dry out.
At the other end of the spectrum, clay particles are so small that the air spaces between them are extremely tiny. Often, water will just puddle on top of clay soils and take ages to drain away. (This is why they use clay to line ponds and landfills.) However, water will eventually soak into the pores and because of the physics of water molecules and soil particles, the clay will actually “hold onto” the water for a much longer time.
Anybody who has walked through wet clay can attest to the fact that wet (even damp) clay sticks tightly to boots, shovels, etc. One impact of this characteristic is that clay soils readily compact and compacted soils do not allow roots to penetrate easily.
To test if you have clay soil, wet a small lump of it and press it between your finger and thumb and try to form a ribbon of soil. In pure clay, the small clay particles will stick tightly together and you should be able to press out a ribbon a few centimeters long. This Ribbon Test is used by soil scientists to get a quick idea of the soil texture.
Just as some plants have evolved in sandy soils, so too have some evolved in areas of heavy clay. Cardinal flower (Lobelia cardinalis) and Swamp Milkweed (Asclepias incarnata) are two that come to mind.
Everything in Between
Soil scientists have developed a Soil Texture Triangle which, once you learn how to read it, will allow you to identify which of the 12 soil types you have based on the clay, silt and sand composition of your soil.
Let’s say you get your soil tested and it comes back that you have 45% sand, 20% clay and 35% silt. Where these 3 lines cross on the triangle indicates your named soil type – in this case, loam. (The angle of the numbers indicates which line to follow.)
If you want to determine your own soil texture, one method you can use is what is called a jar test.
- Start by getting a shovel full of soil from your garden (you can take samples from different areas, if you want, to get a general feel for your entire yard though it is unlikely that there will be significant differences in an area as small as a town lot). Make sure you take your sample at least six inches (15 cm) down – not just off the surface – as you want to get a complete picture of your soil.
- Sift your soil to remove debris, rocks and large organic matter and fill a clear jar about 1/3 full of this “cleaned” soil.
- Add water almost to the top of the jar and add a small amount of water softener or liquid dish soap. Put the lid on the jar and shake vigorously for a few minutes until the soil/water mixture is a uniform slurry. Let the mixture stand for 48-72 hours for the soil to settle out (it will separate out into layers with sand at the bottom and clay at the top).
- After the mixture has set for a couple of minutes, you can mark the side of the jar with a black marker. This represents the sand component.
- After the mixture has set for about two hours, mark the side of the jar. This represents the silt layer.
- Finally, after 2-3 days, the clay will finally have settled out and you can mark that level.
- Using a ruler, measure the depth that each layer makes up. To calculate the percentage of each element, divide the height of that element by the total height of the soil in the jar.
With this information, you can now determine which plants will like the soil you have in your garden. After that, you can determine light and moisture levels to narrow down your selection.
Values from my yard’s soil sample: Total soil = 42 mm
Sand = 21/42 = 50%, Silt = 15/42 = 36%, Clay = 6/42 = 14%
Reading these values of the Triangle, we find that my soil is classified as loam (and very close to being a sandy loam).
Next Month – Beyond Sand, Silt and Clay – Organic Matter, Loam, Soil pH and other magic