Dirt Rocks! Physical Qualities of Soil

Are you excited to learn more about soil? I hope you are! Today we'll talk about three physical characteristics of soil: texture, structure, and color. Last week we did an intro on soil science, so I recommend you read it first for a little more background.

Click to view larger
Image courtesy of USDA

Soil Texture

Soil texture refers to the ratios of different sized soil particles. These particles come in three types: sand, silt, and clay. Sand particles are the largest and are visible to the naked eye, and they have a gritty texture. Silt particles fall in the middle, and silty soils feel smooth. Clay particles can only be seen with an electron microscope, and a soil with lots of clay will feel sticky. Soils are given a texture from the texture triangle (see image on the right, click to view larger) based on the percentage of sand, silt, and clay in the sample. Texture can be determined through mechanical analysis, but usually we just estimate by feel. Most central Indiana sites will have a clay, silty clay, or clay loam texture. Soil texture matters because it influences the way the soil interacts with water, nutrients, and plant roots.

Image by Maria Gulley

Image by Maria Gulley

Soil Texture and Water

Water flows and stays differently in different soil textures because of the range of pore sizes. Imagine a bowl of oranges (sand), a bowl of gumballs (silt), and a bowl of marbles (clay). The oranges don't pack in very tight, so the spaces in between are large. The gumballs pack tighter to leave smaller spaces, and the marbles leave the smallest spaces. In sandy soils, water drains quickly, but it doesn't easily move through the tiny spaces between clay particles. Even after a soil has drained from an initial flooding, clay will hold more water since the water clings to the surface of the particles after the spaces have mostly emptied. A lot of tiny pores provide more surface area than a handful of big pores. You can see this illustrated in the image on the left, where the blue represents the layer of water that stays behind on soil particles. We want some large pores though, because these leave space for air to fill back in after some of the water has drained out. A mix of all three particle sizes provides the best amount of drainage and water retention.

Soil Texture and Plant Roots

A plant's ability to take root and remain stable is also influenced by soil texture. In a sandy soil, growing root tips easily move through the larger pores around the grains of sand. On the other hand, sand shifts more readily, and the plant may be uprooted. Clay is the opposite: small pores make it more difficult for roots to establish, but once they do the plant is firmly in place (as you can no doubt confirm if you've ever tried to dig up an old shrub or stump in our Indiana clay!).  Once again, a mix of different particle types offers the best compromise for the plant for ease of rooting and firm establishment.

Soil Structure

Sand, silt, and clay percentages alone don't determine physical behavior of soils; soil structure is important too. Soil particles stick together to form aggregates, and the pattern these aggregates form is called soil structure. While soil texture is rarely altered by human activity (unless you're physically mixing in different materials, like people often do in vegetable gardens), soil structure is very responsive to human pressures.

By Bijltjespad - Own work, Public Domain
translated into English from original Dutch)

The chart to the right shows the main types of soil structure (click to view larger). A good growing soil will have granular structure near the surface and blocky structure further down with prismatic structure down below the root zone. Unfortunately, it is common to find platy soils from compaction where humans have been active. Platy soils hold water longer than is healthy, and they limit root growth. To limit compaction, avoid driving heavy equipment in planting areas, vary your mowing pattern, and don't handle soil while it's wet.

Once soil structure has been ruined, restoring it takes work. Mixing in organic material like compost, manure, or leaf mold can help normal aggregates develop again (note: do not mix in raw wood chips or sawdust, as these materials will cause a drop in soil fertility). Aeration is a common practice for reducing compaction in lawns. When a lawn is aerated, a specialized machine with a roller covered in hollow tines is rolled across the yard to removed plugs of soil to allow more air and water to reach plant roots. You can learn more about soil compaction in the last lesson of the series, which focuses on common problems in urban soils.

Soil Color

Soil color doesn't really do much, but it can tell you a lot. The colors you need to know about here in the Indianapolis area are black/dark brown, light or reddish brown, and gray. I spent a moment on color in the introductory post, but now we'll go into more detail about it.

Black or dark brown soil is rich in organic material and provides lots of nutrients for plants. Your soil may have a layer of dark soil a few inches deep near the top, as in the photo below, or if you grow vegetables you might regularly add compost or other organic material to boost your crops and end up with dark brown. This color is nice to see, but it is by no means necessary for most of the landscaping plants we grow in our region. Most plants we use are adapted to thrive at a lower level of fertility or can have their needs met with annual fertilization.

Light yellowish or reddish brown is what we see in a well-drained native soil around here. The brown color actually comes from iron in the soil turning into iron oxides (rust) when air is present. This is what you want to see in a good soil in most parts of central Indiana. On the left, you can see that color immediately under the dark brown surface color.

Gray soil (see the deepest part of the hole next to the shovel on the left) is an indicator that you have bad drainage. The same iron that turns brown in a well-drained soil remains gray because there is not enough oxygen present, which means there's not enough for healthy plant roots either.  A soil that is gray from lack of oxygen is referred to as gley or gleyed. A poorly drained soil may also have patches of brown and gray mixed together if the problem is not too severe. You can see some of this in the light band of soil and in the bottom of the hole in the picture on the left. Gleyed soil can be the result of compaction, excess irrigation of a heavy soil, or a low point in a wet property. Sometimes the soil can be improved by relieving compaction, reducing irrigation, or grading, but sometimes you're just out of luck for that spot.

Tune in next time for an overview of soil chemistry and biology! If you dreaded chemistry in high school or college, I promise this won't be as intimidating. For more information about soil texture, structure, and color, check out the links below.


Additional Resources
Physical Properties of Soil and Soil Water (Plant and Soil Sciences eLibrary)
Soil Color Never Lies (The European Geosciences Union)
What to Do about Compacted Soil (Michigan State University)
Aerating Lawns (Clemson University)
Soil Drainage: Analysis and Treatment Considerations (Bartlett Tree Experts)

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