Soil Texture Triangle| Earth systems and resources| AP environmental science| Khan Academy

Today we're going to talk about soil, and you've probably noticed that there are many different kinds of soils. The soil near a beach looks and feels very different than the soil in a forest. Part of the reason for that difference is something called soil texture.

Soil texture, so when soil is formed, different types of rock break down because of the wind, rain, and the weather, and they become differently sized particles. The combination of these differently sized particles creates soil texture. These differently sized particles can be broken down into three groups: we have sand, we have silt, and we have clay. Sand is made up of the larger and heavier particles of soil, which is around two millimeters to 0.05 millimeters—this is a 20th of a millimeter in diameter—and it feels very gritty to touch.

Clay, on the other hand, is the smallest particles of soil; it's around 0.002 millimeters and smaller—that's a 500th of a millimeter, so really tiny. If you rubbed clay between your fingers, it would feel smooth, and you wouldn't be able to feel the individual particles of soil. Unlike sand, you can't even see clay's particles with the naked eye. Most people are familiar with sandy soils and clayey soils, but silt is right in between; it's made up of particles between the sizes of clay and sand. So it's between 0.05 millimeters and 0.002 millimeters.

When you think of silts, imagine baking flour—it's powdery, and it can be carried easily by wind and water. But soil is more complicated than just these three soil types. If you grabbed a handful of soil from outside your home, it would probably be a combination of all three soil types along with some organic material. A soil's unique texture, that is its combination of silt, sand, and clay, affects how plants will grow. So gardeners often want to know about their soil texture so they know what kinds of plants to grow in that area.

This is where a helpful diagram called the soil texture triangle comes in. The soil texture triangle allows us to place any soil sample into one of 12 different soil texture categories. The 12 different categories are broken up based on the percentage of silt, clay, and sand in the soil. To see how this works, let's do an example.

Let's say a gardener determined that our soil was 30% sand, 40% silt, and 30% clay. So how can we figure out what kind of soil we have? First, we need to pick a side of the triangle to start with. I'll start with sand. We know we have 30% sand, so we go along the side of the triangle until we find the 30 mark right here. We want to draw a line from this point to the side of the triangle that this arrow is pointing to; the arrow is pointing towards the clay side. So we draw the line from the 30% sand mark through the triangle to the clay side like this.

By drawing this line, we already know that our soil sample will fall into a category along this line. So it could be silt loam, loam, clay loam, or clay. But to figure out exactly where it is, let's draw another line. Let's do silt; the soil has 40% silt. We'll draw our line through the triangle towards the sand side because of this arrow. So again, we do the same thing. We go along the percent silt line, we find the 40 mark, and we're going to draw a line from 40% to the other side of the triangle.

You can see when we draw that line, these two lines intersect at the clay loam point. But to double check, let's draw a line from the 30% clay mark to the other side of the triangle like this. As you can see, these three lines all intersected at the same point right here in clay loam, so we know that we have clay loam. Notice that the percentage of clay, silt, and sand all add up to a total of 100. We could have picked any two of these lines, just two, to find our answer. We don't need to draw three lines every time.

Let's do another example. Let's say our gardener has another plot with soil that contains 58% sand, 27% silt, and 15% clay. So I'm going to go along the percent sand part and find 58. So that's pretty close to 60. I'm going to draw a line from 58% to the other side of the triangle. Sometimes the example question doesn't give us round numbers to work with, so when that's the case, I like to use a ruler or a straight edge just to make sure that my lines are in the correct spot.

Next, I'm going to use the 15% clay. So I find the 15% clay mark, and we draw a line from 15% to the other side of the triangle, following the direction of the lines within the triangle. We can see that our two lines intersect at this point, so we have sandy loam.

The gardener has these two plots: clay loam and sandy loam. When the gardener looks at her two plots, she might want to take into account the different kinds of soil. Certain kinds of plants do better in clay loam soil, while others would do better in the sandy loam soil. This is partly because clayey soils hold onto moisture for longer than sandy soils do. So the plants in the gardener's sandy loam plot would do better if they had longer root systems to access water deeper in the ground or if they stored water inside themselves to access later.

The gardener could use a gardening book or the internet to see what kinds of plants would do best in each of her two plots. Thanks for watching, and happy soil identification!