How to Read the Soil Texture Triangle

Walk through it

This starts on the Clarion topsoil USDA maps in Story County, Iowa: 45% sand, 34% silt, 21% clay^[1]. Press the button to draw each guide line in turn, or type your own numbers.

How the triangle is laid out

The chart is USDA’s textural triangle from the Soil Survey Manual: it shows the percentages of clay, silt and sand in the 12 basic texture classes^[2]. Each corner stands for a soil that is all one particle: sand at the bottom left, silt at the bottom right and clay at the top. Each edge is a scale for one particle. Clay climbs the left edge, silt climbs down the right edge toward its corner, and sand runs along the bottom, growing from right to left.

Every soil is one point inside the triangle, because its three shares total 100%^[2]. Move toward a corner and that particle’s share grows; move toward the opposite edge and it shrinks.

Worked example

Take the Clarion topsoil above: 45% sand, 34% silt, 21% clay^[1].

1. Find 21 on the clay axis (the left edge) and follow it straight across. Every point on that level line has 21% clay.
2. Find 45 on the sand axis (the bottom edge) and follow it up and to the left, parallel to the right edge.
3. Find 34 on the silt axis (the right edge) and follow it down and to the left. All three lines meet at one point.
4. That point sits inside loam. That is the texture class.

Common mistakes

• Following a line the wrong way. Each axis reads in one direction only: clay runs level across from the left edge, sand climbs toward the right edge, and silt climbs toward the left edge. Slide along the wrong one and the lines cross in the wrong region.
• Inventing a class. There are exactly 12 classes, so names like “sandy silt loam” aren’t on the chart^[2]. Use the name of the region your point lands in.
• Numbers that don’t add to 100. If your three shares don’t total 100, you can’t plot them^[2]. Work out the missing one: silt = 100 − sand − clay. With 45% sand and 21% clay, silt is 100 − 45 − 21 = 34%.
• A point that lands on a line. Boundaries belong to one class by the rules: the loam / clay-loam line is 27% clay, and clay loam is defined as 27% clay or more, so 35% sand, 38% silt, 27% clay reads as clay loam^[2]. A jar test can’t measure closely enough to settle which side of a line you’re on, so treat a point that close as sitting between the two classes.

What the lines between classes are

Each line inside the triangle is one of the Soil Survey Manual’s class rules. Loam, for example, is 7 to less than 27 percent clay, 28 to less than 50 percent silt and 52 percent or less sand^[2], and the edges of the loam region sit at exactly those values. Cross the 27% clay line and the same mix becomes a clay loam, because clay loam starts at 27% clay^[2].

Some rules weigh two particles at once. Sand, for instance, needs more than 85% sand and silt plus one and a half times the clay below 15^[2], which is why the sand region’s inner edge slants. All 12 rules are written out under the soil texture triangle calculator, and every class has its own page in our guide to soil types.

Where most US soils land

US soils cluster in a few regions. Of the US topsoil acres USDA’s surveys classify, silt loam (33.0%), loam (20.8%) and sandy loam (20.4%) cover the most^[1]. Those three regions sit side by side across the lower middle of the chart.

Practice

Read each one off the triangle, then check your answer. Every soil here sits well inside its class, so there are no judgement calls; the answers come from the Soil Survey Manual’s class rules^[2].

1. 1. 69% sand, 5% silt, 26% clay
   Show answer

   sandy clay loam

2. 2. 74% sand, 25% silt, 1% clay
   Show answer

   loamy sand

3. 3. 65% sand, 29% silt, 6% clay
   Show answer

   sandy loam

4. 4. 8% sand, 71% silt, 21% clay
   Show answer

   silt loam

5. 5. 42% sand, 9% silt, 49% clay
   Show answer

   clay

6. 6. 95% sand, 4% silt, 1% clay
   Show answer

   sand

Want a printable set with an answer key? Use the worksheet.

Where to get your numbers

• A jar test: settle your soil in a jar, measure the layers, and work out the percentages^[3].
• A soil lab: a particle-size test reports the sand, silt and clay and the USDA class^[4].
• Your address: USDA has already recorded sand, silt and clay for mapped soils^[1]. Look one up and plot it on the texture triangle.

For what the three particles are and how they feel, see sand, silt and clay.

Frequently asked questions

How do you read a soil texture triangle?

Find where your clay and sand percentages cross. Follow the clay percentage straight across from the left edge, follow the sand percentage up and to the left from the bottom edge, and the region where the two lines meet is your texture class. The silt line, read down from the right edge, runs through the same point because the three shares add to 100^[2].

Why do the percentages have to add up to 100?

Texture is the proportion of sand, silt and clay in the mineral particles smaller than 2 mm, so the three shares always total 100%^[2]. Knowing any two gives you the third.

What if my point falls on a line?

A point sitting exactly on a boundary belongs to one class by the USDA Soil Survey Manual’s rules, which our code follows. For example, the loam / clay-loam line is 27% clay: clay loam is defined as 27% clay or more, so a point at exactly 27% reads as clay loam^[2]. A jar test isn’t precise enough to settle which side of a line you’re on, so treat a point that close as sitting between the two classes.

Where do I get sand, silt and clay percentages?

From a soil lab’s particle-size test, from a jar test you do at home, or from the soil USDA has already mapped at your address. Enter one on the home page.

Is sandy silt loam a texture class?

No. The Soil Survey Manual has exactly 12 texture classes, and sandy silt loam isn’t one of them^[2]. A soil you’d describe that way plots as sandy loam, loam or silt loam.