How the samples and the triangle are made
The triangle is drawn from the class definitions in the USDA Soil Survey Manual, not traced from a picture: each region is the set of mixes its definition allows, so the lines sit exactly where the rules put them. Every sample is checked against the same definitions and kept only if it is at least 3 percentage points from every class line. A reading or a plotted point that is off by a little still lands in the same class. Points to read sit where gridlines cross, so careful reading gives exact numbers.
A worked example
In set SQEEZE (AP / High school), sample B is 30% sand, 39% silt and 31% clay. Find 31 on the left edge and follow the clay line straight across. Find 30 on the bottom edge and follow the sand line up and to the left. They meet at 39% silt, inside the clay loam region. The USDA definition of clay loam is 27 to less than 40 percent clay and more than 20 to 45 percent sand, and 31% clay with 30% sand fits. Sample C has 35% clay but only 11% sand, so it is a silty clay loam. Open set SQEEZE
What each level asks
| Level | Samples | Lab data |
|---|---|---|
| Middle school / Earth science | Two per question, in steps of 5%; points to read on the numbered lines, right within 2 points | Jar test layers in millimetres |
| AP Environmental / High school ag | Three per question, any whole percent; points to read where the 2% gridlines cross, right within 1 point | Jar test marks, or hydrometer readings with a blank |
| College soils | As AP, plus which sample stores the most water plants can use | Hydrometer readings with a temperature correction and a blank |
Questions
How do you use the soil texture triangle?
Find the percent clay on the left edge and follow that line straight across. Find the percent sand on the bottom edge and follow its line up and to the left. The sample is where the two lines meet, and the region it lands in is its texture class. The silt line passes through the same point.
Which way do the lines go on a soil texture triangle?
On this triangle, the usual USDA layout, follow a point's clay line across to the left edge, its silt line up to the right-hand edge, and its sand line down to the bottom edge. The arrows show which way each edge's numbers increase. Some books number the other end of each line, so check the tick marks.
What if a point lands exactly on a line?
The USDA definitions say which side each line belongs to. Loam has 7 to less than 27 percent clay, so a soil with exactly 27% clay is not a loam. This site never puts a sample within 3 percentage points of a class line, so every answer is clear.
How do you find the missing percent?
Sand, silt and clay always add up to 100%. Take the two you know away from 100: a soil with 42% sand and 18% clay has 100 − 42 − 18 = 40% silt.
Which soil texture holds the most water?
Clay soils hold the most water after draining, because their many tiny pores hold it against gravity, but water moves through them slowly. Sandy soils drain fastest and hold the least. Loams and silt loams usually store the most water that plants can use, because clay holds part of its water too tightly.
How does the hydrometer method work?
Soil is dispersed in water and a hydrometer is read at 40 seconds, when the sand has settled, and at 2 hours, when only clay is still in suspension. Each reading, less the blank, divided by the soil mass and times 100 gives the percent silt plus clay, then the percent clay. Sand is what is left.
Judging land by feel for an FFA or 4-H land judging contest? Land Judging Key, in the list of tools below, covers that card.
Sources, checked October 25, 2026: USDA NRCS, Soil Survey Manual, Agriculture Handbook 18 (2017, PDF), chapter 3: texture class definitions (pages 122 to 124), Table 3-1, saturated hydraulic conductivity (page 219); USDA NRCS, Soil Quality Indicators: Available Water Capacity (PDF); Ohio State University Soil Fertility Lab, 2-Hour Hydrometer Method (July 2020, PDF); Clemson Cooperative Extension, Soil Texture Analysis: The Jar Test (HGIC 1656, revised December 2023); AP Environmental Science Course and Exam Description, effective fall 2026 (PDF), topic 4.3.