Soil Texture Triangle

Soil texture

Texture is the relative proportion, by mass, of sand, silt and clay in the fine earth — the part of the soil with particles smaller than 2 mm. Gravel, cobbles and organic matter are left out, so the three fractions always add up to 100%.

It is one of the most stable soil properties, unchanged by management, which is why it is used to predict how a soil behaves hydraulically and mechanically.

Particle-size scales

Where one fraction ends and the next begins depends on the scale. Soil science (USDA) puts the sand–silt limit at 0.05 mm; the Unified Soil Classification System (USCS) splits fines off at the No. 200 sieve.

This app uses the USDA scale. Results from geotechnical labs should be converted to it before classifying; in the USCS, silt and clay are told apart by plasticity, not by size.

How the fractions are measured

In particle-size analysis the sample is dispersed chemically (sodium hydroxide or hexametaphosphate) and mechanically. Sand is separated by sieving; silt and clay, too fine for sieves, are separated by sedimentation with the pipette or hydrometer method.

Sedimentation relies on Stokes' law: a sphere falling through a viscous fluid reaches a terminal velocity proportional to the square of its diameter.

At 20 °C a 0.05 mm grain falls 10 cm in under a minute, while a 0.002 mm particle takes about 8 hours. Sampling the suspension at a known depth and time gives the mass of particles finer than each diameter.

Reading the texture triangle

The triangle is a ternary diagram: each side is the scale of one fraction, from 0 to 100%, and each corner is a soil made of that fraction alone. Since sand + silt + clay = 100%, any two values fix the point; the third is always the remainder.

Each fraction is read along the grid lines parallel to the side opposite its corner. Where the three lines meet, the point falls in one of the regions, and that region is the sample's textural class.

The class boundaries are linear inequalities on the fractions. For example:

• Sand: silt + 1.5 × clay < 15%
• Loam: 7 to 27% clay, 28 to 50% silt and at most 52% sand
• Clay: at least 40% clay, at most 45% sand and under 40% silt

The USDA triangle has 12 classes. The Brazilian SBCS triangle, used in the Portuguese version of this app, is the same but splits clay at 60%, adding a very clayey class.

When the total isn't 100%

Lab results rarely add up to exactly 100%, because of losses during analysis or rounding. The app scales the three fractions proportionally to 100% before classifying; totals outside 100 ± 0.5% are flagged so you can check the data.

Why it matters, and its limits

Texture governs particle surface area and pore size, and with them much of how a soil behaves:

• Water retention and hydraulic conductivity — see the water retention curve app
• Plasticity, cohesion and compressibility
• Susceptibility to erosion and compaction
• Cation exchange capacity and retention of nutrients and contaminants

Texture does not describe clay mineralogy, structure, organic matter or gravel, though. Two soils in the same class can behave very differently; for geotechnical work, use a classification that includes the Atterberg limits, such as the USCS.