Soil Texture Triangle and Irrigation

A soil texture triangle gives a common language for sand, silt and clay in a laboratory sample. It is useful for comparing fields and choosing what additional water measurements to collect; it does not measure a root zone's water holding, infiltration or irrigation need.

Use the USDA Soil Texture Triangle Calculator to classify fine-earth fractions, then use measured water-retention information for a scheduling decision.

The triangle uses the fine-earth fraction: soil particles smaller than 2 mm by mass. Exclude gravel and stones. USDA separates sand (2.0 to 0.05 mm), silt (0.05 to 0.002 mm), and clay (smaller than 0.002 mm); the three must total 100% for one sample.

• Sand: sand-dominated.
• Loamy Sand: sand-rich with more fine particles.
• Sandy Loam: a sandy mixture.
• Loam: a mixed region.
• Silt Loam: silt-rich.
• Silt: strongly silt-dominated.
• Sandy Clay Loam: clay-bearing and sand-rich.
• Clay Loam: intermediate clay-bearing loam.
• Silty Clay Loam: clay-bearing and silt-rich.
• Sandy Clay: clay-rich with substantial sand.
• Silty Clay: clay-rich with substantial silt.
• Clay: clay-dominated.

These are regions, not direct hydraulic measurements. A shared edge can touch more than one class, so retain original fractions and inspect the calculator's boundary disclosure.

When only two matching fine-earth fractions are measured, check the third with clay = 100 − sand − silt. This is a check, not permission to mix whole-soil gravel with a fine-earth report.

Available water capacity is the difference between field capacity and permanent wilting point across the actual root zone. Structure, compaction, bulk density, organic matter, salinity, horizons, coarse fragments and restricted rooting can make fields in one texture class behave differently.

Use independently justified water measurements for irrigation scheduling. Use the Lime Requirement Calculator only with its pH and buffer inputs: a texture class does not provide an amendment recommendation.

• The example classifies one representative fine-earth laboratory sample; it is not a water-retention test or irrigation prescription.
• Small total discrepancies can be rounding; the calculator offers opt-in normalization and does not silently alter laboratory values.
• Confirm sampling depth, horizon, laboratory method, rooting depth and local agronomic guidance before changing practice.

• Including gravel as sand: use only fractions below 2 mm.
• Calling Loam equal thirds: it is a USDA polygon, not a fixed recipe.
• Ignoring boundary results: inspect touching classes near an edge.
• Using texture as field capacity, wilting point or infiltration: measure or independently justify those inputs.

The class names, fine-earth basis and triangle context follow USDA-NRCS materials. The worked fixture is a composition classification, not a hydraulic or liming claim.

• USDA Natural Resources Conservation Service, Multi-Point Texture Triangle workbook — USDA textural classes and triangle reference.
• USDA Natural Resources Conservation Service, National Agronomy Manual, Part 508 — Soils — particle-size and soil-property context.