Three Particles, Twelve Classes
Every soil is a mixture of three mineral particle sizes: sand (0.05–2 mm), silt (0.002–0.05 mm), and clay (less than 0.002 mm). The USDA texture triangle maps the relative proportions of these three fractions onto a ternary diagram, classifying the soil into one of 12 texture classes — from sandy clay to silty loam to pure clay. This classification is the starting point for predicting nearly every physical property of soil.
Water and Air
Texture controls the balance of water and air in soil. Sand grains create large pores that drain quickly under gravity, while clay particles generate tiny pores that hold water tightly by capillarity. Silt provides an intermediate behavior. The texture class directly predicts field capacity (water held after drainage), wilting point (water unavailable to plants), and available water capacity — the difference that sustains crops between rains.
The Role of Organic Matter
Organic matter acts as a texture modifier. Just a few percent of humus can dramatically increase the water-holding capacity of sandy soils and improve the structure and aeration of clay soils. Organic matter also provides cation exchange capacity — the soil's ability to hold and release nutrient ions like potassium, calcium, and magnesium. This simulation accounts for organic matter's influence on hydraulic estimates.
From Lab to Field
In the laboratory, particle-size analysis uses the hydrometer or pipette method, both based on Stokes' law: larger particles settle faster in a water column. In the field, soil scientists use the ribbon test — squeezing moistened soil between thumb and finger. A long, flexible ribbon means high clay; a gritty feel that won't ribbon means high sand. These simple tests, combined with the texture triangle, guide irrigation scheduling, fertilization, and construction decisions worldwide.