Soil texture is the relative proportion of different particle size fractions in a soil, specifically sand, silt, and clay, and it is one of the most fundamental physical properties used to classify soils in stormwater management and hydrology.
These three particle types are defined by size. Sand particles are the largest, typically ranging from about 0.05 to 2.0 millimeters in diameter, and they feel coarse and gritty. Silt particles are intermediate in size, about 0.002 to 0.05 millimeters, and have a smooth, flour-like texture when dry. Clay particles are the smallest, less than 0.002 millimeters, and exhibit cohesive, sticky behavior when wet due to their plate-like structure and high surface area.
Soil texture is commonly expressed using standardized classification systems such as the USDA soil texture triangle, which categorizes soils into classes like sandy loam, silty clay, or loam based on the percentage of each particle size fraction. Unlike soil structure, which can change over time due to compaction or biological activity, soil texture is largely determined by the soil’s parent material and remains relatively constant.
In the context of stormwater management, soil texture has a direct influence on several critical processes. It strongly controls infiltration rates, with sandy soils allowing rapid water movement and clay-rich soils restricting it. It affects water holding capacity, as finer-textured soils retain more moisture due to higher surface area and capillary forces. It also influences erosion potential, since silty and fine sandy soils are more easily detached and transported by runoff, while clay soils tend to resist detachment but can contribute to turbidity once suspended.
Additionally, soil texture plays a key role in pollutant transport and treatment. Fine-textured soils, especially those with higher clay content, tend to adsorb nutrients, metals, and other contaminants more effectively, whereas coarse-textured soils allow faster percolation and may provide less opportunity for pollutant removal.
Because of these relationships, soil texture is a critical parameter in the design and performance evaluation of stormwater practices such as infiltration basins, bioretention systems, and vegetated swales, where it helps determine suitability, expected drainage behavior, and long-term maintenance needs.