Only the percent sand, silt, and clay are used for soil textural determination, material larger the 2 mm are described as rock fragments. Rock fragments are described separately on a pedon description. Textural modifiers are used when rock fragments occupy more than 15 percent by VOLUME of the horizon (gravelly sandy loam for example).
Organic material and organic horizons are excluded from soil textural determination. Organic horizons (Oi, Oe, Oa) are described based on the degree of decomposition (fibric, hemic, sapric, etc.) and other factors. Mineral horizons that have considerable (> 8%) organic matter can be described with a mucky modifier (mucky silt-loam).
Importance: Soil texture is one of the most important soil property particularly when dealing with water movement through the soil. Soil mechanical and engineering properties are also influenced by soil texture.