Soil Composition and Texture
Soil composition and texture (i.e., particle-size distribution) vary, giving rise to different soil names. In this survey course, the names are not so important – but having a base knowledge of them can be helpful.
Learning Goals
- Know what the common composition of soil is (but that this varies!)
- Recognize the soil ternary diagram and the textural names: sand, silt, clay, and loam (a mix)
- Learn the basics of what goes into a soil classification
Soil composition
This is a “standard” or “textbook” soil compositoin. As you look at soils, you will see some with different amounts of organic vs. inorganic (i.e., mineral) material. This is an important diagnostic feature! Think about how a desert soil and a soil developed in a forested temperate climate might differ. The fractions of water and air may also change depending on the aridity of the environment, and the total air and water fraction wiil change as a function of soil compaction.
USDA
Soil texture
Ternary diagram
This “ternary diagram”, so named because it has three sides and hence compares three components, defines the names of soils based on their relative content of clay, silt, and sand.
USDA
Grain size
“Clay”, “silt”, and “sand” are defined by grain size. The Wentworth (1922) grain-size scale codifies common words describing sizes of grains into quantified size ranges.
From USGS Open-File Report 2006-1195: Typos for sizes: 33.1 mm –> 38.1 mm; 0.545 mm –> .594 mm
- Clay is smaller than 1/256 mm (~4 μm)
- Silt is larger than clay but smaller than 1/16 mm (62.5 μm)
- Sand is larger than silt but smaller than 2 mm
Beyond sand is gravel, which comprises pebbles, cobbles, and boulders. These are not part of the soil classification, but are important to geomorphology.
Video: Field Soil Classification
Soil composition and texture are very important for soil classification. Although this is not a goal of the class, the following video does a really did a good job of building field classification of soils into a comprehensible procedure. (Only undefined term that I noticed: “peds” are naturally forming clumps of soil that are separated from one another by planes or zones of weakness: https://www.soil-net.com/legacy/advanced/soil_formation5.htm.)
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