7 Soil Texture and Structure
Soil texture and structure are considered “master variables”, meaning that texture and structure directly influence a large number of other soil properties. For example, in comparing a clayey soil and a sandy soil, one would expect the clayey soil to have larger specific surface area, more cation exchange capacity, more total porosity, less macroporosity, and more organic matter than the sandy soil. Thus, by simply knowing the texture of the soil, inferences can be made in regard to many soil properties. Here, soil texture will be determined quantitatively using the hydrometer method, and estimated using the texture by feel method. Different soil structure types will also be observed. What you learn about texture and structure in these activities will be used later during the soil pit field trips to describe soil profiles in the field.
Learning Objectives
- Differentiate the three soil separates (sand, silt, and clay) based on their particle size diameters.
- Determine the percentages of sand, silt, and clay in selected soil samples using data collected from the hydrometer method of particle size analysis.
- Estimate the textural class using the texture-by-feel method on selected soil samples.
- Use a textural triangle to determine the textural class of a soil.
- Understand the relationship between particle size and specific surface area.
Materials
- Three soils of known textures
- Sodium hexametaphosphate
- Squirt bottles filled with tap water
- Lab balances accurate to the nearest 0.01 g
- Soil test cylinders, 1L volume (Item #200231000, Kimble ™ Kimax™ Soil Test Cylinders)
- Hydrometers calibrated in units of g/L (Item #13-202-133 Fisherbrand™ Soil Analysis ASTM Hydrometer)
- Digital thermometers accurate to the nearest 0.1°C
- Milkshake mixers and stainless steel milkshake mixer cups (Hamilton Beach HMD400 120V Triple Spindle Commercial Drink Mixer, Hamilton Beach, Glen Allen, Virginia, U.S.)
Recommended Reading
- Eye on Agriculture Today: Soil Texture by Feel (KSREVideos, 2010)
- Soil Profiling: Structure (KSREVideos, 2011c)
- Estimating Soil Texture by Feel (Presley and Thien, 2008)
Prelab Assignment
Using the recommended reading and viewing resources and the introduction to this lab, consider the questions listed below. These definitions/questions will provide a concise summary of the major concepts to be addressed in the lab. They will also serve as the basis for the post-lab quiz and are useful study notes for exams.
- Define and explain the difference between soil texture and soil structure.
- List the sizes for sand, silt, and clay particles using USDA criteria.
- What is a textural triangle?
- What is Stoke’s Law and how is it used in soil science?
- Define specific surface area and explain its relationship to size of soil particles.
Introduction
Soil texture refers to the proportions of sand (2.0 – 0.05 mm in diameter), silt (0.05 – 0.002 mm), and clay (less than 0.002 mm). The relative proportions determ