13 Soil Colloids
Colloids consist of clay minerals and organic matter, and play critical roles in soil chemical, physical, and biological properties. Colloids are very small in size. The majority of charges in soil comes from colloids, making them important for cation exchange capacity, buffering capacity, and retention of nutrients like Ca2+, K+, Mg2+, etc. In addition, the small size of colloids leads to very high surface areas, which facilitates chemical reactions and provides habitat for microbes. Some clays expand and contract with changes in soil wetness. The force from expanded clays can be considerable, and commonly cause structural failures in house foundations and basement walls, roads and bridges, and other man-made structures. Other clays do not expand, and make ideal substances for clay pottery, or tile roofs. The organic matter fraction of colloids are important for the development of soil structure, or as food sources for soil organisms. The soil properties attributed to colloids are numerous, and will be the focus of the following laboratory activities.
Learning Objectives
- Measure the effects of different cations on colloidal properties.
- Identify the soil components controlling ion exchange.
- Determine the cation exchange capacity of selected soil horizons.
Materials
- Petri dishes
- Beakers
- Spatulas
- Pure kaolinite
- Pure bentonite
- Caliper
- 6V battery
- Solid copper wires, 2 total, 25 cm long
- Alligator clips for the wires
- Paper towels
- Test tubes
- Flasks
- Burettes
- Bottle-top dispensers set to dispense 10 mL
- Phenolphthalein indicator solution
- Aluminum chloride solution, 1 M
- Potassium chloride solution, 1 M
- Sodium chloride solution, 1 M
- Calcium chloride solution, 1 M
- Sodium hydroxide solution, 0.01 M
- Simulated soil CEC extract solutions
- Norfolk E horizon, 0.002 M HCl
- 10 mL of extract represents 1 g of soil
- Norfolk Bt horizon, 0.005 M HCl
- 10 mL of extract represents 1 g of soil
- Cecil Ap horizon, 0.010 M HCl
- 10 mL of extract represents 1 g of soil
- Cecil Bt Horizon, 0.007 M HCl
- 10 mL of extract represents 1 g of soil
- Whitestore Bt horizon, 0.015 M HCl
- 10 mL of extract represents 0.5 g of soil
Recommended Reading
- Introduction to the Sorption of Chemical Constituents in Soils (Thompson and Goyne, 2012)
- Cation Exchange Capacity and Base Saturation (Sonon et al., 2017)
Prelab Assignment
Using the recommended readings 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 the term “soil colloid”. What are the main types of soil colloids?
- Describe the sources of charges on soil colloids.
- Define cation exchange capacity.
- What units are used to express cation exchange capacity?
- List the general quantity of cation exchange capacity contributed by kaolinit