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1. Explain what is isomorphous substitution in clay minerals.

During clay formation, an atom of different charge substitutes for silicon in the tetrahedral or aluminum in the octahedral position in the clay structure.

Isomorphous substitution leads to permanent charge on the clay.

3. Using the data below, calculate the gravimetric water content, bulk density and porosity of the soil sample.

GWC = Air dry wt. in air minus oven dry wt. in air/ODW = 180-170/170 = 0.059

Bulk density = Mass/Volume = 170g/170-50.23 = 170/129.77 = 1.31 g/cm³

Porosity = 1- bulk density/particle density = 1 - 1.31g/cm³/2.63 g/cm³ = 0.502

Credit off for lack of bulk density units

4. How does granular soil structure form in A horizons?

The high biological activity adds organic matter and helps to mix inorganic and organic material together. The uneven pressures applied by roots and other living organisms pushes particles together. The "glue" they produce helps hold the particles together.

5. Minerals of the sand fraction in soils have different properties than minerals of the clay fraction. List three differences and explain how each influences soil behavior.

Size, surface area, charge, compostion (Others??)

Size influences how particles pack together, thus pore size distribution and total porosity. It also influences reactivity.

Surface area affects reactivity. More surface area of clay provides lots of surfaces for reactions and living space for microorganisms.

Charge also affects reactivity. Charge on clays is responsible for exchange capacity, retention of nutrients and toxic ions.

Composition is different in some cases and may influence soil fertility and how quickly minerals weather.

6. Smectite and Kandite are two families of clay minerals. List three properties that are different in the two families of minerals.

7. Make two lists of the following colors. Arrange the first list in order from darker to lighter. Arrange the second list from redder to yellower.

5YR 4/6, 2.5Y 2/2, 10R 4/4, 10YR 3/3, 2.5YR 6/8

list 1: 2.5Y2/2, 10YR 3/3, 10R4/4, 5YR4/6, 2.5YR6/8 (from darker to lighter)

list 2: 10R, 2.5YR, 5YR, 10YR, 2.5Y (from red to yellow)

Soils with more structure tend to have more macropores and fewer other pores sizes than poorly structured soils.

9. How does pore size distribution influence the rate at which water flows through soil and the amount of plant available water held by soil?

Fluids flow more rapidly through soils with more macropores, while soils with more micropores and mesopores have higher water holding capacity.

10. What are the two main soil factors that influence the rate of flow of the soil atmosphere by convection? Explain your answer.

Pore size distribution and connectivity. Tortuosity also plays a role.

Convection moves all the soil atmosphere as a mass. The larger the pores and the better connected they are, the more rapid will be the movement.

11. What is the main factor responsible for creating a diffusion gradient of soil gas within the soil atmosphere. Explain how it creates the gradient.

12. How does soil pH influence the permanent charge on clay minerals?

It does not influence permanent charge.

13. Explain the mechanism through which carboxyl groups (COOH) and phenolic groups (OH) on organic matter contribute to pH dependent charge.

At high pH, the hydrogen leaves the structure and enters solution, which leaves the groups with a net negative charge. At low pH, the groups take on an additional proton, thus giving them a positive charge. It is pH dependent charge because it changes with soil pH.