Introduction to Soils Chapter 1

Air quality

Habitat for organisms soil

Engineering considerations problem bigger problem

You will pick up new jargon, like pedogenic = soil forming.

This is called a general soils map. The different colored areas show soil associations. These are not soil types but groups of soil types that occur together in the landscape. This give a sense of the lateral variability in soils.

Soils

Here are some more terms. All the particulate material lying above rock is not necessarily considered soil, only that upper part that shows evidence being transformed by soil forming processes. This is called the solum, and it contains at least one (more or less) horizontally oriented zones called horizons that have developed due to pedogenesis.

The solum is the upper part of the regolith.

Looking down from the top, you should see a dark upper horizon, a light horizon below it, then another dark horizon, a more colorful one, etc. Most soil profiles are not this distinctive and odd. The dark horizon below the surface is what is called a secondary accumulation of organic matter and it exists due to special conditions.

These are the 5 master horizons. They are defined on the next slide. Know them well. Note that not all soils have all five master horizons. They will, however, have at least an A and a C. The A is a pedogenic horizon.

All soils will have an A horizon. Its exists from plants growing in the soil and depositing organic matter on and just below the surface (residue on the surface and roots). However, the A is dominantly mineral, not organic. If there is substantial deposition of organic residue on the surface, an O (organic) horizon exists. Common in forest soils. An E is distinguished if there is a different horizon below it. The light color just means there is not much organic matter. Used to be called an A2 horizon.

The B is a secondary accumulation of clay or salts. It is often more brightly colored that the other horizons. The clay or salt has been washed down to this depth by water. Salts will be found in the subsoil like this where the climate is arid (little water for dissolving and leaching out of the profile). A clayey subsoil is common where there has been a lot of water draining through the profile. So, the A and E are depleted of clay. The C is thought to be what existed when pedogenesis began –unaltered regolith.

The solids are either mineral or organic and the pores are filled with air or water.

. Approximate relative sizes of sand, silt and clay. You see the clay particle?

While some soils contain solids that are larger than the largest sand, these large bodies do not strongly affect biological, chemical and physical process in soils. The fine earth particles do, especially the small ones.

Colloidal particles tend to remain in suspension. As you will see, the clay particles (and organic matter) carry electrostatic charges. These must be balanced by adsorption of oppositely charged ions from solution.

The ultimate origin of soil mineral particles are rocks, and the first rocks come from within the earth. Thus minerals derived from igneous rocks are called primary. Some soils contain an appreciable amount of primary minerals, whereas others are dominated by minerals that were produced from primary minerals. The latter are called secondary.

The small clay particles tend to form aggregates because of their large surface area to mass ratio. However, aggregates contain silt and sand, too. Organic matter is also important in aggregate formation and stability. It tends to glue particles together.

With respect to plant nutrition, recognize that organic matter is not just C, H and O. It may contain appreciable N, P and S that are released when the organic matter decomposes. Also, the electrostatic charges on it are sites that hold nutrient ions like Ca, Mg, and K that may be used by plants.

What do you think? Well, on the average, do you think soil air is wetter (higher humidity) than the aboveground air? I’d say so. Recognize that the soil is full of life, especially microbial life, that is respiring and giving off CO 2. If this life is aerobic, it is also consuming O 2. Diffusion of gases through soil pores, especially when some are filled with water, is a very slow process.

These are the sources of plant nutrients in the soil. See next slide.

Mineral weathering is a slow process (geologic time scale). Decomposition of organic matter with release of N, etc. is much faster but still somewhat slow. On the other hand, the ions that are adsorbed on mineral and organic matter in soil are in equilibrium (more or less) with ions in the soil solution so that depletion of ion X from the soil solution by plant uptake from the soil solution results in release of ion X from mineral and organic colloidal surfaces (this is somewhat of a simplification). Now, how do roots come into contact with nutrients in the soil solution? See next slide.

Plants transpire water (into roots through stems to leaves where it evapor- ates). The mass flow of water to roots (down the gradient, wetter to drier) carries dissolve nutrients to the plant root surface. Also, since nutrients are taken up at the root surface, their concentration is lower there, resulting in a concentration gradient, thus diffusive mass transfer to the root. Finally, the plant root system grows and expands over time.