Lecture 2 - Soil formation, morphology, and characteristics

Page 1 — Soils: Composition and Components

Soils are composed of inorganic minerals and organic compounds.
The inorganic portion includes solid minerals, air, and water.
Solid minerals in soil are categorized by size: \text{gravel}, \ \text{sand}, \ \text{silt}, \ \text{clay}.
Organic compounds consist of plant and animal residues, as well as microorganisms.
Organic compounds exist at various stages of decomposition.

Soil particles, both organic and inorganic, aggregate and form granules and clods.
Water and air fill the soil pores.
Water in the soil acts as a solution (soil solution), dissolving and carrying various ions.
The oxygen in the air is used by plant roots for respiration.
When water fills the pores, it displaces the air within them.

Soil provides a habitat/home for a wide variety of microorganisms and animals.
Among the microorganisms present are fungi and bacteria.
The animals include worms, such as centipedes, earthworms, and nematodes, and insects like ants and termites.
Both soil animals and microorganisms contribute to nutrient recycling by breaking down and decomposing plant and animal matter.
Successful crop growth and yield depend greatly on these nutrient recycling processes.
Reference: Lecomte et al (2018) Front. Environ. Sci. 6:139. doi: 10.3389/fenvs.2018.00139

The Earth consists of three main layers: Crust – the Earth's outermost shell; Mantle – the layer below the crust; Core – the innermost layer.
The thickness of the crust is not uniform and changes depending on location.
On continents, the crust can reach up to 30\ \text{miles} in thickness.
Beneath the ocean floor, it is much thinner, about 2\ to\ 3\ \text{miles} thick.

Oxygen (O) and silicon (Si) together make up approximately 74\% of the Earth’s crust by weight.
In the atmosphere, oxygen is present as \mathrm{O_2}, a stable molecule.
Within soil minerals, oxygen exists as the anion \mathrm{O^{2-}}, carrying a negative charge.
Silicon is found as the cation \mathrm{Si^{4+}}, with a positive charge.
Six other elements, Al, Fe, Ca, Na, K, and Mg, account for about 24\% of the crust’s weight.
Together, these eight elements constitute 98\% of the Earth’s crust by volume, with oxygen and silicon alone making up 94\%.
Reference: Steven Earleis (2015), Physical Geology: https://opentextbc.ca/geology/

Rocks are naturally occurring solid aggregates of minerals.
The three primary types of rocks are: Igneous, Sedimentary, and Metamorphic.
Igneous rocks, such as granite and basalt, are created when molten lava cools and crystallizes.
Sedimentary rocks, such as shale and limestone, form through the accumulation and compaction of sediment, often in water.
Metamorphic rocks, for example, marble, originate when existing igneous, sedimentary, or other metamorphic rocks undergo structural and compositional changes due to intense heat and pressure.

Rocks consist of a variety of minerals.
Minerals are formed as molten rocks cool and crystallize.
Approximately 90\% of the Earth’s primary minerals are silicates, which are made up of oxygen and silicon (\mathrm{SiO_2}).
Specific minerals form when additional elements are incorporated into the \mathrm{SiO_2}.
For example, quartz has the chemical formula \mathrm{SiO_2}, while muscovite is a silicate mineral that contains aluminum and potassium.

Soils vary from one location to another.
- Example: eastern Texas has clay-rich soils, while the soil in western Texas is sandy.
Even within the same area, soils can differ:
- Example: the soil at the top of a hill may be different from that at the base.
Understanding how soils form and interact with their environment helps us use and care for them responsibly.
With proper care and management, soils can remain fertile and productive indefinitely.

Soil formation includes soil formation factors and soil-forming processes.
There are five soil formation factors that determine the type of soil formed:
- 1) parent material
- 2) climate
- 3) organisms
- 4) relief or topography
- 5) time
Soil-forming processes explain how soils are formed:
- physical weathering
- chemical weathering
- leaching
- the accumulation of organic matter

Understanding composition, layering, and processes helps in managing soil fertility, informing agricultural practices, and guiding environmental stewardship.
The nutrient recycling carried out by soil organisms and microorganisms underpins crop yields and ecosystem health.
Knowledge of mineral abundances and rock types supports interpretation of soil properties, mineral availability, and soil-water interactions.
Variability across locations and microenvironments highlights the need for site-specific soil management strategies.