Understanding Soil Formation and Composition: A Comprehensive Guide
What is Soil?
Soil is the uppermost part of the continental crust resulting from weathering, colonization, and evolution of surface rocks. It is the layer that gives rise to vegetation, and producers take inorganic nutrients that make them affordable to consumers, including humans.
How Does Soil Form and Evolve?
The soil formation process is called pedogenesis.
- Weathering of Bedrock: On bare rock, the elements act to break down and alter the rock into a thin layer composed of rock fragments and mineral grains called regolith.
- Colonization: The regolith is colonized by the first living things that intensify the disturbance of the soil with their metabolic activities.
- Progression of Alteration and Increased Thickness: With the help of living organisms, the soil becomes suitable for the germination of seeds of plants with roots.
- Depth and Structure: The roots penetrate cracks and help break up the rock. This leads to the formation of the soil profile with its horizons.
Factors Involved in Soil Development
- Time: Over time, the soil evolves from young to mature. When this occurs, it is said to have reached a climax soil.
- Climate: The climate is the main factor in mature soils. The same rock can produce different soils under different climates.
- Bedrock: The bedrock is the main factor in young soils. Soils can form in any rock but are best developed in those that are more weatherable.
- Living Organisms: With their biological activity and decomposition of their remains, they transform inert matter and form humus.
- Relief: Soils formed on flat surfaces are deeper.
- Human Action: Humans modify the evolution of soil, often causing degradation.
Soil Inorganic Components
- Solids: Formed by minerals related to the bedrock. They are the main component of any soil. They can be inherited (rock fragments) and formed from alteration.
- Liquids:
- Water: According to its mobility, it can be retained or current. If it moves vertically downward, it is called gravitational water (fills in the pores and is not absorbed by the roots). If it can be absorbed by plant roots, it is called capillary absorbable water.
- Ions (Na+), cations (Cl-), and anions: Derived from organic matter mineralization.
- Gas: Composed of soil air due to the respiration of organisms living in it.
Soil Organic Components
- Living Organisms: Living organisms in soil belong to five kingdoms:
- Bacteria: Ubiquitous in soil.
- Fungi: Abundant; they act as decomposers of organic matter and favor the absorption of nutrients.
- Algae: Living in the surface areas of soil.
- Animals: As heterotrophs, they feed on decaying organic matter.
- Plants: Their roots absorb water and minerals necessary for photosynthesis.
- Humus: Can be non-processed (consisting of organic waste) and processed (formed by the decomposition of the above). The proportion of humus decreases with depth.
The Soil Profile
Soil Profile: The set of horizons, i.e., the different layers that make up the ground.
Soil Horizons
- Horizon O: Formed by decaying organic matter.
- Horizon A: Consists of mineral matter and humus; the proportion of humus decreases as one goes deeper. Leaching occurs because the water that infiltrates it dissolves minerals that are dragged down. It is the most important horizon for soil fertility.
- Horizon B: Consisting of mineral matter. Precipitation occurs because dissolved minerals are concentrated from Horizon A.
- Horizon C: Consists of altered minerals from the bedrock.
Main Types of Soils
- Zonal Soil: Mature soils whose characteristics are consistent with the climate zone in which they are. The climate zone depends on:
- Adequate Moisture: When all soil types are dependent on temperature, there is a thermal area with soils such as:
- Polar: Soils are not considered because they have no organic matter.
- Podzols: Occur in cold climates that have a horizon rich in organic matter and a B-horizon rich in cations.
- Brown Earth: Occurs in temperate continental climates, with a horizon rich in humus and an underdeveloped B-horizon.
- Red Earth: Occurs in warm climates with distinct A and B horizons.
- Lateritic: Occurs in equatorial and intertropical climates.
- Dryness: When all soil types are dependent on rainfall, there is an area with soils such as:
- Desert: Occurs in very low rainfall areas.
- Subdesert: Occurs in climates with low rainfall.
- Steppe: Occurs in areas with average precipitation spaced out in time.
- Adequate Moisture: When all soil types are dependent on temperature, there is a thermal area with soils such as:
- Intrazonal Soil: Partially adapted to the climatic characteristics. They are less evolved than zonal soils. Within this group are saturated soils, gleys, and halomorphic soils.
- Azonal Soil: Do not conform to the weather but to the characteristics of the bedrock. They are very poorly developed and have an AC profile that lacks the B horizon. Within this group are the rankers and rendzinas.