Soil Formation and Composition: An In-Depth Look
**Soil: The Foundation of Terrestrial Ecosystems**
Strata: Horizontal levels that descend vertically, characterizing the vegetation of an ecosystem and sustaining life.
Humus: The most fertile layer of soil, formed by decaying organic matter, primarily leaves, and is highly nutritious.
Leaching: The process by which mineral salts are carried from a higher to a lower soil horizon.
Microbiota: Organisms that inhabit the soil, including worms, scorpions, centipedes, and other arthropods, seeking sustenance and contributing to soil health.
- Chilopoda (1 pair of legs)
- Diplopoda (2 pairs of legs)
- Insects (3 pairs of legs)
- Arachnids (4 pairs of legs)
- Crustaceans (5 pairs of legs)
Parent Rock: The underlying geological material that gradually breaks down due to bacterial action, climate change, and plant activity.
Soil: A vital natural resource where life thrives, composed of organic matter (mineral salts), inorganic matter (fossils), air (25%), and water (25%).
Weeds: Plants that grow in undesired locations, competing with cultivated plants.
Examples: pumpkin, mulberry bush, jimsonweed, thistle, Galician milk thistle, tomatillo, *duraznillo*, radish.
Herbs: Plants lacking a woody stem, typically with an annual lifespan.
Examples: pennyroyal, mint, lemon balm, chamomile.
Shrubs: Woody plants typically growing between 1 and 5 meters tall, often with dense foliage.
Examples: *maqui*, *boldo*, *culén*.
Inorganic Components of Soil
- Sand
- Clay
- Water
- Air
- Humus
- Salts
Organic Components of Soil
- Grasses
- Roots
- Insects
- Organic debris
- Fungi and bacteria
Soil: A Slowly Renewable Resource
- Soil is renewable, but its formation is extremely slow (approximately 1000 years for 30 cm of fertile soil).
- The most fertile soil is dark, spongy, and readily absorbs water, often referred to as “Trumao”.
- The amount of fertile soil is decreasing globally.
- Urban land requires essential services like sewage, public utilities, potable water, and efficient transportation routes.
Lithosphere: The Earth’s Solid Outer Layer
The lithosphere encompasses the Earth’s rocks, soil, and sediments, providing a habitat for diverse life forms.
It consists of three main rock types:
- Igneous rocks: Formed from the solidification of magma.
- Sedimentary rocks: Formed by the deposition and consolidation of weathered material from other rocks.
- Metamorphic rocks: Formed through the transformation of existing rocks under heat and pressure.
- The action of bacteria, fungi, and climate change contributes to rock breakdown.
- Rock surfaces can provide suitable habitats for microorganisms.
- Bacterial and fungal activity can solubilize silicates and other minerals through the production of organic acids and chelating agents.
What is Soil?
Soil is the portion of the Earth’s crust where life develops.
In agronomy, soil is a complex mixture of minerals, gases, liquids, organic matter, and living organisms that support plant growth.
How is Soil Formed?
Soil originates from rocks through intricate physical, chemical, and biological processes.
- Parent material
- Climate
- Topography
- Biological activity
- Time
The reduction of rock to regolith (rock fragments) involves:
- Algae and lichens, as phototrophic producers of organic matter, support the growth of chemoheterotrophic bacteria and fungi.
- Chemoheterotrophs excrete organic acids that contribute to mineral dissolution.
- These processes lead to the development of a rudimentary soil profile.
Pedogenesis: The Process of Soil Formation
Soil formation involves:
- Weathering
- Roots
- Fungi
- Bacteria
- Organic acids
- Time
Horizon “O”: The uppermost layer, composed of organic matter from deceased plant and animal remains, forming humus.
Horizon “A”: The eluvial layer, a mineral layer near the surface characterized by high leaching of silicate clays and oxides of iron (Fe) and aluminum (Al).
Horizon “B”: The illuvial layer where leached materials from Horizon “A” accumulate.
Horizon “C”: A layer with low biological activity, marked by the accumulation of calcium and calcium carbonate. Below this lies the regolith and bedrock.
- The bedrock begins to disintegrate.
- Organic matter facilitates disintegration.
- Horizons are formed.
- The developed soil supports dense vegetation.
Soil Formation Process
Volumetric Soil Structure
- Inorganic Solids: 38-45%
- Organic Solids: 5-15%
- Water: 15-35%
- Gas: 15-35%
- Microorganisms: 0.05%
Solid Fraction of Soil
Sand, silt, clay, humus
Component | Diameter (mm) | Number of Particles/g | Surface Area (cm2/g) |
Sand | 2.00 – 0.05 | 90 | 11 |
Silt | 0.05 – 0.002 | 5.78 x 106 | 454 |
Clay | < 0.002 | 9.03 x 1010 | 8,000,000 |
Loam: A soil texture classification containing 7% to 27% clay, 28% to 50% silt, and less than 52% sand.
Soil Classification
- Rocky Soil: Lacks horizons A or B, with bedrock exposed at the surface. These soils are hard, impermeable, and consequently dry.
- Sandy Soil: Porous and permeable to water, these soils do not retain moisture effectively.
- Clay Soil: Possesses a soft texture, is porous, and retains moisture. It is more compact than sandy soil.
- Organic Soil: Rich in organic matter and has a fluffy texture, enabling it to retain moisture and making it particularly fertile.
Questionnaire
- Factors affecting soil formation: Climate, roots, fungi, bacteria, organic acids, time.
- Rock types identified in the soil: Igneous, sedimentary, metamorphic.
- Microbiota includes: Fungi, yeasts, bacteria.
- What is soil? The portion of the Earth’s crust where life develops.
- How is soil formed chemically? Through the interaction of water, air, organic, and inorganic matter.
- Three basic inorganic components of soil: Sand, silt, and clay.
- Horizons identified in the soil: O, A, B, C.
- How are soils classified according to their uses? Agriculture, livestock, forestry, and mountains.
- What does the solid fraction of soil include? Sand, silt, clay, and humus.
- Sort the components according to their diameter (largest to smallest): Sand, silt, and clay.
- What could be found in 1g of agricultural land? Bacteria, fungi, algae, protozoa.
- Enter 3 types of bacteria that can be found in the soil: Bacillus, Pseudomonas, Corynebacterium, Staphylococcus.
- Name groups of nitrogen-fixing bacteria in the soil: Azotobacter, Rhizobium, Clostridium.
- Which molecules in the soil contain nitrogen? Proteins, nucleic acids, and protoplasm.
- What are the 5 components of soil solids from smallest to largest? Clay, silt, sand, gravel, and pebbles.