Soil Composition, Formation, and Ecosystem Dynamics
Classified in Geology
Written on in 
English with a size of 3.33 KB
What is Soil?
Soil is a complex mixture of eroded rock, dead organic matter, and many kinds of living organisms.
Key Factors in Soil Formation
- Parent material: Bedrock, till, and fluvial soil.
- Climate: Temperature and precipitation levels.
- Biotic factors: Plant roots breaking up parent material, decomposers, and bacteria adding nitrogen.
- Topography: Slopes experience greater weathering and erosion (shallow soils), while level areas have deeper soils.
- Time: All of the above processes require significant time.
Soil Particle Sizes and Permeability
| Type | Particle Size | Texture (Wet) | Permeability |
|---|---|---|---|
| Clay | Very fine (< 0.002 mm) | Sticky | Low |
| Silt | Fine (0.002 - 0.05 mm) | Smooth | Moderate |
| Sand | Medium (0.05 – 2.0 mm) | Gritty | High |
| Gravel | Coarse (> 2.0 mm) | Rough | Very High |
The Ideal Soil: Loam
Loam is the best type of soil for growing plants. It consists of a mix of 40% sand, 40% silt, and 20% clay.
Soil Depth Across Ecosystems
Soil depth varies by ecosystem type and depends on climate, weathering, and biotic factors:
- Grassland soils: These are deep because they are derived from the decomposed roots of grasses.
- Temperate forest soils: These are moderately deep and are derived mostly from leaf litter.
- Tropical forest soils: These are shallow because nutrients are quickly taken up by trees and shrubs; most biomass is in the living trees rather than the soil.
Understanding Soil Horizons
O Horizon
The top layer is a mix of decomposing organic matter. In forests or grasslands, this consists of leaf litter; in deserts, it is very thin.
A Horizon (Topsoil)
This layer contains mineral particles and humus (decomposing organic matter). The O and A horizons teem with decomposers like bacteria, fungi, and roundworms. Darker topsoil (brown or black) is nitrogen-rich, while gray, yellow, or red soils are nutrient-poor.
B Horizon (Subsoil)
This layer consists of inorganic minerals leached from upper layers, including iron, aluminum, and calcium compounds. It contains very little organic matter.
C Horizon
This contains weathered rock and gravel. It sits on a base of unweathered parent material called bedrock.
Common Soil Organisms
Nematodes (Roundworms): Members of the Phylum Nematoda have the greatest biomass within the soil ecosystem. They are found in nearly every soil type and aquatic sediment.
Springtails: These organisms possess a specialized abdominal limb called a furcula used for jumping away suddenly.
Mites: These arachnids have variable diets. Most shred decomposing organic matter, while some are predatory, eating smaller organisms including other mites.