Characteristics of Soil
Soil Texture: This refers to the relative proportions of sand, silt, and clay particles. Texture is a permanent characteristic that dictates how well soil drains water and how much surface area is available for nutrient exchange.
Porosity and Permeability: Porosity is the measure of open space between soil particles, while permeability is the ease with which water and air move through those spaces. While clay has high total porosity, its small pore sizes result in low permeability.
Soil Color: The hue of the soil provides clues about its composition; dark brown or black indicates high organic content, while red or yellow suggests the presence of oxidized iron minerals.
Soil Structure: This describes how individual soil particles clump together into aggregates or 'peds.' Good structure improves aeration and allows roots to penetrate deeper into the profile.
O Horizon: The surface layer dominated by fresh and decaying organic matter (litter). It is most prominent in undisturbed ecosystems like forests.
A Horizon: Known as topsoil, this layer contains a mixture of organic humus and mineral material. It is the most biologically active layer and is critical for agriculture.
E Horizon: The zone of eluviation or leaching. As water moves downward, it carries dissolved minerals and clays out of this layer, often leaving it lighter in color.
B Horizon: The subsoil or zone of accumulation (illuviation). It collects the minerals, clays, and nutrients leached from the layers above.
C Horizon: Consists of partially weathered parent material. It contains large rock fragments and has very little organic activity.
R Horizon: The unweathered, solid bedrock that lies beneath the soil profile and serves as the ultimate source of mineral content.
| Property | Sand | Silt | Clay |
|---|---|---|---|
| Particle Size | Large ( - mm) | Medium ( - mm) | Small ( mm) |
| Water Drainage | Very Fast | Moderate | Very Slow |
| Nutrient Holding | Low | Medium | High |
| Aeration | Excellent | Good | Poor |
The CLORPT Mnemonic: Always use the CLORPT acronym to remember the five factors of soil formation (Climate, Organisms, Relief, Parent Material, Time).
Identify the 'E' Horizon: In exam diagrams, look for a light-colored, sandy layer between the A and B horizons; this is almost always the E (eluviation) horizon.
Texture and Drainage: Remember that the 'ideal' soil for most plants is loam, which is a balanced mix of sand, silt, and clay that provides both drainage and nutrient retention.
Human Impact: Be prepared to explain how human activities like deforestation or overgrazing accelerate the natural process of erosion by removing the protective O and A horizons.