Soil
Soil system
A soil system is a dynamic system composed of organic and inorganic matter, water, air, and living organisms that support terrestrial life by providing nutrients, water, and a medium for plant growth.
Soil Profile
The soil develops a stable, layered structure known as a profile. The soil profile consists of several horizons, produced by interactions within the system over long periods.
Horizons:
- O Horizon: Organic matter like decomposed leaves and plant material.
- A Horizon (Topsoil): Rich in organic matter, nutrients, and biological activity.
- B Horizon (Subsoil): Contains minerals leached from the upper layers and less organic material.
- C Horizon: Partially weathered parent material.
- R Horizon: Unweathered bedrock (parent material).
Inorganic and Organic Components of the Soil System
- Inorganic Components:
- Minerals like sand, silt, and clay are derived from weathered rocks.
- Essential nutrients like nitrogen, phosphorus, and potassium.
- Organic Components:
- Humus: Decomposed organic matter that retains nutrients and moisture.
- Living organisms: Microorganisms (e.g., bacteria, fungi) and macroorganisms (e.g., earthworms, insects).
Inputs and Outputs of Soil
- Inputs:
- Dead organic matter (e.g., plant and animal residues).
- Mineral particles from weathering.
- Water and atmospheric gases.
- Outputs:
- Nutrients removed by plants and erosion.
- Leaching: Loss of soluble nutrients to deeper soil layers or groundwater.
In managed soil systems, many inputs are anthropogenic: compost, fertiliser, agrochemicals, irrigation, and salinisation. Natural inputs may originate from within the ecosystem (weathering of underlying parental rock, litter from above-ground vegetation, decomposition) or be derived from other ecosystems (wind-blown and waterborne deposition, guano).
Transfers and Transformations in the Soil System
- Transfers:
- Movement of nutrients, water, and organisms between soil layers.
- Erosion and deposition by wind or water.
- Transformations:
- Decomposition: Organic matter broken down by microorganisms.
- Weathering: Physical and chemical breakdown of parent material.
- Nutrient Cycling: Conversion of organic matter into inorganic nutrients for plant uptake.
The soil system can be shown using a systems diagram.
Role of Soil in Ecosystems
1. Biodiversity:
- Provides habitat for microorganisms, insects, and burrowing animals.
- Essential for the survival of plants, which support terrestrial food webs.
2. Nutrient Cycle:
- Stores and recycles nutrients like nitrogen, phosphorus, and potassium.
- Decomposition releases nutrients for plant uptake.
3. Terrestrial Ecosystems:
- Regulates water flow and prevents flooding.
- Anchors plants, providing physical support.
- Filters and purifies water by trapping pollutants.
Soil Texture Triangle
A soil texture triangle is a graphical tool used to classify soil types based on the percentage of sand, silt, and clay.
- Testing Methods:
- Texture by feel.
- Chemical tests for pH, nutrients, and salinity.
- Permeability tests for water movement.
Differences Between Soil Types
- Sandy Soil:
- Large particles, high porosity, quick drainage.
- Low nutrient and water retention.
- Silty Soil:
- Medium-sized particles, moderate porosity.
- Retains water and nutrients well.
- Clay Soil:
- Small particles, low porosity, high water and nutrient retention.
- Poor drainage and aeration.
