5.1 Soil

Soil and its Role in the Ecosystem

Guiding Questions

  • How do soils play a role in sustaining natural systems?

  • How are human activities affecting the stability of soil systems?

Key Understandings

  1. Dynamic System

    • Soil is a dynamic system within the larger ecosystem with distinct components including inputs, outputs, storages, and flows.

    • It encompasses inorganic and organic components, water, and air.

  2. Soil Classification

    • Soils are classified based on the overall appearance of their profiles and mapped accordingly.

    • Key horizons include the organic layer, mixed layer (topsoil), mineral soil, and parent rock, denoted as O, A, B, and C horizons.

  3. Soil Profile Structure

    • Soils develop a stable layered structure, known as a profile, consisting of multiple horizons caused by interactions within the system over time.

  4. Soil Inputs and Outputs

    • Inputs include organic matter from dead organisms and inorganic minerals.

    • Outputs consist of organic matter losses due to decomposition, mineral losses, and energy loss as heat.

  5. Storage and Transfers

    • Transfers of materials occur across soil horizons, influencing the composition of the soil system.

    • Transformations can change components, affecting sustainability and management practices.

  6. Soils as Ecosystems

    • Soils support terrestrial ecosystems, acting as a medium for plant growth and storing essential nutrients and water.

    • They also foster biodiversity by providing habitats for numerous organisms.

  7. Biogeochemical Cycles

    • Soils play a significant role in recycling elements within biogeochemical cycles, contributing to the ecosystem’s vitality.

  8. Soil Texture and Productivity

    • Soil texture, defined by the proportions of sand, silt, clay, and humus, significantly affects primary productivity and the physical makeup of mineral soil.

  9. Carbon Dynamics

    • Soils can act as carbon sinks, sources, or stores, depending on rates of organic matter input and decomposition.

    • Carbon cycling is influenced by temperature, moisture, and precipitation.

  10. Implications for Sustainability

    • Understanding soil systems is crucial for sustainable management and addressing threats to soil health.

Soil Composition and Characteristics

Definition of Soil

  • Soil is:

    • A dynamic system with inputs, outputs, storages, and flows.

    • A complex mixture of components forming its own ecosystem with distinct soil organisms.

    • Composed of inorganic components (rock fragments, sand, silt, clay) from weathering of parent rock and organic matter (living organisms, decayed material).

Soil Facts

  • Soils store more carbon than the atmosphere and all world plants combined.

  • Approximately 10% of the world’s CO₂ emissions are stored in soils.

  • A tablespoon of healthy soil contains more microorganisms than the entire human population.

  • Underground biomass can surpass above-ground biomass in many ecosystems.

  • Only about 1% of soil microorganisms have been identified, indicating high biodiversity reservoirs.

  • Approximately 95% of food production relies on soil.

  • Topsoil is being lost 10-40 times faster than it is formed; one inch of topsoil can take 500 years to develop.

  • Earthworms can consume as much as 5 tonnes of dry matter per hectare annually.

Soil Components and Processes

Components of Soil

  1. Rock Particles

    • Insoluble components (e.g., gravel, sand) provide the structure of soil derived from underlying rock or transported by erosion.

    • Soluble minerals include nitrogen, phosphorus, potassium, and other essential nutrients filtered through water movement in soils.

  2. Humus

    • Decomposed organic matter contributing to soil color and nutrient content.

    • Holds substantial amounts of water.

  3. Water

    • Essential for transporting minerals; leaching can result from rapid water movement.

    • Excessive water can lead to waterlogging and anoxia.

  4. Air

  • Composed mainly of oxygen and nitrogen; crucial for soil organism respiration and plant roots.

  1. Soil Organisms

  • Include invertebrates and microorganisms that decompose organic matter and recycle nutrients.

  • Burrowing animals enhance soil aeration and mixing.

Soil Processes

  • Transformations of Soil Components

    • Influences include decomposition, nutrient cycling, salinization, weathering, and biological mixing.

Soil Structure and Profiles

Understanding Soil Profiles

  • Soil Profile is formed by digging a trench revealing horizontal layers:

    • O Horizon: Leaf litter and organic matter accumulation.

    • A Horizon: Rich in humus, dark in color due to organic matter.

    • B Horizon: Deposited nutrients and minerals leaching from upper layers, enriched in clay and salts.

    • C Horizon: Weathered rock layer where soil material originates.

    • R Horizon: Bedrock or parent material beneath soil.

Formation Processes

  • Water movement influences mineral transport and sorting of soil horizons.

  • In areas with high evaporation, salinization can occur as minerals are left behind after water evaporation.

  • In wetter climates, leaching of minerals from soil layers into groundwater is common.

Soil Types and Classification

Classification Systems

  • Soil types are classified using various systems (USDA, FAO, Canadian); examples include:

    1. Mollisols: Fertile temperate grasslands, rich in organic matter.

    2. Alfisols: Found in deciduous forests, productive with clay retention.

    3. Oxisols: Highly weathered tropical soils, low fertility.

    4. Gelisols: Found in tundra regions, characterized by permafrost and slow decomposition.

Factors Influencing Soil Formation

  1. Time: Soils can take decades to thousands of years to form, highly influenced by the other factors.

  2. Climate: Precipitation and temperature govern soil chemical reactions, leaching and evaporation.

  3. Organisms: Biological contributions to nutrient cycling and ecosystem dynamics.

  4. Geomorphology: Landscape features affect drainage and erosion patterns.

  5. Geology (Parent Material): Source of minerals, influences soil properties based on material type (e.g. volcanic vs calcareous).

Soil Sustainability and Human Impact

  • Soil Degradation: Intensive agricultural practices threaten soil health through compaction, monoculture, pollution, and neglect.

  • Carbon Emissions: Soil contributes to greenhouse gases; agricultural practices may accelerate carbon release.

  • Importance of Topsoil: Vulnerable to erosion and critical for plant growth; its loss can lead to reduced agriculture productivity.

Summary of Key Points

  • Soil is an intricate system that sustains life, plays roles in ecosystems, and is crucial for agriculture.

  • Understanding soil types, profiling, and the influences on soil formation is necessary for sustainable practices.

  • Human actions significantly affect soil stability and health, highlighting the need for responsible management approaches in agriculture and land use.