Lecture 7: Soil System Notes

Definitions of Soil

  • Soil is a dynamic, three-dimensional natural body on the Earth's surface where plants grow.
  • It consists of minerals, organic material, living organisms, water, and air.
  • Soil properties develop due to the integrated effects of climate, topography, and living organisms acting upon parent material.

Function of Soil

  • Plant Growth Medium: Soil provides physical support and supplies water and nutrients to plants.
  • Habitat for Organisms: It hosts a diverse community of organisms, including bacteria, fungi, invertebrates, and vertebrates, essential for decomposition and nutrient cycling.
  • Environmental Interactions: Soil interacts with climate, atmosphere, and the broader environment.
  • Buffering System: Soil can resist changes in its properties and overcome drastic changes, acting as a buffer for pH and ionic composition.
  • Influence on Vegetation: Soil influences the types of crops and weeds that can grow.

Soil Components

  • Four major phases:
    • Solid
    • Liquid
    • Gases
    • Living components

Soil Phase (Solid)

  • Composed of mineral particles (from rocks) and organic matter.
  • Makes up 50% or more of the soil volume.
  • Organic matter typically comprises 0.5 to 6% of soil volume.

Liquid Phase (Soil Solution)

  • Primarily water, occupying 10-50% of soil volume.
  • Contains dissolved minerals and organic substances.
  • Supplies water and nutrients to plants.

Gases Phase

  • Primarily air, occupying 10-50% of soil volume.
  • Fills pore space not occupied by water.
  • Adequate air (oxygen) is essential for aerobic microorganisms and plant roots.

Colloidal Components

  • Very small particles, mainly clay and organic matter.
  • High surface area and negative charge.
  • Critical for the soil's ability to hold and exchange positively charged ions (cations), influencing nutrient availability.

Living Components

  • Diverse community of organisms such as bacteria, fungi, invertebrates, and vertebrates.
  • Play crucial roles in soil health and function.

Texture

  • Soil texture is defined by the relative proportion of mineral particle sizes: sand, silt, and clay.
  • These particles are sorted into size classes.
  • The specific combination of sand, silt, and clay percentages determines the soil texture class, which can be visualized using a standard soil texture triangle.
  • Texture can be broadly categorized as:
    • Coarse (sandy)
    • Fine (clay)
    • Medium (loam): a mixture of sand, silt, and clay.

Soil Characteristics

  • Soil properties differ significantly from one soil to another.
    • Texture
    • Structure
    • Aggregate stability
    • Porosity
    • Water holding capacity
    • Aeration
    • Nutrient holding capacity
  • Cation exchange capacity (CEC), largely influenced by colloidal clay and organic matter, is a crucial characteristic affecting the soil's ability to retain and exchange cations, which are often essential nutrients.
  • Organic matter significantly enhances CEC.
  • Soil also responds to climate, topography, and natural vegetation.
  • Soil biota activity is influenced by moisture, aeration, temperature, and organic matter.
  • As a buffered system, soil resists changes in pH and ionic composition.

Soil and Weeds

  • Soil conditions play a significant role in determining the success and composition of weed populations.
  • Factors such as moisture, aeration, fertility level, and pH influence weed growth.
  • Some weeds can serve as indicators of specific soil conditions, such as saline or acidic soil.
  • Weeds can adapt and thrive in soil conditions that might be unsuitable for crops.
  • Certain invasive weeds have the capacity to alter soil properties.
  • Weed control practices have a major impact on the soil system.
    • Tillage: A common method that physically disturbs the soil, affecting soil structure, erosion, and the distribution of weed seeds within the soil profile.
    • No-Tillage Systems: Minimize soil disturbance, altering weed populations and species composition compared to tilled systems, often relying more on herbicides.
  • Surface soil conditions, which are affected by weed control methods, can influence weed seed germination.