physical

• Soil composition:

  • Soil consists of mineral matter (45%), air (25%), water (25%) and organic matter (5%)
  • The amount of air and water in soil is about 25% each, but this can fluctuate with heavy rainfall or prolonged periods of dry weather
  • During heavy rainfall, the water level in the soil rises and the air level drops
  • In periods of drought and low rainfall, the water level in the soil falls and the amount of air increases
• Mineral matter:

  • The five types of mineral particles found in soil are gravel, coarse sand, fine sand, silt, and clay
• Organic matter:

  • Organic matter consists of the remains of plants and animals
  • Humus is the dark-colored, decomposed plant and animal matter found in soil, rich in nutrients and contributes to soil structure
• Physical properties of soil:

  • Texture
  • Structure
  • Porosity
  • Colour
  • Temperature
• Soil texture:

  • A measure of the proportion of sand, silt, clay found in a sample of soil
  • Loam soil contains equal amounts of sand, silt, and clay
  • A soil triangle can be used to determine and classify a soil sample when the percentage of sand, silt, and clay is known
• Soil structure:

  • Describes the arrangement of soil particles within soil
  • A good soil structure has a large volume of pores, with approximately half filled with air and the other half with water
  • Sand, silt, and clay are primary particles that form clusters called aggregates/peds, determining soil pore space
• Factors affecting structural development:

  • Freezing and thawing
  • Wetting and drying
  • Soil organic matter
  • Plant root activity
  • Animal activity
  • Cultivation and tillage
• Soil compaction:

  • Occurs when soil particles are pressed together and aggregates are destroyed, reducing total pore space
• Soil porosity:

  • Refers to the total volume of soil occupied by soil pores
  • Air is needed for plant root respiration
  • CO2 diffuses into the atmosphere and O2 diffuses into the soil if there are sufficiently large pores
  • If large pores are absent, diffusion cannot take place, affecting plant respiration
• Soil water:

  • Hygroscopic water (adsorbed water) forms a thin film around a soil particle and is held on the surface by force of attraction
  • Capillary water is held in the pores within soil aggregates and between them
  • Gravitational water is moved through the soil by gravity
• Water availability in the soil:

  • Field capacity: the amount of water in soil after gravitational water has drained away
  • Permanent wilting point: the point at which no more capillary water can be removed from soil by plant roots
  • Available water capacity: the amount of water between field capacity and permanent wilting point available for absorption by plant roots
• Soil colour:

  • Indicates humus and mineral content
• Soil temperature:

  • The rate of chemical reactions doubles with every 10°C rise in temperature
  • Effects of soil temperature include germination rates, growth of crops, water and mineral uptake, root growth, crop development, and length of growing season
• The impact of erosion, sedimentation, and weathering on soil:

  • Soil erosion can be reduced by min-till operations, drainage, and catch crops
  • Soil erosion removes topsoil from the land by wind, water, and tillage erosion
  • Sedimentation is the result of erosion, leading to the loss of topsoil rich in nutrients and organic matter