UNIT 1: Soil

Form 2 Agriculture Lessons for JCE: Grow Your Knowledge

The main components of soil are:

Importance of Soil Water:
- Supports plants, making their cells turgid.
- Helps dissolve plant nutrients for transport up the plant.
- Raw material for photosynthesis.
- Necessary for microbial activity (decomposition).
- Facilitates seed germination.
- Transpiration has a cooling effect on plants.
Types of Soil Water:
- Superfluous Water: Found in large air spaces (macropores), held by gravitational forces. Leads to waterlogging when spaces are saturated; not readily available for plant use.
- Hygroscopic Water: Thin film on soil particle surfaces, held by strong forces; not available for plant use.
- Capillary Water: Occupies micropores, held by strong adhesive and cohesive forces; readily available and beneficial to plants.
Experiment 1.1: To determine the percentage of water in a soil sample
- Procedure: Weigh dish, add soil (e.g., 50g), heat to 105°C, stir, cool, re-weigh.
- Calculation: % water lost = (Mass of water lost / Original mass of soil) × 100%.
- Conclusion: Water is a constituent of soil.

Description: Mixture of gases (oxygen, carbon dioxide, nitrogen) found in spaces between soil particles. Amount depends on water content. Ploughing increases air spaces.
Importance of Air on Crop Production:
- Helps plant roots and soil animals respire.
- Nitrogen is converted into nitrates, improving soil fertility.
- Facilitates aeration in the soil.
- Good aeration removes excess carbon dioxide from the soil.

Description: Made up of decayed plant and animal matter, forming humus.
Importance of Organic Matter (Humus) in Crop Production:
- Binds soil particles, forming crumbs (improves soil structure).
- Loosens up the soil, creating spaces for water and air.
- Holds water and nutrients essential for plant growth.
- Improves soil colour, which influences soil temperature.
- Improves soil fertility by releasing nutrients.
Experiment 1.3: To find the percentage of humus in a soil sample
- Procedure: Weigh dry soil sample. Heat strongly to burn off humus. Re-weigh burnt soil.
- Calculation: % organic matter = ((Mass of dry soil – Mass of burnt soil) / Original mass of dry soil) × 100%.
- Conclusion: Humus/organic matter is a constituent of soil.

Description: Products of weathering (sand, silt, clay) – these are the soil fractions which vary in size. Also contains essential minerals (calcium, potassium).
Experiment 1.2: To show that soil contains mineral matter of different sizes
- Touch and Feel Method:
  - Procedure: Rub soil between thumb and finger.
  - Results: Clayey soil feels fine/smooth; sandy soil feels coarse/rough.
  - Conclusion: Clay has smaller particles, sand has larger particles.
- Sieve Method:
  - Procedure: Grind soil, pass through sieves of decreasing mesh sizes (e.g., 2mm, 0.2mm, 0.002mm).
  - Results: Different particle sizes (coarse sand, fine sand, silt, clay) are retained at different sieve levels.
  - Conclusion: Soil is made up of different sized particles.
- Sedimentation Method:
  - Procedure: Mix soil with water and sodium carbonate (disperses particles) in a measuring cylinder, shake, and let stand.
  - Observation: Particles settle into layers based on size and weight (largest/heaviest at bottom, lightest/smallest at top).
  - Conclusion: Soil is made of different sized particles.

Description: Soil contains various organisms, from large (earthworms, ants, termites) to microscopic (saprophytic bacteria, Rhizobium bacteria).
Importance of Living Organisms on Crop Production:
- Burrowing animals aid in soil formation by mixing soil particles.
- Fix nitrogen in the soil (e.g., Rhizobium bacteria in legumes).
- Improve aeration in the soil.
- Facilitate microbial activities (decomposition), turning organic substances into humus.
Experiment 1.4: To show the presence of micro-organisms in the soil
- Procedure: Suspend fresh garden soil in a muslin bag in lime water (Flask A). Suspend burnt soil in another muslin bag in lime water (Flask B – control). Cork tightly.
- Results: Lime water in Flask A turns milky (due to CO2 from respiration of microorganisms). No change in Flask B (burning kills organisms).
- Conclusion: Soil contains microorganisms.

A soil profile refers to the vertical arrangement of different layers of soil. Examining it reveals:

Superficial Layer (O Horizon):
- Made up of organic substances like dead plant matter (leaves, wood) and animal matter.
Horizon A or Topsoil:
- Topmost part of the soil.
- Dark in colour due to richness in organic matter/humus.
- Typically 5–30cm deep.
- Has abundant plants and living organisms.
- Well aerated.
- Contains most plant roots.
Horizon B or Subsoil:
- Also known as a zone of accumulation as most leached minerals from Horizon A are found here.
- Red or reddish-brown due to fewer organic substances.
- Contains few plant roots and other living organisms.
- Less aerated than topsoil.
- More compact; hardpans can form here.
Horizon C or Weathered Rock:
- Contains weathered rocks, broken from the parent rock beneath.
- Contains both leached minerals and those originating from the parent rock.
Horizon D or Bedrock:
- Composed of unweathered rocks.
- Source of the inorganic (mineral) materials of the soil.
- Determines the ultimate colour and mineral content of the soil above.

The soil profile significantly influences crop production by:

Soil Depth: Affects the soil’s ability to hold water, air, and nutrients, directly impacting root growth and plant access to resources.
Compactness: Helps a farmer determine soil compactness, which influences aeration, drainage, and root penetration.
Parent Rock Composition: Identifies the mineral content of the parent rock, indicating potential nutrient availability in the soil.