Soil Science, Soil Profile, Soil Types & Erosion Control
Physical and chemical composition of soil, soil profile horizons, physical properties (texture, structure, porosity, water-holding capacity), sandy vs clayey vs loamy soils, soil erosion causes, types, and conservation methods.
In the Guinea Savannah zone of the Upper East and Upper West Regions, severe sheet and gully erosion during intense torrential downpours strips topsoil, prompting farmers to adopt stone bunding, zai pits, and vetiver grass contour hedges to trap moisture and soil.
Key Syllabus Learning Objectives
- 1Identify the 5 major components of fertile soil and explain their agricultural functions
- 2Draw and describe the vertical horizons of an idealized soil profile
- 3Compare sandy, clayey, and loamy soils in terms of particle size, drainage, capillarity, and nutrient retention
- 4Explain the agents, stages, and modern physical/biological methods of controlling soil erosion
Soil Physical Properties: Drainage, Capillarity & Aeration
- Drainage Experiment: Equal masses of sand, loam, and clay placed in filter funnels over measuring cylinders; equal water added. Sand drains most rapidly with highest filtrate volume; clay drains slowest.
- Capillarity Experiment: Glass tubes packed with different soils dipped into water. Water rises fastest and highest in clay soil over time due to extremely fine, continuous capillary pores.
- Soil Air: Crucial for cellular respiration in root hairs. Waterlogged clay soil expels soil air, suffocating roots and causing root rot.
Cocoa trees flourish exclusively on deep, well-drained forest loams in the Western Region, perishing rapidly if planted in waterlogged heavy clay.
Sandy soil has superior drainage but poor water retention; clay soil exhibits high capillarity and water retention but poor aeration.
Mechanisms and Mitigation of Soil Erosion in Ghana
- Four Stages of Water Erosion: (1) Splash (raindrop impact shatters aggregates) → (2) Sheet (uniform removal of thin topsoil layer) → (3) Rill (formation of small distinct channels) → (4) Gully (deep chasms that destroy farm tracks).
- Cover Cropping: Fast-growing creeping legumes (e.g. Centrosema pubescens, velvet bean) cover the soil surface, cushioning raindrop impact and binding soil particles with fibrous roots.
- Contour Plowing: Plowing ridges horizontally across slopes perpendicular to runoff flow creates micro-dams that slow water velocity, encouraging infiltration.
Planting cashew trees and vetiver grass barriers across slopes in the Bono East Region prevents productive farm lands from developing impassable erosion gullies.
Vegetative cover provides the most cost-effective protection against soil erosion by dissipating raindrop kinetic energy.
- In soil profile diagrams, remember that the A horizon (topsoil) is darker than the B horizon due to organic humus content.
- When distinguishing between drainage and capillarity, remember: drainage measures downward gravity flow; capillarity measures upward tension movement.
- Name at least two cover crops recognized in West Africa: Centrosema pubescens, Pueraria phaseoloides, or Mucuna pruriens.
- Confusing soil texture (permanent physical proportion of sand/silt/clay) with soil structure (arrangement of soil particles into aggregates/peds).
- Stating that clay soil has no pore spaces (it actually has more total pore volume than sand, but individual pores are microporous).
- Plowing along the slope instead of across the slope (plowing up and down hills accelerates gully formation).
- Can I draw a well-labeled diagram of an idealized soil profile showing horizons O, A, B, C, and R?
- Can I describe an experiment to determine the percentage of air in a fresh soil sample?
- Can I rank sand, loam, and clay in order of increasing water-holding capacity?
- Can I explain the differences between sheet erosion and gully erosion?
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