Welcome to Section 2.4: Soil Erosion and Soil Conservation!
Hello future agriculturist! Soil is the foundation of farming, but it's also very delicate. In this chapter, we learn about the biggest threat to our valuable soil: erosion, and critically, how we can stop it. This is a crucial topic because once the fertile topsoil is gone, it can take thousands of years to replace!
2.4(a) Describing Soil Erosion: Agents and Types
Soil Erosion is defined as the removal of the top layer of soil (the topsoil) by natural agents like water or wind. This topsoil contains the majority of the nutrients and organic matter (humus) needed for healthy plant growth, so losing it is disastrous for farming.
The two main agents that cause erosion are Water and Wind.
Agent 1: Water Erosion (The Four Stages)
Water erosion usually occurs in a progressive sequence, meaning one type of erosion leads to the next, more severe type.
Step 1: Splash Erosion
- What is it? When a raindrop hits bare soil, it acts like a tiny bomb, dislodging soil particles and splashing them into the air.
- Did you know? A single heavy storm can dislodge millions of soil particles!
Step 2: Sheet Erosion
- What is it? After the soil is loosened, the running water moves over the land like a thin, uniform sheet. This water carries away the dislodged soil particles.
- Why is it dangerous? It’s often hard to spot because it removes a thin layer evenly across the field.
Step 3: Rill Erosion
- What is it? As the sheet flow increases in speed and volume, it concentrates into tiny, defined channels or streams called rills.
- Analogy: Think of the small streams of water running down a paved road after rain—only these are carrying soil! Rills can be easily removed by normal ploughing.
Step 4: Gully Erosion
- What is it? If rills are left unchecked, they deepen and widen into large channels called gullies.
- Impact: Gullies are too large to be crossed by farm machinery and permanently damage the land, rendering it useless for cultivation.
Agent 2: Wind Erosion (Moving Particles)
Wind erosion happens primarily in dry, open areas where the soil is light (like sandy soils) and loose.
Wind moves soil particles in three ways (A great mnemonic is SSC: Suspension, Saltation, Creep):
- Suspension: Very fine particles (like dust) are lifted high into the air and can travel for hundreds of kilometres (like a dust storm).
- Saltation: Medium-sized particles bounce along the soil surface, lifted momentarily by the wind, causing abrasion and dislodging more soil. This is often the most significant type of wind erosion.
- Surface Creep: Large, heavy particles roll or slide along the ground surface, pushed along by the momentum of the saltating particles.
Quick Review: Water erosion involves Splash, Sheet, Rill, Gully. Wind erosion involves Suspension, Saltation, Creep.
Causes of Soil Erosion (Why it gets worse)
Soil erosion is a natural process, but humans often speed it up drastically through poor farming and land management practices.
1. Removal of Natural Vegetation
- Deforestation: Cutting down forests exposes the soil directly to rain and wind. Tree roots are essential for holding the soil together.
- Overgrazing: Too many animals eating the grass leave the land bare. The hooves also compact the soil, making it less absorbent, which increases water runoff.
2. Inappropriate Cultivation Methods
- Ploughing up and down the slope: If you plough vertically down a hill, you create ready-made channels for water to rush down, accelerating rill and gully erosion.
- Continuous Monoculture: Planting the same crop repeatedly can deplete organic matter, leading to poor soil structure that is easily broken up by wind and rain.
- Leaving the soil bare: Especially during dry or windy seasons, exposed soil is unprotected.
3. Natural Factors
- Steep slopes (Topography): Water flows faster on steep slopes, increasing its erosive power.
- Heavy rainfall: High-intensity rain provides more energy for splash erosion and greater volume for runoff.
- Strong Winds: Common in dry, arid (desert-like) regions, where wind can easily pick up fine soil particles.
Prevention and Control of Soil Erosion
Preventing soil erosion is critical for sustainable agriculture. Control measures aim to achieve two main goals:
- Reduce the speed and volume of water/wind over the soil.
- Keep the soil covered and bound together.
Don't worry if this seems like a lot! We can break the methods down into two types: Vegetative (Biological) and Mechanical (Structural).
A. Vegetative (Biological) Control Methods
These methods use plants or plant residue to protect the soil.
1. Contour Ploughing
- How it works: Instead of ploughing up and down a hill, the farmer ploughs along the contours (lines of equal elevation) of the slope.
- Effect: Each furrow acts as a small dam, trapping rainwater and slowing down runoff, allowing more time for infiltration.
2. Strip Cropping (or Contour Strip Cropping)
- How it works: Alternating strips of different crops, often a clean-tilled (row) crop (like maize) with a close-growing, soil-holding crop (like grass or a legume).
- Effect: The dense, soil-holding crops act as barriers, catching soil particles and reducing wind speed across the field.
3. Cover Crops and Mulching
- Cover Crops: Planting crops specifically to cover the soil when the cash crop is not growing (e.g., planting legumes after harvesting maize).
- Mulching: Covering the soil surface with organic materials (crop residues, straw, leaves) or plastic sheets.
- Effect: Both methods absorb the energy of falling raindrops (reducing splash erosion) and shield the soil from wind. Mulch also keeps the soil moist.
4. Afforestation/Reforestation
- Planting trees or replacing forests that have been cut down. Tree roots bind the soil firmly and the canopy protects the ground from heavy rain impact.
B. Mechanical (Structural) Control Methods
These involve building physical structures on the land.
1. Terracing
- How it works: Building step-like platforms or "benches" along steep slopes.
- Effect: This breaks the long slope into a series of short, level steps. This dramatically reduces the speed of water flow and allows water to collect and soak into the soil on the level platform. This is highly effective but labour-intensive.
2. Construction of Bunds and Stone Lines
- How it works: Building low earth mounds (bunds) or placing lines of stones along the contours of the land.
- Effect: These structures act as physical barriers to runoff water, forcing it to slow down and deposit any carried soil particles behind the bund/line.
3. Check Dams (for Gully Control)
- How it works: Small temporary dams built across the bottom of a gully, often using rocks, wire netting, or brushwood.
- Effect: They slow the flow of water within the gully, preventing the gully from deepening further and encouraging soil deposition to fill the gully over time.
Quick Review: Key Takeaways
Summary of Erosion and Conservation
- Erosion Threat: The loss of nutrient-rich topsoil.
- Water Types (worst to best): Gully (worst) ← Rill ← Sheet ← Splash.
- Main Cause: Lack of protective vegetative cover.
- Conservation Principle: Slow down water/wind and cover the soil.
- Key Methods: Contour Ploughing and Terracing are essential for slopes.