Soil Erosion and Conservation Lecture Notes

Soil erosion, its impacts, types, factors influencing it, and various soil conservation methods. It covers both the theoretical aspects, such as the definition and types of soil erosion, and practical approaches to mitigating its effects through different conservation techniques

Soil Erosion

The wearing away of the land surface by running water, wind, ice, or other geological agents, including such processes as gravitational creep and landslide.

Soil Erosion Consequences

Identified by the World Bank (1989) as the Philippines’ worst environmental problem.

Yearly Volume of Erosion Caused by Degradation (Philippines)

1,000,000,000m3 of material

Percentage of Philippine Croplands Vulnerable to Erosion

75% of the Philippines’ croplands

Number of Provinces in the Philippines Already Severely Eroded

13 provinces

On-Site Impacts of Soil Erosion

Reduction in soil depth, structure deterioration, nutrient loss, decline in soil fertility, loss of production, decline in soil biota.

Off-Site Impacts of Soil Erosion

Flooding, sedimentation of irrigation systems and water reservoirs, reduced hydroelectric output, destruction of coral reefs.

Geological Erosion

Wearing away of the earth’s surface by water or other natural agents under natural environmental conditions of climate, vegetation, undisturbed by man.

Accelerated Erosion

Erosion much more rapid than normal, primarily as a result of human activities or animals; often 10 to 1,000 times as destructive as geological erosion.

Most Important Detachment Agent in Soil Erosion

Rainfall

Detachment-Limited Erosion

More material than is supplied by detachment

Transport-Limited Erosion

More material is supplied than can be transported

Deposition of Transported Particles

Occurs when sufficient energy is no longer available to transport the particles.

Impact of Raindrops (Three Important Effects)

It detaches soil, destroys granulation, and causes transportation of soil.

Influence of Raindrops on Soil Surface

Surface soil pores filled with soil particles, reducing infiltration; surface flow begins due to lack of infiltration.

Specific Kinds of Soil Erosion (By Water)

Sheet Erosion, Rill Erosion, Gully Erosion

Sheet Erosion

The removal of a fairly uniform layer of soil from the land surface by runoff water.

Rill Erosion

Numerous small channels of only several centimeters in depth are formed, occurring mainly on recently cultivated soils.

Gully Erosion

Water accumulates in narrow channels and removes soil to considerable depths, ranging from 0.3-0.6m to 23-30m.

Erosivity Factors for Rainfall

Intensity, duration, mass of raindrop, diameter of raindrop, and velocity of raindrop

Erodibility Factors

Large particles are resistant to transport, fine particles are resistant to detachment.

Least Resistant soil particles to detachment and transport

Slit and fines sands

Slope Factor in Soil Erosion

Function of slope length and slope steepness (angle/gradient).

Slope Length

The distance from the point of flow to the point of runoff.

Major Role of Vegetation in Reducing Erosion

Interception of raindrops so that their kinetic energy is dissipated rather than imparted to the soil.

Most Efficient Plant Cover for Erosion Control

Forests are the most efficient (usually with dense ground litter), but dense grass growth can be almost as efficient.

Human Activities That Contribute to Soil Erosion

Reduction of vegetation cover (tillage, burning, overgrazing, mining, logging, road construction).

Universal Soil Loss Equation (USLE)

A = 0.224 * R * K * L * S * C * P

The USLE was developed at the…

USDA National Runoff and Soil Loss Data Center at Purdue University led by Walter H. Wischmeier and Dwight D. Smith

Limitations of the USLE

It does not explicitly represent hydrologic and erosion processes, predicts only sheet and rill erosion, and has considerable interdependence between variables

Soil Properties That Tend to Result in High K Values (USLE)

High contents of silt and very fine sand, expansive clay minerals, tendency to form surface crusts, impervious soil layers, blocky/platy/massive structure.

Soil Properties That Tend to Make Soil More Resistant to Erosion (Lower K Values)

High SOM content, non-expansive clay, strong granular structure.

Soil Conservation Objectives

Control of soil erosion, maintenance of soil fertility and productivity, avoidance of soil toxicities.

General Objectives for Soil Conservation

Dissipate raindrop impact, increase infiltration rates, reduce surface runoff velocities, reduce soil erodibility.

Surface Covers for Soil Conservation

Most effective measure in reducing erosion; dissipates energy, impedes water flow, increases infiltration, reduces runoff.

Cover Crops

Temporary vegetative cover of fast-growing annuals grown primarily to protect bare soil.

Ground Cover

Perennials or regenerating annuals specifically established to protect soil from erosive agents.

Mulching

Spreading plant residues/organic materials on the ground between crop rows or around tree trunks to protect bare soil.

Geo Textiles

Use of coconets to protect slopes.

Physical Barriers for Soil Conservation

Land treatments or physical structures constructed along the contour of the land to intercept and/or divert runoff.

Bench Terracing

Slope is converted into series of level steps supported by steep risers with a horizontal cultivated area on the step.

Contour Rock Walls

Fence-like structures made up of rocks and/or stones piled along the contour of sloping lands.

Contour Bounds

Embankments of stones, grasses, or compacted soil constructed along the contour to slow runoff and trap eroded soil.

Pole Barriers / Contour Fences

Ipil-ipil stems bundled into rows across the slope and staked to form low barriers.

Contour Hedgerows

Hedges of leguminous trees/shrubs or grasses grown on the contour at a defined interval; crops cultivated in alleys.

Natural Vegetative Strips (NVS)

Naturally occurring vegetation like grasses and herbs left in narrow strips to serve as barriers.

Contour Composting

Banks or barriers of trash and/or earth constructed across the slope to check surface runoff and soil erosion.

Diversion Canals

Natural/artificial drainage channel to accommodate runoff.

Contour Canals

Holes dug intermittently along water channels to intercept water flow, catch soil particles, slow down runoff, and increase infiltration.

Grassed Waterways

Channel constructed across the slope to catch upslope runoff and divert it safely to a nearby gully/waterway/river.

Check Dams

Obstruction walls across the bottom of a gully to reduce velocity and prevent deepening or widening of the gullies.

Soil Traps

Small canals dug along contour line to allow water to pass through and trap soil sediments.

Zero Tillage

Sowing crops directly into the residue of the previous crop without cultivation.

Minimum Tillage

Sowing crops on lightly cultivated soil.

Contour Plowing

Plowing, harrowing, and furrowing along the contour of the land.

Multi-Storey Agroforestry System

Multi-layered canopy system.