CHAPTER 4 - Soil Conservation and Management

Erosion Process and Mechanisms

I. Introduction
For a better understanding of the course, the students will learn about soil conservation and
management and how the process and mechanics of soil erosion. This module will focus on
the terminologies which will be used most of the time in the discussion of the course.
II. Learning Outcomes:
At the end of the module the students should be able:
1. To explain soil erosion process and mechanics
2. To enumerate different types of soil erosion
3. To understand the adverse effects of soil erosion.

III. Learning Content

Of the total land area (30 million hectares) of the Philippines, about 8 million hectares
of the alienable land are badly eroded. Approximately one million of agricultural lands with 8-15
percent slope are also subject to serious erosion during heavy rains. These lands are planted
to various crops. If the present agricultural practices employed by the farmers are not changed,
most of these lands are soon become unproductive.
Soil erosion in our country is very pronounced owing to high rainfall intensities,
improper land use and poor soil and farm management practices. Soil erosion is undesirable
process in agriculture. Tremendous amounts of surface soil rich in plant nutrients are lost
through erosion. Erosion is the source of sediment which degrades the quality of the water
resources in which it is entrained and frequently degrade the site where it is deposited.
 Soil erosion the detachment of soil materials and subsequent transport by an agent
to an area of deposition. Water is the most important agent of erosion in the humid tropical
areas. The two general types of erosion are:
1. Geological or natural erosion = a type of soil erosion at natural rates, unaffected by
human activities. A soil loss of about 1mm/yr has been reported under natural
forest/vegetation. However, such erosion rate is fairly matches y soil formation which is
estimated to be approximately 1mm/yr.
2. Accelerated erosion = this refers to the water or wind erosion at more rapid than normal,
usually associated with human activities. (Soil loss of 10 mm/year which is about 100
mg/ha/yr at a bulk density of 1mg/m³).
Mechanics of soil erosion process : there are three process involved during soil erosion by
water, namely:
a. Soil detachment by rainfall - falling raindrops which can reach a terminal velocity of such as
20 feet per second cause detaches soil particles in all directions.

b. Entrainment or transportation of sediments

c. Sediment deposition - the process of sediment settling out under the action of gravity. This
rate depends on particle size, being rapid for sand and slow for clay particles. Deposition is
a selective process.
 Figure 1. Four types of soil erosion on an exposed slope. (Source:Ref.33)

Types of erosion
1. Raindrop erosion - this type of erosion results as soil particles are detached due to the
impact of raindrops and splashed at a longer distance in the downslope than in the
upslope direction.
2. Sheetwash erosion - the uniform removal of soil in thin layers from sloping land,
resulting from sheet or overland flow occurring in thin layers.
3. Rill erosion - the removal of soil by water from small but well defined channels formed
when there is concentration of sheetwash flow
4. Gully erosion - the removal of soil by water from channel formed when rills combine
and develop to the extent that they cannot be eliminated by normal tillage operation.
5. Stream bank erosion - a kind of erosion that occurs along the banks of streams or
rivers. The water cuts the lower banks causing the upper parts to fall or collapse.
6. Scour erosion - this type of erosion is influenced by the velocity and direction of flow,
depth and width of channel, and soil texture.
7. Gravity erosion - generally termed “mass wasting” because large masses of soil move.
Water is usually involved, although flowing water is not the main moving force. The
movement may be instantaneous: landslides and soil creep
Types of Gravity erosion
1. Fall - occurs when gravity pull apart of a cliff or steep hillsides vertically downward
2. Rotational slides or slumps - actually rotate around a point in the center of slump.
3. Compound slide - a combination of fall and land rotational slide.
4. Transitional slide - occurs when a break causes a mass to slide along the faces of the
supporting mass.
Process:
 Mass wasting is a balance between the gravity pulling a mass downhill and the friction
holding the mass in place. Anything that decreased the friction or increase the mass helps
move the mass downhill: “water can do both”. Water lubricates the sliding plane and adds
mass to the block to be moved.
Saturated soils have a positive pore water pressure that enhances the possibility of
slides. Positive pore water pressure (outward) – soil grains will not be in contract and soil will
lose strength and move.
Factors affecting gravity erosion
1. Soil properties
2. Slope
3. Vegetation

Shrubs and trees with deep roots reduce landslides in two ways:
a. Plants use water, thereby decreasing the pore water pressure and mass and increasing
friction.
b. Roots bind soil together, and thick tap roots help anchor slope in place. Roots also
stretch and help slow movement.

Loss of plant cove leads to soil erosion

Factors affecting soil erosion
1. Rainfall is the climatic element that mainly affects erosion in the humid tropics. In
general, rainfall intensity (and not the amount) is positively correlated to soil erosion if all
other factors are held constant. The potential ability of the rain to cause erosion is called
erosivity. The rainfall characteristics are:
a. Intensity – describe the amount of rainfall per unit time expressed in mm/hr or inches/hr.
 Important parameter of measuring the kinetic energy of raindrops
b. Energy of precipitation
c. Quantity
d. Raindrop size

1. Soil erodibility refers to the vulnerability or proneness of the soil to erosion. The soil
properties affecting erodibility are as follows:
a. Soil texture, the fineness or courses of soil particles. Soil erosion by raindrop impact is
affected by texture which in turn also affects transportability. For instance, high clay
content induces surface runoff and increases transportability but decreases
detachability.
b. Structure, the arrangement of primary particles into aggregates that affects soil erosion
either directly by increasing infiltration or indirectly through the function of stable soil
aggregates brought about by increased organic matter concentration.

1. Vegetation or plant cover serves to cushion the impact of raindrops on the soil surface
and to slow down the velocity of runoff. The major effects of vegetation in reducing soil
erosion (Frevert et al, 1966) are:
a. Interception of rainfall by absorbing the energy of raindrops and thus reducing runoff.
b. Retardation of erosion by decreased runoff velocity
c. Physical restraint of soil movement
d. Improvement of aggregations and porosity of roots and plant residue
e. Increased biological activities.

1. Relief (Slope), the degree of deviation of a surface from horizontal, measured in a

numerical ratio, percent or degree. The two characteristics of slope the affects splash
erosion:
a. Degree (Steepness), the steeper the slope the greater the amount of soil loss from erosion
by water.
b. Slope length, horizontal distance downslope from the points where overland flow begins
and to where deposition starts.

1. Human activities such as: shifting cultivation, improper tillage, indiscriminate lumbering,
overgrazing, burning, road construction, etc.

ADVERSE EFFECTS OF SOIL EROSION

In general, damages brought about by soil erosion could either be “on-site effects” or
“off-site effects”. The former occurs right there in the field whereas the latter is usually a
consequences of the former
 “On-site Effects of Soil Erosion”
a. Particle selectivity, erosion increase the fraction of course primarily soil particles in the
in-situ soil while selectively removing fine particles, organic matter nutrients and
chemical associated with fine particles.
b. Surface sealing and hardening, raindrops can wash fine soil particles into the soil pores,
forming a seal up of 1 cm thick. This seals restricts water infiltration, thereby increasing
runoff and decreasing soil moisture. As a seal dries, it forms a crust, a dense layer of
soil that can impede seedling emergence and reduce plant population
c. Nutrient loss, nutrients attached to soil sediments are lost during erosion. Likewise,
dissolved nutrients are also lost in runoff, and productive capacity of the soil is reduced.
d. Loss of soil depth, erosion reduces rooting depth and soil moisture storage. When the
erosion rate exceeds the arte of topsoil formation, tillage will mix subsoil with topsoil the
often degrades soil as medium for crop production
Secondary changes:
a. Water and nutrient availability. Removal of top soil exposes the subsoil with poor
water storage capacity and conductivity from reduced pore space. Nutrient availability is
usually less in subsoil which is exposed by erosion.

b. Soil pH and lime requirements. Erosion tends to increase acidity and lime
requirements of soils in 3 important ways:
b.1. Replacement of topsoil with more acidic subsoil as erosion occurs.
b.2. Selective removal of base-forming elements (K, Ca, & Mg) from the topsoil
b.3. Removal of applied lime before it reacts to neutralize soil acidity particularly of
heavy rains occur after liming.
c. Soil Tilth
A physical condition of soil as related to its ease of tillage, fitness as a seedbed, and
its impedance to seedling emergence and root penetration. The mixing of topsoil with
subsoil often result in deteriorated soil tilth because of the poorer tilth of subsoil.
“Off-site Effects of Soil Erosion”
a. Eroded soil materials may bury crops or lower the fertility of the adjacent bottom lands.
b. Eroded soil is deposited in drainage channels, irrigation canals, or runs into ponds
reservoirs, tributary streams and rivers
c. Sediments deposits raised the level of river beds, which in turn reduces the capacity of
the channel to hold water so that river bank may overflow and valuable low-lying
agricultural lands including roads and bridges are damaged by flooding.
IV. Teaching and Learning Activities
The activity will be done by the students during their laboratory period.
Activity 1:
Download a pdf copy of module 1: Erosion Process and mechanics. Read and after which,
answer the following guide questions. Write your answer in a clean bond paper (handwritten).

Study questions:
1. Explain how erosion process occurs.
2. Is it possible to alter the slope condition? Support your answer
3. Differentiate geologic erosion from accelerated erosion.

Activity 2:
Give what is being asked from the following questions.

Assessment Task

A. Multiple Choice: Select from the following the correct answer.

a. soil erosion i. on-site effect

b. geologic erosion j. off-site effect
c. accelerated erosion k. stream bank erosion
d. raindrop erosion l. scour erosion
e. sheetwash erosion m. rainfall
f. rill erosion n. vegetation
g. gravity erosion o. relief
h. gully erosion

1. It occurs right there in the field and the reduction of soil quality which result from soil
hardening, nutrient loss from the upper layer of the soil and reduced water holding
capacity.
2. Eroded soil is deposited in drainage channels, irrigation canals, or runs into ponds,
reservoirs, tributary streams and rivers.
3. A kind of erosion that occurs along the banks of streams or rivers. The water cuts the
lower banks causing the upper parts to fall or collapse.
4. This type of erosion is influenced by the velocity and direction of flow, depth and width
of channel, and soil texture.
5. The climatic element that mainly affects erosion in the humid tropics.
6. Serves as cushion the impact of raindrops on the soil surface and to slowdown the
velocity of runoff
 7. The degree of deviation of a surface from horizontal, measured in a numerical ratio,
percent or degree.
8. Detachment of soil materials and subsequent transport by an agent to an area of
deposition
9. Type of soil erosion at natural rates, unaffected by human activities.
10. This refers to the water or wind erosion at more rapid than normal, usually associated
with human activities.
11. This type of erosion results as soil particles are detached due to the impact of raindrops
and splashed at a longer distance in the downslope than in the upslope direction.
12. The uniform removal of soil in thin layers from sloping land, resulting from sheet or
overland flow occurring in thin layers.
13. The removal of soil by water from small but well defined channels formed when there is
concentration of sheetwash flow
14. Generally termed “mass wasting” because large masses of soil move
15. The removal of soil by water from channel formed when rills combine and develop to the
extent that they cannot be eliminated by normal tillage operation

B. Essay:
1. Explain what is “mass wasting”.
2. Enumerate and discuss different factors affecting soil erosion

V. Flexible Teaching Learning Modality (FTLM)

The following teaching learning modality will be used in this course: a) provision of module to
each student; b) on-line teaching via video conferencing; c) activities; and d) use of social
media.
VI. ASSESSMENT TASK
This module has two assessments:
 Answer to study questions
 Answer to assessment task (Activity 2)
VII. REFERENCES
BRADY,N.C. and R.R. WEILL. 2000. The Nature and Properties of Soils. 11 ed. Macmillan, New
th

York.
DUNNE, T. 1977. Evaluation of erosion conditions and trends. In: Food and Agriculture
Organization of the UN. Guidelines for Watershed Management. FAO Conservation Guide No. 1,
FAO, Rome. Pp. 53-83.