MODULE IN SCI 101 – EARTH SCIENCE

LESSON 4: SOIL RESOURCES

Soils are fundamental to life on Earth but human pressures on soil resources are
reaching critical limits. Careful soil management is one essential element of sustainable
agriculture and also provides a valuable lever for climate regulation and a pathway for
safeguarding ecosystem services and biodiversity.

LAYERS OF THE SOIL

There are different types of soil, each with its own set of characteristics. Dig down
deep into any soil, and you’ll see that it is made of layers, or horizons (O, A, B, C, R). Put the
horizons together, and they form a soil profile. Like a biography, each profile tells a story
about the life of a soil. Most soils have three major horizons (A, B, C) and some have an
organic horizon (O). The horizons are:

O -(humus or organic) Mostly organic

matter such as decomposing leaves. The
O horizon is thin in some soils, thick in
others, and not present at all in others.
A -(topsoil) Mostly minerals from parent
material with organic matter incorporated.
A good material for plants and other
organisms to live. The hummus makes the
topsoil soft, porous to hold enough air and
water. In this layer, the seeds germinate
and roots of the plants grow. Many living
organisms like earthworms, millipedes,
and centipedes, bacteria, and fungi are
found in this layer of soil.
B - (subsoil) Rich in minerals that leached
(moved down) from the A or E horizons
and accumulated here. It is comparatively
harder and compact than topsoil. It is lighter
in color than the topsoil because there is less
humus in this layer. This layer is less
organic but is rich in minerals brought
down from the topsoil. It contains metal
salts, especially iron oxide in a large
proportion. Farmers often mix horizon-A and horizon-B when ploughing their fields.
C - (parent material) The deposit at Earth’s surface from which the soil developed.
R - (bedrock) A mass of rock such as granite, basalt, quartzite, limestone or sandstone that
forms the parent material for some soils – if the bedrock is close enough to the surface to
weather. This is not soil and is located under the C horizon. It contains no organic matter and
made up of stones and rocks, so it is very hard. 

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KINDS OF SOIL AND THEIR CHARACTERISTICS
Soil is a natural resource that can be categorized into different soil types, each with
distinct characteristics that provide growing benefits and limitations. Identifying the type of
soil you require for a project is paramount to support the healthy growth of plant life.
Soil can be categorized into sand, clay, silt, chalk and loam types of soil based on the
dominating size of the particles within a soil.

1. Sandy soil. Sandy Soil is light, warm, dry and tend to

be acidic and low in nutrients. Sandy soils are often
known as light soils due to their high proportion of
sand and little clay (clay weighs more than sand).
These soils have quick water drainage and are easy to
work with.
They are quicker to warm up in spring than
clay soils but tend to dry out in summer and suffer
from low nutrients that are washed away by rain. The
addition of organic matter can help give plants an
additional boost of nutrients by improving the
nutrient and water holding capacity of the soil.

2. Clay Soil. Clay Soil is a heavy soil type that benefits

from high nutrients. Clay soils remain wet and cold
in winter and dry out in summer. These soils are
made of over 25 percent clay, and because of the
spaces found between clay particles, clay soils hold a
high amount of water. Because these soils drain
slowly and take longer to warm up in summer,
combined with drying out and cracking in summer,
they can often test gardeners.

3. Silt Soil. Silt Soil is a light and moisture retentive

soil type with a high fertility rating. As silt soils
compromise of medium sized particles they are well
drained and hold moisture well. As the particles are
fine, they can be easily compacted and are prone to
washing away with rain. By adding organic matter,
the silt particles can be bound into more stable
clumps.

4. Chalk Soil. Chalk soil can be either light or heavy

but always highly alkaline due to the calcium
carbonate or lime within its structure. As these soils
are alkaline they will not support the growth of
ericaceous plants that require acidic soils to grow. If
a chalky soil shows signs of visible white lumps then

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 they can’t be acidified and gardeners should be resigned to only choose plants that
prefer an alkaline soil.
5. Loam Soil. Loam soil is a mixture of sand, silt and
clay that are combined to avoid the negative
effects of each type. These soils are fertile, easy to
work with and provide good drainage. Depending
on their predominant composition they can be
either sandy or clay loam. As the soils are a perfect
balance of soil particles, they are considered to be
a gardeners best friend, but still benefit from
topping up with additional organic matter.

FACTORS AFFECTING SOIL FORMATION

Soils differ from one part of the world to another, even from one part of a backyard to
another. They differ because of where and how they formed. Five major factors interact to
create different types of soils:

1. Climate, temperature and moisture influence the speed of chemical reactions, which
in turn help control how fast rocks weather and dead organisms decompose. Soils
develop faster in warm, moist climates and slowest in cold or arid ones.
2. Organisms—Plants root, animals burrow, and bacteria eat – these and other
organisms speed up the breakdown of large soil particles into smaller ones. For
instance, roots produce carbon dioxide that mixes with water and forms an acid that
wears away rock. 

Termites can generate mounds in the soil that are three stories tall!!!
3. Relief (landscape)—The shape of the land and the direction it faces make a difference
in how much sunlight the soils gets and how much water it keeps. Deeper soils form
at the bottom of a hill because gravity and water move soil particles down the slope.

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 Soils are different depending on the location in the slope that they are located.
4. Parent material—Every soil “inherits” traits from the parent material from which it
formed. For example, soils that form from limestone are rich in calcium and soils that
form from materials at the bottom of lakes are high in clay. Every soil formed from
parent material deposited at the Earth's surface. The material could have been bedrock
that weathered in place or smaller materials carried by flooding rivers, moving
glaciers, or blowing winds. Parent material is changed through biological, chemical
and environmental processes, such as weathering and erosion.

These are soils forming in real time from the side of the volcano. They form into
rock first, then weather into fertile soil.

5. Time—All of these factors work together over time. Older soils differ from younger
soils because they have had longer to develop. As soil ages, it starts to look different
from its parent material. That is because soil is dynamic. Its components—minerals,
water, air, organic matter, and organisms—constantly change. Components are added
and lost. Some move from place to place within the soil. And some components are
totally changed, or transformed.

COMPOSITION OF SOIL

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 Soil composition is an important aspect of nutrient management. While soil minerals
and organic matter hold and store nutrients, soil water is what readily provides nutrients for
plant uptake. Soil air, too, plays an integral role since many of the microorganisms that live in
the soil need air to undergo the biological processes that release additional nutrients into the
soil.
The basic components of soil are minerals, organic matter, water and air. The
typical soil consists of approximately 45% mineral, 5% organic matter, 20-30% water,
and 20-30% air. These percentages are only generalizations at best. In reality, the soil is
very complex and dynamic. The composition of the soil can fluctuate on a daily basis,
depending on numerous factors such as water supply, cultivation practices, and/or soil type.

There are three soil components – Clay, Sand, and Silt

Clay is the smallest mineral component. These tiny flat particles fit closely together to
create the greatest surface area of all soil types. Clay soil contains needed nutrients and also
stores water well. So well in fact, that drainage is slow in clay soil. It is also the slowest to
warm in the spring.
Sand makes up the largest particles in soil structure. These are rounded, rather than
flat and allows for larger space between the particles. Water drains quickly from the soil that
has a lot of sand and the nutrients drain faster too. If your soil is mostly sand the plants will
need more water and fertilizer.
Silt represents the middle size pieces. It is made up of rock and mineral particles that
are larger than clay but smaller than sand. Individual silt particles are so small that they are
difficult to see. To be classified as silt, a particle must be less than .005 centimeters (.002
inches) across.
Loam is the combination of these three particles and is considered the ideal garden
soil.
The solid phase of soil, which includes minerals and organic matter, are generally
stable in nature. Yet, if organic matter is not properly managed, it may be depleted from the
soil. The liquid and gas phases of the soil, which are water and air respectively, are the most
dynamic properties of the soil. The relative amounts of water and air in the soil are constantly
changing as the soil wets or dries.

SOIL CONSERVATION
Soil Conservation is the name given to a handful of techniques aimed at preserving
the soil. Soil loss and loss of soil fertility can be traced back to several causes, including
over-use, erosion, salinization, and chemical contamination. Unsustainable subsistence
farming and the slash and burn clearing methods used in some less developed regions can
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often cause deforestation, loss of soil nutrients, erosion on a massive scale, and sometimes
even complete desertification.
Soil erosion removes the topsoil that is necessary for organic matter, nutrients,
microorganisms that are required for plants to grow and shine. Soil conservation is one such
step that protects the soil from being washed away. The soil then ends up in aquatic resources
bringing in pesticides and fertilizers used on agricultural land.
Soil loss is not only a problem for the farmer, with the loss of organic matter and
fertility, it is also an environmental problem. Sediment entering streams can destroy fish
habitat and water quality especially when soil particles contain contaminants such as
pesticides or nutrients.
Soil conservation practices are tools the farmer can use to prevent soil degradation
and build organic matter. These practices include: crop rotation, reduced tillage, mulching,
cover cropping and cross-slope farming.

1. Crop Rotation is a tool that enables

farmers to increase soil organic matter
content, soil structure and rooting depth.
This is accomplished by growing
secondary crops which enhance soil
health. Root crops are particularly
destructive to soil structure because of the
extensive shattering of soil aggregates
during seedbed preparation and harvest.
For this reason, root crops should not be
grown more than once every three years.

2. Cover Cropping and Mulching are

effective at reducing soil erosion by leaving
a cover over the soil which reduces soil
displacement associated with the impact of
raindrops hitting soil particles. They also
reduce the volume and velocity of runoff
over the soil. Mulching consists of applying
organic material over the exposed soil. Hay
makes the best mulch, but it is important to
ensure that the hay is harvested
before weeds are mature. Straw can
also be used.

3. Conservation Tillage is field

operations aimed at preserving soil
aggregates, organic matter and surface

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 residue from previous crops. Residue management is a facet of conservation tillage
that is designed to leave crop residue on the soil surface to prevent erosion. The
amount of residue on the soil surface depends on the amount of residue left from the
previous crop and the tillage performed.

SCI 101 – EARTH SCIENCE

LESSON 4: SOIL RESOURCES

Name: Yr. and Sec: Rating:

Exercises/Drills:

I. Multiple Choice: Encircle the letter of the correct answer.

1. Which of the following is an example of sediment?

a. Clay, gravel, sand, pieces of shell b. Only Clay
c. Only pieces of shell d. Only sand

2. What is SOIL made of?

a. Only weathered rocks and minerals b. Mostly decayed plants and animals (organic stuff)
c. Only wet sediments d. Mostly sediments, organic material, air and water

3.  Planting different crops each year (corn one year, beans the next) is called what?
a. Terracing b. No Till-farming
c. Wind Breakers d. Crop Rotation

4. Is soil a renewable or nonrenewable natural resource?

a. renewable b. nonrenewable

5. What is the name of the layer of soil just under the grass?
a. topsoil b. subsoil c. bedrock d. parent rock

II. Essay: Briefly explain why is topsoil so important to farming and gardening.

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 SCI 101 – EARTH SCIENCE
LESSON 4: SOIL RESOURCES

Name: Yr. and Sec: Rating:

Evaluation: The Mason Jar Soil Test

1. Use a clear, clean, empty jar with a tight lid. A pint or quart Mason jar works
fabulously.
2. Fill the jar about half full of garden soil. You can use soil from different areas of the
garden to get an overall view or make a test for each garden bed.
3. Fill the jar nearly to the top with water. Leave room for shaking.
4. Tighten the lid and shake the jar for several minutes so that all the particles are in
suspension.
5. Set your mason jar soil test aside for 5 hours, so the particles have a chance to settle.
They will separate into clay, silt, and sand layers

Observe and describe each jar (Clay, Silt, and Sandy). Use extra sheet if necessary. Paste
at least 5 photos of you doing the experiment and label each photo.

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Additional Readings:
Status of the World’s Soil Resources http://www.fao.org/3/a-bc590e.pdf

References:
Books
Refran, et al (2016) Earth Science.Quezon City, Philippines. Vibal Group Inc. pp. 20-28
Seeds, M.A. & Backman, D. (2016) Earth Science. Manila, Phillipines. Rex Bookstore, Inc.
pp. 109-142

Website
Soil Horizons https://www.soils4teachers.org/soil-horizons
Soil and Soil Profile
https://www.toppr.com/guides/science/soil/soil-and-soil-profile/#:~:text=The%20main
%20layers%20of%20the,three%20layers%2C%20is%20mature%20soil.
Soil Types https://www.boughton.co.uk/products/topsoils/soil-types/
Soil Formation https://www.soils4teachers.org/formation
Mason Jar Soil Test https://preparednessmama.com/jar-soil-test/
Soil Conservation https://www.conserve-energy-future.com/methods-of-soil-
conservation.php

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