Colorado Agriscience Curriculum
Section:
Plant & Soil Science
Unit:
Soils
Lesson Number:
PS U6 L3
Lesson Title:
Understanding Soil Horizons
Colorado Agricultural Education Standards:
Ag. II 10.9: The student will demonstrate an understanding of soil fertility and its effect
on crop production.
Colorado Science Standards:
4.1: Students know and understand the composition of Earth, its history, and the natural
processes that shape it.
Student Learning Objectives (Enablers)
As a result of this lesson, the student will …
1. Identify soil horizons
2. Understand a soil profile
3. Identify factors that cause a soil profile to change.
Time: Instruction time for this lesson: 50 minutes.
Resources
AgriScience Lesson Plan Library
Colorado Land Classification Manual
Tools, Equipment, and Supplies
Soil Cup Recipe
2 cups of milk
1 - 4 serving size chocolate pudding
8 oz cool whip
Oreo crumbs or cookies (to keep from having to smash Oreos – the Oreo pie crusts break up well
and save time
Clear plastic cups
Spoons
1. Make up the pudding.
2. Combine the pudding and cool whip.
3. Split off about 1/3 of pudding mixture and add in more Oreo crumbs (this will be the
darker top soil)
4. Put a layer of plain crumbs in the bottom of each class.
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5. Next put a layer of the pudding/cool whip combination.
6. Last put a smaller layer of the pudding/cool whip/Oreo crumb combination.
7. Gummy worms on top are fun and can be used to reinforce the idea of organic matter.
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Overhead projector or computer with projector
PowerPoint: PS U6 L3 Instructor PowerPoint
1 copy per student of PSU6L3StudentEvaluation
Student Notebooks
Writing tools
Key Terms: The following terms are presented in this lesson and appear in bold italics:
 Horizon
 Translocations
 Profile
 Lithology
Interest Approach
Show slide 1 of PS U6 L3 PowerPoint.
Without saying anything – raise your hand if you believe you know what this picture is
representing. Wait at least 15 – 20 seconds for students to observe the picture and think. Pick one
student to respond. Look for the answer of a soil profile. Good job. The picture is a soil profile.
What does the word profile mean in everyday language? Work with the students to determine it
means a snapshot, side view or cut-out of something. Soil profiles are the same things – they are
a cut-out of what is below the level we typically see. Give me some examples of where you have
seen a soil profile. Answers will vary: canyon roads, construction sites, hillsides. These are all
good examples. Let’s look at some various soil profiles. Show the next 2 slides. Why do you
think these profiles look different? Get the students to think. Soils from various parts of the
country, various parent material, various organic matter, minerals, drainage, etc. Those are all
correct. Today we are going to look more closely at determining what makes up a soil profile.
Summary of Content and Teaching Strategies
Utilize the PS U6 L3 PowerPoint for students to take notes.
Objective 1: Identify soil horizons. & Objective 2: Understand a soil profile.
Today I would like you to take notes as we look at soil profiles.
If the entire picture here is called a soil profile, what to you think each individual part you can
see is called? Good guesses – each individual layer is called a horizon. Hence soil horizon means
the same thing as soil layer. A change in the horizon is determined by the change in structure,
texture or other factors.
I am going to set out some visual aids at this time. You are not to touch your visual aid until
further notice. Thank you!
Set out the soil cups on the table in front of each student.
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What would be the entire name of the visual aid you have in front of you if you where looking at
it from the side? Demonstrate - Soil Profile. That is correct – soil profile. What would be the
name for each of the 3 distinctively different sections within the profile? That is correction – soil
horizons.
Slide # 4
Soil Profile
•sequence of horizons from the surface down through the horizons to the parent material
Slide #5
Soil Horizon
•Layer approximately parallel to the soil surface, with characteristics produced by soil forming
processes. If you dig very deeply into the soil, you will see that soil lies in distinct layers. These
layers are called soil horizons.
We are now going to look at the individually horizons and what the properties of each horizon
are.
Soil Horizons
Slide #6
Let’s start off simple. The 3 major horizons are:
A = dark colored zone of organic matter accumulation lying on the surface of the soil profile. It is
generally darker than the lower layers. The A horizon is where life in the form of plant roots,
bacteria, fungi and insects are most abundant. It is often called “top soil”.
B = this horizon lies just below the A horizon and is often called the sub-soil. It is often finer in
texture than the A horizon. It is usually higher in clay and has more pronounced structure than the A
horizon.
C = this horizon is located below the “true soil”. This horizon is often called parent material. It has
not been weathered enough to be called soil. This zone may show an accumulation of calcium and
magnesium salts causing it to be light in color.
Slide #7
As we get more technical in identifying various soil horizons three kinds of symbols are used to
designate horizons. Capital letters (A,B,C,E,O,R) identify master horizons, and are used singly or in
combination. For example, an AC horizon has characteristics of both A and C horizons, and is often
described in profiles lacking a B horizon.
So now let’s talk about the other horizons we did not include above.
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Slide #8
O - Organic horizon of decomposed and undecomposed organic matter. Present in forested soils.
Absent in grassland, cultivated and many other soils.
Slide #9
E - Mineral horizon that is usually light in color due to the leaching out of iron, aluminum, clay
and organic matter. Typically present in forested soils.
Slide #10
R - Underlying consolidated bedrock such as granite or shale. Absent under many soils.
Slide #11
Lower case letters are used as suffixes. For example, an Ap horizon designation indicates that the
horizon is an A but that it has been plowed. Arabic numerals are used as suffixes to indicate
divisions within a horizon (e.g. C1,C2) and as prefixes to indicate a change in lithology within the
soil profile (e.g. B, 2C).
Slide #12
Use this slide to put it all together for the students. Stress order.
Slide #13-14
Objective 3: Identify factors that cause a soil profile to change.
What can cause soil profiles to change? Does anyone have any idea? Call on a variety of students
until several correct answers have been gleaned. Good thoughts. You are correct – there are a
variety of factors that may act alone or combine to change the soil profile over time. This change
may happen very slowly or may be more rapid. Soils mostly change in time in response to their
environment. The environment is influenced by those soil forming factors we previously studied.
Most causes can be classified into four main categories.
1. Additions: Materials such a fallen leaves, wind-blown dust, or chemicals from air
pollution that may be added to the soil.
2. Losses: Materials may be lost from the oil as a result of deep leaching or erosion from the
surface.
3. Translocations: Materials may be moved within the soil. This can occur with deeper
leaching into the soil or upward movement caused by evaporating water.
4. Transformations: Materials may be altered in the soil. Examples include organic matter
decay, weathering of minerals to smaller particles or chemical reactions.
Each of these processes occurs differently at various depths. As a soil ages, horizontal layers
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develop and changes result.
Review/Summary
Utilize the soil cup visual aids at this time.
Hold up one soil cup. Okay – now that you have learned about the various horizons in a profile –
let’s discuss our soil profiles that are in the room today. Hold up your hand if you can tell me
what horizon this bottom dark colored layer would be. R horizon is correct answer. Now – who
knows what this middle horizon would be classified as – be sure to look at what it contains. The
B horizon is the correct answer. And lastly – who can tell me what this top horizon would be
classified as. Students will generally answer A horizon. However, a case can be made for Ap
since it would have been disturbed! Good job today learning about profiles and horizons. You
may now enjoy your “soil cups”.
Application
Extended classroom activity:
It is a great tool to have a soil pit someone near the Agriculture Facility. If this is
possible, walk the students to the pit and discussion the various horizons in the profile.
Have them take note of the texture, structure and color differences.
FFA activity:
Have the students participate in the Land Classification Career Development Event.
SAE activity:
Have a student find a location close to their home and determine what horizons are
present in their soil.
Evaluation
At the start of the next class period, give a pop quiz with two simple questions. Because of
the “soil cups” they ate – they should remember these two important concepts.
1. What is a soil profile?
2. What is a soil horizon?
The attached worksheet can also be used as a test or extension of this lesson.
Answers to Assessment:
Pop Quiz Answers
1. Soil profile – a cut-out view of the horizons in the soil – generally shows several
horizons.
2. Soil horizon: a horizontal section of the profile with similar characteristics and
properties
Worksheet Answers
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Part One: Matching
1=d, 2=c, 3=a, 4=b, 5=e, 6=f
Part Two: Completion
1. transformation
2. topsoil
3. organic
4. R
5. A
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Part Three: Short Answer
1. Use slide 1
2 Materials in the soil may be lost as a result of leaching or erosion.
3 A Horizon—darker, contains microorganisms and will be relatively uniform in texture
(probably medium or moderately fine).
C Horizon—lighter in color, contains parent material, which may be sandy and gravelly.
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PS U6 L3 Student Evaluation
Name:_____________________________
Date: ___________
Part One: Matching
Instructions. Match the term with the correct response. Write the letter of the term by
the definition.
a. soil profile
b. translocation
c. substratum
d. solum
e. illuviation
f. subsoil
_______1. Area where most plant roots grow.
_______ 2. C horizon. Usually composed of the parent material
of the soil and has had little soil forming activity.
_______ 3. A vertical cross-section of the soil.
_______ 4. Process of materials moving within the soil horizons.
_______ 5. The accumulation of chemicals, clay, iron, and other
materials in the B horizon.
_______ 6. Often referred to the B horizon.
Part Two: Fill in the blank
Instructions. Complete the following statements.
1. As materials such as organic matter and minerals are altered in the soil, this process
is called ____________________________.
2. The A horizon is often referred to as ___________________________.
3. The O horizon is often composed of an _________________ layer made up of
decayed plant and animal debris.
4. The ____________ horizon is underlying bedrock, such as limestone, sandstone, or
granite found underneath the C horizon.
5. The _____________ horizon is the one best suited for growth of plant roots.
Part Three: Short Answer
Instructions. Use the space provided to answer the following questions.
1. Draw a soil profile and label each of the three major horizons.
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2. Explain how “losses” might occur that will cause a change in the soil profile.
3. Explain how the A horizon is different from the C horizon in terms of color and
content.
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