Soil Science
Student’s Book
FET FIRST
Level 2
W. Burger
FET First Soil Science NQF Level 2 Student’s Book NQF Level 2
FET First
©Willem Burger
© Illustrations and design Macmillan South Africa (Pty) Ltd, 2006
All rights reserved. No part of this publication may be reproduced,
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or by any means, electronic, photocopying, recording,
or otherwise, without the prior written permission of the
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of the Copyright Act, 1978 (as amended).
Any person who does any unauthorised act in relation to this
publication may be liable for criminal prosecution and civil
claims for damages.
First published 2007
08 10 09 07
1 3 5 7 9 10 8 6 4 2 0
Published by
Macmillan South Africa (Pty) Ltd
P O Box 32484
2017 Braamfontein
Gauteng
South Africa
Typeset in DIN and Palatino 11/13
Cover design by BrandTalk, a subsidiary of African Access
Illustrations by Blake Moorhouse
Design by Brandtalk
The publisher would like to thank the following for permission to use
photographs in this book:
Dreamstime, Franco Esposito, Yotophoto.
The publishers have made every effort to trace the copyright holders.
If they have inadvertently overlooked any, they will be pleased
to make the necessary arrangements at the first opportunity.
eISBN: 9781431020669
ISBN 13: 9780853205074
WIP: 1789
It is illegal to photocopy any page of this book
without written permission from the publishers.
Contents
Topic 1: Some aspects of atoms and
molecules
Module 1 Basic concepts in chemistry
Unit 1: Atoms and elements
Unit 2: Molecules, compounds and mixtures
Unit 3: Ions
Module 2 Changes of state
Unit 1: Changes of state: evaporation and condensation
Unit 2: Dew point and relative humidity
2
3
7
14
16
17
25
Topic 2: Soils and their components
Module 1 The composition and development
of soil
Unit 1: What is soil?
Unit 2: The weathering of rock
Unit 3: Soil development and soil horizons
Module 2 Soil texture and structure
Unit 1:
Unit 2:
Unit 3:
Unit 4:
Unit 5:
Soil texture
The effect of texture on the properties of soil
Soil structure
Soil sampling
Soil temperature and its influence on plant growth
34
35
43
47
56
57
61
66
70
74
Topic 3: What plants require from the
soil for growth
Module 1 Major plant nutrients and
their uptake
Unit 1: The nutrients neccessary for plant growth
Unit 2: Uptake of water and nutrients
Addendum 1: Student’s Portfolio of Evidence
Guideline
82
83
91
95
Topic 1:
Some aspects of
atoms and molecules
1
Module 1
Basic concepts in chemistry
Overview
At the end of this module you will be able to …
● explain that matter consists of atoms that combine to form
molecules
● explain differences between elements, compounds and mixtures
and give examples of each
● explain that atoms gain or lose electrons to become charged ions
● conduct simple chemical experiments
2
Topic 1, Module 1
Unit 1:
Atoms and elements
Introduction
In this unit we you will study some basic chemistry that you will need
to understand soil science. You will learn about the basic structure of
atoms and molecules. You will also learn about elements, compounds
and do some laboratory experiments.
Atoms
All matter consists of atoms. Atoms are made of three main particles:
protons (p+), neutrons (n0) and electrons (e-). The nucleus of an atom
is at the centre and is made of protons and neutrons. The electrons
are in the space around the nucleus. Protons and neutrons have
1
nearly equal mass. The mass of an electron is about 2000 of the mass
of a proton. (We can ignore this mass because it is negligible.)
A proton has a small positive electrical charge and an electron has
an equal, but opposite negative charge. Each atom is electrically
neutral therefore the number of protons (in the nucleus) is equal to
the number of electrons (in the space around the nucleus).
Although the mass of an atom is concentrated in its nucleus, the
volume of an atom comes from the space that the electrons occupy.
The diagram shows the hydrogen atom (H) with 1 proton
and 1 electron
e-
Words &
Terms
An atom
is the sm
a
particle
into whic llest
h an elem
can be d
ent
ivided.
Words &
Terms
Negligib
le mean
s to
make a d
ifference o small to
.
Electron
Nucleus
p+
n
Proton
Neutron
Hydrogen
Unit 1: Atoms and elements
3
The diagram below shows a helium atom (He). The nucleus has
2 positive protons (p+) and 2 neutral neutrons (n0). There are
2 electrons (e-) in the space around the nucleus. The atom is
electrically neutral in its normal state.
e-
Nucleus
p+
n
Electron
Neutron
n
p+
Proton
eHelium
In chemistry, the electrons determine how atoms combine to form
molecules. So, we leave the nucleus with its protons and neutrons out
to simplify our diagrams. For example, we draw a carbon atom, which
has six protons, six neutrons and six electrons like this:
C
Carbon
4
Topic 1, Module 1
The volume of an atom is made up by the space that the electrons
occupy as they move around the nucleus. However, the mass of an
atom is concentrated in its nucleus. The mass of the electrons is
negligible. So, the total mass of an atom is equal to the mass of the
protons and the neutrons in its nucleus. We call this number the
atomic mass number and its symbol is A.
The atomic number is the number of protons in the nucleus of the
atom. The symbol for the atomic number is Z. So, for example, the
atomic number of hydrogen is one and the atomic number of carbon
is six.
Elements
There are over 90 types of atoms in nature. All the substances in
nature are made up of atoms. Some substances are made up of only
one type of atom. A substance that is made of only one type of atom is
called an element. The table shows the main elements that we deal
with in Agricultural Sciences.
We use letters as symbols for each element.
Element
Symbol
Atomic number
Hydrogen
H
1
Carbon
C
6
Oxygen
O
8
Nitrogen
N
7
Phosphorous
P
15
Potassium
K
19
Sulphur
S
16
Sodium
Na
11
Calcium
Ca
20
Magnesium
Mg
12
Iron
Fe
26
Assessment activity
Short test on atoms and elements
1. Explain the following terms in your own words:
a) Atom
b) Element.
2. Study the diagram of a nitrogen atom on the next page.
a) What is the atomic mass number?
b) What is the atomic number?
c) How many protons?
d) How many neutrons?
e) How many electrons?
Unit 1: Atoms and elements
5
e-
Electron
en
p+
p+
e-
n
n
p+
p+
p+
p+
n
n
n
n
p+
-
e
Nitrogen
e-
Proton
eNeutron
Nucleus
e-
3. Magnesium is an element that plants need to make the green pigment chlorophyll. Without
magnesium, the leaves turn yellow. The atomic number of magnesium is 12. Draw a diagram to
represent an atom of magnesium. Show the electrons, protons and neutrons.
Assessment activity
Assignment on fertilisers
1. Plants need a range of elements in the correct quantities to grow well. Farmers use fertilisers to
add certain elements to the soil to ensure healthy growth of crops. Research three different
fertilisers and write a report that covers the following points for each one:
a) What elements does the fertiliser contain?
b) What is the fertiliser used for?
c) How is the fertiliser applied?
Assess yourself
Assess your understanding of the following:
1 = not achieved 2 = not yet competent 3 = competent 4 = outstanding
1
1. Atoms
2. Elements
3. Atoms and elements in fertilisers
6
Topic 1, Module 1
2
3
4
Unit 2:
Molecules, compounds and
mixtures
Molecules
In nature, not many substances are made up of single atoms. Instead,
they are made up of atoms that have joined together with other atoms
to form molecules. A molecule consists of two or more atoms joined
together by a chemical bond. A chemical bond is a force that holds
two atoms strongly together by interactions between their electrons
and protons.
Substances made of molecules that consist of only one type of atom
are called elements. For example, oxygen gas is made up of molecules
that consist of two oxygen atoms bonded together. Hydrogen gas
consists of two hydrogen atoms bonded together. Nitrogen gas
consists of two nitrogen atoms bonded together.
Learning activity
Preparing and testing for hydrogen
You will need: a few small pieces of zinc (Zn); dilute hydrochloric acid (HCl); two test tubes; a
splint; a Bunsen burner or spirit burner.
Unit 2: Molecules, compounds and mixtures
7
Collect the
hydrogen gas
Burning splint
Dilute HCl
Bubbles
Hydrogen gas
Zinc
Preparing and testing for hydrogen
Method
To prepare hydrogen, follow these steps:
1. Place the zinc in a test tube.
2. Pour in about 5ml of dilute HCl
3. Observe bubbles coming out of the liquid.
To test for hydrogen, follow these steps:
1. Hold the second test tube upside down over the first test tube to collect the gas bubbles that are
given of. (Hydrogen is lighter than air and floats upward.)
2. Keep the second test tube in position for about two minutes to push the air in the test tube
downwards and out.
3. Hold the flame of a burning splint to the mouth of the upside down test tube, then turn both the
burning splint and the test tube right-side up.
4. If you hear a ‘pop’ sound the gas is hydrogen.
5. What is your conclusion?
We use chemical formulae to abbreviate the types and numbers of
atoms in one molecule of a substance. For example, the chemical
formula of oxygen is O2. This tells us that one molecule of oxygen
consists of two oxygen atoms. The chemical formula of water is H2O,
which shows that a water molecule consists of two hydrogen atoms
and one oxygen atom.
8
Topic 1, Module 1
Compounds
Compounds are substances that consist of molecules made up of
different types of atoms bonded together. For example, carbon dioxide
(CO2) consists of molecules made of one carbon atom bonded to two
oxygen atoms. Water (H2O) consists of molecules made of one oxygen
atom bonded to two hydrogen atoms.
Carbon dioxide (CO2)
Water (H2O)
A compound has different properties from the chemical elements that
it is made of. It looks and behaves differently. For example, table salt
or sodium chloride (NaCl) is a compound made of the elements
sodium and chlorine. Sodium is a soft substance that looks like white
rubber but fizzes violently if it is dropped into water. Chlorine is a
very poisonous green gas. However, these two dangerous elements
combine to form harmless white crystals of table salt that is safe to eat.
Investigating chemical reactions that form compounds
A compound is formed when the atoms of the elements take part in a
chemical reaction. During the chemical reaction, the atoms of the
elements form chemical bonds that hold them together as molecules.
To separate the atoms from each other again requires another chemical
reaction to break the chemical bonds that hold them together.
In the next activity you will investigate rusting. Rust is a brown
chemical compound that forms when the iron metal undergoes a
chemical reaction with water and air. Rusting involves the breaking of
chemical bonds and the formation of new compounds.
Unit 2: Molecules, compounds and mixtures
9
Assessment activity
Practical chemistry: Investigating rusting
You will need: four small glass or transparent plastic bottles with airtight lids. The bottles should
be labelled 1, 2, 3 and 4; four steel nails; steel wool or sandpaper; a small amount of calcium
chloride (CaCl2); a small amount of table salt (NaCl); a little cooking oil.
Lid
Oil
Nail
CaCl2
Cool boiled
water
Air
Air
Tap water
Tap water
containing NaCl
Investigating rusting
Method
1. Boil about 200ml of water and let it cool. (This removes any dissolved oxygen from the water.)
2. Clean each nail with sandpaper or steel wool until it is shiny and place one nail in each bottle.
3. Bottle 1: Add a little CaCl2 to the bottle in order to dry the air. Close the lid tightly.
4. Bottle 2: Completely cover the nail with the cooled boiled water. Pour a layer of oil on the
surface of the water (to keep the air out). Close the lid tightly.
5. Bottle 3: Pour ordinary tap water into the bottle, and leave some of the nail sticking out of the
water. Close the lid tightly.
6. Bottle 4: Do the same for bottle 4 as for bottle 3, but add a little NaCl to the tap water before
closing the lid tightly.
7. Place the four bottles on a shelf and leave them for five days.
8. Record your observations (after 5 days) in a table like the one below.
Bottle number
How much rust is present?
(a lot of rust / some rust / no rust)
1
2
3
4
9. What conclusions can you draw from your observations? (Remember that we want to know
what conditions are necessary for rust to form.)
10
Topic 1, Module 1