Unit
Year levels
5–6
Acidity in the Soil
Curriculum Links
Science
growth and survival of living things are
• The
affected by the physical conditions of their
•
•
environment (ACSSU094).
Scientific knowledge is used to inform
personal and community decisions
(ACSHE217).
With guidance, pose questions to clarify
practical problems or inform a scientific
investigation, and predict what the findings
of an investigation might be (ACSIS231).
About this activity
Soil testing is very popular in Kitchen Garden Schools, and it is
enormously useful in the garden – not just for planning where
plants will go, but also to involve students in an understanding of
the needs of plants and the reasons behind our treatment of the
soil (with compost and other additives such as lime). It goes a small
way towards explaining why some plants love to grow together and
others won’t tolerate the same conditions. You can even explain to
students why we don’t put citrus and onions into our worm farms,
using a basic understanding of pH levels to show that these are
acidic items that make worms uncomfortable .
Demonstrating pH levels in substances
Many people have heard of this pH demonstration using red
cabbage water, but have put it aside as ‘too hard’. Do try it! It really
is easy – particularly if you have bits of red cabbage from the garden
– and it is lots of fun. As with any experiment, try it out first on
your own; it’s easy to do at home or in the kitchen, and doesn’t need
special equipment.
See the notes in Lesson 3 about which substances to try testing
so that you get a full range from acid to alkaline. Test a variety of
substances but don’t forget to test the soil from your garden, plus
perhaps your compost bins, the worm farm and even under specific
plants (label the samples, e.g. ‘Under the rhubarb’, ‘Under the
banana tree’, ‘Chicken coop’, ‘Wet compost, middle bin’ and so on ...)
and allow time for the sediment to settle.
You can use the demonstration to lead to a discussion about acidity,
alkalinity and pH levels in the soil, the requirements of plants or
the way our taste buds sense acidic foods (vinegar, lemon juice –
very sour!) as opposed to alkaline foods (chalky substances – soft
and sweetish). When testing the soil using this method, you could
check your findings against the results of one of the readily available
commercial soil testing kits on the market.
Several Kitchen Garden Schools have told us that they get the best
value out of the activity when they introduce it with questioning
and discussion with the class first – because once the students see
the change in colours in the solutions, it’s a little hard to bring them
back to thinking about plants for a few minutes!
Stephanie Alexander Kitchen Garden Foundation © 2011
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Acidity in the Soil | Lesson 1
Discussion Ideas
Year levels 5–6
Curriculum Links
Science
•
•
The growth and survival of
living things are affected by
the physical conditions of their
environment (ACSSU094).
Scientific knowledge is
used to inform personal
and community decisions
(ACSHE217).
Location
The kitchen or the classroom
Duration:
15 minutes
Vocabulary
acid/acidic
alkali/alkaline
base/basic
hydration
neutral
pH scale
solution
substance
trace elements
Getting started
start, you might discuss how we, as gardeners, try to create the ideal
• To
environment for our plants to ensure that they grow well.
the basic needs of plants: sunshine for energy, water for hydration,
• Revisit
soil for nutrients.
Our own experiences
how smart gardeners recognise these different needs and adjust the
• Discuss
water, soil and sunshine (sun/shade) to suit the plants they want to grow.
students’ experiences and examples of things we have done to alter
• Collect
the soil in our garden (such as adding compost and spreading mulch, chicken
poo or lime).
• You could also discuss taste as a way of introducing acids.
• Ask students to give examples of foods that taste bitter, salty, sweet and sour.
they name some sour foods (lemon juice, vinegar and dishes with these
• Can
ingredients such as salad dressings)? These foods are acids.
do we know about acids? Can we name any other acids? Which recipes
• What
have we used acids in, and why do you think we used them? (Sometimes acid
is used for the sour flavour and sometimes to take advantage of the way it
emulsifies with oils, like olive oil and vinegar shaken vigorously together in a
dressing. It can also be used in a marinade to break down and help tenderise
ingredients such as meat or fish.)
opposite to acidic substances are alkaline substances, sometimes called
• The
alkali or base substances.
substances often taste chalky, mealy and sweet. Do students know of
• Alkaline
anything alkaline? It’s hard to think of ingredients, but bicarbonate of soda is
very alkaline (you don’t want to taste it as it’s too strong to be pleasant).
• Create a class list of vocabulary and examples as you go.
Extensions / Variations
Have the students look at gardening reference material, such as books or the
Gardening Australia website (www.abc.net.au/gardening).
for ‘acid soil’ and also for ‘alkaline soil’, and plants that prefer one
• Search
or the other.
• Do any of the plants mentioned grow in our garden?
they do, does this tell us anything about how and why we grow particular
• Ifplants
together?
• What does this make students think about plant families?
do we do to adjust the soil acidity to suit our plants? (Such as adding
• What
lime for some plants; compost or manures for others.)
• Students may extend this concept in part by exploring crop rotation.
Stephanie Alexander Kitchen Garden Foundation © 2011
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Acidity in the Soil | Teacher Resource
The pH scale
The pH scale is an international standard that has been tested and agreed on. Having a standard
measurement of acidity enables us to talk about acidity or alkalinity of substances in a consistent way.
The name pH refers to ‘potential Hydrogen’ or ‘power of Hydrogen’ – which means that it is a
measure of the number of hydrogen ions.
The scale runs from very acidic at 0 to very alkaline at 14. Neutral water is at 7 – this is a measure of
pure water (but most of our tap water has traces in it that shift the pH slightly, nearer to 6).
The pH (potential Hydrogen) scale
Concentration of hydrogen ions
compared to distilled water
pH rating
Sample substances at this level
10 000 000
pH = 0
battery acid, hydrochloric acid
1 000 000
pH = 1
sulphuric acid
100 000
pH = 2
lemon juice, vinegar
10 000
pH = 3
orange juice, fizzy drinks
1000
pH = 4
tomato juice
100
pH = 5
bananas, healthy lake water, coffee
10
pH = 6
milk, urine, saliva
1
pH = 7
pure water
1/10
pH = 8
sea water, eggs
1/100
pH = 9
bicarbonate of soda
1/1000
pH = 10
milk of magnesia
1/ 10 000
pH = 11
ammonia
1/ 100 000
pH = 12
soapy water
1/ 1 000 000
pH = 13
bleach, oven cleaner
1/10 000 000
pH = 14
liquid drain cleaner
Stephanie Alexander Kitchen Garden Foundation © 2011
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Acidity in the Soil | Lesson 2
Soil Health and pH Levels
Year levels 5–6
Curriculum Links
Science
•
•
The growth and survival of
living things are affected by
the physical conditions of their
environment (ACSSU094).
Scientific knowledge is
used to inform personal
and community decisions
(ACSHE217).
Location
The kitchen or the classroom
Duration:
15 minutes
Getting started
• Revisit the class list of activities and vocabulary from the previous lesson.
students looked up acid-loving and alkaline (lime-loving) plants, discuss
• Iftheir
findings.
Soil health and plant health
can be acidic or alkaline – acidic soil is sometimes called ‘sour’ and
• Soil
alkaline soil is sometimes called ‘sweet’ soil.
plants are very sensitive to acidity and will die if exposed to very acidic
• Most
or very alkaline soil.
make soil more acidic when we add compost, manure and other organic
• We
material.
make it more alkaline when we add lime (ground limestone or dolomite
• We
lime).
The pH scale
at students’ lists of examples of acidic foods. If someone says lemon
• Look
juice is ‘very acidic’ and someone else says orange juice is ‘very acidic’, does
that mean they are exactly the same? (No.)
Why does pH matter?
d,
When the soil is too aci
er
oth
to
ds
bin
s
oru
sph
pho
y
wa
a
in
l
elements in the soi
ts
roo
nt
pla
ts
ven
that pre
need
from taking it in. Plants
’t get
can
y
the
and
phosphorus,
ething
som
to
ck
stu
is
it
en
it wh
pens
else. The same thing hap
t
tha
nts
me
ele
ce
to other tra
t is too
tha
l
soi
so
d,
nee
nts
pla
.
acidic starves the plants
s
Acidic soil also inhibit
t ideally
tha
ia
ter
bac
ial
efic
ben
roots
nt’s
pla
grow around the
fix
to
nt
pla
the
p
and hel
nitrogen in the soil.
These little changes can
low
make a healthy plant yel
le to
and weak, and susceptib
ts.
pes
diseases and
• Ask: Okay then, how do we communicate what the acidity is?
international agreement is called a standard, or a scale, and it helps us
• An
to talk about a measure like acidity in confidence that we are talking about
exactly the same measure.
the pH scale and discuss students’ experiences of some of the
• Introduce
substances on the list.
that we are going to test some of the things in our kitchen but also
• Explain
the soil in our garden.
plants prefer to grow in more acidic soil than others (blueberries,
• Some
potatoes, radishes, carrots, beans and peanuts are some of the plants that
love slightly acidic soil between 6 and 5.5 on the scale). Many Australian
indigenous plants prefer slightly acidic soil, as much of the soil here is
naturally acidic. Most of our vegetables prefer soil that is very slightly on the
acid side of neutral (pH 6 to 7 – remember 7 is neutral). (An obvious point to
make here is that these vegetables come from other countries, where the soil
is slightly different.)
• Ask: Why would we need to know the acidity of the soil in our garden beds?
}}
To ensure that we plant the kinds of plants that will grow best and
produce beautiful fruit and vegetables for our garden.
}}
To see if we need to add anything to the soil to help the plants that are
already there.
Stephanie Alexander Kitchen Garden Foundation © 2011
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Acidity in the Soil | Lesson 3
Hands On! Doing the pH Test
Year levels 5–6
Curriculum Links
Science
•
With guidance, pose questions
to clarify practical problems
or inform a scientific
investigation, and predict what
the findings of an investigation
might be (ACSIS231).
Resources
• red cabbage (½ or more of one
•
•
•
•
•
•
•
•
•
small cabbage)
knife
chopping board
kettle for boiling water
a large bowl, jug and strainer
that can handle boiling water
several very clean clear glass
jars (same size is best)
measuring spoons
½ tsp each of bicarbonate of
soda, lemon juice, vinegar and
other materials to test
torch (optional)
white paper (optional)
Location
The kitchen, garden or classroom
Duration:
20–30 minutes
Before the lesson
chop ½ a red cabbage into small pieces. Shredded, as for coleslaw,
• First,
works well. You can use the other half of the cabbage in cooking.
a litre of water and pour the boiling water on the cabbage. (Try a large
• Boil
pyrex bowl.)
• Let it sit until it cools. You will see that the water is a rich purple-red colour.
the cabbage, keeping the purple-red liquid in another bowl or a jug.
• Strain
You could do the preparation to this point before class. The pale cabbage can
go in the compost now.
sure you keep all the purple water – it’s best if it’s in a white or see• Make
through jug so that the students can see what colour you started with (plastic
should be okay because it’s cool now). You can keep this cold purple water in
the fridge for 2–3 days if you need to.
Doing the demonstration
up your clean, clear glass jars or cups in a space where students can see
• Set
them.
can put white scrap paper under them so that the colour change will be
• You
obvious, and you can also write the substance on the paper next to each jar –
e.g. ‘Bicarbonate of soda’.
each jar, measure ½ teaspoon of each of the substances that you want to
• In
test: ½ tsp bicarbonate of soda, ½ tsp lemon juice, ½ tsp tartaric acid etc.
1 tablespoon of fresh water and stir until the powder has become a paste.
• Add
(Important: Use a clean spoon for each one, otherwise you will contaminate
each substance and ruin the results!)
Predicting results
• Ask students to predict what will happen before you mix each one in.
60 mL of the purple cabbage water and add it to each of the cups
• Measure
or jars in sequence. You will see a sudden change of colour in each one as the
purple liquid mixes in.
||Pink
is acid; red slightly acid.
||Dark
blue is neutral.
||Greenish
yellow is alkaline.
the students to arrange the jars in a row according to how acidic they
• Get
think they are. Don’t show them the full scale just yet.
they need prompts, talk about mixing light or paint colours (blue and
• Ifyellow
make green, therefore a green liquid goes between a yellow and a
blue liquid).
• Below is the scale for the class to check their assumptions.
• Discuss the process and how they made decisions.
Stephanie Alexander Kitchen Garden Foundation © 2011
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Acidity in the Soil | Lesson 3
p2
pH scale for Red Cabbage Water
pH number
1–2
3–4
5–6
7–8
9–10
11–12
Colour
Pink
Dark Red
Violet
Blue
Blue–Green
Greenish–Yellow
Notes
The activity works best if you have a variety of substances with varying levels of
acidity.
Try these as the basic set:
• bicarbonate of soda
• lime or lemon juice
• vinegar
• tartaric acid (not cream of tartar).
Try also:
• powdered chalk
• a crushed clove of garlic
• mashed or puréed apple, banana, potato
• a dash of milk
soil, compost from the worm farm, soil from under a native plant
• garden
or tree.
Mix powders with a bit of water by stirring or shaking well just before adding
the cabbage water.
To test soil, put about ¼ cup soil in the bottom of the jar, add ¼ cup water and
shake vigorously. Add red cabbage water, shake again and let it settle until you
can see the colour of the liquid. (This settling can take about half an hour so you
might prepare and label these first. Hold them up to the light or shine a torch
through them on a white background to show the colour clearly.)
Try testing more than one spot of the garden and discussing with students why it
would vary.
Stephanie Alexander Kitchen Garden Foundation © 2011
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