School Science Lessons
Understanding food plants
2012-05-12 SPwp
Please send comments to: J.Elfick@uq.edu.au
Table of Contents
5.0.0 Understanding food plants
Preface
5.19 Acid soils and alkaline soils
5.22 Control of pests and diseases
5.24 Crop management
6.46 Crop rotation
5.21 Diseases of plants
5.18 Fertilizing the soil
5.12 Flower and fruit formation
5.11 Flowers
5.13 Fruit, Kinds of fruit
9.9.18 Hydroponics, soil-less culture solutions
5.20 Insect pests of plants
5.07 Leaves
5.01 Plant body
5.25 Plant nutrients from plant ash
5.16 Planting material
5.02 Photosynthesis
5.03 Photosynthesis, sunlight is necessary for photosynthesis
5.04 Photosynthesis, oxygen gas is formed during photosynthesis
5.10 Root hairs
5.10.01 Root rhizosphere
5.09 Roots
5.14 Seed, Parts of a bean seed
5.15 Seed germination
5.08 Stems
5.17 Weed control
Preface
The aim of this set of agriculture teaching notes is to teach students how food plants work and the
reasons for the methods used to look after crops in the school gardens. These teaching notes will also
help students to understand the other agriculture teaching notes that deal with crop projects. Each lesson
has a set of Revision Questions at the end of the lesson. The suggested answers are in [square brackets].
5.01 Plant body
See diagram 9.53: Parts of a plant
You will need some small plants that have flowers, branches and roots. Bring them to the classroom.
Give one plant to each pair of students. Point to the parts of the plant body.
The plant body consists of two parts: The shoot above the ground, and the root below the ground.
The shoot is the stem with the leaves.
The stem is divided by thick parts called nodes.
Leaves grow from the nodes.
Roots may grow from nodes when the nodes touch the wet ground, e.g. sugar cane or para grass. Roots
grow at the root tips.
A bud is a baby shoot. The terminal bud is at the end of the shoot and either makes the shoot grow or
forms a flower. The axillary buds grow from the nodes. They grow in the angle (axil) between the leaf and
the stem. Axillary buds can grow to form lateral shoots called branches. The buds at the ends of the
branches can either make the branch grow or form a flower. Axillary buds make the plant grow short and
bushy or they can form flowers.
Leaves make food in the sunlight using air and water. The stem carries food and water between the leaves
and the roots. The stem holds up the leaves, and may store food, e.g. sugar cane.
Roots take in water and plant nutrients from the soil. Roots hold the plant in the ground and may store
food, e.g. sweet potato. Plant nutrients are chemicals the plants need.
Flowers can form fruits containing seed. The seeds can grow to form new plants.
5.01 Plant body, revision questions
What parts are found in the shoot? [Stem, leaves (and buds)]
What are the two parts of the plant body? [Root and shoot]
What is a bud? [A baby shoot]
Which bud makes a plant grow straight and tall? [The terminal bud]
Which buds make a plant grow bushy and short? [Axillary bud]
Which buds can produce flowers? [The terminal bud and axillary buds]
What are plant nutrients? [Chemicals in the soil that plants need]
What is the function of the stem? [It carries substances between leaves and roots, holds up leaves, and
may store food.]
What is the function of the roots? [Roots take in water and plant nutrients from soil,
old the plant in the ground, and may store food.]
What is the function of the flower? [To form a fruit containing seed that can grow to become a new plant
5.02 Photosynthesis, light is needed for photosynthesis
All food comes originally from green leaves in the sunlight.
A process is a change in something, or a way in which something is made. So you can say that
photosynthesis is the process by which green plants make food.
Show the students the prepared demonstrations.
Where does all your food come from? [Green plants in the sunlight.]
In sunlight green, plants use carbon dioxide gas from the air, water, and plant nutrients from the soil to
make food. This process is called photosynthesis. ("Photo" means light, "synthesis" means putting together).
Plant nutrients are chemicals in the soil that plants need, e.g. plant nutrients are needed to make the green
colour that absorbs the sunlight energy.
This energy remains stored in the food. The first food made in green plants is the simple sugar, glucose.
Photosynthesis equation
Carbon dioxide gas + water + sunlight energy --> simple sugar (food) + oxygen gas.
The oxygen gas produced by photosynthesis goes out into the air.
All your food comes originally from photosynthesis in a green plant because your food is either plants, or
animals that have eaten plants.
Animals breathe in oxygen gas and breathe out carbon dioxide gas that is a waste. Plants use the carbon
dioxide gas during photosynthesis and give out oxygen gas.
To make food crops grow well they need:
1. Sunlight, but baby plants need some shade,
2. Water, but soil must not be waterlogged (filled with water)
3. Plant nutrients, but they must be the kinds of chemicals the plants need,
4. Green leaves. The leaves must not be damaged by insects or disease, and they must not turn brown or
yellow.
5.03 Photosynthesis, sunlight is necessary for photosynthesis
See diagram 9.145.1: Silver paper on leaf
Three days before the lesson fold a piece of silver paper from a cigarette packet to form a band around
a leaf on a tree growing in the sunlight.
After three days, pick the leaf off the tree and remove the silver paper.
Drop the leaf in boiling water to kill it.
Put the leaf in methylated spirit to remove the green substance called chlorophyll.
Place the leaf in an iodine solution.
The part of the leaf not covered by the silver paper turns blue black colour.
The part of the leaf covered by the silver paper does not turn blue black because there was no sunlight for
photosynthesis to make starch. Students could form their own initials out of silver paper and "write" them
on a leaf.
Do the last part of the demonstration in front of the students.
The simple sugars produced by photosynthesis are soon changed to starch in the leaf.
When iodine solution is added to starch it changes from a white colour to a blue black colour.
A leaf with a silver paper band around it is left on a tree for three days. The leaf is then picked and tested
for starch.
The part of the leaf shaded by the Silver paper shows no starch present. The part of the leaf not shaded
shows starch.
Conclusion: Sunlight is necessary for photosynthesis.
5.04 Photosynthesis, oxygen gas is formed during photosynthesis
See diagram 9.145.1: Waterweed in the light and in the dark
Demonstration: Oxygen gas is given off during photosynthesis
Try out the demonstration a few days before the lesson.
The demonstration works best if you can start it the day before the lesson.
Or
Start the demonstration at the beginning of the lesson and allow students to see it again in the next lesson.
A suitable waterweed is Elodea.
The demonstration works better if you add some sodium bicarbonate (baking soda) to the water.
Tests for oxygen gas
Light a thin piece of wood then blow out the flame leaving the wood glowing red. If you put this into
oxygen, the glowing wood will burst into flame.
Show the students the demonstration.
Put green waterweed in a test-tube and invert it under water. The test-tube contains no air or bubbles in
it. This is left in the sunlight. Another piece of waterweed is placed under a similar test-tube. This is left
in the dark. After some hours look at the waterweeds again. The waterweed in the light has bubbles of
oxygen gas coming from it. No bubbles of oxygen gas come from the waterweed in the dark. The gas
was oxygen because it made a glowing piece of wood burst into flames.
Conclusion: During photosynthesis oxygen gas is given off.
9.0.4 Photosynthesis, revision questions
What does the word "photosynthesis" mean? ["Putting together using light energy"]
What are the four things green plants need to make food? [Sunlight, carbon dioxide gas, water plant
nutrients]
What are two kinds of food made by green plants? [Simple sugars, starch]
Which gases do green leaves give out? [Oxygen gas]
What are two ways in which plants help us? [They make food that you eat. They make oxygen gas that
you breathe.]
Write the photosynthesis equation: [light energy + carbon dioxide gas + water --> simple sugar +
oxygen gas]
5.07 Leaves
See diagram 9.66.2: Shapes of leaves
Collect samples of leaves with different shapes. Some leaves should be the leaves of your crops.
Show the students the leaves you have collected.
Leaves are usually flat and thin so that they can catch plenty of sunlight and have little holes in the lower
side to let gases and water vapour move in and out.
The bushy plant the leaf has three parts: 1. leaf, 2. petiole, and 3. leaf blade.
The grass leaf has three parts: 1. leaf blade, 2. leaf sheath, and 3. ligule.
Draw the leaves of the different crop plants you are growing and describe their shape.
5.07 Leaves, revision questions
Part 1 Draw Diagram: Leaf of a bushy plant and grass leaf on the chalkboard.
Write the correct labels.
Part 2
Draw the different shapes of leaves on the chalkboard and name them
Part 3
Why are most leaves flat and thin? [To catch plenty of sunlight]
What are the parts and functions of the leaf of a bushy plant? [The leaf base attaches the leaf to the stem.
The petiole turns and holds up the leaf blade like a handle. The leaf blade takes in sunlight to make food.]
What are the parts of a grass leaf? [Leaf blade, leaf sheath and ligule]
What is the difference between veins in a bushy plant leaf and veins in a grass leaf? [Veins are arranged in
a network in a bushy plant leaf. Veins are arranged in parallel in a grass leaf.]
What is the difference between a leaf and a leaflet? [A leaf is attached to the stem, and in the angle
between the leaf and stem is the axillary bud. A leaflet is part of a leaf. There is no axillary bud between
the leaflet and the stem.]
5.08 Stems
See diagram 9.57.2: Section of cut wood | See diagram 9.57.1: Wood sections
The day before the lesson, stand a young plant in a jar containing enough red or blue ink to cover the
roots. Find a stump of a tree or coconut that has been cut down recently. Get a piece of sugar cane, a yam,
a taro corm, a bit of banana corm and a ginger or tumeric rhizome.
You will need a razor blade for this lesson.
Show the students the young plant standing with roots in red ink. Take the plant out and wash it. Then cut
the stem and open it with a razor blade to show the ink inside. What does this prove? [Liquids move up
the stem.]
Show the students the cut stump of a tree. What is the function of the wood? [To let the plant become
large and strong]
Eat some sugar cane. What does this show? [Stems can be used to store food.]
Show the students the yam, taro and bit of banana corm.
Stems have three functions: transport of food from leaves to roots,
transport of water and plant nutrients from roots to leaves,
support of leaves and branches. In a young stem only a few leaves can be supported. In a large tree many
leaves and branches can be supported,
food storage, e.g. sugar cane.
5.08 Stems, revision questions
What are the three functions of stems? [Transport of food, water and plant nutrients between the leaves
and the roots, support of the leaves and branches, storage of food ]
What is the function of bark? [It protects the stem.]
Fill in the table in columns 2 and 3:
Stem description Name of stem Example.
Swollen stem base [Corm] [Banana or taro]
Swollen horizontal underground stem [Rhizome] [Ginger or tumeric]
Swollen rounded underground stem [Stem tuber] [Yam]
5.09 Roots
See diagram 9.73.1: Two main kinds of roots
Before the lesson, dig up a small bushy plant such as a mung bean and a grass such as para grass. Wash
the soil off the roots.
Show the students the bushy plant roots and grass roots.
Bushy plants have a main root called a tap root (or a primary root) and smaller lateral roots. These roots
grow very deep.
Grasses and palms have no main root, only many fibrous roots. These are thin roots and do not grow
deep.
5.09 Roots, revision questions
Draw a plant with tap root and lateral roots, and give one example. e.g. bean plant.
Draw a plant with fibrous roots, and give an example, e.g. para grass.
Complete columns 2 and 3 of the table:

Kind of Root 		Function of this kind of root? Example of this kind of root
Tuberous root [Stores food ] [Sweet potato]
Aerial root [Breathes above water] [Mangrove]
Prop roots [Holds up stem] [Maize (Corn)]
Swollen tap root [Stores food] [Radish]
5.10 Root Hairs
See diagram 9.73.2: Root hairs 1 | See diagram 9.73.3: Root hairs 2 | See diagram 9.75: Root hairs 3
Remind the students of the need to transplant carefully so as not to damage the roots or root hairs
Five days before the lesson put some bean seeds on wet paper or cotton wool on a plate. Cover with a
saucer or another plate to keep the seeds damp. After germination you will see small root hairs growing
from the side of the root. Use seed packaged in silver packets because they will be protected from attack
by fungus.
Most plants take water and plant nutrients into their roots through tiny root hairs (but coconuts do not
have root hairs). The root hairs are very small, have thin walls and are easily damaged.
If plants do not get enough water the leaves will wilt then dry up and later the plant will die.
If you damage the root or root hairs during transplanting: insects or disease damage the root,
e.g. bacterial wilt disease of tomatoes,
there is not enough water in the soil,
the soil water is salty.
If there is too much water in the soil you say that the soil is waterlogged.
Roots will die in waterlogged soil because they need some air to breathe.
Soils should be well drained so that there is some air in them to give oxygen to the root hairs.
5.10.01 Root rhizosphere
The rhizosphere is the region just around the root hairs and fine roots that is influenced by root hair
secretions and the local micro-organisms. In the rhizosphere the following processes may occur:
1. Excretion of H+ that exchanges for nutrient Mg2+, Ca2+, NH4+, K+.
2. Release of carbon dioxide from root hair cells by respiration.
3. Oxidation of nitrogen and sulfur to nitrate and sulfate ions.
4. Excretion of organic acids by the root hairs. The organic acids may complex metals and increase the
mobility of Al3+ and Fe3+.
5. Depletion of oxygen gas around the root hairs to reduce the redox potential and so favouring Fe2+
over Fe3+.
6. Removal of water by root uptake.
7. Change in soil permeability.
5.10 Root hairs, revision questions
What is the function of root hairs? [They take up water and plant nutrients.]
Why are root hairs so easily damaged when you transplant? [Because they are very small and have thin
walls]
What are the three causes of wilting? [Damage of roots by man, e.g. when transplanting, damage of roots
by insects or disease. not enough water in soil or salty water]
Why do plants die in waterlogged soil? [Because the roots cannot get air to breathe]
Where do the water and plant nutrients go, after they are taken in by the root hairs?
[Root hairs --> roots --> stem --> leaves]
5.11 Flowers
See diagram 9.98.7: Flower parts and flowers
Before the lesson pick enough large flowers so there will be one flower for each pair of students.
Flowers to use:
Poinciana (Delonix) flower
This large tree is a legume and forms large pods. It has five sepals that are thick and green on the outside
and red on the inside, five separate petals (The upper one is more showy.) 10 stamens, and an ovary like
a bean pod.
Hibiscus flower
This large red flower has five sepals, five red petals, many stamens attached to a red tube separate from
the style that is inside it, five stigmas and so five female parts. The flower does not usually produce seed.
Sweet Potato flower (Ipomoea)
It has five sepals joined, five petals joined to form a purple trumpet, five stamens attached to the petals,
and female part. It can form little black seeds.
Other suitable flowers are tomato, chilli, wing beans, peanut.
Give the flowers to the students. What are the shapes, colour and smells of the flowers? Shake some
pollen out of the flowers.
Most flowers contain male parts and female parts. Insects are attracted to flowers by their shape, colour
and smell. They fly from flower to flower carrying pollen on their bodies and legs. Pollen comes from the
male parts. Pollen makes the female part form a fruit and seeds. This is called fertilization.
Hold the flower by the stalk and touch the following parts:
Receptacle
Where the stalk gets wider at the base of the flower. It is a platform for the other flower parts.
Sepals
Like little green leaves on the outside of the flower. They project the young flower in a flower bud.
Petals
Coloured parts inside the sepals. They attract insects.
Stamens
The male parts. Each stamen consists of a little stick called the filament that holds up a bag called the
anther. The anthers produce pollen. Can you see the pollen coming out of the anthers?
Stigma
The top of the female part in the middle of the flower. It is sticky and can catch the pollen.
Style
The long tube of the female part below the stigma. Pollen can grow down inside the tube to fertilize the
ovary.
Ovary
The swollen base of the female part that contains ovules. After fertilization, the ovary will form a fruit and
the ovules will form seeds.
Count the number of sepals, petals and stamens. Then they should pull off some sepals and petals, and
open the ovary to see the ovules.
5.11 Functions of the parts of flowers
See diagram 9.98: Dicotyledon half flower | See diagram 54.9.2: Chilli flower (dicotyledon)
See diagram 9.98.1: Lily flower (monocotyledon)
For this lesson you will need chilli or tomato flowers, one for two students, and razor blades.
You will also need a chilli or tomato fruit.
Before the lesson, practice cutting a half flower. To do this hold the flower upside down and cut down
the middle of the stalk between two of the sepals and cut the flower in half. Now check that the stigma,
style and ovary are cut exactly in half. You will need a magnifying glass to see all the ovules in the ovary.
Can you count them?
Give a flower to each pair of students. Name each part of the flower as you point to them. Show the
students how to cut a half flower.
Touch each part of the half flower.
Students should also touch each part of their half flower and tell you is name and function:
6. Stalk
5. Receptacle: platform for other parts
1. Sepals: protect flower bud
2. Petals: attract insects
4a. Stigma: female part, pollen sticks to it
4b. Style: pollen grows down it
4c. Ovary: pollen fertilizes it to form a fruit
7. Ovule: pollen fertilizes it to form seed
3b. Filament: part of male stamen, holds up anther part
3a. Anther: part of male stamen, produces pollen
8. Pollen: can fertilize the ovary and ovules to produce fruit and seed.
After fertilization, sepals, petals, stamens, stigma, and style usually die and fall off.
5.11 Flowers, revision questions
Part 1. Show the students a flower, e.g. wing bean. Point to the different parts and then write down the
name of each part. [Flower stalk, stigma, receptacle, style, sepal, ovary, petal, ovules, stamens, pollen]
Which part produces pollen? [Stamens]
Which part will become a fruit (if fertilized)? [Ovary]
What is the name of the base of the flower? [Receptacle]
What protects the bud? [Sepals]
What attracts insects? [ Petals]
What does pollen grow down? [Style]
What fertilizes the female part? [Pollen]
What will become a seed? [Ovule]
What is like a stem? [Stalk]
What does pollen stick to? [Stigma]
5.12 Flower and fruit formation
See diagram 9.4.2: Tomato flower and fruit formation
Collect some green tomatoes and tomato flowers, one of each for each group of four students.
You could also use chilli or eggplant (aubergine) fruit and flowers.
You will need some razor blades and a magnifying glass.
Examine the flowers and fruit, cut them down the middle with razor blades. BE CAREFUL! Show them
the ovary (fruit) and ovules (seed). The remains of the sepals, petals, stamens, stigma, style, may be seen
if they have not dropped off.
After pollen sticks to the stigma and grows down the style into the ovary, the ovary swells to form a fruit.
The ovules in the ovary become seeds.
Pollination is when the pollen touches the stigma and sticks to it. Big flowers are pollinated by insects.
Small flowers such as grass are pollinated by the wind.
Fertilization is when the male pollen reaches the ovary and ovules. The pollen grows down the style to the
ovary.
Tell the students to draw their cut tomato flowers and fruit
After fertilization:
The flower stalk becomes the fruit stalk.
The sepals, petals, stamens, stigma and style usually die.
The ovary becomes the fruit.
The ovules become seed.
5.12 Fruit, revision questions
What is pollination? [When the pollen touches the stigma and sticks to it.]
Where does the pollen go after pollination? [It grows down the style into the ovary.]
What do the pollen grains do in the ovary? [They fertilize the ovules.]
What happens after fertilization? [The ovules form seeds and the ovary forms a fruit.]
What happens if flowers are not pollinated? [The flowers do not set fruit and then die.]
What happens if insects do not come to the flowers of some plants? [The flowers will not be pollinated
and the plants will not form fruit or seed.]
Papaya can be hand pollinated. How do you do this? [Dust the pollen from the male flowers on the
stigmas of the female flowers.]
Which flowers are wind pollinated? [Grass flowers.]
What happens to sepals, petals, stamens, stigma, style, after fertilization? [They usually die and drop off.]
What is the difference between pollination and fertilization? [Pollination is the carrying of pollen to the
stigma. Fertilization is pollen growing down the style to the ovules in the ovary, the pollen fertilizes the
ovules that then become seed.]
5.13 Kinds of fruit
See diagram 9.100.3: Succulent fruit | See diagram 9.100.4: Berry, drupe legume
See diagram 9.100.5: Pome, apple | See diagram 53.7: Coconut fruit |
See diagram 58.1: Papaya fruit
Collect examples of the eight different kinds of fruit. When preparing the lesson, use only examples of
fruit that the students have seen.
Show the students the different examples of fruits you have collected.
A fruit is a swollen ovary with one or more seeds inside.
Kinds of fruit:
Grain
A single seed is tightly closed in the ovary wall. e.g. corn (maize) or wheat.
Capsule
A dry fruit that opens to let the seeds out, e.g. cassava or sweet potato or okra.
Pod
A dry fruit that opens on two sides to let the seeds out. e.g. legume (cowpea, wing bean, peanut)
Berry
Fruit wall in two layers - outer layer is a tough skin, inner layer is thick and juicy, e.g. tomato, chilli,
papaya, guava, banana.
Citrus fruit
Like a berry with thick peel and oil glands, e.g. orange, lime, pomelo.
Gourd
Like a berry with a hard outer skin, e.g. pumpkin, melon, snake gourd.
Drupe
Fruit walls in three layers: outer thin, middle thick, inner very hard - called a shell or stone,
e.g. coconut, mango, coffee.
Multiple fruit
Many fruits stuck together to form one fruit, e.g. pineapple, breadfruit.
5.13 Kinds of fruit, revision questions
Part 1 Names of different kinds of fruits Like a berry with thick peel. [Citrus fruit]
Many fruits together to form one. [Multiple fruit]
Dry fruit opens to let seeds out. [Capsule]
Single seed tightly held in ovary wall. [Grain]
Like a berry with hard outer skin [Gourd]
Wall in three layers. [Drupe]
Dry fruit opens on two sides. [Pod]
Fruit wall in two layers - thin outer layer, thick inner layer [Berry]
Part 2 Examples of different kinds of fruits Citrus fruit [Orange]
Pod (give two examples) [Wing bean, cowpea]
Gourd (give two examples) [Melon, pumpkin]
Grain [Corn]
Drupe [Coconut]
Multiple Fruit [Pineapple]
Capsule (give two examples) [Sweet Potato, cassava]
Berry (give two examples) [Papaya, chilli]
5.14 Parts of a bean seed
See diagram 9.113: Soaked bean seed
Two days before the lesson put some bean seeds in water. Use enough seed for each student.
Also, have one dry bean seed ready for each student.
Leave the bean seeds in water in the classroom so the students can observe the seed coats swell and
become wrinkled, then later become smooth when the insides of the seed swells.
Give each student a dry bean seed.
See: the hard shiny seed coat,
the scar where the seed was originally attached to the fruit,
the tiny hole (micropyle) at the end of the scar to let water and air in. The baby root (radicle) grows out
through this hole.
Give each student a bean seed soaked in water. The soaked seed is larger and softer because water has
gone through the tiny hole. When the water goes into the seed coats, they are first wrinkled and later
smooth when the whole seed swells.
Cut open the seed coat with a finger nail or razor blade and see the baby plant inside. The two main parts
in halves are the cotyledons swollen with stored food. Between the cotyledons is the baby shoot (plumule)
and the baby root (radicle).
Tell the students to draw their seeds or draw Diagram 16
Seed coats (outside)
scar
hole (micropyle)
Baby plant (embryo)
Baby plant shoot (plumule)
Baby plant root (radicle)
Seed leaves (2 cotyledons) (store of food)
5.14 Seeds, revision questions
What are the two things you can see when you look at the seed coat? [Scar and hole called micropyle]
What are their functions? [The scar was where the seed was attached to the ovary wall. The micropyle
lets water and air in, and the radicle (first root) grows out through the hole.]
What are the three parts of a newly formed seed? [Baby plant (embryo) seed coats, food store in the
cotyledons ]
What are the three parts of a baby plant? [The shoot called the plumule, the seed leaves called the
cotyledons, the root called the radicle]
When you put a bean seed in water why does the seed coat first become wrinkled? [Because water
soaks into it and makes it swell.]
Why does the seed coat later become smooth? [Because water then soaks into the inside of the seed,
which swells and pushes out the wrinkles.]
5.15 Seed germination
See diagram 9.113.2d: Bean seeds germinating
Two weeks before this plant 5-10 bean seeds at different levels as the diagram. This can be done behind a
piece of glass or in a glass aquarium, but keep the seeds in the dark.
Before the lesson, dig up germinated bean seeds at different stages of germination. In the type of
germination in the diagram, called epigeal germination, the cotyledons come out of the ground. In the
other type of germination, hypogeal germination, the cotyledons remain below the ground.
Show the students the seeds at different stages of germination.
Seeds are alive but they breathe very slowly. When a seed is placed in the damp ground and water enters
the hole the seed becomes very active, breathes more quickly and a new plant starts to grow. This is
called germination. The baby plant uses the stored food in the seed until it can produce green leaves and
make its own food.
Stages in germination of a bean seed: The seed swells and the radicle (baby plant root) grows out the
hole in the seed coat.
The stem below the cotyledons grows into a loop.
This stem straightens and the cotyledons and the first leaves are pushed up out of the ground.
Conditions for good germination are as follows:
1. Water
2. Air to let the baby plant breathe
3. Healthy seed
4. Correct depth of planting
5. Correct variety kind of seed for tropical countries
The problem of poor germination is usually caused by sowing seeds too deeply, or over-watering or
under-watering.
If the seed is planted too deeply, the tiny developing plant exhausts the food supply in the endosperm
long before the shoot and leaves have broken through the soft surface and it dies. The general rule is to
plant a seed at a depth that equals twice its width. Water newly-sown seeds with a mixture of one fifth of
a teaspoon of Epsom salts in a litre of water to aid the germination process. The magnesium in the salts
will help stimulate the enzymes that make the food in the endosperm more readily available to the young
seedling.
Small seeds should be only lightly covered with soil, but larger seeds will be planted at a greater depth.
The surface of the soil dries out more quickly than it does a few millimetres deeper. So small seeds that
are surface-sown should be kept moist while the larger seeds should be given a good soaking at planting
time and then watered again only after they break through the ground. If you water the larger seeds too
much, they will rot in the moist soil. The seed of many vegetables and flowering annuals are F1 hybrids,
where the plant breeders have combined two different strains each with desirable characteristics.
However, F1 hybrids usually do not set seed or the seeds are not viable. To select tomato seed for
planting, squeeze the pulp and the seeds on absorbent paper, e.g. a paper towel, and spread it evenly
over the paper. Remove as much pulp as possible and lay the remainder in the sun for two weeks to dry
the germination inhibiting enzymes. Lay the paper towel with seeds uppermost on seed-raising mix and
lightly cover with fine seed mix. Water lightly until the seedlings emerge.
5.15 Germination of seeds, revision questions
Are seeds alive? Do they breathe? [Yes, they are alive. They breathe very slowly.]
What are the four things that happen during germination? [1. Water enters the seed. 2. The seed
becomes active. 3. The seed breathes more quickly. 4. A new plant starts to grow.]
What are the three stages of germination? [1. The seed swells and the radicle grows out. 2. The stem
grows into a loop the stem straightens. 3. The cotyledons and leaves are pushed up out of the ground.]
5.16 Planting material
See diagram 9.81: Bulb | See diagram 9.82: Corm | See diagram 9.83: Rhizome
See diagram 9.93.1: Stem cuttings | See diagram 9.93.2: Cuttings
Collect examples of the different kinds of planting material to show the students in the classroom.
Find pictures of planting material mentioned in the lesson from your agricultural reference library.
Show the students the different kinds of planting material and the pictures from the agricultural reference
library.
Planting material means seeds or parts of plants that can grow to become a new plant. The parts of plants
may be pieces of stems or special kinds of stems.
Seeds are produced in the ovary of a flower after pollen from the stamen (father) fertilizes the ovary
(mother). So seeds are like the children of the stamen and the ovary. The pollen that fertilizes the ovary
may come from the same flower as the ovary (self-pollination) or come from a different flower on a
different plant (cross-pollination). The seeds (children) will have a mixture of the different characters of the
parent plants. So the seeds will be like brothers and sisters. When they grow, they will all look like the
parents and like each other but none will look the same. Plants that grow from parts of plants will grow to
look the same as the parent plant.
Parts of plants used for planting material:
Stem cutting
When the nodes of some plant stems touch damp soil, they will produce roots. If these stems are cut
between the nodes then each node can form roots and shoot and grow into a new plant, e.g. sweet
potato or cassava. Stems or stem tips are cut and planted so that some nodes are under the ground
and some are above the ground. The nodes under the ground produce roots and the nodes above the
ground produce leaves.
Corm
A corm is a swollen stem base, e.g. taro and banana. The corm will have small buds growing from the
nodes. The corm can be cut into "bits" which are pieces of corm with buds. Each "bit" can be planted to
grow into a new plant.
Stem tuber
A stem tuber is a swollen stem or branch that grows in the ground, e.g. yams. Each "eye" in the tuber is
really a very small leaf and axillary bud. The tubers can be cut into pieces, each containing an eye, and
planted. Each piece will grow into a new plant.
Root tuber
Root tubers cannot be used for planting material because there are no nodes or axillary buds in roots.
However, at the neck of the tuber where the root becomes stem axillary buds can sprout, be separated,
and then used as planting material, e.g. sweet potato or cassava.
Bulb
A bulb is a shoot with fleshy leaves that store food, e.g. spring onion. Some plants can produce little
daughter bulbs that can be separated and planted to grow into big plants.
Shoot
Some underground stems produce shoots from their axillary buds which can be separated by cutting, then
planted out, e.g. sword suckers of banana corms, tops and slips of pineapples.
Rhizome
A rhizome is a shoot which grows horizontally underground. It can be easily broken into pieces which
each piece containing axillary buds. The axillary buds can form new roots and shoots, e.g. ginger or
tumeric, and many grasses.
Aerial sucker
Aerial suckers are branches which grow in the air than touch the ground. Where a node touches the
ground, a new plant can grow, e.g. pineapple aerial suckers, taro suckers, yam bulbils.
5.16 Planting material, revision questions
Types of planting material. Stem cuttings [Sweet potato]
Stem cuttings [Cassava]
Corm [Banana, Taro]
Stem tuber [Yam ]
Root tuber [Sweet potato]
Bulb [Spring onion]
Shoots [Banana sword suckers]
Shoots [Pineapple tops]
Rhizome [Ginger or turmeric]
Aerial suckers [Pineapple, taro suckers or yam bulbuls]
Which young plants are more like their parent plants? Plants grown from seed or those grown from parts
of plants? [Those grown from parts of plants.]
What is a "bit"? [A piece of banana corm with a bud in it.]
What is an "eye"? A very small leaf and axillary bud in a tuber.
5.17 Weed control
Make a collection of weeds such as nut grass, sensitive plant, purslane, guava, rattle pod, pig "grass",
lantana, broomweed, water hyacinth.
Show the students examples of weeds you have collected. How do weeds harm the crops?
Weeds are plants that lower the yield of garden plants. Weeds make the garden plants grow less and
have smaller fruit.
Weeds harm crops by: taking in water from the soil, then the garden plants do not have enough water,
taking plant nutrients from the soil, then the garden plants do not have enough nutrients,
shading the young crop plants then the garden plants do not have enough sunlight for photosynthesis,
having pests and diseases on them, which can later infect the garden plants,
mixing with the crop and making it hard to harvest, e.g. sensitive plant growing among sweet potato.
Weeds live in vegetable gardens, plantations or pastures. Weeds may be of three types: Very strong
plants that are no use to us, e.g. lantana, common guava,
Plants that are weeds only when they grow in crops. However, they may also be used by themselves,
e.g. purslane is a common garden weed but it can be eaten as a green vegetable.
Plants from old crops are weeds because they may have pests and diseases on them, e.g. old maize
(corn) plants left over from a previous crop may carry disease.
Weeds can be controlled by the following:
1. Pulling out by hand
2. Hoeing
3. Cutting down with a bush knife
4. Shading by leaves of plants, e.g. sweet potato (kumara) or cover crops, e.g. Pueraria
5. Spraying with chemicals called herbicides, e.g. "Paraquat" for broadleaf weeds, "Dalapon"
("Dowpon") for grass weeds. Herbicides are dangerous, expensive and hard to use properly.
5.17 Weeds and their control, revision questions
What are five ways that weeds lower the yield of garden plants? [Take in their water, take in their plant
nutrients, shade them, have pests and diseases that may infect garden plants, mix with the crop]
What are five ways of controlling weeds? [Pulling out by hand, hoeing, cutting down, shading, spraying
with herbicides]
5.18 Fertilizing the soil
See diagram 6.0.0: Digging the soil | See 9.14.0 Composting | See 12.14.5: Superphosphate
Note that there are three methods of fertilizing the soil but the word "fertilizer" usually refers to artificial
fertilizer.
Be ready to show the students a bag of fertilizer, e.g. Muriate of potash that contains potash or sulfate of
potash, which contains potash and sulfur. "Potash" is an old name for potassium oxide.
Collect same well rotted compost in a jar to show the students.
Show the students the well rotted compost in a glass jar and the fertilizer bag. Read the words on the bag.
There are three ways in which a deficiency of plant nutrients can happen:
1. There is a natural deficiency because there was not much of the plant nutrient in the original rock from
which the soil was made, e.g. soils made from coral rock are deficient in many plant nutrients.
2. The plant nutrients have been taken out of the soil by crops. When a crop is harvested, some plant
nutrients are lost.
3. The plant nutrients have been washed out by water.
There are two ways of increasing plant nutrients in the soil:
1. Stop farming the land for some time. Then plant nutrients will slowly be added to the soil from soil
particles and rotten plants. This is called fallow.
2. Add fertilizer to the soil.
There are four methods of fertilizing:
1. Dig compost into the soil. Compost is made from plants, manure, and food scraps kept in a heap and
allowed to go rotten before being put in the soil.
2. Grow green manure. Legume crops such as cowpea have little white lumps on their roots that add
nitrogen to the soil. If you dig a legume crop into the ground, it is called green manure.
3. Add liquid manure. Fresh (or fowl) manure can damage young vegetables. Put the manure in a 44
gallon drum and cover with water. After one week, use this manure water on the plants.
4. Add Artificial fertilizer such as muriate of potash contains potash. sulfate of potash contains potash and
sulfur. These fertilizers are made in factories. Other artificial fertilizers are superphosphate that contains
phosphorus and urea that contains nitrogen.
5.18 Fertilizing the soil, revision questions
What are three reasons why soils can be deficient in plant nutrients? [The original rocks were deficient in
the plant nutrients. Crops have taken the plant nutrients away. Water has washed the plant nutrients
away.]
What are two ways of adding plant nutrients to the soil? [Stop farming the land for some time (fallow).
Fertilize the land.]
What are four methods of fertilizing? [Compost, liquid manure, green manure, artificial fertilizer]
Name one artificial fertilizer. [Muriate of potash or urea or superphosphate or sulfate of potash or NPK]
5.19 Acid soils and alkaline soils
See diagram 6.65.3: Nitrogen cycle
Soil pH is a measure of the acidity of the soil, on a scale from 1 to 14, the pH scale. A neutral substance
such as pure water has a value of 7. Strong alkaline solutions have a pH near 14. Strong acids,
e.g. hydrochloric and sulfuric acid, have a pH value close to 1.
Most soils have a pH range of 4.0 to 9.5. Most plants prefer a pH of between 5.5 and 7.5 and it is
desirable to keep the soil's pH in this range.
Incorrect soil pH affects the availability of many plant nutrients and so affects plant growth. For example,
phosphorus is an important nutrient for plant growth but if the pH drops below 5, the availability of
phosphorus sharply decreases because the phosphorus in the soil, present as phosphates, forms insoluble
compounds at low pH, stopping them from dissolving in water, so plants cannot absorb the phosphorus.
At low pH values many soil bacteria will not survive so changing the turnover rate in nutrient cycles,
including the important nitrogen cycle. The soil pH requirements of plants vary. Some plants grow well
under acidic conditions, while others grow best in a more alkaline soil; e.g. potatoes and watermelons
like slightly acidic soils, while apples and lucerne grow well in slightly alkaline soils.
Soil acidity increases through the removal of some basic nutrients from the soil, e.g. calcium leaching and
the excessive use of fertilizers, particularly nitrogen fertilizers. To correct low soil pH, use application of
lime (calcium compounds) or dolomite (calcium and magnesium compounds). Use 1-5 tonnes / ha of lime
to increase the pH of the topsoil to pH 6.5 but it can take several years to take effect.
You can improve the fertility of your garden soils by treating them so that they are not too acid or too
alkaline.
The pH scale measures whether substances are acid or alkaline. pH 1 very strong acid that can burn you,
e.g. battery acid
pH 6 weakly acid, e.g. soda water
pH 7 neutral, neither acid nor alkaline, e.g. water
pH 8 weakly alkaline, e.g. soap
pH 14 very strong alkali that can also burn you.
Acids have a sharp raw taste, e.g. unripe oranges or bush limes.
Alkalis have a slippery feel, e.g. soap, saliva.
Plants can absorb plant nutrients best when pH is 6 to 7. In soils formed from coral rock the pH will be
too high. In swampy land the pH will be too low.
To lower the pH add rotten compost. To raise the pH add lime.
In this lesson show how to use a "Soil pH Test Kit".
You will need: Coral soil or coral sand, well drained dark soil, swamp soil
Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline. Soils that are not
well-drained are too acid. Soils made from coral rocks are too alkaline.
You can test the soils using a colour test. If the colour of soil in the test turns yellow orange, the soil is
too acid.
If colour if soil in the test turns blue purple, the soil is too alkaline.
If colour if soil in the test turns dark green, the soil is not too acid nor too alkaline.
Collect just enough soil from just under the surface of the soil to cover your little finger nail, and place on
a white plate.
Shake two drops of the indicator on the soil and mix to a paste with the stick.
Sprinkle some special white power on the paste.
Wait a few minutes then match the colour of the powder with the colour chart. Do this for swampy soil,
coral soil, dark well drained soil.
Acids have a sharp sour taste and can dissolve substances, e.g. in a car battery.
Alkalis have a slippery feel and can dissolve substances, e.g. soap.
Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline. Soils in swampy
ground are too acid for most plants. Soils made from coral sand are too alkaline for most plants.
Make soil less acid by adding burnt shells hammered to a powder, and by draining the soil. Make soil less
alkaline by adding rotten plants from a compost heap.
Good soil is dark in colour from the rotten plants and is well drained. To make sure that the soil is not too
acid and not too alkaline the agriculture field officer can do a soil test.
5.19 Acid soils and alkaline soils, revision questions Why should soils not be too acid or too alkaline?
[So the plants can absorb plant nutrients from the soil.]
What sort of soil is too acid for most plants? [Soils in swampy ground and soils that are not well drained.]
What sort of soil is too alkaline for most plants? [Soils formed from coral rock or coral sand.]
How do you make soils less acid? [Add powdered burnt shells and drain the soil.]
How do you make soils less alkaline? [Add rotten plants from a compost heap.]
Who can do a test to tell us whether soil is too acid or too alkaline? [An agriculture field officer]
What arc two things you notice about a good soil? [It is dark in colour from rotten plants.
It is well-drained, not swampy.]
5.20 Insect pests of plants
See diagram 9.303: Insect pests
Before starting to teach this and the next lesson, make a collection of pests and diseases from
your garden. They can usually be preserved in methylated spirits.
Also, collect diagrams of different pests. make sure that your garden is a good example of pest and
disease control.
Check your spray equipment and pesticides.
A field visit may be useful for this lesson.
Show the students the examples of pests and diseases that you have collected from the garden.
To obtain the greatest possible production and cash income from your crops you must control loss by
pests and diseases.
Pests are animals that eat your crops or infect your crop plants with diseases. Pests may live in the
ground, be attached to infected plants or may fly or walk to your crop.
Diseases are any change in the plant such as spots or holes in the leaves, wilting of the plant, rotting of the
living plant. They are caused by living things that may be so small that they cannot usually be seen by eye.
Diseases are carried by wind and water, infected plants, and pests.
The main pests of crops:
1. Nematode worms or eel worms
They are small worms pointed at both ends and have S-shapes. They attack plant roots and may live in
lumps on the roots. They attack papayas, tomatoes, and bananas. Killing nematodes or eel worms in the
soil is very difficult.
2. Snails and slugs
Slugs are like snails with no shell. They eat plant leaves and stems. They can be controlled by picking off
by students or by spraying with "Cuprox".
3. Spiders
Tiny red spiders called mites attack the leaves of beans and eggplant. A chemical that kills them is called
an acaricide, e.g. wettable sulfur.
4. Flies
Small white worms called larvae eat flowers and leaves, e.g. bean fly larvae.
5. Caterpillars.
Some butterflies lay their eggs on the leaves and later caterpillars hatch out and eat the leaves. They attack
cabbages, tomatoes and beans.
6. Mole crickets and grasshoppers
They attack Chinese cabbage and lettuce.
7. Beetles
Beetles have hard shiny outer wings, e.g. Pumpkin beetle on pumpkins and sweet potato, Rhinoceros
beetle on coconut palms.
8. Bugs
Bugs have soft wings, e.g. vegetable bugs.
Chemicals used to kill insects are called insecticides, e.g. Carbaryl
9. Rats
They eat root crops and fruit. They can be controlled by destroying their nests and using poisonous bait,
e.g. Warfarin.
5.20 Insect pests of plants, revision questions
What are the seven main kinds of pests of crops? [Nematode worms or eel worms, snails or slugs,
spiders or mite, flies (larvae) caterpillars, bugs or beetles or aphids, mole crickets or grasshoppers, rats]
5.21 Diseases of plants
See diagram 6.0.1: Sterilizing soil |9.0.0 Fungi |3.44 Bacteria | 9.213 Viruses
Before the lesson walk out in the garden and look for examples of infected plants to show the students.
Show the students different examples of plants infected by diseases.
When a disease attacks a plant you say the plant is infected with the disease.
The three main causes of disease in crops:
1. Fungus diseases
A fungus is like a plant with a body of threads, is not green, and can attack and digest green plants.
Many fungi live in the damp soil.
1.1 Damping off fungus disease
This fungus attacks germinating seeds and makes all the seedlings fall over and die. It can be prevented by
heating the soil to kill all the fungi before planting seeds. This is called soil sterilization.
1.2 Collar rot fungus disease
It attacks the stem near the ground. Leaf and stem fungus diseases. There are many of these diseases.
They appear as yellow or brown spots that may later join, then the leaf or stem dies.
1.3 Powdery mildew
It looks like white threads on leaves or stems of pumpkins and cucumbers.
Chemicals that kill fungi are called fungicides, e.g. "Benomyl", "Captan".
2. Bacterial diseases
Bacteria are tiny living things that are much too small to be seen. They can attack plants and cause them to
rot. They live in damp places. They cause many diseases, e.g. wilting of tomato plants, rotting of the heart
of lettuce. These diseases cannot usually be cured, but they can be prevented by soaking seeds in very
hot water for a short time.
3. Virus diseases
Viruses are even smaller than bacteria and these diseases cannot be cured. Viruses may cause yellow
patches on leaves and later the plant dies, e.g. in taro and bananas. These diseases cannot be cured but if
the virus is carried by insects they may be controlled. The best way to deal with bacterial or virus diseases
is to burn the infected plant or part of the plant.
4. Diseases are carried to plants in four ways:
4.1 Wind can carry diseases a short distance.
4.2 Disease can be carried in running water or in the splash of rain drops.
4.3 Infected plants can pass disease to healthy plants if the plants are touching.
4.4 Disease can be carried by insect pests that can then infect healthy plan
Diseases of plants, revision questions
1. Give an example of a virus disease. [Yellow patches on taro or banana leaves]
2. What are the four ways in which diseases are carried to plants? [By wind, by water or raindrop splash,
by infected plants or soil, by insect pests]
3. What does "infected" mean? [It means that a disease is attacking a plant.]
5.0.21 Diseases of plants, revision questions
What are the three causes of disease in crops? 1. Fungus (plural: fungi), 2. Bacterium (plural: bacteria),
3. Virus (plural: viruses)]
What is the name of a chemical that kills a fungus? [Fungicide, e.g. benomyl]
How can some bacterial diseases be prevented? [By soaking seeds in hot water]
How can bacterial or virus diseases be cured? [Usually they cannot be cured]
How can bacterial or virus diseases be controlled? [By burning the infected plant]
Give an example of a fungus disease. [collar rot or stem disease or powdery mildew bacterial disease
wilt of roots or rot of the heart of lettuce]
5.22 Control of pests and diseases
6.9.18.0 Pesticides | 16.13.6.0 Fungicides
In this lesson teach the students that you must be looking at your crops ever day for signs of pests and
diseases. Then you must be ready to decide what to do about the pests or diseases. Often the pest or
disease is harmless or does not do much damage so it is best to either leave it alone or just pick off insects
or diseased leaves by hand. Yet you must still teach the students to look at the crops every day.
If the pest or disease is damaging your crop so much that you will not get enough to eat from it, or you
cannot sell it, then be prepared to use pesticide sprays or dusts: Contact the agricultural field officer and
ask his advice.
Clean your spray pump and try it with water first. Check that you have enough of the right kind of spray.
The only really safe pesticides to use in schools are as follows: Benomyl, Captan, Mancozeb, Maneb,
Methoxychlor, Quintozene, Sulfur, Zineb. Mix the pesticide according to the instructions on the label. Do
the spraying yourself or supervise an older student very closely. Keep the students away from the spray.
Do not spray on a windy day. Wash your hands and face (using soap) after spraying. Clean out the
sprayer after use, do not leave any pesticide in the sprayer. Store pesticides and chemicals in a safe place
away from students.
The teacher should show the correct way of spraying or dusting of pesticides in the garden
Show the students examples of good preparation.
Pests and diseases can be controlled by good preparation before you plant the crop. Good management
after you have planted the crop. There are five methods of good preparation for control of pests and
diseases.
Use healthy planting material. Seeds and cuttings should be clean and free of insects or disease spots.
Imported seeds in sealed silver packets will be free of disease.
Select planting material and seeds from plants known to be resistant to disease. In village gardens some
kinds of yams resist disease and some are attacked by disease.
Prepare the land properly so the plants can grow well - dig the soil deeply and dig drains near by so the
roots can grow strongly in the drained soil. Fertilize the plants to provide enough plant nutrients. Sick
plants are damaged more by pests and diseases than healthy plants.
Dig out and burn all plants from the previous crop, weeds, and other stray plants, such as male papaya
trees. If you get rid of all unnecessary plants, then pests and diseases will not live on them ready to attack
your crop. Destruction of diseased or useless plants is called garden hygiene.
Soil for seed beds or seed boxes can be sterilized by heating the soil in an oil drum all day to kill pests and
diseases.
The five methods of control are as follows:
1. Use healthy planting material
2. Select resistant planting material
3. Good land preparation
4. Garden hygiene
5. Sterilize soil for seed beds and boxes
5.22 Pests and diseases, revision questions
What are the five methods of control of pests and diseases by good preparation for the crop?
[Use healthy planting material, select planting material known to be resistant to pests and disease,
prepare the land properly, garden hygiene, sterilize the soil.]
5.24 Crop management
Show students some examples from your garden of: care of the crop, interplanting, need for control of
pests and disease. Show them how to control by hand. Keep a record of their garden visits and
observations in their note books.
Care of the crop
Keep soil cultivated between plants, well drained and free of weeds. This will allow the crop to grow
strongly and not lose any water and plant nutrients to weeds.
Use mulch to protect the soil but do not let it touch the plant stems because some disease may be in the
mulch.
Add some compost or artificial fertilizer to provide plant nutrients to keep the crop healthy.
Interplanting
Interplanting can help plants to help each other so use a mixture of different kinds of plants in a garden,
For example:
Row 1 Row 2 Row 3 Row 4
sweet potato maize (corn) sweet potato maize (corn)
climbing bean maize (corn) pumpkin .
If the same kinds of plants are separated from each other by other kinds of plants, it is harder for pests and
disease to spread from one plant to another. Also some plants can help each other by shading weeds or
repelling insects, e.g. marigolds will protect other plants from nematode worms, radishes help other
vegetables.
Control by hand
Insects such as caterpillars, diseased plants and parts of plants can be removed by hand and burnt. Crops
should be looked at every day for signs of pests and disease.
Garden hygiene
Do not leave diseased plants in the garden - pull them all out and burn them. Also look at compost heaps
and mulch for signs of insects that attack plants, e.g. Rhinoceros Beetle, mole crickets.
Control by spraying and dusting
Chemical sprays and dusts called pesticides can be used. However, they are expensive and have to be
handled with care because they are poisonous to people. Use sprays or dusts only if it is essential,
e.g. Carbaryl, Maldison, Dimethoate, Captan, Mancozeb, Acephate.
5.24 Management, revision questions Give two reasons why it is good to interplant crops. [Less spread
of disease, some plants can help each other]
If you interplant corn (maize) with climbing beans how do the beans help the corn? [The beans shade
weeds and give nitrogen plant nutrients to the corn.]
How does the corn help the beans? [Shades weeds and provides something for the beans to climb up.]
What should you do if you see a few diseased leaves on a plant? [Pull them off and burn them.]
What should you do if the whole plant is diseased and using pesticides is not profitable? [Pull it out and
burn it.]
What are the two disadvantages of spraying with pesticides? [They are expensive They must be handled
with care because they are poisonous to people.]
What must you do to your crops every day to guard against pests and diseases? [Look at them carefully.]
Why do you need to control pests and diseases? [Because they decrease the production of a crop.]
5.25 Plant nutrients from plant ash
Collect some white ash from burnt wood and bring it to the classroom. The black ash is carbon. Show
the students the white ash you have collected. Let them taste some. It tastes a bit salty. The ash contains
plant nutrients. Show the students a bag of fertilizer let them read the names written on the bag. Do not
let the students taste the fertilizer from the bags.
Plant nutrients are chemicals that plants take in from the soil. Some people call them plant foods. These
chemicals are needed by the plant to keep it alive, to make food, and make the plant body. If there are
not enough plant nutrients in the soil, the plant will be weak, grow slowly, and have yellow or brown
leaves. It may die.
The most important plant nutrients are as follows:
1. Nitrogen for plenty of strong green leaves
2. Phosphorus for root growth and making fruit
3. Potash (potassium oxide) for healthy plants
Other important plant nutrients are as follows:
4. Sulfur and iron for green leaves
5. Magnesium and calcium for healthy plants
There are other plant nutrients needed in very small amounts, which may be important for some plants,
e.g. manganese, boron.
Most plant nutrients originally come from the rocks that formed the soil. Other plant nutrients in the soil
have come from plants that have died then rotted in the soil. If a soil does not have enough of any plant
nutrient, e.g. potash, you say it is deficient in potash.
5.25 Plant nutrients, revision questions
What are plant nutrients? [Chemicals which plants absorb from the soil.]
What are they used for? [To keep plants alive and to let plants make food]
Name the three most important plant nutrients. [Nitrogen, phosphorus, potash (potassium oxide)]
Name other plant nutrients. [sulfur, iron, calcium, or magnesium]
Where do most plant nutrients come from? [The rocks from which the soil was made] Where do other
plant nutrients come from? [Plants that have rotted in the soil.]
History
These teaching materials were written by Dr J. Elfick in the Solomon Islands for the Australian
Development Assistance Bureau. The following persons helped with the original version of this text:
Dr Guy Evans, Goroka Teachers College, Papua New Guinea; Dr C. E. Mulquiney, Armidale College of
Advanced Education, Australia; Mr B. Deutrom, Ministry of Education, Papua New Guinea; Mr Geoffrey
Creek, Secondary Teachers College, Apia, Western Samoa; Mr B. Bennett, Solomon Islands Teachers
College, Honiara, Solomon Islands; Mr A. Hunt, Su'u National Secondary School, Solomon Islands.
The diagrams were by Mr J. A. Sutherland, University of New England, Australia.