School Science Lessons
Sweet Potato
Project
Updated: 2009-04-11
Please send comments to: J.Elfick@uq.edu.au
See: Interesting
Websites
See:
Sweet Potato, Queensland Department of Agriculture
Preface
Before teaching this project, discuss the content of the lessons with
a field officer of the Ministry of Agriculture and get advice on
planting
material, planting distances, site for planting, approved mulch,
composting,
and control of pests and diseases. Use only the procedures,
agricultural
chemicals and insecticides recommended by the local field officer of
the
Ministry of Agriculture. If you cannot control insects by
hand-picking,
ask the Ministry of Agriculture to recommend a chemical spray. All
insect
sprays are dangerous. Show the students how to use them safely. Do not
get the spray onto your hands. Do not breathe in the spray. Wash your
hands
well after using spray. Keep the spray container in a safe place where
students cannot get it. Spray on a day of no wind but if you must spray
when there is a wind, spray down wind. Make sure the spray does not
blow
on other people.
1. Introduction to Sweet Potato
2. Leaves
3. Stem
4. Tubers
5. Flowers
6. Planting material
7. Planting
8. Growth studies
9. Fertilizers
10. Pests of Sweet Potato
11. Climate for growing Sweet Potato
12. Harvesting Sweet Potatoe
13. Storing tubers
14. Costs and profits
15. Sweet potato varieties
16. Food value of Sweet Potatoe
17.
Understanding the records
1. Introduction to Sweet Potato,
Ipomoea
batatas
See diagram 61.1: Sweet potato plant, leaves,
flowers
1.1 The sweet potato comes from South America but is
now
grown very widely in many tropical and temperate countries, e.g USA. It
is now grown more in South West Asia than anywhere else in
the
world. It is the chief food crop in Papua New Guinea and is very
important in the Solomon Islands and New Hebrides. This crop is less
important
on other islands like Tonga and the Cook Islands, but it could be grown
more. In Papua New Guinea it is called kaukau. In Fiji it is called
kumara
and
in some Polynesian islands its name is kumala.
1.2 Sweet potatoes are very easy to grow. It is probably easier to grow
than any
other food plant. They are dependable, and will always give a crop.
They are very quick in growth. Some varieties can give a crop in
as little as 3 or 4 months.
Sweet potatoes will grow well in many situations from those that
are wet to those that are dry. It grows well at sea level, and some
varieties
can grow at over 1500 metres.
The plant does not suffer from many diseases.
The crop produces a good yield if it is given some care.
1.3 The emerging shoots, tips, stalks and unformed leaves can be eaten
in stir fies or salads. White varieties can be cooked as chips (French
fries) boiled or roasted. Grated sweet potato can be used to thicken
soup and curries, and also be used as a filler in cakes,
breads and fritters.
Dried sweet potatoes can be made into a flour and used is batters and
pancakes.
1.4 The tubers and leaves can also be fed to pigs.
1.5 The crop can be harvested a little at a time. This means that there
may be fewer losses of tubers.
2. Leaves
See diagram 61.2: Leaf shapes
1. The leaf is the most important part of the plant because it is here
that the food is made. It is stored in the tubers after it is made. The
leaves of sweet potatoes have a long thin leaf stalk or petiole. The
shape
of the leaf itself is very variable. Some leaves are entire, i.e. have
no lobes but are just one shape. Other leaves have three, four or five
lobes. Most of the lobes have a pointed shape. Look at the different
leaf
shapes in the picture. There may be different leaf shapes on one plant.
2. Collect sweet
potato leaves of different shapes. Trace the outline of 3 leaves of
different
shapes.
3. Stem
1. The stem of the sweet potato plant is thin, bends easily,
has little lumps along it called nodes.
The leaves are attached to its younger parts. Where leaves drop
off, a leaf scar is lef. Buds on the stem are just above the position
of each leaf scar. Every fifth leaf is directly in line with another
leaf below it.
2. The stem it carries the food from the leaves down to the tubers.
If a piece of stem is broken off and one end is buried, new leaves
will grow from the buds above the ground.
Roots will grow from buds on the part of a stem buried under the
soil.
When a stem grows over the surface of the soil, some roots may grow
down from some nodes that are touching the soil.
4. Tubers
See diagram 9.87: Tuber
A outer periderm B may produce secondary roots C the "crown" end may
produce shoots
1. The tubers of sweet potatoes vary greatly in size, shape and colour
of the flesh and of the skin. The commonest shape of a tuber is a long
spindle shape as you see in the picture. Other shapes are also common,
some are nearly round, others are oval. The outside of the tuber has
small
pits on it. There is a bud at the bottom of each pit and it can grow
out
into a branch. Sometimes tubers have long grooves in them. These are
not
good because much peel is lost from this part. When you see a sweet
potato
variety with tubers that are always marked by deep grooves, you do not
plant
from this variety. There are two other ways in which the tubers may
vary
from one another.
2. The colour of the skin and of the flesh may be white, cream, yellow
or purplish, or another colour.
3. The taste of the tuber varies a lot. Some tubers are very good to
eat; other tubers do not have such a good taste.
Cook different varieties and taste them.
5. Flowers
1. The flowers are white or purplish in colour. They are formed
singly or in small clusters in the axils of the leaves. The 5 petals
are
joined to make a tube. There are 5 small green sepals under the petals.
Sometimes the flowers make seeds. These are contained in a small
capsule
that splits open when it is ripe. There may be 2 to 4 small brown or
black
seeds with sharp corners. The seeds are not usually used for sowing
sweet potatoes.
2. Describe the colour and shape of the flowers.
3. Describe the seeds. Find seeds in the seed capsules.
6. Planting material
See diagram 61.6: Planting material
1. Use pieces of stem and leaves from the middle part of the
stem. The best pieces of stem to use are those that have a new shoot
growing
in a leaf axil.
The most commont method is to plant tip cuttings.
2. Leave stem cuttings for a week
until the leaves have died, then plant the stem piece.
3. Leave a bundle of stem cuttings in the shade and covered with grass
or banana leaves to make
roots growing out from the covered nodes.
4. Keep tubers in weak light in the shade until they start to sprout,
chitted. Cut off
pieces
of the tuber with the sprouts attached to them, then plant the pieces.
5. When the
sprouts
are long enough, they can be broken off the tuber and planted.
7. Planting
See diagram 61.7: Sweet potato gardens
1. Planting on the flat
The garden is prepared by digging up the soil, burying
weeds, and making the soil loose. This method of planting is not used
very
much because it does not make good drainage.
2. Planting on ridges or moulded beds 30 cm apart
Sometimes ploughs are used to throw earth up into ridges
and the cuttings are planted along the top of the ridges. Sometimes
ridges
are made by hand. This method is satisfactory because it makes the soil
loose and because it makes good drainage for the soil.
3. Planting on mounds
The best method of planting is to form small
mounds and plant the cuttings in the top of the mounds. The mounds can
be made by hand, but sometimes mounds are formed from ridges made by a
plough first. When a mound is made, it may be about 25 to 90 cm wide
across
the top and it may be heaped until it is 15 to 40 cm high. When making
a mound, bury grass or other plant material in the mound so that the
soil is very loose to allow the tubers to grow. The soil in the mound
has good drainage, so it will not become too
wet.
Do not dig animal manure into the beds unless it is
well-composted. Excess nitogen fertilizer produces leaves but no
tubers. The buried plant material will make the soil warm and this will
make the sweet potatoes grow better.
4. Use cuttings cut about 20 cm from the growing tip of the runner.
Trim all the leaves off except for the two small emerging leaves.
Another method
is to make cuttings from pieces of stem that have 5 nodes and to bury
the
bottom 3 nodes and leave 2 of them above ground.
Plant the cuttings 30 cm apart.
5. The depth of planting cuttings also varies. In some places cuttings
are buried 30 cm deep, but in other places they are only buried 6 cm
deep.
Planting too deep makes it too hard to harvest the tubers. In some
places
the people plant the cuttings so they are straight up in the soil. In
other
places they plant the cuttings so they are at an angle of 300 or 600 to
level. In some places the bottom part of the cutting is buried
horizontally
and the top part is bent upwards.
6. Usually more than one cutting is planted at each place. Planting 2
to 5 cuttings in one place on a mound is usual.
Plant a new row every week to get a continuous supply. Dig a sweet
potato garden, planting 3 cuttings in
each mound. Use cuttings that have 5 nodes and plant them so the bottom
3 nodes
are under the soil and 2 are above the soil.
8. Growth studies
See diagram 61.8:
Sweet potato diary
Keep records of the growth of the crop.
1. Note how many stems come from each cutting.
2. Note how fast do the stems grow, i.e. what length of stem is grown
each day.
3. Note when the tubers start to form by uncovering the
roots of one plant each week. The normal life of the plant is 4-5
months
at sea level but 5-6 months planted high up in the mountains.
4. Note when the tubers have formed properly
and the plant is near the end of its years growth. At this stage some
leaves may turn yellow.
9. Fertilizers
See 6.9.14
Composting
Sweet potatoes respond well to fertilizers especially those that
contain
nitrogen and potassium. It is a good idea to put some fertilizers on
one
part of the garden: then students can see the result of using them.
The fertilizers to use are as follows:
1. Ammonium sulfate. If you cannot get this fertilizer, use urea or
IBDU.
2. Chloride or potash or sulfate of potash.
Too much of them or it will burn the plants. Use about one matchbox
full of mixed fertilizer and spread
it over one square metre. Do not put the fertilizer too close to
the plants. Sprinkle it over the ground but keep it 10 cm away from the
plants and dig it lightly into the soil.
10. Pests of Sweet Potato
See diagram 61.10: Sweet potato weevil
Sweet potato weevil, Cyclas formicarius
Eggs are laid in the root and the larvae burrow through the tubers
making a sodden rotten mess. The female weevils chew small holes in the
surface of the stems or
tubers.
They then lay their eggs in these cavities. The eggs hatch and a small
larva comes out of the egg. The larvae bore into the stems or tubers.
The
larvae have no legs, are white in colour and are nearly a third of an
inch
or over one centimetre long. After 3 weeks the larvae turn into a
resting
stage called a pupa. Later the adult weevil comes out of the pupa. The
adult weevil is shiny blue and red in colour and is less than 0.5 cm
long. The
adult
weevils can feed on the leaves, vines, roots and the outside of the
tubers.
The worst damage is done by the larvae. If the larvae have been living
in the stem, they often go down into the tubers later. The weevils may
be very bad in dry weather because when it is dry, cracks come in the
soil
and then going down to the tubers is easy for the weevils.
Control sweet potato weevil by using crop rotation and crop hygiene.
After harvesting all the tubers, pull out all the remaining roots and
stems and burn them. Termites can infect old growth. Do not leave any
tubers in the soil after harvest. Do not plant
any more sweet potatoes for at least a year in that place so that they
cannot go from the parts of the old plants into
the
new plants. Do not use planting material from this place because it may
contain
weevils. Tubers with
weevils in them can be fed to pigs.
Burn the old parts of the plants where weevils may be
living.
11. Climate for growing Sweet Potato
Sweet potatoes will grow in many warm climates. The crop likes to have
temperatures of 24oC or
more
with plenty of sunlight and warm nights. The plants will not grow well
when the temperature is below 10oC. Sweet potatoes can be
grown
in a cold climate if there is a summer season of 4 to 5 months with no
frosts. Sweet potatoes need 500 mm of rainfall while they are growing,
but they need 750, 1000 mm of rain for the whole year. It is good if
there
is not much rain when the crop is nearly ready to dig, but it, is bad
if
the dry time comes 50 days after planting becaue then the storage
roots
are forming.
12. Harvesting Sweet Potatoes
Test whether the sweet potato crop is ready
for harvesting
1. When many leaves turn yellow and begin to fall off the plant it
is a sign that the tubers are ready to dig.
2. When a small piece of
tuber
is cut off with a knife and the juice does not go black quickly, then
the
tuber is ripe. It is easy to damage the tubers when you are digging
them
up. Dig into the side of the mound carefully with a garden fork and
lift.
13. Storing tubers
1. Sweet potato tubers go bad very easily, so storing them for a long
time is not usual. However, there is a way of storing them for short
periods.
This is by "curing" them first. Curing is done by giving them a high
temperature
of 29oC and keeping them in moist air for 4 to 7 days. Keep
them in a room for curing, but there must be air
flowing
through this place. After about a week of curing, this hardens the skin
of the tubers and this keeps out the fungus that makes them go bad.
2. After
curing for about a week, put the tubers where where
it is cool and the temperature is only about 16oC.
Temperature
below 10oC is too cold for the sweet potatoes and they will
be damaged by the cold. In some places the people store the tubers in
pits
made in the ground in a cool place. They may keep them for times up to
4 months in the pits, but usually you can only store them for shorter
periods,
e.g. 4 weeks.
3. Test if the tubers
have
been properly cured:
4. The skin of the tubers does not slip easily when pushed.
5. The buds may have a purplish colour.
6. The skin looks dry and spongy.
14. Costs and profits
1. Establishment costs are the things you buy which will probably last
for a long time, e.g. spades, hoes, sprays. Divide
the establishment costs by the number of years you think these things
will
last and you add this amount to the costs, e.g. 3 years.
2. Production costs are things that you must buy each year that you
grow the sweet potatoes, e.g. fertilizers and
insecticides.
3. (Profit = returns - establishment costs - production costs).
15. Sweet potato varieties
1. Where sweet potatoes are grown, there are usually many varieties.
Often
these varieties have local names. Know about these
varieties and which ones are the best to grow. Try to get some leaves
and
tubers of as many varieties as you can.
2. Note the features of the different varieties:
2.1 Shape of leaves
2.2 Colour of tuber skin
2.3 Colour of tuber flesh
2.4 Shape of tuber
2.5 How long it takes to grow
2.6 Taste
of cooked tubers
16. Food value of sweet potatoes
The tubers are full of sugar and starch. This means
they are a good energy food. However, sweet potatoes only
contain
a very little protein food. They are rich in iron and
calcium,
minerals and in vitamins, e.g. vitamin A.
History
These teaching materials were originally written and illustrated by
Mr J. A. Sutherland, Faculty of Education, University of New England,
Armidale,
Australia and later edited by Dr J. Elfick, School of Education,
University
of Queensland, Brisbane, Australia.