School Science Lessons
Yam Project
Updated: 2008-12-15
Please send comments to: J.Elfick@uq.edu.au
See: Interesting
websites
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
Table of contents
1.0. Introduction
2.0 Different kinds of yams
3.0 Leaves of yams
4.0 Stems
5.0 Tubers
6.0 Bulbils and flowers
7.0 Planting
8.0 Keeping records
9.0 Staking
10.0 Weeding
11.0 Fertilizers
12.0 Fungus diseases
13.0 Pests of yams
14.0 Climate for growing yams
15.0 Varieties of yams
16.0 Harvesting yams
17.0 Storage of yams
1.0 Introduction to the yam project
See also
6.9.14 Composting
1. Yams are important in the lives of some of the people for yam
festivals
at harvest time and food. Teach this project if yams are an important
crop
locally. Teach with real plants or parts of plants. Students should do
the actual work of growing the yams. Plant yams at the end of the dry
season
and before the tubers begin to make long sprouts, i.e. plant early
varieties
in August and late varieties before the end of September. You could
start
the teaching in early June before planting time.
2. Yams are very important in all parts of Melanesia, Papua New
Guinea,
the Solomon Islands, New Hebrides and New Caledonia. They are grown
mostly
by the coastal people, not by people living inland. Yams are also
important
in the high islands of Micronesia, in Ponape, Kusaie, Truk and Yap.
They
cannot be grown on the coral atolls. Yams are less important in most
parts
of Polynesia, but some are grown in Tonga, the Cook Islands, French
Polynesia,
Samoa and also in Fiji. Yams are called by different names in different
islands. One of the commonest names in the Solomon Islands and Fiji is
uvi. In some islands a man is important if he can grow the biggest
yams.
Many people think that magic words must be used when planting yams. In
some islands the people put special stones near the planting place
because
they believe magic stones will help the yams to grow.
2.0 Different kinds of yams
See diagram 63.2:
Winged Yam
The yam family (Dioscoreaceae), monocotyledon, are twining plants.
The stem is not strong so it needs to have something strong that it can
twist around to climb up. There are many different kinds (species) of
yams
and each kind of yam has many varieties.
1. Dioscorea alata (D. rubella) "common yam", Greater
yam, Water yam,
Winged yam, White yam, Guyana arrowroot, Ten-months yam,
Name-de-Agua
It is the
most common yam. It is a large climber with winged stems produce large
single tuber. The stem is square and has four thin pieces of stem or
"wings"
at each corner. Climbs from left to right, i.e. the stem always twines
to the right, as in the diagram. Each variety winged yam has only one
tuber
to each plant but the shape of these tubers varies in different
varieties.
2. Dioscorea bulbifera,
Potato yam, Aerial yam, Bulbil-bearing yam, Air-potato, bulbil yam,
wild
yam
It is
a climber with round stems bearing edible aerial tubers called bulbils
in the leaf axils. The stem twists to the left, and the plant makes
many
tubers up in the stem above the ground. It may also make some tubers
under
the ground.
3. Dioscorea
esculenta, the lesser yam, pan is a small climber
producing
more than 1 small tuber. The lesser yam has a round spiny stem which
always
twists to the left, i.e. climbs from right to left. The tubers are
small
and rounded with groups of them to each plant. The leaves are
alternate.
4. Other yams include:
Dioscorea batatas, Chinese
yam, Cinnamon vine, Shan-yao
Dioscorea cayenensis, Yellow
Guinea yam, Yellow yam, Twelve-months yam, Lagos yam
Dioscorea composita (D.
tepinapensis) Barbasco
Dioscorea dumetorum (Helmia dumetorum) Bitter yam,
Cluster yam, Trifoliate yam
Dioscorea elephantipes
elephant's foot, Hottentot bread
Dioscorea floribunda
Dioscorea numularia
Has very spiny stem and climbs from left to right.
Dioscorea
pentophylla Has a leaf with 3-5
lobes, spiny stem and climbs from right to left.
Dioscorea praehensilis, Bush
yam, Forest yam
Dioscorea preussii
Dioscorea rotundata, White
yam, White guinea yam, Guinea yam, Eboe yam
Dioscorea trifida, Cush-cush
yam, Aja, Yampi, Mapuey, Cush-cush, Yampee
3.0 Leaves of yams
See diagram 63.3: Leaves of yams
The leaves of yams vary in the shape of the leaf, the length of the
leaf stalk, the thickness of the leaf stalk, the leaves are in pairs
opposite
one another, or are alternate (first one side then another), the
thickness
of the leaves. Use leaves of 2 or 3 different shapes to name the parts
of the leaf and label leaf stalk, veins and lamina. Trace the shape of
two different leaves, the small veins and the parts of the leaves.
4.0 Stems
See diagram 63.4:
Twining yams
Use pieces of stem from the winged yam and the lesser yam. Look at
the stem of a winged yam and draw it. Draw a stem that twines to he
right
and one that twines to the left. Look at how leaves are arranged on the
stem. Draw the twining of the stems in two kinds of yams. Look at the
arrangement
of leaves on the stems and draw one leaf attached to the stem. Cut
across
the stem of a winged yam and draw the shape of the stem. Stems of yams
twine to the right or to the left. The stem of the winged yam has four
thin wings at the corners of the stem. Stems may have leaves in
opposite
pairs or leaves alternate on the stem.
5.0 Tubers
See diagram 63.5: Yam tubers
Teach this lesson in the dry season after the yams have been dug. Use
yam tubers of different shapes. Note the shape of the whole tuber, the
colour of the flesh, the colour just under the skin, the appearance of
the outside skin. Bring different shapes of tubers into the classroom.
Draw some tuber shapes. The tubers of yams are of different shapes. The
colour of the flesh and the taste of the flesh may be different. The
colour
of the flesh just under the skin may vary in different varieties. Each
yam plant may make one large tuber, but some varieties make several
smaller
tubers.
6.0 Bulbils and flowers
See diagram 63.4: Yam bulbils
1. Bulbils and flowers may be seen on the stem. Some kinds of yams
form small swollen bulbils like small tubers formed above ground in the
axils, i.e. the angle between the leaf stalk and the stem. Bulbils can
be eaten like tubers. Bulbils are the main ways of storing food for
Dioscorea
bulbifera. Bulbils can sometimes make shoots and roots if a stem
cutting
is taken with the bulbil. Bulbils usually form when the plant has been
growing for a long time and is near the end of its growth for that
year.
Bulbils can be used as planting material.
2. Yams do not usually make flowers. The male and female flowers are
formed on different plants, i.e. yams are dioecious. Flowers of yams
are
very small and usually do not make seeds but they may make small seed
cases
called capsules containing small seeds.
7.0 Planting
See diagram 63.7: Planting yams
1. After the yams have made tubers, the plants lose some of their
leaves. Then the tubers have a resting time usually in the dry part of
the year that lasts for 3 to 4 months. Then the tubers will start to
make
sprouts. Plant the tubers before sprouting because the sprouts may be
broken
off the tuber during planting. Early varieties of yams are usually
planted
in August. The late varieties are usually planted before the end of
September.
2. The planting material may be any of the following:
2.1 Whole tubers. If the tubers are small, they are not cut, but
planted
whole.
2.2 Cut pieces of tuber. The tubers may be cut into three pieces,
heads,
middles and tails.
2.3 The heads are the pieces that grow into the best plants. There
are many buds in this part of the tuber.
2.4 After the tubers have been cut into pieces, it is best to leave
them for a week so that the cut part can heal and get hard.
3. To stop the cut pieces of tuber from rotting put fungicide on to
the cut surface.
4. To prepare the yam garden, dig a long trench and put some compost
into it. Then cover the compost with topsoil and heap it up to make a
mound.
Plant the yams in this mound. Make the mounds about 1 metre apart and
plant
the yams about 45 cm apart along the ridge with the tops is 5 cm below
the surface. Cover the place where the yam was planted with dead grass
to keep the soil moist around the yam piece.
1. What time of the year are yams planted? It is important to plant
before sprouting.
2. Different kinds of planting material, whole tubers, heads, middles
and tails.
3. Leave the cut pieces for a week then put Captan on the pieces.
The village people use wood ash.
4. Make a garden.
5. Planting and capping.
6. Cut some yams into pieces.
1. Yams are planted at the end of the dry
season, 3 or 4 months after
harvesting.
2. Planting material may be whole small tubers, or heads, middles
and tails cut from large tubers.
3. Tubers are left for a week before planting.
4. Make a garden. The size depends on the amount of planting material.
5. Dig trenches and put compost in them.
6. Heap up soil to make ridges.
7. Plant the yam pieces.
8. Put a capping of dead grass over the planting places.
8.0 Keeping records
See diagram 63.8.1: Yam growth | See diagram 63.8.2:
Yam diary
Keep good records of the growth of the yams. Make a table to include
the date the yams were planted, the date when the shoots first came,
how
high the plant was each week, how many centimetres the plant grows each
day.
Choose one planted yam and make notes on its growth. Put a stake in
the ground for the yam to grow up. Every day for one week make a mark
on
the stake showing how high the yam has grown. In the diagram see the
growth
of one yam.
1. You will learn more about yams if you keep careful records.
2. Make a table for keeping records.
3. Make the first entry of the date yams were planted.
Use growth records to calculate the average daily growth of the yam
shoot. The yam in the diagram grew nearly 8 cm a day in wet weather but
only about 4 cm a day when the weather was dry. In some countries yams
grow very much faster than this.
9.0 Staking
See diagram 63.9: Staking Yams
Yams have weak stems. They cannot stand up by themselves. Yams need
to have something to climb up so that they can reach the sunlight. When
the leaves of yams are in full sunlight they make more food and the
tubers
grow bigger. There are many ways of making supports for yams:
1. Sometimes people push long pieces of branches into the soil near
the planting piece. These are called stakes. The trouble with this kind
of support is that it can be blown over by the wind.
2. Sometimes people put pieces of dead bushes on the soil over the
planting place. Then the yams can grow over these bushes to a height of
30 to 60 cm. The trouble with this kind of support is that it is not
high
enough, and the yam leaves may not get enough sun.
3. A better way of staking yams is to make "A frames". These are posts
or pieces of bamboo or other stems pushed into the soil so they lean
towards
each other. They are tied together at the top, and other long pieces
are
tied across the tops of the A frames to keep them still in the wind.
Look
at the picture.
4. If a man wants to grow many yams so he will have a lot to sell
in the market, he may make special wire supports for them as you see in
the picture. First he must plant the yams in ridges. Then he must get
some
strong posts and bury them 120 cm in the soil. He puts the posts in the
hollows between two ridges. Then he must tie wires to the end posts, at
the top and at the bottom. He uses wires and pegs to stop the end posts
from moving.
1. Staking is needed for yams.
2. Different ways of staking for yams.
3. Cut some stakes to make "A frames" for your yam garden.
4. Put the A frames in the soil.
10.0 Weeding
It is very important for the yam garden to be weeded, especially in
the early stages when the yams are quite small. Weeds stop the growth
of
the yams by stealing water, light and plant foods in the soil. Weeding
can be done in several ways:
1. In small gardens it is best to weed the yams by hand.
2. If the gardens are larger, the soil may be sprayed with Atrazine
at the rate of 3 kilograms per hectare. This spraying should be done
when
the soil is still weed free just after planting the yams. This chemical
will kill any weeds that come up for the first three months.
3. After the yams have come up, patches of weeds may be sprayed with
the weed killer Paraquat, but care must be taken not to let the spray
get
on the yams.
1. Bad effects of weeds on yam growth.
2. Weeding is most important in the early stages of growth.
3. Hand weeding.
4. Use of Atrazine. Because this kills weeds before they have come
up, it is called a pre-emergence spray.
5. Paraquat is a very strong weedicide so use it with care.
6. Weed the yam garden by hand.
1. Weeds steal water and plant foods from yams. They may also keep
sunlight off the plants.
2. There are 3 ways of killing weeds:
2.1 Hand weeding
2.2 Using pre-emergent sprays, e.g. Atrazine
2.3 Using very strong weed killers, e.g. Paraquat.
11.0 Fertilizers
Yams planted in very good soil or soil which has just been cleared
from bush usually do not need any fertilizers. If fertilizers are used,
sometimes the yams grow much better but sometimes it is hard to see if
the fertilizers have been good for the yams. If the soil is not very
good,
or if it has had yams or other crops growing in it before, it is good
to
put some fertilizers on the soil. Phosphorus and potash fertilizers
should
be sprinkled on the soil near the yam tubers at planting time. The
amount
to use is about 50 kg per hectare. Eight weeks after planting, when
the
yam shoots are growing up the stakes, then nitrogen fertilizer should
be
used at the rate of 100 kg per hectare. If the garden is quite small,
these
fertilizers should be put on at the rate of one matchbox full of the
fertilizer
sprinkled over the soil around each planting piece.
1. Yams grow better if they have a lot of plant food.
2. No fertilizer is needed on new ground.
3. Phosphorus and potash fertilizers can be put on the soil at
planting
time.
4. Nitrogen fertilizer can be used about 8 weeks after planting.
5. Spread some fertilizer on the yams.
1. Yams need no fertilizers on new ground.
2. On old gardens use phosphorus and potash fertilizers at planting
time at 50 kg per hectare.
3. Eight weeks after planting nitrogen fertilizer can be used at 100
kg per hectare.
1. Yams grow better if they have a lot of
plant food.
2. No fertilizer is needed on new ground.
3. Phosphorus and potash fertilizers can be put on the soil at
planting
time.
4. Nitrogen fertilizer can be used about 8 weeks after planting.
5. Spread some fertilizer on the yams.
12.0 Fungus diseases
See diagram 63.12: Fungus diseases
There are two fungus diseases of yams that are found in many countries
on the winged yam. The first of these diseases is a dieback disease
caused
by a fungus, Colletrotrichum
sp. This disease starts when water from
heavy rains splashes the small seeds or spores of the disease from the
soil up onto the young leaves. The fungus does not grow so well on the
older leaves. The first signs of this disease are small round brown
dead
spots on the leaves. There is a yellow colour on the outside of the
spots.
Then later the dead spots grow in size until the whole leaf has died.
Then
the whole branch with its stem and leaves may die back. Of course this
means that the tubers will not grow properly.
1. Yams need no fertilizers on new ground.
2. On old gardens use phosphorus and potash fertilizers at planting
time at 50 kg per hectare.
3. Eight weeks after planting nitrogen fertilizer can be used at 100
kg per hectare.
There are two fungus diseases of yams that are found in many countries
on the winged yam. The first of these diseases is a dieback disease
caused
by a fungus called Colletrotrichum. This disease starts when water from
heavy rains splashes the small seeds or spores of the disease from the
soil up onto the young leaves. The fungus does not grow so well on the
older leaves. The first signs of this disease are small round brown
dead
spots on the leaves. There is a yellow colour on the outside of the
spots.
Then later the dead spots grow in size until the whole leaf has died.
Then
the whole branch with its stem and leaves may die back. Of course this
means that the tubers will not grow properly. This is a bad disease
because
it cannot be cured, and there are not many things we can do to stop it.
Some of these things are as follows:
1. Do not keep the tubers from diseased plants to use as planting
material.
2. Do not keep on planting yams in this soil, but use a rotation.
Plant another crop next year in this place.
3. Look for varieties of yams that do not get this disease. These
varieties will be resistant to the disease. They are the best varieties
to grow.
4. If you know this disease may be in the soil, then you could spray
the plants when they are very young with a chemical called Benlate.
This
may keep the fungus away from the plants.
5. Another thing that can be done is to plant the yams at a time when
there may not be many heavy rain storms. The danger of attack is less
once
the yams have grown higher, because the older leaves do not get the
disease
so easily.
Another fungus disease of yams is caused by a fungus, Rhizoctonia sp.
This fungus gets into the roots of tubers of the yams and rots them.
There
may be signs of the disease low down on the stem near the soil. Dark
coloured
spots on this part of the stem are called 11 collar rot". This fungus
is
carried by the tuber. If there are any marks of this disease on the
leaves,
they are on one side of the leaf only. The most important way of
stopping
this disease is not to use tubers from diseased plants as planting
material.
The big tubers are probably good to use for planting, but eat the small
tubers which may have this disease.
1. Fungus is very small and can only be seen
properly with a microscope.
2. The signs of the disease are small round brown spots which get
bigger until the leaves die. Ways of stopping the disease include do
not
keep diseased tubers for planting, use a rotation, do not plant yams
next
year in this soil, use resistant varieties, spray young plants with
Benlate.
3. Collar rot disease.
4. Tubers from diseased plants must not be used for planting material.
Dieback is a bad disease of yams. It is caused by a fungus. This fungus
makes many small seeds called spores. These spores are splashed by the
rain from the soil onto the young leaves. The fungus makes spots come
on
the leaves. Then the leaves die. The ways of stopping this disease
are as follows:,
do not plant tubers from diseased plants. Do not plant yams in this
soil
next year but use a rotation. Try to find resistant varieties of yams
to
plant. Spray young plants with Benlate. Collar rot is another bad
disease
of yams. It makes brown marks low down on the stems. The only way of
stopping
this disease is to use healthy big tubers as planting.
13.0 Pests of yams
See diagram: Yam beetle
1. The yam beetle is a pest that starts as a curled grub that changes
into a beetle. Yam beetles may spoil the tubers by eating holes into
them.
Then the tuber rots.
2. If the yam beetle is bad in your district you can wash the planting
pieces in lead arsenate and Bordeaux mixture. This will stop the grubs
from eating into the tubers and spoiling them.
3. Another thing to do is a rotation, do not plant yams twice in the
same soil, but use some other plant next year.
14.0 Climate for growing yams
1. Yams are hot weather plants and cannot stand frost.
2. The best temperature for yams is 30oC, and they do not
grow well if the temperature is below 20oC. Very high
temperatures
are bad for yams.
3. Yams need to have enough water in the soil, especially between
the 14th and 20th weeks of growth.
4. Yams grow well if there is a dry season lasting 2, 5 months
followed
by a wet season with at least 1150 mm of rain.
5. Yams like low altitudes, but can grow well if the land is not too
high above the sea.
6. Yams need a temperature of 30oC. They do not grow well
if the temperature is less than 20oC.
7. Yams need good rain especially between the 14th and 20th weeks
of growth.
15.0 Varieties of yams
Know how to tell the different varieties of yams. You cannot do this
without seeing some varieties. You can either bring some leaves and
tubers
in to the classroom, but taking students outside to see yams is
probably
best. The best time to teach this lesson is at the end of the yam
season,
just before the yams are harvested.
1. Yam varieties have different shaped leaves, different shaped
tubers,
different colours in the flesh of the tubers, different in their taste.
2. These are some leaves of yam varieties. Write the names of
varieties
under each.
3. Yam varieties have different shaped tubers. Some varieties of yams
grow quickly, e.g. They are ready to dig in 7 months to 10 months.
16.0 Harvesting yams
1. Go outside to dig yams. Note the signs that the yams are mature
and ready to dig.
2. Explain to students the need for care in digging yams so that the
tubers are not damaged or bruised.
3. Weigh the tubers produced by 10 plants. Calculate the average
weight
of tubers produced by each plant.
4. Students take the tubers into a shady place.
1. Our yam tubers weighed . . . kg each.
2. This means that we could get about - tonnes of tubers from a
hectare of yams.
3. Care is needed in harvesting yams. Do not bruise or damage the
yams.
17.0 Storage of yams
See diagram 63.17: Sprouting yams
Yams keep better than any other root crop. Sometimes yams can be stored
for 3 or 4 months.
1. Yams can be stored better than any other root crop.
2. The yams must first be dried by spreading them out under a house.
3. Then they can be stored in any cool dry place. In some islands
the people keep the yams in a special yam house.
4. Do not put damaged yams with the others. They should be eaten.
5. Yams can be stored until they start to sprout, Then they should
be planted. It is bad if the sprouts get too long and are broken off.
Another
sprout will not grow from this place.
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.