Soils2

School Science Lessons
Soils 2 SPP
2012-05-12 SPwp
Please send comments to: J.Elfick@uq.edu.au

Table of Contents
6.42 Artificial fertilizers
6.43 Chalk (lime) content of the soil
6.34a Chemical fertilizers
6.46 Crop rotation
5.35.1 Deficiency symptoms and fertilizing the soil
5.35 Fertilizer trial
6.33 Fertilizing soil (Primary)
6.33.1 Potash fertilizer
5.35.2 Fertilizing the soil
6.37 Fertilizing the soil, methods
6.9.15.0 Fertilizing the soil (Primary)
6.9.15.2 Green manure
6.40 Legumes for the soil
6.41 Make compost
6.44 Nutrient cycles
6.37.1 Nutrition from the soil
6.38 Plant foods
6.9.15.1 Plant nutrients, primary, secondary and micronutrients
6.39 Plants need nitrogen, nitrogen cycle

S15. Coral soils
6.47 Water lens in atolls.
6.48 How soils form in atolls
6.49 How atoll soils change

5.35 Fertilizer trial
If a soil does not have enough of a particular plant nutrient, e.g. nitrogen, you say that the soil is deficient
in nitrogen. The only sure way to find out whether a soil is deficient in any plant nutrients is to ask a field
officer of the Department of Agriculture to send some soil to a laboratory for chemical testing. However,
you can tell if the soil is deficient in plant nutrients by examining your crop plants carefully. If a plant does
not look healthy because there is not enough plant nutrient you say that it shows deficiency symptoms.
If a soil does not have enough of a particular plant nutrient, e.g. nitrogen, you say that the soil is deficient
in nitrogen. The only sure way to find out whether a soil is deficient in any plant nutrients is to ask a field
officer of the Department of Agriculture to send some of the soil to a laboratory for chemical testing.
However, you can tell if the soil is deficient in plant nutrients by examining your crop plants carefully. If a
plant does not look healthy because there is not enough plant nutrient we say that it shows deficiency
symptoms.
Be able to set up a fertilizer trial to test whether fertilizer makes crops grow better.
You will need: Piece of land already dug, wooden pegs. Ask an agricultural officer for advice on which
fertilizer to use for a trial. Use corn seeds or pineapple suckers or other planting material.
1. It is important to do experiments in agriculture. One kind of experiment is the fertilizer trial. The fertilizer
trial is designed to try out different kinds of fertilizer in different amounts to see if fertilizer makes crops
grow better.
2. Show how to mark out with wooden pegs the corners A. B. C. D and the mid points I, II, III, IV.
The rows should all be the same size. The seeds or cuttings should be all planted the same distance apart
in the rows.
3. Fertilizer is applied to the experiment plots A and D. No fertilizer is applied to plots B and C called
control.
4. Draw the trial in an exercise book, write down the date.
5. Compare the crops in A and D with plots B and C plots. Does the fertilizer improve the crop yield?

5.35.1 Deficiency symptoms and fertilizing the soil
A mineral element is essential if it is necessary for the reproduction of the plant. The sixteen essential
elements are as follows: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium,
sulphur, iron, manganese, zinc, copper, boron, molybdenum and chlorine. The macro nutrients, 1 to 150
g per kg of plant dry matter required, are N, P, K, Ca, Mg, and S. The micro-nutrients, 1 to 1.00 mg per
kg of plant dry matter required, are Fe, Zn, Mn, Cu, B, Mo and Cl. Chloride is a micro nutrient but it can
accumulate in the plant if present in high concentrations in the soil solution. Mineral deficiency symptoms
are sometimes confused with damage caused by insect pests, disease, salt stress, pollution, light and
temperature injury and herbicide damage. Also, toxicity of Mo or Se is similar to P deficiency and Fe
deficiency in a mango is similar to chloride toxicity.
1. Nitrogen N
The plants are small with few leaves that are pale green or yellow. The lower leaves look burnt and die
early.
Nitrogen deficiency symptoms are the appearance of uniform yellowing of leaves including the veins, this
being more pronounced on older leaves in blueberry, fescue, chilli and sugarcane. The leaves become
stiff and erect. In dicotyledons the leaves detach easily under extreme nitrogen deficiency. Cereal crops
show characteristic V-shaped yellowing at the tip of the lower leaves. Small and pale green leaves with
dull appearance occur in sweet potato. If nitrogen deficiency persists, decreased foliage and shoot growth
occurs in black pepper and sapota.
2. Phosphorus P
The plants are small and do not grow well. The leaves are a blue-green colour and are usually purple
underneath. The lower leaves die early. They take a long time to produce fruit that are small and
badly-shaped.
Phosphorus deficiency causes leaves to remain small, erect, and unusually dark green with greenish red in
sweet potato, blue-green in chilli, brown in bird's foot trefoil and with purple tinge in sugar maple,
blueberry and sugar cane. The under side of the leaves develop a bronze appearance. Phosphorus stress
causes restriction of root growth in black pepper and increase of anthocyanin pigment in leaves of barley
and thale cress (mouse-ear cress).
3. Potassium K
The plants have small main shoots but many side shoots. The leaves have dead white areas on the leaf
edges and later die.
Potassium deficiency causes yellowing of leaves starting from the tips or margins of leaves extending
towards the centre of leaf base. The yellowing is interveinal and irregular in the leaves of tomato and
blueberry. These yellow parts become necrotic (dead spots) with leaf curling in tobacco, sugar maple,
sapota and sugarcane. There is a sharp difference between green, yellow and necrotic parts.
4. Sulfur S: The plants are small with pale green upper leaves.
Sulfur deficiency causes leaves to become yellow in black pepper, potato, and wild cabbage, and so
appear similar to nitrogen deficiency. However, the symptoms are first visible on younger leaves. The
affected leaves are narrow and the veins are paler and chlorotic than in the interveinal portions of the leaves,
especially towards the base with marginal necrosis in sugarcane.
5. Calcium Ca
The plants are small with unusually shaped leaves. The shoot tips may die.
Calcium stress causes chlorosis of young leaves along the veins of birdsfoot trefoil and blueberry. A
persistent deficiency causes bleaching of upper half leaf followed by leaf tip curling black pepper and
sugarcane. The growing bud leaf becomes chlorotic white with the base remaining green, and the distortion
of the tips of shoots, i.e. dieback, in peach seedlings. Similarly, calcium stress causes brown spots on
leaves, reduced expansion and premature leaf senescence in soybean. Calcium stress during fruiting
increases susceptibility to blossom end rot in tomato. Calcium stress also causes bitter pit in apple, leaf
tip burn in cabbage and lettuce, black heart of celery, cavity spot of carrots and vitrescence in melons.
6. Magnesium Mg
The leaves have green veins but are pale yellow between.
Magnesium deficiency causes yellowing, but this differs from the yellowing of leaves caused by nitrogen
deficiency. The yellowing takes place between veins of older leaves and veins remain green, followed by
necrosis of tissues in bird's foot trefoil, melons, black pepper and blueberry. Magnesium deficiency may
be caused in tomatoes by high levels of ammonium ion in a nutrient solution. Magnesium deficiency of
citrus causes yellow areas near the midrib.
7. Iron Fe
The younger leaves look yellow. the lack of healthy green colour in the leaves is called chlorosis and this
may be due to a deficiency in one or more plant nutrients.
8. Boron, B
Boron deficiency causes yellowing or chlorosis of youngest leaves and stems, starting from the base to the
tip, rosettes of terminal shoots in potato, browns heart in radish, crown choking in coconut and leaf tip
burn that elongates and become white-brown in rice. In extreme cases, the terminal bud dies.
9. Chlorine, Cl
Chlorine deficiency causes discrete patches of pale green chlorotic tissue between the main vein near the
tip of the leaf, first on the older leaves, and downward cupping of older leaves of kiwi fruit. The leaflets of
the youngest leaves shrivel completely and the older leaflets develop a brown necrosis that starts near the
tip and extend backwards, particularly at the margins in red clover.
10. Copper Cu
Copper deficiency causes visible foliar symptoms appear on young leaves as chlorosis changing to
necrosis, the rolling, wilting and twisting of leaves in wheat and the later affected leaves appearing papery
and twisted in rice.
11. Manganese, Mn
Manganese deficiency causes the principal veins and smaller leaf veins to remain green, but the interveinal
portion becomes chlorotic followed by necrosis and browning of interveinal tissue in melons. The affected
young leaves remain small and abscise before older leaves in birdsfoot trefoil. Manganese deficiency of
citrus causes light yellow-green between veins.
12. Molybdenum, Mo
Molybdenum deficiency causes a general yellowing, marginal and interveinal chlorosis, marginal necrosis,
rolling, scorching and downward curling of margins in poinsettia cultivar and in various field, horticulture
and forage crops. The deficiency of molybdenum in cauliflower causes the "whiptail" disorder.
13. Nickel, Ni
Nickel deficiency causes reduced plant growth and older leaves turn chlorotic similar to nitrogen deficiency.
Similar symptoms appear in tomato, soybean oilseed rape, zucchini when grown on urea-based nutrient
solutions not supplemented with nickel.
14. Zinc, Zn
Zinc deficiency causes the leaves to become narrow and small in chilli, the lamina becomes chlorotic in
sweet potato, sour orange seedlings and chickpea, while veins remain green. Subsequently, dead spots
develop all over the leaf including veins, tips and margins under severe deficiency, shoot growth is reduced.
Khaira disease in rice is caused by zinc deficiency. Shoot elongation is reduced and a tuft or rosette of
distinctly narrow leaves is produced at the shoot terminal in apple and pear, called "little leaf" or "rosette".

5.35.2 Fertilizing the soil
1. To fertilize means to make the soil richer by adding plant nutrients which will make the crops grow
better and produce a greater yield when they are harvested.
2. There are 4 reasons why you must fertilize the soil:
1. Some soils have been formed from rocks which did not have enough of the chemicals which make
plant nutrients. In some tropical countries the parent rock had little potash in it and so the soil formed from
that rock will be deficient in potassium.
2. Soils may lose plant nutrients when rainwater washes through them. This is called leaching. Soils which
have an open texture and do not have enough clay and rotten plant material in them lose a lot of plant
nutrients by leaching.
3. When a crop is harvested and taken away some plant nutrients are taken away as well. You must
replace these plant nutrients if the soil is to remain fertile and produce good crops. Some plants need more
of a particular plant nutrient than others and they take away a lot of that plant nutrient when they are
harvested.
4. Fertilizing increases the yield of a crop and allows us to feed more students or make more profit if you
sell the crop. The work done in making compost and adding it to the soil is rewarded by an increase in the
amount of food for the school kitchen. The cost of buying fertilizer is rewarded by an increase in profit.
3. The plants get only a small amount of their total plant nutrients from the soil, most of them come from
the air, but crops will not grow well unless you fertilize the soil to give the plants all the plant nutrients they
need.
4. The amounts of different plant nutrients in a typical plant: Plant nutrients from the soil 7%, Hydrogen
from soil water 7%, Carbon from carbon dioxide gas in the air 42%, Oxygen from air and soil water 44%

6.9.15.0 Fertilizing the soil
Preparation
This topic is concerned with why you must fertilize the soil and the different ways of fertilizing.
School food gardens have only a limited area and the amount of physical effort is limited by the size of
them, the time available and how much physical activity you think is reasonable to expect from students.
So it is important to have a maximum use of all kinds of fertilizers to make the school food gardens as
productive as possible.
The following lesson is based on the resource material at the end of the lesson.
Method
1. Show the students some grey white plant ash or heat some plant material very strongly until only grey
white ash remains. Let the students taste this ash. It tastes salty. Tell them that they are tasting some of the
plant nutrients which are chemicals that the plants take in from the soil. Tell them that all plants need plant
nutrients and that if there are not enough of the right plant nutrients in the soil then you must put some in the
soil. Putting plant nutrients in the soil is called "fertilizing" the soil.
2. Fertilizing the soil means to add extra plant nutrients to the soil so that the plants will grow better.
3. There are 4 reason for fertilizing the soil.
3.1 The rock the soil was made from did not have enough plant nutrients in it.
3.2 Soil loses plant nutrients when water washes through it.
3.3 When crops are harvested and taken away the plant nutrients in them are taken away as well.
3.4. Fertilizing the soil increases the yield of the crop and so gives us more to eat or more profit.
4. There are 5 ways of fertilizing the soil:
4.1 Dig in well rotted compost.
4.2 Dig in a green manure such as coupe when the flowers have formed.
4.3 Dig in grey white plant ashes.
4.4 Pour liquid manure around the plants.
4.5 Use chemical fertilizers made in factories.

6.9.15.1 Plant nutrients, primary, secondary and micronutrients
Plant nutrients may be found in the rock that the soil is made from, or in the soil particles, or held loosely
in the soil usually attached to the very small clay particles. Plant nutrients that are loosely held like this can
easily be used by plants. The roots of the plants can take these nutrients from the soil. These plant
nutrients are available to the plants. However, sometimes plant nutrients are in the soil but the plants
cannot use them they may not be available to the plants. Quite often this happens because the soil is too
acid or too alkaline. Most plant nutrients are available to plants when the soil is slightly acid. If the soil is
too acid, you can add some lime to make it more alkaline. Dark swampy soils are often too. acid, so you
can make them less acid if you put drains in the soil and make it less wet. Soils made from coral rocks or
coral sand or limestone are often too alkaline. You can make these soils less alkaline by mixing the soil
with some acid, or by mixing some rotten compost with the soil or by adding ammonium sulfate fertilizer
with the soil.
1. Primary plant nutrients, N, P, K
One or more of these elements usually limits the yield of a crop, i.e. if there were more of this plant nutrient
in the soil the yield would be greater. These are the most important plant nutrients. They make from 2 to
6% of the dry weight of plants. One or more of these usually limits the yield of a crop, i.e. if there were
more of this plant nutrient in the soil, the yield would be greater. These are the most important plant
nutrients. They make from 2-6% of the dry weight of plants.
2. Secondary plant nutrients: S, Fe, Ca, Mg
These elements are needed in smaller amounts than the primary plant nutrients to make the plant body for
normal growth. There is usually enough of these in the soil for good crop yields.
3. Micronutrients (sometimes called trace elements), B, Cl, Co, Cu, Mn, Mo, Zn
Plants need these chemical elements in very tiny amounts for their normal growth.

6.9.15.2 Green manure
Many legumes such as cowpea and lablab bean are grown until the flowering stage and then dug into the
soil. When this plant material rots it leaves nitrogen and other plant nutrients in the soil both from the
leaves and root nodules. The root nodules contain bacteria called Rhizobium which take in nitrogen gas
from the air and change it into organic compounds. However, it is unlikely that green manure crops alone
can keep the soil fertile. You will probably have to use other fertilizers. The best green manure crops have
lots of soft green leaves and grow upright so it is easy to dig them into the soil before the seeds form. The
day before digging in a green manure crop wet the soil if it is dry. After digging in, keep the soil damp but
not wet. Dig the bed over again after 3 weeks and it should be ready for sowing with a new crop in
another 3 weeks.

6.33 Fertilizing soil
See diagram 6.65.1: Soil nutrient cycle 1 | See diagram 6.65.2: Soil nutrient cycle 2
See diagram 6.65.3: Nitrogen cycle
Be able to state the four reasons for fertilizing soil.
Use a drawing of the soil nutrient cycle, evidences of fertilizer activities and soils in the surroundings.
To fertilize means to make the soil richer by adding plant nutrients that will make the crops grow better
and produce a greater yield when they are harvested.
The four reasons why the soil must be fertilized:
1. Some soils have been formed from rocks that did not have enough of the chemicals that make plant
nutrients. In some tropical countries the parent rock has little potassium in it and so the soil formed from
that rock will be deficient in potassium.
2. Soils may lose plant nutrients when rainwater washes through them. This is called leaching. Soils that
have an open texture and do not have enough clay and rotten plant material in them lose many plant
nutrients by leaching.
3. When a crop is harvested and taken away, some plant nutrients are taken away as well. These plant
nutrients must be replaced if the soil is to remain fertile and produce good crops. Some plants need more
of some particular plant nutrients than others. You take away many those nutrients when the plants are
harvested.
4. Fertilizing increases the yield of a crop and allows us to feed more people or make more profit if the
crop is sold. The work done in making compost and adding it to the soil is rewarded by an increase in
profit. The plants get only a small fraction of their total plant nutrients from the soil. Most of the nutrients
come from the air. However, crops will not grow well unless they have all the plant nutrients they need.

Lesson
1. Today you will learn the four reasons why you must fertilize the soil. We will learn from a diagram.
First, here is the soil. The soil contains plant nutrients such as nitrogen, phosphorus and potassium. We
can see rocks deep in the soil. Are all the rocks the same? [No.] Do all rocks give the same nutrients?
[No.]
In some places the rocks are usually old coral reefs or volcanic rocks. These rocks do not contain much
potassium, so the soil needs more.
2. What happens to the soil nutrients? [They go into the sweet potato plant.] What else goes into the
plant, it is shown in circles. [Sunlight, carbon dioxide, water.]
3. How do the nutrients get out of the plant? There are two ways. [The pig eats the plant or the plant
dies.] Where are the nutrients now? [In the pig or in the rotten leaves on the ground.]
4. How do the nutrients get back to the soil? [Rotten leaves go into the soil or via the pig's faeces or
urine.] Do all the plant nutrients get back to the soil? [No.] Where do they go? [Pigs and tubers go to the
market and some nutrients are lost by leaching.
5. How do you grow more sweet potato and pigs? [By using fertilizer.]
Extra Activity: Make a chart of the nutrient cycle.

6.33.1 Potash fertilizer
Potash is an old name for potassium oxide, K₂O, but nowadays a "potash" fertilizer may contain any one
of, or any mixture of, potassium chloride, potassium sulfate, potassium magnesium sulfate or potassium
nitrate, but never potassium oxide itself. Formerly, the potassium content of fertilizers was expressed as
potash equivalents as a way of comparing the amount of potassium from the various potassium salts in it.

6.34a Chemical fertilizers
Be able to explain how chemical fertilizers should be used.
Use Illustrations and real examples of chemical fertilizers, technical information, artificial fertilizers.
It is unlikely that children will be using chemical fertilizers but you can explain as follows.
1. Chemical fertilizers can increase yields so much that all village gardeners should think about using them.
2. Chemical fertilizers are expensive because they are all imported.
3. The use of chemical fertilizers must be exactly as recommended by the agricultural officers. Otherwise,
money will be wasted. Recommendations are based on type of crop and soil grouping.

Soil Grouping:
1. Coral beach sand soils are weakly weathered, alkaline, have moderate amounts of phosphorus and low
amount of potassium.
2. Mineral beach sand soils are weakly weathered, weakly acid, and have moderate amounts of
phosphorus and potassium.
3. Alluvial soils are near rivers or on lower slopes, weakly weathered, weakly acid or alkaline, with good
amounts of phosphorus and moderate amounts of potassium.
4. Yellowish, brownish, reddish clay soils are mainly on coral limestone are weathered, acid, with good
amounts of phosphorus but low amounts of potassium.
5. Reddish clays on volcanic rocks are strongly weathered, acid, low on phosphorus and potassium. From
these descriptions of the soil groupings you can see that most gardens may not have enough potassium in
the soil. You will need a fertilizer bag. Find out the cost of fertilizer in the local area.
1. Have you seen chemical fertilizer being used? Show the children a fertilizer bag or packet. Read the
label for them. What is the price of this fertilizer?
2. Explain why these fertilizers are needed but why use them carefully based on the advice of an agricultural
officer.
Extra Activity: See technical information over. Visit a plantation or project to see the use of chemical
fertilizer.

6.37 Fertilizing the soil, methods
1. There are three methods of fertilizing the soil but the word "fertilizer" usually refers to artificial fertilizer.
Examine a bag of fertilizer, e.g. Muriate of potash or sulfate of potash, which contains potassium and sulfur.
"Potash" is an old name for potassium oxide., langbeinite (K₂Mg₂(SO₄)₃
Collect same well rotted compost in a jar. Examine the well-rotted compost in a glass jar and the fertilizer
bag. Read the words on the bag.
2. There are three ways in which a deficiency of plant nutrients can occur:
2.1 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.
2.2 The plant nutrients have been taken out of the soil by crops. When a crop is harvested, some plant
nutrients are lost.
2.3 The plant nutrients have been washed out by water.
3. There are two ways of increasing plant nutrients in the soil:
3.1 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.
3.2 Add fertilizer to the soil.
4. There are four methods of fertilizing:
4.1 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.
4.2 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.
4.3 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.
4.4 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.

6.37.1 Nutrition from the soil
1. The rate of plant growth reflects the ability of plants to extract nutrients from rocks. Grind samples of
quartzite, schist, basalt, limestone. Plant radish seeds in each sample and note rates of plant growth.
2. Good agricultural soils have low levels of "exchangeable" sodium. With high exchangeable sodium,
aggregates breakdown to form a dispersed layer causing waterlogging and later particles dry to form hard
clay. Use swelling clay from a dry clay pan, e.g. montmorillonite. Pack clay into 2 tubes. Add sodium
chloride to one tube and calcium chloride to the other tube. Pass water through both tubes and note the
different rates of water passing.
3. Put a layer of cotton wool in five Petri dishes then add:
3.1 50 mL of normal nutrient solution,
3.2 50 mL of nutrient solution without nitrogen,
3.3 50 mL of nutrient solution without potassium
3.4 50 mL nutrient solution without iron
3.5 50 mL of deionized water.
Put 10 small same size plants on the cotton wool in each dish. Put the dishes in an empty fish tank with a
glass top to form a moist chamber. Look at the growth of the plants every two days. After two weeks
there is an obvious difference in the growth of the plants in the various dishes.
The plants in 3.1 are thriving best of all, while the plants in 3.5 are the worst. The plants in 3.2 are almost
as badly developed as those in 3.5. The plants in 3.3 are better developed. The plants in 3.4 are as large
as the plants in 3.3 but are yellow-green, chlorotic.
4. Collect white ash from burnt wood. The black ash is carbon. Show the white ash you have collected.
Let students taste it. The taste is salty. The ash contains plant nutrients. Show 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: nitrogen - for plenty of strong green leaves, phosphorus
- for root growth and making fruit, potash (potassium oxide) - for healthy plants, potash (potassium
oxide) - for healthy plants. Other important plant nutrients are as follows: sulfur and iron for green leaves,
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. Composition of mature maize plant dry matter: Oxygen 46.43%, Carbon 43.57%, Hydrogen 6.24%,
Nitrogen 1.46%, Phosphorus 0.20%, Potassium 0.92%, Calcium 0.23%, Magnesium 0.18%,
sulfur 0.17%, Iron 0.08%, Silicon 1.172, Aluminium 0.11%, Chlorine 0.14%, Manganese 0.04%,
Trace elements 0.093%
Ten elements are essential for the growth of a green plant. Carbon (C), Hydrogen (H), Oxygen (O),
Nitrogen (N), Sulfur (S), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg) and Iron (Fe).
Plants take in carbon dioxide from the air and hydrogen and oxygen from the water in the soil. Plants
absorb other elements with the soil water as salts.

6.38 Plant foods
1. Plants need two kinds of plant foods:
1.1 Main plant foods called nitrogen, phosphorus and potash.
1.2 Minor plant foods and trace elements. The word "trace" means a very little.
One of these traces is Iron and you know that people sometimes bury pieces of old iron under coconut
trees. When plants gather plant foods from the soil, they take these foods into their own bodies the roots,
stems, leaves and flowers. Most of the plant foods are stored in the plants above the soil. Even when a
plant dies or a leaf falls off, the plant foods are still there. Some plant foods are in the soil and some are
stored in the stems and leaves of plants. Some plant foods are lost when people harvest and eat the plants.
These plant foods leave their bodies in the toilet. Some plant foods are lost when plant leaves and stems
are burnt. Some plant foods are lost when animals eat them, e.g. Pigs kept in pens or houses.
2. You can return plant foods to the soil in these ways:
2.1 Dig dead leaves and stems of plants into the soil.
2.2 Burn plants and put the ash in the soil.
2.3 Collect manure from chickens and pigs to make compost for growing plants.

6.39 Plants need nitrogen, nitrogen cycle
See diagram 6.65.1: Cycle of nutrients | See diagram 6.65.3: Nitrogen cycle
Nitrogen is the most important plant food. All animals and plants need nitrogen. Plants and animals will
not grow well if they do not have enough nitrogen. Nitrogen gas in the air, but most plants and animals
cannot use it. Nitrogen occurs in fish, animals like chickens and pigs, animal wastes, plants called legumes
and nitrogen fertilizers, e.g. urea. Some foods, e.g. bananas, papaya (pawpaw) and breadfruit, contain
very little nitrogen. Students do not grow fast if their parents give them only these foods and boiled white
rice but not much fish or meat. Legumes are the pea and bean plants. Legumes are different from other
plants because they have small lumps on their roots called nodules. The nodules can catch the nitrogen gas
from the air in the soil and use it to build their bodies. So the bodies of legume plants contain much nitrogen.
Nitrogen is lost when heavy rain falls on the soil. However, rain will not wash away the nitrogen if it has
much humus in the soil to hold the nitrogen. When leaves and plants burn, some nitrogen goes back in to
the air as a gas. Nitrogen is added to the soil when people use compost for their plants, when legumes
grow in the soil or when leaves and stalks of legumes are used to make compost, and when people add
animal manure to the soil.
Nitrogen is lost when: heavy rain washes it out of the soil, plants are burned by fire, animal manure and
urine do not go back to the soil.
Nitrogen can be added to the soil when: you put compost on their plants, you grow legumes in the soil or
use them to make compost, you put animal manure around plants or use it to make compost.
You can keep nitrogen instead of losing it. Nitrogen can go from the soil to plants, to animals and then
back to the soil again.

6.40 Legumes for the soil
See diagram 9.72: Root nodules | See diagram 9.72.1: Legume plants
See diagram 9.72.2: Legume flower | See diagram 9.209: T. S. Root nodule
Legumes used for food are commonly called peas and beans. A bacterium (plural bacteria) called
Rhizobium can get into the roots of legumes. Here they cause lumps called root nodules where they live.
The bacteria can take the nitrogen gas from the air and put it into their bodies. Rhizobium can "fix"
nitrogen from the air. Very few other living thing can fix nitrogen. Some of this nitrogen goes into the stems
and leaves of the legume plant. When the leaves fall off, some nitrogen is added to the soil. Other plants
can then use the nitrogen to make them grow better. When the legume plants die, the nitrogen fixed by
the Rhizobium can still be available to growing plants. If you cut legumes and put them into compost it
will be very much better. To make good compost you must add something that contains much nitrogen.
Legumes are very good to feed to animals because legumes contain much nitrogen.

6.41 Make compost
1. Before teaching this lesson, ask a field officer from the Ministry of Agriculture about compost heaps.
In some places the Department of Agriculture does not approve compost heaps because they can be home
for insect pests. Prepare to make compost heaps about 2 m × 2 m long and about 1 m high. Many plants
do not grow well in coral soils because they are not good soils. The way to make good soil is to put much
organic matter into it. Organic matter is anything that contains plant or animal material that was once living,
e.g. dead leaves and animal manure. When you put organic matter into the soil bacteria turn them into dark
humus, another kind of organic matter.
2. The reason that organic matter in the soil is good for plants is that it has two functions:
2.1 It holds water very well and can give this to plants.
2.2 It holds plant foods very well and can give these to plants.
3. To make a compost heap use leaves of different plants, e.g. beach bean Canavalia rosea, jack bean
Canavalia ensiformis, chicken manure, pig manure and fish scraps. You can sprinkle a little nitrogen
fertilizer over the compost layers but this is expensive. Build the compost heap by making layers of dead
leaves, black soil, and some manure or other nitrogen containing substances. Do this again so you have
many thin layers one on top of the other. Then water the compost heap to make it damp. Then cover it
with dead coconut leaves to keep the hot sun from making it dry. After five weeks, turn the compost
layers over onto another place. Mix up all the layers. Then water it again and cover it with coconut leaves.
After another five weeks, do this again. In about three months the compost will be ready to use. If it has
been a dry time, it may take a little longer to be ready. You can then mix with some soil - half of each -
and use the compost to make a garden bed.

6.42 Artificial fertilizers
See 12.14.5: Superphosphate production
Artificial fertilizers are expensive so you can use them only if your agriculture project has good rainfall and
is close to a market. A fertilizer is a substance that is very rich in plant foods. Simple fertilizers contain
only one kind of plant food, e.g. if the fertilizer urea contains only nitrogen. Mixed fertilizers contain
several plant foods. The three main plant foods are nitrogen, N, phosphorus, P, and potash, K₂O, and are
contained in the following simple fertilizers. Urea contains only nitrogen. Ammonium sulfate contains only
nitrogen. Superphosphate contains phosphorus and calcium. Sulfate of potash contains only potash.
Chloride of potash only potash. Mixed fertilizers are named by numbers. You always use these numbers in
the same order: nitrogen, phosphorus, potash, or N, P, K. Thus, 100 kg of the mixed fertilizer 20-14-14
contains 20 kg of nitrogen, 14 kg of phosphorus, 14 kg of potash. Other mixed fertilizers are 9-25-25,
13-13-13 and 13-1-21. Do not put too much fertilizer on the soil, but just sprinkle it on lightly. Do not put
fertilizer too close to the plant stem but under the outer leaves. Put some mixed fertilizer on half a vegetable
bed to see the effect of the fertilizer.

6.43 Chalk (lime) content of the soil
The chalk (lime) content of the soil is important for plants. It affects the quality of the soil, e.g. its acidity,
heat retention capacity, water balance and aeration. Calcium, an antagonist of potassium, plays a direct
role in swelling processes and is also a plant nutrient. The soil contains salts which plants have taken and
used as nutrients. 1. Put a small amount of each soil sample on a watch glass. The soil sample may be
fresh or air dried and should cover an area on the watch glass 2 -3 cm in diameter. Add 3-5 drops of 5%
hydrochloric acid to the soil sample using a pipette. The intensity of the reaction that occurs is an
approximate indication of the chalk content of the soil. Take soil samples from as many different places as
possible. Compile a table of results.

6.44 Nutrient cycles
See diagram: 6.0: Nutrient cycle 1 | See diagram 6.0: Nutrient cycle 2
When you harvest a crop you are taking away nutrients from the soil. These nutrients must be replaced if
the soil is to remain fertile.
When plant and animal material is being added to the soil (arrow 4 and arrow 8) they contain not only
nutrients but also substances such as sugars produced by photosynthesis.
No. 1 The plant roots take in plant nutrients from the soil and rocks.
No. 2 The plant uses the plant nutrients to make it grow and for photosynthesis in the leaves.
No. 3 Some plant nutrients are stored in the sweet potato (kumara) tuber.
No. 4 Dead leaves and stems containing plant nutrients fall to the ground and rot in the soil.
No. 5 The plant nutrients from the rotten leaves and stems can be taken in again by the roots.
No. 6 A pig eats the sweet potato (kumara) tuber and some leaves.
No. 7 Most of the nutrients are used to make the pig grow.
No. 8 Some nutrients leave the pig in the faeces and urine.
No. 9 Nutrients from the faeces and urine can be taken in again by the plant roots.
No. 10 The sweet potato tuber is harvested and taken away or the pig is taken away to be eaten.
The nutrients in the sweet potato tuber and in the pig cannot be put back into the soil, they are lost.
The lost plant nutrients can be replaced by the following:
1. Fallow gives time for more plant nutrients to come from the soil, See arrow No. 1.
2. Green manure adds nitrogen and other plant nutrients from the body and nodules of legume plants.
3. Fertilizing with rotted compost. fertilizing with animal manure.
4. Fertilizing with artificial fertilizer.

6.46 Crop rotation
Legume root
Collect examples of plants used in crop rotations in the school gardens. Plants can seem different yet be
in the same family. Plants from the same family have similar flowers, e.g. legume family, pumpkin family.
One way to control plant pests and diseases is to follow a rotation. In a rotation you do not let the same
crops follow in the same piece of land.
An example of a crop rotation:
Crop 1 corn (maize) or sorghum (grain crop)
Crop 2 sweet potato (kumara) or cassava or yam or taro (root crop)
Crop 3 Chinese cabbage or lettuce (leafy crop)
Crop 4 Mung bean or snake bean or peanut or cowpea (legume crops) or crotalaria or pueraria or
centrosema (legume cover crop)
In the rotation you may have a fallow when you grow no crop, or a green manure fallow when you grow a
legume crop and dig it into the soil to rot before the next crop is planted. The legume crop will fertilize the
soil when the root nodules and the rest of the plant rots and add plant nutrients such as nitrogen to the soil.
Rotations control disease because the same kinds of plants or plants from the same families of plants will
have the same pests and diseases. So if you let two different plants from the same family of plants follow in
the rotation, the pests and diseases from the first crop will attack the following plants in the next crop.
Some food crops in their families:
1. Bean family (legumes): mung bean, peanut, snake bean, winged bean, cowpea, crotalaria, pueraria,
centrosema
2. Pumpkin family: pumpkin, melon, cucumber, snake gourd
3. Tomato family: tomato, eggplant, chilli, tobacco
4. Taro family: taro, Chinese taro, wild taro
5. Cabbage family: cabbage, radish, Chinese cabbage
These are two other reasons why a rotation should be followed:
1. Different kinds of plants take up different kinds of and amounts of plant nutrients from the soil. So a
rotation allows a soil to be more fertile.
2. Different kinds of plants have different kinds of roots. So a rotation helps the soil to keep a good
structure.

Crop rotation, revision questions
Give an example of a 4 crop rotation. [crop 1: corn (maize), crop 2: sweet potato (kumara),
crop 3: Chinese cabbage, crop 4: cowpea]
What is green manure? [A legume crop which will be dug into the soil to rot.]
What does the green manure add to the soil? [Mainly nitrogen and other plant nutrients from the rotted
root nodules and body of the plant]
What are the three reasons why you should use rotations? [Control pests and diseases Allows soil to be
more fertile Allows soil to keep a good structure]
Give examples of two members of the pumpkin family [Melon, Snake gourd]
Give examples of 2 members of the tomato family. [Eggplant (Aubergine) Chilli]
Give examples of two legume cover crops. [Crotalaria, Centrosema]

6.47 Water lens in atolls
See diagram: Atoll water lens
The water lens deep under the soil contains freshwater. The coral rock of the island is full of small holes.
So seawater can go right through the coral rock and sand under the island. However, when it rains, the
freshwater pushes the salt water out and makes the water lens. You can dig wells to find this freshwater.
The water lens is on the same level as the mid tide level, but is slightly higher in the middle of the island.
Freshwater is not as heavy as salt water and it floats on top of it. The lens in thinner near the shores. The
lens water rises and falls with the tides. If no rain for some time, the salt water comes into the water lens
and makes the lens water salty.

6.48 How soils form in atolls
See diagram: Forming an atoll 1 | See diagram: Forming an atoll 2 | See diagram: An atoll and its
peripheral reef (cross-section)

6.49 How atoll soils change
When soils change they may become better or worse for plants to live in. Before the lesson, look for
examples of soil changes near your school. Also, in this lesson the students record the plants growing in
different soils to show that many plants only grow in one kind of place and kind of soil. So plants can
indicate the kind of soil under them.
Coral soils may change in many ways as follows:
1. The dead leaves of plants fall onto the soil and rot. This gives the topsoil a dark colour.
2. Strong winds may blow sand over the top of the soil and cover it. A new dark topsoil layer may then
form over the old layer. Sometimes in a profile you can see the old buried soil.
3. Burning grass will leave black charcoal (carbon) in the soil. You may see layers of charcoal in the soil
profile.
4. The light grey stones of floating pumice may be washed onto the island. You may see layers of this rock
in a soil profile. This pumice layer can provide some plant foods for coconuts and other plants.
5. Birds may gather in one place and leave their droppings (faeces) there. The droppings contain plant
foods and people may collect them for fertilizer (phosphate fertilizer).
6. Humans can change soils too, making them worse, by burning the grass, or making then better, by
adding compost.

When soils change the plants may also change:
1. Some plants can live in salt spray blown in from the sea, e.g. Pandanus, coconuts, salt bush, but some
plants do not like salt spray, e.g. breadfruit.
2. Some plants can live in a drought, e.g. salt bush, and Pandanus but some plants may die in a drought,
e.g. coconuts.
3. Some plants are found on the ocean side and some plants are mostly found on the lagoon side of an
island.
4. Go to the ocean side and list plants growing there. Then go to the lagoon side and list plants growing
there.

History
These lessons 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 or made available to UNESCO by PHYWE SYSTEME
GMBH, Robert-Bosch-Breite 10, D-37070, Gottingen, Germany and edited by Dr J. Elfick, School of
Education, University of Queensland, Brisbane, Australia, or are based on the lessons in the New
UNESCO source book for science teaching, Third impression 1979, ISBN 92-3-101058-1, and edited
by Dr J. Elfick, School of Education, University of Queensland, Brisbane, Australia, working under
UNESCO Contract No. 8347201, 2001-12-15. Experiments 32 to 40 were written by Dr J. Elfick,
School of Education, University of Queensland, Brisbane, Australia. The experiments in this file were
reviewed and edited by soil scientist Dr R. C. Bruce in July, 2005.