School Science Lessons
Soils 2
Updated: 2009-04-11
Please send comments to: J.Elfick@uq.edu.au
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Table of Contents
6.37 Fertilizing the soil
6.37.1 Nutrition from the soil
6.38 Plant foods
6.39 Plants need nitrogen, nitrogen cycle
6.40 Legumes for the soil
6.41 Make compost
6.42 Artificial fertilizers
6.43 Chalk (lime) content of the soil
6.44 Nutrient cycles
6.46 Crop rotation
S15. Coral soils
6.47 Water lens in atolls.
6.48 How soils form in atolls
6.49 How atoll soils change
6.37 Fertilizing the soil
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 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. 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, 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 X 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) 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 only phosphorus. 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
See diagram 5.6.5: Legume root | See diagram 9.72.2 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. On 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, egg plant, 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 sea water 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.