School Science Lessons
(Soils2)
2024-09-16
Soil fertility, Soil tests
Contents
16.8.0 Coral island soils
6.9.0 Soil fertility
6.10.0 Soil tests
6.9.0 Soil fertility
6.9.01 Artificially-prepared fertilizers
6.9.02 Chalk (lime), content of the soil
6.9.3 Crop rotation
6.9.4 Factors affecting plant growth, Latin Square
6.6.1 Fertility of different soils
6.9.5 Fertilizer trial
6.9.6 Fertilizing soil
6.9.7 Fertilizing the soil, methods
6.9.8 Reasons for fertilizing soil
6.9.9 Green manure
6.9.10 Improve soil fertility and pH
6.9.11 Legumes for the soil, make compost
6.9.12 Nutrient cycles
6.9.13 Nutrition from the soil
6.9.14 Plant foods
6.9.15 Plant nutrients
6.9.16 Prepare potash from ash
6.9.17 Nitrogen cycle
6.9.18 Salinity, leaf versus root application of water
6.9.19 Soil forms from rocks by heating
6.9.20 Soil forms from rocks by mechanical action
6.9.21 Soil improvement
6.9.22 Soil structure
6.9.23 Soil tilth
6.9.24 Soil watering, deep pipe irrigation
6.9.25 Weathering soils
6.9.26 Soil fertility decline
6.10.0 Soil tests
6.10.0 Soils tests
6.10.1 Calcium : magnesium ratio soil test
6.10.2 Cation exchange capacity soil test (CEC)
6.10.3 Conductivity
6.10.4 Exchangeable cations soil test
6.10.5 Nitrate nitrogen soil test
6.10.6 Organic carbon soil test
6.10.7 pH soil test
6.10.8 Phosphorus soil tests, Bray test and Colwell test
6.10.9 Tests for soil organic matter
6.10.10 Trace elements soil test
6.8.0 Coral island soils
6.8.1 Atoll water lens
6.8.2 How soils form in atolls
6.8.3 How atoll soils change
6.9.01 Artificially-prepared fertilizers
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.
Straight fertilizers, simple fertilizers, contain only one kind of plant food.
Urea, ammonium nitrate and ammonium sulfate fertilizer contain only nitrogen.
Chloride of potash and sulfate of potash contains only potassium.
Superphosphate contains only phosphorus.
Single superphosphate (SSP), contains 14-18% P2O5, in the form of Ca(H2PO4),2.
Mixed fertilizers contain several plant foods.
The three main plant foods are nitrogen, N, phosphorus, P, and potash, K2O.
NPK grade formula
Study the grade formula of artificial fertilizers.
If the fertilizer contains 13% nitrogen, 13% phosphorus and 21% potassium
100 grams of the fertilizer would contain 13 g nitrogen, 13 g phosphorus and 21 g potassium.
The grade formula is NPK =13:13:21.
Other artificial fertilizers: muriate of potash (NPK = 0:0:50), superphosphate (NPK = 0:9:0), sulfate of ammonia (NPK = 21:0:0), urea (NPK = 46:0:0),.
6.9.02 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.
Experiment
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.
6.9.3 Crop rotation
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 do not let the same crops follow in the same piece of land.
An example of a crop rotation:
Crop 1 Grain, e.g. maize or sorghum
Crop 2 Root crop, e.g. sweet potato or cassava or yam or taro
Crop 3 Leafy crop, e.g. Chinese cabbage or lettuce
Crop 4 Legume, e.g. Vigna radiata (mung bean, green gram), or Vigna unguiculata ssp. Sesquipedalia (snake bean, yard-long
bean), or Arachis hypogea (peanut, groundnut), or Vignaunguiculata (cowpea), or Crotalaria (rattlepod), or Pueraria(kudzu),
or Centrosema (butterfly pea)
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.
If plants from the same plant family follow in the rotation, the pests and diseases from the first crop are likely to attack the following plants in the next crop.
Food crops in their families:
Bean family (legumes),: mung bean, peanut, snake bean, winged bean, cowpea, Crotalaria, Pueraria, Centrosema
Pumpkin family: pumpkin, melon, cucumber, snake gourd
Tomato family: tomato, eggplant, chilli, tobacco
Taro family: taro, Chinese taro, wild taro
Cabbage family: cabbage, radish, Chinese cabbage
These are two other reasons why a rotation should be followed:
* 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.
* Different kinds of plants have different kinds of roots.
So a rotation helps the soil to keep a good structure.
6.9.4 Factors affecting plant growth, Latin Square
In the above experiment, pots of seedlings can be given different levels of treatment, e.g. 1 to 5, and each level replicated three times as replicates A, B and C.
For example, an experiment to test the effect of a treatment of five different levels of salinity on sweet basil and try each level in three replicates.
The concentrations of salt in the water in g / L are 0, 10, 20, 30, 40 and 50.
Note the concentration of 0 g / L is the control. So using a total of 15 pots (5 treatments x 3 replications), could be set out as in the following diagram.
The different concentrations of salty water are added carefully to each pot.
Treatment
 Concentration
 Replicate
A
 Replicate
B
 Replicate
C
1
 0 g / L
 1a
 1b
 1c
2
 1 g / L
 2a
 2b
 2c
3
 2 g / L
 3a
 3b
 3c
4
 10 g / L
 4a
 4b
 4c
5
 20 g / L
 5a
 5b
 5c
However, the experiment as set out above can be spoilt by "nuisance factors".
The outer pots could get more breeze or light.
The pots facing north could get more sunlight.
The pots in the centre could get more water when being sprinkled.
Any nuisance factors can be avoided by arranging the pots in a Latin Square, for equal numbers of treatments and replicates, or
a Latin Rectangle, for when the number of treatments is more or less than the number of replicates.
A Latin Square design is a method of placing treatments in a randomized pattern within a block.
Treatments appear once in each row and column.
Replicates are also included in this design.
Latin Square
The following diagram shows one of the many Latin Rectangle arrangements for a 5 x 3 array.
Replicates
 Replicates
 Replicates
1a
 2b
 5c
2a
 4b
 1c
3a
 1b
 2c
4a
 5b
 3c
5a
 3b
 4c

6.9.5 Fertilizer trial
If a soil does not have enough of a particular plant nutrient, e.g. nitrogen, the soil is deficient in nitrogen.
The only sure way to find out whether a soil is deficient in any plant nutrients is by chemical testing.
If a soil does not have enough of a particular plant nutrient, e.g. nitrogen, the soil is said to be deficient in nitrogen.
You can tell if the soil is deficient in plant nutrients by examining your crop plants carefully.
Experiment
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?
6.9.6 Fertilizing soil
| See diagram 6.65.1: Soil nutrient cycle1
| See diagram 6.65.2: Soil nutrient cycle 2
| See diagram 6.65.3: Nitrogen cycle
| 9.14.0 Composting, C/N ratio, humus, worm farms
| 12.14.5 Superphosphate production:
Use a drawing of the soil nutrient cycle, evidences of fertilizer activities and soils in the surroundings.
To fertilize soil is to make the soil richer in plant nutrients that will make the crops grow better and produce a greater yield when they are harvested.
Plants get only a small amount of total plant nutrients from soil, but crops will not grow well unless you give the plants all the plant the nutrients they need.
The four reasons why the soil must be fertilized:
* Some soils have been formed from rocks that did not have enough of the chemicals that make plant nutrients.
They have a natural deficiency of plant nutrients.
In some tropical countries the parent rock has little potassium in it.
The soil formed from coral rock will be deficient in potassium.
* 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.
* 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.
* 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 five ways of fertilizing the soil:
* Dig well rotted 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.
*. Dig a green manure into the soil.
Legume crops, e.g. 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.
* Dig grey-white plant ash into the soil.
* Pour liquid manure around the plants.
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.
* Use chemical fertilizers made in factories, e.g. muriate of potash (potassium chloride, KCl),.
* Stop farming the land for one growing season.
The plant nutrients will slowly be added to the soil from soil particles and rotten plants.
This practice is called fallow.
Experiment
Plant nutrients
Do this experiment in the school garden
1. Burn some crop plants until all the black carbon is gone and only some grey-white plant ash remains.
Let the students taste this ash.
It tastes salty.
They are tasting some of the plant nutrients which are chemicals that the plants take in from the soil.
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.
Fertilize the soil so that the plants will grow better.
6.9.7 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.
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:
* There is a natural deficiency, because there was not much of the plant nutrient in the original rock from which the soil was made.
For example, soils made from coral rock are deficient in many plant nutrients.
* The plant nutrients have been taken out of the soil by crops.
When a crop is harvested, some plant nutrients are lost.
* The plant nutrients have been washed out by water.
3. There are two ways of increasing plant nutrients in the soil:
* 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.
* Add fertilizer to the soil.
4. There are four methods of fertilizing:
* 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.
* 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.
* 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.
* 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.
6.9.8 Reasons for fertilizing soil
Use a drawing of soil nutrient cycle, evidences of fertilizer activities and soils in the surroundings.
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 you are harvested.
1. The four reasons why the soil must be fertilized:
* 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 has little potash in it and so the soil formed from that rock will be deficient in potassium.
* Soils may lose plant nutrients when rain water 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.
* 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 a particular plant nutrient than others and you take away a lot of that nutrient when you are harvested.
* Fertilizing the soil increases the yield of a crop and allows us to feed more pupils 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.
6.9.9 Green manure
See: 9.70: Rhizobium in legumes
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.
6.9.10 Improve soil fertility and pH
See: pH (Commercial)
Improve the soil fertility by:
* adding plant nutrients as well rotted compost, unless they do not allow compost,
* adding animal manure or water in which animal manure is soaking,
* adding green manure, dig in a legume crop such as cowpea when the flowers are forming,
* adding grey white wood ash, which contains potash (potassium oxide),
* adding manufactured fertilizer,
Ammonium sulfate for nitrogen and sulfur,
Muriate of potash for potash,
Sulfate of potash for potash and sulfur,
Superphosphate for phosphorus and sulfur,
Mixed fertilizer (NPK), for nitrogen, phosphorus and potash.
* Improve the pH
Most plants like soil to be a little bit acid (pH 6 to pH 7).
If soil is too acid (pH 5 to pH 1), add lime such as burnt crushed shells or coral sand.
If soil is too alkaline (pH 8 to pH 14), add well rotted compost or ammonium sulfate.
6.9.11 Legumes for the soil, make compost
| 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 muchbetter.
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.
Make compost
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.
The reason that organic matter in the soil is good for plants is that it has two functions:
1. It holds water very well and can give this to plants.
2. It holds plant foods very well and can give these to plants.
Experiment
To make a compost heap use leaves of different plants:
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.
6.9.12 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 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 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:
* Fallow gives time for more plant nutrients to come from the soil, See arrow No.
* Green manure adds nitrogen and other plant nutrients from the body and nodules of legume plants.
* Fertilizing with rotted compost and animal manure.
* Fertilizing with artificial fertilizer.
6.9.13 Nutrition from the soil
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.
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.
Composition of mature maize plant dry matter:
O 46.43%, C 43.57%, H 6.24%, N 1.46%, P 0.20%, K 0.92% Ca 0.23%, Mg 0.18%, S 0.17%, Fe 0.08%, Si.0.17, Al 0.11% Cl 0.14%, Mn 0.04%, Trace 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.
Experiment
1. Put a layer of cotton wool in five Petri dishes then add:
50 mL of normal nutrient solution,
50 mL of nutrient solution without nitrogen,
50 mL of nutrient solution without potassium,
50 mL nutrient solution without iron, and
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.
Examine 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.
2. 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 makefood, 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.
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, we say it is deficient in potash.
Plants absorb other elements with the soil water as salts.
6.9.14 Plant foods
(Teachers should note that although we use the term "plant food:", it refers to minerals in the soil that plants use to make "food", i.e. carbohydrates, proteins, and fats.)
1. Plants need two kinds of plant foods:
* Main plant foods called nitrogen, phosphorus and potash.
* 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:
* Dig dead leaves and stems of plants into the soil.
* Burn plants and put the ash in the soil.
* Collect manure from chickens and pigs to make compost for growing plants.
6.9.15 Plant nutrients
1. The non-mineral nutrient elements are hydrogen, H, oxygen, O, carbon, C from air and water used in photosynthesis to form starches and sugars.
The mineral nutrient elements from the soil and dust are dissolved in water and absorbed through the plant's roots.
The macro nutrients are divided into the primary nutrients and secondary nutrients.
The primary nutrients are nitrogen, N, phosphorus, P, potassium, K, and are used by plants in large amounts.
The secondary nutrients are calcium, Ca, magnesium, Mg, and sulfur, S.
Ca and Mg are added to the soil when lime is applied to acidic soils.
Sulfur is available from the decomposition of organic matter in the soil.
The micro nutrient elements are boron, B, copper, Cu, iron, Fe, chloride, Cl-, manganese, Mn, molybdenum, Mo, and zinc, Zn.
2. Trace elements are chemical elements present in organisms in very low concentrations.
The term trace elements is used to designate a number of chemical elements contained in soils, rock, minerals, and water, but precise quantitative criteria for distinguishing trace elements from major elements have not been established.
Some major elements of soils and rock, e.g. aluminium and iron, are trace elements for most animals and plants, as well as for humans.
The mobility of trace elements and their availability to plants are greatly affected by the acidity of the soil, the humidity, and the content of organic matter.
Some soils are rich in mobile forms of boron and copper (0.4-1.5 and 4-30 mg / kg), and some are poor in them (0.02-0.6 mg / kg),.
There is an insufficiency of molybdenum, cobalt, manganese and zinc in some soils.
An insufficiency or excess of trace elements in soil leads to an insufficiency or excess of the elements in plant and animal organisms.
A deficiency of molybdenum suppresses the blooming of cauliflower and some legumes.
Copper insufficiency disrupts grain production of cereals and fruition of citrus trees.
Boron deficiency cause poorly developed receptacles and the absence of flowering in peanuts, the withering of buds in apple and pear trees, drying of racemes on grapevines, and withering of peanuts and cabbage.
However, excess boron causes rotting of roots, chlorosis, and formation of galls.
3. 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.
4. 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.
5. 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.
6. Micro nutrients (sometimes called "trace elements"), B, Cl, Co, Cu, Mn, Mo, Zn.
Plants need these chemical elements in very tiny amounts for their normal growth.
6.9.16 Prepare potash from ash
Potash is an old name for potassium oxide, K2O, Nowadays a "potash" fertilizer may contain potassium chloride, potassium sulfate, potassium magnesium sulfate, potassium nitrate, but not potassium oxide itself.
Formerly, the potassium content of fertilizers was expressed as potash equivalents to compare the amount of potassium from the various potassium salts in it.
Langbeinite, K2Mg2(SO4),3, may be in "potash" fertilizers
Use lots of white wood ash from burning large pieces of wood until only the soft ash is left.
Glass jars or bottles.
Filter paper or any soft paper or cotton wool.
Filter funnel empty coconut shell with a small hole in it, burners and burner stands or a small fire.
1. What was left after you burned the wood? [Carbon.]
How can you make ash? [By burning wood in a very hot fire.]
2. Put the ash in a beaker or jar so that it is half full.
Put some water in and stir up the ash and water very well.
Filter the liquid from the jar into another jar or bottle.
Put some filtered liquid on a tin lid.
Heat the liquid so that it evaporates.
What is left? [Some white substance.]
3. What does the white substance taste like? [Salty.]
Where does the white substance come from? [The wood ash.]
The wood ash contains something like salt called potash.
Potash is a very good fertilizer for the gardens.
4. What kind of substance was found in the wood? [Carbon.]
Where does the wood get the salty substance from? [From the soil.]
5. If the people near their school get salt from ash.
6. How you can test whether potash is a good fertilizer or not. [Fertilizer trial.]
6.9.17 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.
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 from legumes in the soil or from leaves and stalks of legumes in compost, and from animal manure added 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.
6.9.18 Salinity, leaf versus root application of water
See: Salinity (Commercial)
The effect of leaf versus root application of water to a plant and the effect of salinity on crops.
In Australia dry land salinity is caused by the removal of deep rooted trees and shrubs and their replacement with crops that do not use as much water.
Irrigation salinity is caused by heavily drawing on the water table, bringing water to the surface that contains high level of sodium chloride and other salts.
Experiment
Investigate the effect of different concentrations of sodium chloride in water on two methods of application:
1. Foliar and root applications by sprinkler systems
2. Direct root applications by drip irrigation systems on a young sweet basil crop.
The two manipulated variables are as follows:
* The continuous variable of salt concentration and
* The categorical variable of the method of application, sprinkler or drip.
Use concentrations: 0 ppt (parts per thousand, or g / L), 1 ppt, 2 ppt, 10 ppt, 20 ppt.
Record the cumulative harvestable leaf number per stem for foliar and root applications and direct root applications of NaCl solution.
As salinity increases past 0 ppt, harvest drops 0-100%.
Direct root application tests performs better than foliar and root application tests, because of more advanced restrictive mechanisms of the roots.
So use drip irrigation and a minimized use of sprinkler systems, because saline water on leaves seriously damages crops.
6.9.19 Soil forms from rocks by heating
Experiment
Heat rocks then pour cold water on them.
The rocks often break up both when being heated and when being rapidly cooled.
One stage in the formation of soil is the breaking up of rocks under changes of temperature.
6.9.20 Soil forms from rocks by mechanical action
Roots can penetrate between layers of rooks and push them apart.
Find some soft rocks such as shale or weathered limestone in yourlocality.
Crush and grind them into small particles.
Soil forms from rocks by acids produced by plant roots and decaying plant material, humus.
Experiment
Find soils with a deep layer of humus and test the pH.
6.9.21 Soil improvement
Bring to the classroom samples of sandy soil, clay soil, and a good soil.
1. Show the students handfuls of:
* sandy soil that slips between the fingers,
* clay soil that is sticky, you can make it into a ring,
* good soil that is dark in colour, and forms crumbs when you open your hand (crumb structure).
Let the students feel the three soils in their hands.
6.9.22 Soil structure
See diagram 6.8.0: Good structure, a soil aggregate, capillary water
| See diagram 6.8.1: Plough pan
A good structure is shown by the soil forming crumbs in the opened hand, because the particles stick together.
You can improve soil structure by digging well rotted compost into the soil.
Use animal manure if they do not allow compost in your region.
Experiment
1. To observe soil structure, carefully dig up a block of soil and put it in your hands with fingers closed around the block.
Open your hands and observe how the soil particles stick together.
Good structure is shown by the soil forming crumbs in the opened hand, because the particles stick together.
2. Soil structure are describes as follows:
* Columnar structure is caused by vertical cracks so the sample consists of columns
* Blocky structure is caused by vertical and horizontal cracks to form blocks, 2-5 cm in diameter
* Granular structure is caused by vertical and horizontal cracks to form blocks about 0.5 cm in diameter.
This structure is best for root penetration, drainage and aeration
* Plate-like structure is caused by cracks more horizontal than vertical to form plate-like blocks of compacted soil
3. You can improve soil structure by digging well rotted compost into the soil.
Use animal manure if they do not allow compost in your region.
4. The soil structure refers to the way soil particles (sand, silt and clay), and organic matter are grouped together to form soil aggregates.
Soils without structure include:
* Soils in sand dunes consisting of single particles of sand that do not cling together.
* Soils consisting of particles that stick together in one solid mass (massive soils),.
* Soils with good structure break up easily into soil aggregates with definite shapes and sizes.
5. The soil particles hold together, because of a binding agent in the soil to bind the individual particles of sand, slit and clay together.
The most common binding agent is a colloid.
Other binding agents are oxides of iron and aluminium.
6. Colloids can be formed from very small clay particles (inorganic colloid), or from dead and decaying organic matter (organic colloid).
Colloids bind soil particles together, attract plant nutrients to the surface and retain soil moisture.
7. A soil with good structure will easily take in water through the spaces between the soil aggregates.
Water storage capacity of the soil, is a measure of the water is stored inside the soil aggregates + water stored in the large spaces between the soil aggregates.
4. Soil aeration is a measure of the space formed when excess water drains through the soil spaces to be replaced with air.
5. A soil with good structure loses less soil particles on the surface from being washed away by water or blown away by wind.
6. Porosity of the soil is the space between the soil aggregates + the space within the soil aggregates.
7. Soil structure is destroyed by:
* Too much cultivation that breaks up soil aggregates resulting in a dust layer on the soil surface that may restrict air and water penetration and can blow or wash away
* Compaction either by creating a plough pan or from the pressure of the tractor tyres.
Plough pans reduce water and root penetration.
Excessive cultivation, may produce a compaction layer that limits root development, because crop roots and moisture should penetrate the soil profile.
Vehicle traffic compacts soil not only under tyres, but also nearby and at depth.
The heavier the vehicle or the more passes the vehicle makes over the area of ground, the greater the compaction.
8. Not maintaining the organic matter content of the soil.
Cattle and sheep trampling, especially around watering points.
9. Methods for improving soil structure
Soil structure can be maintained by changing management practices:
* Limit the frequency of cultivation
* Cultivate only when the soil is not too wet
* Limit tillage traffic.
10. Lost soil structure can be regained by:
* Using an implement for deep ripping to destroy a plough pan, an expensive procedure.
* Adding organic matter, by growing a green manure crop, or not burning off stubble then ploughing it in
* Using a pasture phase for 4-5 years as part of a crop rotation plan
* Using runoff control structures, e.g. contour banks on sloping or steep land.
11. Soil structure decline
Over the years of cropping, the soil can also gradually lose some structure.
This is called structural decline.
The soil aggregates breakdown leads to problems with the entry of water and air at the surface, or
the growth of plant roots and movement of water and air through the subsoil.
It can also lead to problems of nutrient storage and changes in population levels of organisms in the soil.
Experiment
Carefully dig up blocks of soil from different areas and put it in your hands with fingers closed around the blocks.
Open your hands and observe how the soil particles stick together.
6.9.23 Soil tilth
Tilth means how well roots can pass through the soil.
Roots can pass best through soil that is loose, contains air and water, and is free of stones.
To improve tilth you dig the soil to a depth of 15-30 cm, take out stones then smooth the surface flat with a rake.
Tilth also refers to the ease of planting or growing a crop or harvesting a root crop.
So the tilth can be assessed only by using an agricultural implement, e.g. a plough.
Tilth is affected by soil structure, soil moisture, aeration and drainage.
Plowing may cause loss of soil structure by compaction and so decrease the tilth for a following crop.
Experiment
Observe ploughing by farmers and ask them for their assessment of the tilth of the soil.
6.9.24 Soil watering,, deep pipe irrigation
Deep pipe irrigation encourages tree roots to grow down.
It uses an open vertical or near vertical pipe to deliver irrigation water to the deep root zone.
It encourages a much larger root volume than other forms of irrigation and helps develop a plant that is better adapted to survive after watering is ended.
By delivering irrigation water through deep pipes rather than on the surface, tree roots grow down and benefit any shallow-rooted intercropping annuals.
Weed competition is reduced by avoiding surface irrigation.
Deep pipe irrigation works just as well on steep slopes as on level ground.
Use 2.5-3 cm diameter pipes placed vertically 30-50 cm deep in the soil near a young tree 2.5-7.5 cm away for seedling trees up to 12 cm away for larger trees.
A screen cover of 1 mm mesh can be added to keep animals out.
Several pipes can be used for a larger tree if necessary arranged around the tree symmetrically.
A series of 1-2 mm holes should be spaced 5-7.5 cm apart down the side of the pipe nearest the plant.
This spacing allows water to weep into the soil at all levels, not only at the bottom, to help early root growth.
If shallow-rooted plants from containers are planted next to a deep pipe without weep holes, the roots may not contact the wetted soil.
Similarly, a young seedling can dry out if a drip emitter is usedto deliver water into the pipe, even if it has weep holes.
Growing plants in deep containers can reduce these problems.
Deep pipes can be filled from hoses, watering cans or fitted with a drip emitter.
If a drip emitter is used then the deep pipe can be smaller, down to 1 cm diameter.
To ensure that the water seeps through the pipe at all levels, the drip water rate must be fast enough to fill the pipe.
Alternatively, the pipe can be tilted with the weep holes downwards, so that the water runs over and through them.
A battery-powered remote timer combined with a water tank can be set up at a remote site to irrigate once a week that should lead to good tree survival.
The advantages over buried drip systems include ease of access to the drippers in case of a blockage.
Deep pipe irrigation can be used with low quality water (though not necessarily with drip emitters if they are likely to block up),.
The deep pipes can be collected at the end of the season for re-use.
Experiments have shown that deep pipe irrigation systems are very effective and more efficient than surface drips or conventional surface irrigation.
Much larger effective rooting volumes are developed and the plants are better adapted to survive future dry spells.
In very dry regions, long term survival and growth can be improved by micro catchments to increase effective rainfall with tree shelters to reduce water demand.
6.9.25 Weathering soils
See: 4.32 Weathering rocks (Primary)
See diagram 6.04: Weathering
Weathering of rocks and of soil particles never stops.
More than one type of weathering may operate simultaneously.
Weathering may result from either physical or chemical processes, helped by biological activity.
1. Physical weathering is the breakdown of rocks into smaller particles by mechanical action and includes frost, abrasion and temperature changes.
* Frost action occurs when water in cracks and crevices may break the rock as it freezes and expands
* Abrasion occurs when rocks roll or wash away from where they are lying and bouncing rock pieces knock parts off each other.
The broken bits and the freshly exposed surfaces can then be further broken down by other types of weathering.
Grains of sand can be blown by the wind against rocks to rub of bits of rock.
* Temperature changes cause rocks to expand with heat and contract with cold.
Rocks are made of different materials and each material expands and contracts at a different rate to set up stresses.
They cause cracks, rock splitting off, and even the outside layer breaking away from the core rock.
Heat can come from the sun or bush fires.
The outer rock experiences greater temperature changes than the inner rock.
2. Chemical weathering is a change in composition of the rock material by the chemical action of water, gases in the air and other chemicals.
The chemicals work on the surface of both rock (parent material), and the particles within a soil.
* Carbonic acid forms when carbon dioxide dissolves in water.
The carbon dioxide comes from the air in the atmosphere and soil, and the respiration of soil organisms and plant roots.
When the acid contacts rock materials or minerals below the surface of the soil, it dissolves some substances, leaving others behind.
Those that cannot be dissolved become the inorganic basis of soil.
H2O (l), <--> H+ (aq), + OH-(aq)
2H+ (aq), + CO32- (aq), <--> H2CO3 (aq), carbonic acid
CO2 + H2O <--> H3O+ + HCO3-
HCO3- + H2O <--> H3O+ + CO32-
3. Organic acids come from organisms living in the soil, some excreted by organisms and some from their decay.
The acids help to breakdown rock material chemically below the soil surface.
4. Oxidation is a process by which oxygen from the air reacts with the rock materials to form new substances.
For example, the red colour of some soils is due to an oxide of iron.
5. Hydrolysis is the process of decomposition of minerals by chemical reaction with water and is the most common chemical weathering process.
6. Plants, animals and micro-organisms help the physical and chemical weathering processes.
The tunnel of a burrowing rabbit allows extra water and air to enter the soils to increase chemical weathering, turns the soil over and exposes new surfaces for weathering to occur.
Micro-organisms, e.g. fungi, bacteria, algae, release chemicals into the soil that can breakdown rock material and transform minerals into forms that the micro-organisms can use for nutrients.
Plant roots grow in cracks in the rocks.
As the roots get bigger, they make the crack wider and can split the rocks.
7. We study soils, because soils give us food.
Soils may be not very fertile, but you can improve them if you know how to do it.
Many plants will not grow well in coral soils, e.g. bananas will not grow in coral sand under the coconuts, because coral soils are poor soils.
Soils are not all the same so go outside to look at the soil surface in different places.
The differences may include the following:
Colour: [white, grey, black, reddish]
Particles: [sand only, stones only, sand and stones, fine sand, mud]
Grass cover: [thick cover, few grasses, none]
Other plants: [few, none, many, names of plants]
Dead leaves: [many, a few, none]
Nearby coconuts: [many, few, none]
Go outside to look at the soil surface at six different places and record their appearance.
6.9.26 Soil fertility decline
Over years of cropping, the crops gradually use up the store of soil nutrients.
Some nutrients are lost in the removal of plants (as grain, hay, or vegetables), or animals from the area.
Other nutrients are lost by, wind or water erosion, particularly if the topsoil is lost.
The soil fertility gradually declines, but the process is so slow it is often hard to detect.
Soil tests can reveal exactly which nutrients are in short supply and how much is needed to make the soil fertile again.
The correct land management strategy can then be devised using options such as fertilization, crop residue retention and legume rotation.
Experiment
Obtain samples of soils from a flower or vegetable garden from a wood, from a place where foundations are being dug, from a sandy place, from a clay bank.
Place the samples in separate flowerpots or glass jars.
Plant seeds in each type of soil and give each plant the same amount of water.
Note the type of soil in which the seeds sprout first.
After the plants have started to grow, note the soil sample in which they grow best.
Record rates of growth of plants in different soils.
6.6.1 Fertility of different soils
1. A soil is fertile when it can continually supply plants with enough nutrients for them to grow well.
If a plant is to grow well it has to have large quantities of six nutrients (nitrogen, phosphorus, potassium, sulfur, calcium and magnesium) and smaller quantities of nine trace elements.
Plant nutrients are stored in the soil by being chemically held onto the surface of colloids, so a soil with a lot of colloids can store a lot of nutrients.
Gradually the stored nutrients dissolve into the soil water where the plant roots take them up.
Topsoil is the major store of nutrients so it is important to protect topsoil from being blown or washed away.
Most of the plant roots are in the topsoil, especially when the plants are young.
Only tree roots can take up nutrients from deep down in the subsoil.
2. To correct soil fertility fertilizers can supply nutrients to a soil where they are missing or in short supply.
A soil low in colloids where nutrients have been leached out (lost), can be improved by increasing the organic matter level.
A soil with pH too low or too high may have enough nutrients, but they do not dissolve quickly enough for the plants to use, so if we correct the pH first the nutrient supply will improve.
The diameter of most soil particles < 2 × 10-6 m and expose a large external surface area per unit mass.
The external surfaces of soil particles usually carry negative charges and so attract and adsorb cations to the particle surface, e.g. H+, Ca2+, Mg2+, and A13+.
Also, water molecules are associated with soil particles are attracted to the adsorbed cations.
Each clay particle is composed of a series of layers, sheets of aluminium, silicon, magnesium, and iron atoms surrounded and held together by oxygen and hydroxy (OH), groups.
However, the clay particles usually have a net negative surface charge.
Similarly, humus consists of negatively charged particles surrounded by cations.
The negative charges of humus particles are associated with bases from ionized carboxylic acids and phenol groups.
3. Cation exchange in soils
Particles soil involve fixed negative charges, both in clay mineral soil and in organic soil.
Thus soil charge balance tends to occur between fixed negatively charged groups and relatively mobile positively charged cations.
Water moving through soil forms a soil solution of ions dissociated from the surface of soil particles.
Cations are exchanged between soil particles and the soil solution.
The cation exchange capacity of a soil is a measure of the quantity of negatively charged sites to which cations can be held by ionic bonds.
Cation exchange affects the pH of the soil solution.
In acid soils, the cations that are present will be mainly acid cations, e.g. H+, Al3+, Al(OH),2+, Fe3+, Fe(OH),2+.
In alkaline soils, the cations that are present will be mainly "base cations", e.g. Ca2+, Mg2+, Mn2+, K+, and Na+.
When acid is added to the soil solution, hydrogen can exchange with these "base cations" to remove hydrogen cations from the soil solution.
So the soil pH does not change much as long as there are more basic soil cations than added H+.
Neutral soils will contain a balance of both acid cations and base cations.
For most plants, optimum growth occurs when Na+, Ca2+, Mg2+ and K+ occupy 80% of the cation exchange sites.
The pH of the soil is then 6.3 - 6.6.
Hydrogen cations in the soil compete for binding sites with these four cations.
When the pH of the soil drops the hydrogen ion concentration of the soil solution is increased, more hydrogen ions are competing for exchange sites.
So there is a higher concentration of hydrogen cations on the exchange sites and a lower concentration of these cations on the exchange sites.
These cations, now displaced into the soil solution, may be leached away by rainwater if not taken up by plants.
One of the principle long-term effects of acid rain is this loss of cations by leaching.
4. Fertility of different soils
Experiment
Obtain samples of soils from a flower or vegetable garden, from a wood, from a place where foundations are being dug, from a sandy place, from a clay bank.
Place the samples in separate flowerpots or glass jars.
Plant seeds in each type of soil and give each plant the same amount of water.
Note the type of soil in that the seeds sprout first.
After the plants have started to grow, note the soil sample in that they grow best.
Record rates of growth of plants in different soils.
5. Fertility of subsoil and topsoil
Experiment
Obtain a sample of good topsoil from a garden.
Obtain another sample of soil from a depth of about 50 cm.
Place the samples in separate flowerpots and plant seeds in each.
Keep the amount of water, the temperature, and the light equal for each sample.
Note that soil produces the healthier plants.
6.10.0 Soils tests
See: Soil tests, (Commercial)
If produce is sold off the farm, an annual test is needed, because crop removal rapidly depletes the soil of nutrients.
If animals are grazed, a test every 2 to 3 years ensures nutrients are in balance.
Soil sample kits may be purchased that instruct you how to collect soil samples to be mixed together in a bag.
So the report will be an average of the samples, so test separately any areas with special problems.
When collecting soil samples, note the condition of the soil surface, depth of topsoil, soil structure and penetration of plant roots, because laboratory tests do not give information on soil compaction, structure decline, erosion or subsoil problems.
6.10.1 Calcium : magnesium ratio soil test
Preferred level > 3
If the quantity of calcium (meq / 100 g), / the quantity of magnesium (meq / 100 g), < 2 plants may not take up potassium, and the soil structure may break down due to dispersion.
If dolomite (2 parts calcium to 1 part magnesium), is applied regularly, the soil's calcium : / magnesium ratio will fall, because too much magnesium is applied compared with calcium, so add calcium in the form of gypsum or lime.
High calcium : magnesium ratios up to 20 / 1 do not affect plant yields.
6.10.2 Cation exchange capacity soil test (CEC)
Preferred level above 10 CEC is a measure of the ability of the soil to hold the nutrients calcium, magnesium and potassium.
Good fertile soils with high clay content and moderate to high organic matter levels usually have a cation exchange capacity of 10 or higher.
6.10.3 Conductivity
See: Soil tests, (Commercial)
Conductivity is a measure of the specific conductance of an electrolyte solution.
By using the AC resistance between electrodes, it measures the ionic content in a solution.
Also it indirectly measures total dissolved solids, TDS.
Conductivity SI unit: siemens per meter, S / m
Typical values of conductivity: Deionized water 5.5 μS / m, Drinking water 5-50 mS / m, Sea water S / m.
Industrial distilled water, density 1.000 g / mL at 3.98 C (lit.), conductivity 0.05 μS / cm (micro Siemens / cm),]
Industrial deionized water, as above, conductivity 4.3 μS / cm at 20 oC.
Deionized water has no ions, except for H+ and OH- ions, but it may be not as pure as distilled water if it contains organic molecules.
Conductivity (salt), soil test, preferred level < 0.15 dS / m (EC1:5)
Electrical conductivity is a measure of salts in the soil.
A productive soil's conductivity should < 0.15 dS / m (decisiemens per metre),.
Different plants may be sensitive to tolerant to salt stress.
The degree of stress is less in clay soils than in sandy soils so soils affected by salt should also have a saturation conductivity test (ECse).
See: Salinity, (Commercial)
Salinity stress may be caused by too much fertilizer, salty irrigation water or saline ground water.
Salts can be leached out with rainfall or low salinity irrigation water without affecting soil pH.
If rainfall is high there is usually no problem with soil salinity, except in some low, poorly draining soils close to tidal rivers.
6.10.4 Exchangeable cations soil test
The major cations are calcium, magnesium, potassium, sodium and aluminium.
These cations are held in the soil by organic matter and clay.
The preferred % of exchangeable cations as a proportion of CEC, and suggested quantities are as follows with the quantity expressed in meq / 100 g (meq% or cmol / kg)
Cation
 Preferred %
 Quantity
Calcium
 -
 >5
Magnesium
 -
 >1.6
Potassium
 26
 >0.5
Sodium
 01
 <1.0
Aluminium
 0
 0

% = Quantity (meq / 100 g), / CEC figure, X 100.
Do not include the level of hydrogen cations in the total CEC figure.
Other cations are reported, e.g. Mn, may indicate a toxicity problem.
High levels of aluminium are toxic to some plants, especially in acidic soils.
High sodium levels can indicate sodicity problems (soil structureproblems), or salinity problems.
See: Salinity (Commercial)
To convert quantities in parts per million (ppm), or mg / kg, use the following conversions to obtain meq figures:
Calcium / 200, Magnesium / 120, Potassium / 390, Sodium / 230, Aluminium / 90.
6.10.5 Nitrate nitrogen soil test
Nitrate levels may fluctuate widely, depending on the season or rainfall.
Soils used for pasture need a level >10 mg / kg and soils used for horticulture need a level >20 mg / kg.
6.10.6 Organic carbon soil test
Preferred level: above 2%
Organic carbon, a measure of the organic matter in the soil, includes undecomposed plant litter, soil organisms and humus.
Soil organic carbon stores important nutrients, stabilizes soil structure and feeds soil microbes.
If soil organic carbon declines use green manure crops, minimum tillage, mulching or strategic grazing.
6.10.7 pH soil test
See: pH (Commercial)
Preferred pH level (CaCl2), 5.0-5.5
The (CaCl2), after the pH figure shows that the pH was measured in a solution of calcium chloride so the pH is 0.5 - 0.8 lower than if measured in water.
A test result of pH 5.0 - 5.5 (CaCl2), is suitable for most agricultural and horticultural purposes.
Soil pH levels above 5.5 are usually costly to maintain.
Soil pH below 5.0 can be raised by applying lime to the soil.
6.10.8 Phosphorus soil tests, Bray test and Colwell test
The Bray test phosphorus levels vary with land use, 1520 mg / kg for dry land pastures, 2530 mg / kg for irrigated and improved
pastures, 3050 mg / kg for tree crops, 50+ mg / kg for vegetables.
The Bray test is more suitable for acid soils, because phosphorus tends to tie up with aluminium, and iron and become unavailable to plants in acid soils.
Keep pH at around 5 if the soil is to benefit from added phosphorus.
The Colwell test levels vary from 20 to 100 mg / kg depending on soil texture.
6.10.9 Tests for soil organic matter
Digging in compost, animal manures or green manures helps increase the levels of organic matter and the soil's biological activity and fertility.
The improved soil allows water to penetrate efficiently, supplies nutrients slowly to root zones of plants, and helps root development.
Also, the greater number of organisms, e.g. earthworms, helps cultivate and improve the soil.
A simple test for the amount of organic matter in your soil tests the biological activity, a direct result of the level of organic material.
The tests show whether the organic materials added to the soil are having much effect, how long their effect takes to decline, and whether some areas in the gardens are much lower in organic matter than others.
The test is based on the length of time it takes for soil organisms in the soil to rot a piece of fabric.
Where there is a higher level of organic matter in the soil, there will be a corresponding higher level of micro-organisms and the fabric will rot faster.
Measure the rotting of the fabric with a simple load-bearing test.
Choose several areas that you want to measure, e.g. compare the results of one garden bed where you have added mulch recently to another that you have added nothing to for a year.
Use pure cotton fabric, e.g. as an old sheet, and cut it into strips about 30 cm long and 3 cm wide.
Use a waterproof fabric pen to label one end of each strip.
Push a sharp spade 15 cm deep into the selected area of soil.
Make a slot in which to insert the cotton strip.
Fold the strip in half over the blade of the spade and use the blade to push it back into the slot you have just cut.
When you draw the spade back out, leave 2 cm of fabric protruding out of the soil.
Take several samples at each selected site.
Leave the fabric in the soil for weeks, and then remove all the strips on the same day.
Completely rotted strips were left in too long, so that tests must be done again.
By having strips in the same area, remove one each week to check the rotting.
The higher the level of biological activity in your soil, the more the fabric will have rotted, and therefore, the weaker it will be.
To test the strength of the partially rotted fabric strips, fold a strip over the handle of a bucket and use the strip to raise the bucket just off the ground.
Then slowly pour water into the bucket until the strip breaks.
Record how much water is in the bucket.
Repeat the test over periods of time, and collect and compare theresulting data.
6.10.10 Trace elements soil test
Preferred level in (mg / kg), = Arsenic <20, Boron 0.54, Cadmium <1, Copper 250
Lead <35, Molybdenum 2, Mercury <1, Nickel 120, Silicon >10, Sulfur 1020, Zinc 120.
Iron, manganese and zinc are usually available in acid soils.
Zinc is fixed by iron on red basaltic soils, and since boron leaches easily, deficiency may be common in horticultural crops.
Molybdenum is less available in acid soils, so it needs to be added to the soil, particularly for pastures and vegetable crops.
A trace element can be determined by a plant tissue test.
6.8.1 Atoll water lens
See diagram: 6.66.4 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.
6.8.2 How soils form in atolls
See diagram 6.66.1: Forming an atoll 1.
See diagram 6.66.2: Forming an atoll 2.
See diagram 6.66 3: An atoll and its peripheral reef (cross-section).
6.8.3 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 forfertilizer (phosphate fertilizer),.
6. Humans can change soils too, making them worse, by burning thegrass, or making then better, by adding compost.
7. When soils change, the plants may also change:
* 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.
* Some plants can live in a drought, e.g. salt bush, and Pandanus, but some plants may die in a drought, e.g. coconuts.
* Some plants are found on the ocean side and some plants are mostly found on the lagoon side of an island.
8. Experiment
Go to the ocean side and list plants growing there.
Then go to the lagoon side and list plants growing there.