School Science Lessons>
Agriculture
2025-10-03
Soil science
See Websites: Soil
Contents
Soil Science 1.
Soil fertility
Soil minerals
6.0.1 Types of soils
6.1.0 Soil acidity
6.2.0 Soil biology
6.3.0 Soil chemistry
6.4.0 Soil composition
6.5.0 Soil conservation
6.7.0 Soil profiles
6.8.0 Soil waters

6.1.0 Soil acidity
6.1.1 Acid soils and alkaline soils
6.1.2 Soil pH preferred ranges
6.1.3 Soil pH
Experiments
6.1.4 Hydrangea flower colour change
6.1.5 Soil pH of potting mix
6.1.5 Soil pH lower with agricultural sulfur
6.1.7 Soil pH higher with agricultural lime / dolomite
6.1.8 Tests for pH of soil samples
6.1.9 Tests for pH of soils, soil test kit
6.1.10 Test soil pH, acid soils and alkaline soils
6.2.0 Soil biology
6.2.1 Life in the soil
6.2.2 Soil nitrogen bacteria
6.3.0 Soil chemistry
6.3.1 NPK grade formula
6.3.2 Oxidation of iron
6.3.3 Salinization
6.3.4 Soil salts
6.4.0 Soil composition
6.4.1 Soil colour
6.4.2 Soil components
6.4.3 Soil formation
6.4.4 Soil horizons of a soil profile
6.4.5 Soil particle size
6.4.6 Soil temperature
6.4.7 Soil texture
6.4.8 Soil texture by mechanical analysis, texture triangle
6.4.9 Soil texture
6.4.10 Soil texture measure by squeezing the sample
6.4.11 Soils contain air
6.4.12 Surface / volume ratio of particles
6.4.13 Wind deposits
6.4.14 Nutrition from the soil
6.5.0 Soil conservation
6.5.1 Good management practices
6.5.2 Plants can prevent soil erosion
6.5.3 Raindrops affect soils
6.5.4 Running water changes soils
6.5.5 Soil erosion
6.5.6 Splash sticks
6.5.6 Water erosion
6.5.8 Wind erosion
6.7.0 Soil profiles, soil horizons
6.7.1 Soil profiles
6.7.2 Soil changes with depth
6.8.0 Soil water
6.8.1 Angles between mud cracks
6.8.2 Soil contains water
6.8.3 Soil permeability
6.8.4 Soil water-holding capacity
6.8.5 Soil water
6.8.6 Soil water moves down through soil
6.8.7 Soil water moves up in three ways
6.8.8 Soil water content
6.8.9 Capillary action in soil and deposition by groundwater
6.5.10 Infiltration and capillary action by groundwater
6.8.11 Water from leaves
6.8.12 Freezing water expands
6.0.1 Types of soils
Plants need soil to support them and hold them firmly, provide water, provide mineral salts.
Experiment
Obtain samples of soil from as many places as possible and put them in glass jars.
Try to get samples of soil from swamps, hill sides, woods, meadows, dunes, river flats, and other localities.
In this way you will gather sandy, loamy, and clay soils with a range of colours and decayed matter or humus contents.
Study the samples and examine particles from each sample with a magnifying glass.
Note the colour, particle sizes, grass cover, other plants, aggregation of soil particles into lumps and the hardness of the lumps, inclusions of stones.
6.1.1 Acid soils and alkaline soils
See diagram 6.65.3: Nitrogen cycle
Soil pH is a measure of the acidity of the soil, on a scale from 1 to 14, the pH scale.
A neutral substance such as pure water has a value of 7.
Strong alkaline solutions have a pH near 14.
Strong acids, e.g. hydrochloric and sulfuric acid, have a pH value close to 1.
Most soils have a pH range of 4.0 to 9.5.
Most plants prefer a pH of between 5.5 and 7.5, with a pH of 6.2 to 6.5 ideal for most food plants.
It is desirable to keep the soil's pH in this range.
Incorrect soil pH affects the availability of many plant nutrients and so affects plant growth.
For example, phosphorus is an important nutrient for plant growth
However, if the pH drops below 5, the availability of phosphorus sharply decreases, because the phosphorus in the soil, present as
phosphates, forms insoluble compounds at low pH stopping them from dissolving in water, so plants cannot absorb the phosphorus.
At low pH values many soil bacteria will not survive so changing the turnoverrate in nutrient cycles, including the important nitrogen cycle.
The soil pH requirements of plants vary.
Some plants grow well under acidic conditions, while others grow best in a more alkaline soil, e.g. potatoes and watermelons like slightly acidic soils, while apples and lucerne grow well in slightly alkaline soils.
Soil acidity increases through the removal of some basic nutrients from the soil, e.g. calcium leaching and the excessive use of fertilizers, particularly nitrogen fertilizers.
To correct low soil pH, use application of lime (calcium compounds), or dolomite (calcium and magnesium compounds).
Use 1-5 tonnes / ha of lime to increase the pH of the topsoil to pH 6.5, but it can take several years to take effect.
You can improve the fertility of your garden soils by treating them so that they are not too acid or too alkaline.
Clay soils need a much greater amount of lime to shift the pH than sandy soils.
The pH scale measures whether substances are acid or alkaline.
pH 1 very strong acid that can burn you, e.g. battery acid
pH 6 weakly acid, e.g. soda water
pH 7 neutral, neither acid nor alkaline, e.g. water
pH 8 weakly alkaline, e.g. soap
pH 14 very strong alkali that can also burn you.
Acids have a sharp raw taste, e.g. unripe oranges or bush limes.
Alkalis have a slippery feel, e.g. soap, saliva.
Plants can absorb plant nutrients best when pH is 6 to 7.
In soils formed from coral rock, the pH will be too high.
In swampy land, the pH will be too low.
Experiment
To lower the pH add rotten compost.
To raise the pH add lime.
In this lesson show how to use a "Soil pH Test Kit".
You will need: Coral soil or coral sand, well drained dark soil, swamp soil.
Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline.
Soils that are not well-drained are too acid.
Soils made from coral rocks are too alkaline.
You can test the soils using a colour test.
If the colour of soil in the test turns yellow orange, the soil is too acid.
If colour if soil in the test turns blue purple, the soil is too alkaline.
If colour if soil in the test turns dark green, the soil is not too acid nor too alkaline.
Collect just enough soil from just under the surface of the soil to cover your little finger nail, and place on a white plate.
Shake two drops of the indicator on the soil and mix to a paste with the stick.
Sprinkle some special white power on the paste.
Wait a few minutes then match the colour of the powder with the colour chart.
Do this for swampy soil, coral soil, dark well drained soil.
Acids have a sharp sour taste and can dissolve substances, e.g. in a carbattery.
Alkalis have a slippery feel and can dissolve substances, e.g. soap.
Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline.
Soils in swampy ground are too acid for most plants.
Soils made from coral sand are too alkaline for most plants.
Make soil less acid by adding burnt shells hammered to a powder, and by draining the soil.
Make soil less alkaline by adding rotten plants from a compost heap.
Good soil is dark in colour from the rotten plants and is well drained.
To make sure that the soil is not too acid and not too alkaline the agriculture field officer can do a soil test.
6.1.2 Soil pH preferred ranges
1. For the best growth of plants it is essential that the acidity (measured by pH), of the potting mix or soil is suitable for the plants you want to grow.
Most soils are either slightly acid or slightly alkaline.
A few are neutral (between acid and alkaline).
Some soils are very acid and some are very alkaline.
Neutral soils have a pH of 7.
Acid soils have pH values of less than 7.
Alkaline soils have pH values of more than 7.
Plant growth is affected by soil pH.
Few plants grow well in soils with pH values below 6.4.
Plants whose native habitats had very acid soils grow best in soils of pH 6.5 to about 6.
Most do not grow well on neutral and alkaline soils.
Most other plants grow best in soils whose pH values are about 6 to 7.
Plants whose native habitats had alkaline soils will grow on slightly acid soils, but they will also grow well on alkaline soils.
Most plants grow well in potting mixes when the pH of the mix is in the range 5.5 to 6.4.
Plants from areas with very acid soils prefer a potting mix with a pH in the range 6.5 to 5.5.
Plants adapted to acid soils are often unable to get enough of the essential nutrients iron and manganese from alkaline soils.
Their young leaves show yellowing (chlorosis), and growth is poor.
Severe deficiency leads to death.
By contrast, plants adapted to alkaline and slightly acid soils can be harmed by the amounts of dissolved aluminium and manganese present in very acid soils.
They probably cannot take up enough of the essential element calcium.
1.1 Soils of pH 6.5 to 6, potting mixes of pH 6.5 to 5.5: Camellia, Rhododendrons, Azalea, Gardenia, Erica, Macadamia, Juniper Spruce, Japanese Maple
1.2 Soils of pH 5.8 to 7.5, potting mixes of pH 5.3 to 6.5: Most vegetables, bedding plants, commonly grown shrubs and trees.
1.3 Soils of pH 7 and higher, potting mixes of pH 6 to 6.7: Many cacti and succulents.
Plants native to arid areas.
Grow roses and citrus that have been grafted onto rootstocks that tolerate these soils.
6.1.3 Soil pH
See diagram 6.65.3: Nitrogen cycle
1. The acidity of a soil is measured by the pH scale.
Soil pH is a measure of the acidity of the soil, on a scale from 1 to 14, the pH scale.
Numbers are used after pH to show whether the soil is acid or alkaline:
pH 1-2 very strongly acid, pH 5-6 weakly acid, pH 7 neutral, pH 8-9 weakly alkaline, pH 13-14 very strongly alkaline.
A neutral substance such as pure water has a value of 7.
Strong alkaline solutions have a pH near 14.
Strong acids, e.g. hydrochloric and sulfuric acid, have a pH value close to 1.
Most soils have a pH range of 4.0 to 9.5.
Most plants prefer a pH of between 5.5 and 7.5, with a pH of 6.2 to 6.5 ideal for most food plants.
It is desirable to keep the soil's pH in this range.
Incorrect soil pH affects the availability of many plant nutrients and so affects plant growth.
For example, phosphorus is an important nutrient for plant growth, but if the pH drops below 5, the availability of phosphorus sharply decreases.
This occurs because the phosphorus in the soil, present as phosphates, forms insoluble compounds at low pH, stopping them from dissolving in water, so plants cannot absorb the phosphorus.
At low pH values many soil bacteria will not surviv,e so changing the turnover rate in nutrient cycles, including the important nitrogen cycle.
The soil pH requirements of plants vary.
Some plants grow well under acidic conditions, while others grow best in a more alkaline soil; e.g. potatoes and watermelons like slightly acidic soils, while apples and lucerne grow well in slightly alkaline soils.
Soil acidity increases through the removal of some basic nutrients from the soil, e.g. calcium leaching and the excessive use of fertilizers, particularly nitrogen fertilizers.
To correct low soil pH, use application of lime (calcium compounds) or dolomite (calcium and magnesium compounds).
Use 1-5 tonnes / ha of lime to increase the pH of the topsoil to pH 6.5, but it can take several years to take effect.
You can improve the fertility of your garden soils by treating them so that they are not too acid or too alkaline.
Clay soils need a much greater amount of lime to shift the pH than sandy soils.
The pH scale measures whether substances are acid or alkaline.
pH 1 very strong acid that can burn you, e.g. battery acid
pH 6 weakly acid, e.g. soda water
pH 7 neutral, neither acid nor alkaline, e.g. water
pH 8 weakly alkaline, e.g. soap
pH 14 very strong alkali that can also burn you.
Acids have a sharp raw taste, e.g. unripe oranges or bush limes.
Alkalis have a slippery feel, e.g. soap, saliva.
Plants can absorb plant nutrients best when pH is 6 to 7.
In soils formed from coral rock the pH will be too high.
In swampy land the pH will be too low.
Experiment
To lower the pH add rotten compost.
To raise the pH add lime.
In this lesson show how to use a "Soil pH Test Kit".
You will need: Coral soil or coral sand, well drained dark soil, swamp soil.
Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline.
Soils that are not well-drained are too acid.
Soils made from coral rocks are too alkaline.
You can test the soils using a colour test.
If the colour of soil in the test turns yellow orange, the soil is too acid.
If colour if soil in the test turns blue purple, the soil is too alkaline.
If colour if soil in the test turns dark green, the soil is not too acid nor too alkaline.
Collect just enough soil from just under the surface of the soil to cover your little finger nail, and place on a white plate.
Shake two drops of the indicator on the soil and mix to a paste with the stick.
Sprinkle some special white power on the paste.
Wait a few minutes then match the colour of the powder with the colour chart.
Do this for swampy soil, coral soil, dark well drained soil.
Acids have a sharp sour taste and can dissolve substances, e.g. in a car battery.
Alkalis have a slippery feel and can dissolve substances, e.g. soap.
6 Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline.
Soils in swampy ground are too acid for most plants.
Soils made from coral sand are too alkaline for most plants.
Make soil less acid by adding burnt shells hammered to a powder, and by draining the soil.
Make soil less alkaline by adding rotten plants from a compost heap.
Good soil is dark in colour from the rotten plants and is well drained.
To make sure that the soil is not too acid and not too alkaline the agriculture field officer can do a soil test.
In pure water, the number of H+ ions = the number of OH- ions, so water is neutral.
In an acid soil there are many more H+ions than OH- ions.
In alkaline soils there are more OH- ions than H+ ions.
When a chemical fertilizer is added to a soil, it dissolves in the soil water and breaks up into positive ions and negative ions like the water molecule.
For example, if the soil is too acid, calcium, magnesium, potassium, sulfur and nitrogen are not very available to plants.
In Australia, aluminium and other trace elements may be toxic to plants growing in acidic soils, because these elements inhibit root growth.
Soil acidity can be managed by the application of agricultural lime, but this is not economic for broad scale agriculture.
If the soil is too alkaline, iron, manganese and aluminium are not available to plants.
Copper and zinc are not available when the soil is too acid or too alkaline.
If the fertilizer ions are held too weakly to the soil particles, then they will be easily washed out of the soil by heavy rain, and will be lost to plants.
So if you want to use fertilizers properly, it is best if you can make sure that your soil is slightly acid, but not strongly acid or strongly alkaline.
If the soil is too alkaline, then many plant nutrients like iron, manganese, boron, copper and zinc will not be very available to plants.
It is a good idea to use the fertilizer mixtures recommended by the Department of Agriculture.
It only pays to use imported fertilizers if the plants use it to increase the amount of food they produce.
Do not use the wrong mixed fertilizer, e.g. if you use a mixed fertilizer high in nitrogen on potato, it will only form a lot of leaves and little tubers.
6.1.4 Hydrangea flower colour change
White hydrangeas are always white.
Coloured hydrangeas are blue to pink, depending on the aluminium contents of the soil.
(Hydrangea macrophylla), mop head hydrangea, has blue, red, pink, light purple, or dark purple.
The flowers change colour pink to blue by adding aluminium to soil, Asia, Hydrangeaceae
Above pH 6, have pinker flowers, below pH 5 have blue flowers.
Less alkaline soils have "hydrangea blue" flowers.
To change pink flowers to blue flowers, lower pH with weak vinegar or acid fertilizer or add aluminium to soil, to make soil more acid.
To change blue flowers to pink flowers, aluminium cannot be taken out of soil, so add lime or phosphorus fertilizer to soil to make soil more alkaline.
(Hydrangea hortensia) has bi-coloured flowers.
6.1.5 Soil pH of potting mix
The mix must be moist enough to use for potting.
Experiment
1. Raise pH with dolomite. Add 1 to 1.5 g/L of mix to raise pH by about one unit.
2. Lower pH with sulfur.
Add 0.3 g/L to lower pH by about one unit.
3. Check the pH again after two weeks storage and add more as needed.
Check the pH of mix in pots every few months, because most fertilizers produce acidity.
4. Raise pH with a suspension of hydrated lime (builders' lime),
Suspend 5 g (a heaped teaspoon), in a litre of water.
Pour the suspension onto the mix in the pot.
Use 200 mL for each litre of the mix.
A 130 mm pot contains about 1 litre of mix.
Pot plants again if the pH of the mix is below 6.4. 5.
Lower pH with a solution containing 2 g of iron sulfate per litre of water.
Apply 200 mL per litre of mix and within two minutes heavily water the pot to remove excess salt.
Wait for one week, check mix pH and add more iron sulfate if needed.
6.1.6 Soil pH lower with agricultural sulfur
Experiment
Lower soil pH of slightly alkaline soils (pH below 7.5), with agricultural sulfur.
The large amounts of solid lime often present in alkaline soils with pH values higher than about 7.5, make it almost impossible to make these soils acid.
6.1.7 Soil pH higher with agricultural lime / dolomite
Experiment
Raise soil pH by adding agricultural lime or dolomite.
A 1:1 mixture of the two often gives best results.
6.1.8 Tests for pH of soil samples
1. Take soil samples with a teaspoon.
Add an equal amount of water to each sample, enough to cover the soil.
Shake the samples thoroughly then drain off the liquid or use a filter.
Test the pH of the collected liquid.
2. Use a commercial pH testing kit.
Put a level teaspoon of soil on a white tile.
Add 5 drops of pH due indicator liquid and stir with the rod provided.
Dust the paste with the white powder, barium sulfate.
Wait for one minute.
Read from the colour card the pH value of the colour nearest to that of the sample.
3. Most soils are either slightly acid or slightly alkaline.
Acid soils have pH values of less than 7.
Alkaline soils have pH values of more than 7.
Plant growth is affected by soil pH.
Few plants grow well in soils with pH values below 5.5.
Most other plants grow best in soils with pH values 6 to 7. 4.
Plants adapted to acid soils may not get enough iron and manganese from alkaline soils.
Their young leaves show yellowing, chlorosis, and growth is poor.
Severe deficiency leads to death.
Plants adapted to alkaline and slightly acid soils can be harmed by dissolved aluminium and manganese in very acid soils
and may not get enough calcium.
You can raise soil pH by adding agricultural lime or dolomite.
5. Check the pH of mix in pots, because most fertilizers produce acidity.
Raise pH with a suspension of 5 g of hydrated lime (builders' lime), in a litre of water.
Lower pH with 2 g of iron sulfate in a litre of water.
Within two minutes, heavily water the pot to remove excess salt.
6.1.9 Tests for pH of soils, soil test kit
Experiment
1. The test kit contains one bottle of pH dye indicator and one bottle of barium sulfate solution.
Place a level teaspoon of mixed soil or potting mix on the test plate.
Add 3 to 5 drops of indicator liquid and stir with the rod provided.
Dust the paste with the white powder provided.
Wait one minute.
Read from the colour card the pH value of the colour nearest to that of the sample.
For a garden bed, take at least five samples from holes dug in different parts of the bed.
Each sample is to extend from the surface to a depth of 10 cm.
Test each sample separately.
For farm paddocks, take at least 20 samples from each area.
Mix samples together thoroughly and test as one sample.
For bought and home made potting mix, thoroughly mix the bulk lot.
For mix in a pot, first knock the root ball from the pot.
Remove a wedge of mix representing the whole depth of the root ball.
Mix thoroughly.
For a mix in large tubs, dig down the side of the root ball as deeply as is possible.
Thoroughly mix the sample removed.
2. Show how to use a soil pH test kit
Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline.
Soils that are not well-drained are too acid. Soils made from coral rocks are too alkaline.
You can test the soils using a colour test.
If the colour of soil in the test turns yellow orange.
The soil is too acid.
If colour if soil in the test turns blue purple.
The soil is too alkaline.
If colour if soil in the test turns dark green.
The soil is not too acid nor too alkaline.
Collect just enough soil from just under the surface of the soil to cover your little finger nail, and place on a white plate.
Shake two drops of the indicator on the soil and mix to a paste with the stick.
Sprinkle some special white power on the paste.
Wait a few minutes then match the colour of the powder with the colour chart.
Do this for swampy soil, coral soil, dark well drained soil.
3. Acids have a sharp sour taste and can dissolve substances, e.g. in a car battery.
Alkalis have a slippery feel and can dissolve substances, e.g. soap.
Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline.
Soils in swampy ground are too acid for most plants.
Soils made from coral sand are too alkaline for most plants.
You can make soil less acid by adding burnt shells hammered to a powder, and by draining the soil.
You can make soil less alkaline by adding rotten plants from a compost heap.
Good soil is dark in colour from the rotten plants and is well drained.
To make sure that the soil is not too acid and not too alkaline the agriculture field officer can do a soil test.
6.1.10 Test soil pH, acid soils and alkaline soils
Over years of cropping, the pH of the soil can become too low (acidification),
When the soil has a pH of below 5.5, it is called an acid soil.
One of the reasons why soils become more acid, is that under continual cropping, many commonly used fertilizers have an acidifying effect.
Care should be taken when fertilizers such as ammonium sulfate or mono ammonium phosphate are used.
Loss of some basic nutrients from the soil, e.g. calcium, also causes acidification.
Soils high in clay and organic matter are more resistant to acidification, conversely, once a soil has been acidified, they are more resistant to rehabilitation.
Adding organic matter is preventative, and adding lime is rehabilitative.
Acid soils and alkaline soils
This topic is very important, because you can improve the fertility of your garden soils by treating them so that they are not too acid or too alkaline.
The pH scale measures whether substances are acid or alkaline.
pH 1 very strong acid that can burn you, e.g. battery acid
pH 6 weakly acid, e.g. soda water
pH 7 neutral, neither acid nor alkaline, e.g. water
pH 8 weakly alkaline, e.g. soap
pH 14 very strong alkali that can also burn you.
Acids have a sharp raw taste, e.g. unripe oranges or bush limes.
Alkalis have a slippery feel, e.g. soap, saliva.
Plants can absorb plant nutrients best when pH is 6 to 7. In soils formed from coral rock the pH will be too high.
In swampy land the pH will be too low.
To lower the pH add rotten compost.
To raise the pH add lime.
6.2.1 Life in the soil
See diagram 9.3.16: Berlese funnel
Experiment
1. Put a soil sample in a very clean glass funnel.
Suspend a 100 watt light globe over the funnel.
Put a beaker containing methylated spirit under the funnel.
Turn on the light.
Over the next few days the heat from the light globe will cause living animals to move away from the globe and fall down the funnel
into the methylated spirit.
2. Examine the surface of a square metre of soil.
Note any earthworm mounds, anthills, or other signs of animal activities.
Carefully remove the surface plant life and examine the soil for more signs of animal life.
Note how earthworms cause more air to enter the soil through their burrows.
Their digestion of soil particles changes the composition of the soil and their droppings and dead bodies change the composition of the soil.
3. Compare the germination and growth of plants in normal soil and soil put in a microwave to kill all living things in it.
6.2.2 Soil nitrogen bacteria, nitrogen-fixing bacteria
6.9.17 Nitrogen cycle
8.12.1 Nitrifying bacteria, nitrification
6.9.17 Nitrogen cycle
4.12.3 Nitrogen-fixing bacteria, (Experiment)
4.12.4 Denitrifying bacteria, denitrifiers
6.3.1 NPK grade formula
Experiment
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 examples of artificial fertilizers are as follows:
muriate of potash (NPK = 0:0:50), superphosphate (NPK = 0:9:0), sulfate of ammonia (NPK = 21:0:0), urea (NPK = 46:0:0).
6.3.2 Oxidation of iron
Experiment
1. Put a small piece of steel wool in a container of clean sand and keep it moist.
After some days observe any change in the steel wool and any staining of the sand.
2. Put iron powder on a small piece of pyrite in a container and keep it moist.
After some weeks, note the development of a white crystalline substance, iron sulfate.
Oxidation of iron minerals is generally accompanied by a change of colour to yellow, brown, and red oxides and hydrated oxides.
During rusting, metallic ion changes to Fe(OH),3.xH2O, that is later oxidized to hydrated iron (III), oxide, Fe2O3.xH2O, brown rust.
6.3.3 Salinization
(parts per thousand = approx. g / kg salt)
Although every soil contains some salt, some soils have a lot, because the parent material was high in salt, or, because below ground water carried it into them.
The quantity or concentration of salt in the soil only becomes important when those concentrations in the root zone are so high that they become toxic to plants.
Salt at the soil surface stops most plants growing, leaving the surface unprotected from wind or water erosion.
An area which has high salt concentrations is difficult to manage.
Salt tolerant plants can be grown to protect the soil surface.
However, the difficult part is removing the salt.
Sometimes water can be used to wash the salt down below the root zone (leaching),
The problem with this is that if it rains or the land is irrigated incorrectly, the water table rises, bringing the salt back into the root zone.
Drains can be installed to take the salty water in the subsoil away, but this only transfers the problem elsewhere.
6.3.4 Soil salts, soil minerals in solution
Ask the local water supply service to tell you the total amount of dissolved mineral matter per unit volume in the untreated drinking water.
The soil contains salts to be taken up by plants and used as nutrients.
Experiment
1. Add demineralized water to cover garden soil in a beaker.
Stir the soil and allow any suspended soil to settle.
Filter off the liquid on top of the soil.
Examine a drop of filtrate under a microscope.
Leave the drop to dry and examine the residue under the microscope.
Look for crystals of salts.
2. Add drops of 5% cent hydrochloric acid to a sample of the filtrate.
The intensity of the reaction indicates the chalk content of the soil.
3. Add very dilute hydrochloric acid to pieces or limestone and other rocks.
Leave to stand until the limestone dissolves.
Note the residue of insoluble matter, e.g. quartz.
4. Ask the local water supply service to tell you the total amount of dissolved mineral matter per unit volume in the untreated drinking water.
5. 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 examples of artificial fertilizers are as follows:
muriate of potash (NPK = 0:0:50), superphosphate (NPK = 0:9:0), sulfate of ammonia (NPK = 21:0:0), urea (NPK = 46:0:0),
6.4.1 Soil colour
Colour is the most obvious feature of a soil.
It is usually determined by the kinds and amounts of iron compounds present (red, yellow, blue or green), silicates (white), and the level of organic matter in the soil (black or dark),
Iron compounds give soils a range of colours.
These are a useful indication of the aeration and drainage characteristics of a soil.
Colours range from red in well-aerated and well-drained soils through to brown, yellow, mottled yellow and to the grey and green/blue/grey of poorly drained and waterlogged soils.
Soils that are light-coloured have often had the coloured iron oxides leached, or washed out of them.
Dark soils may derive their colour from organic matter, which darkens the topsoil, or from certain clay types.
Many of the black clay soils expand when wet, and shrink and crack when dry.
These soils are often fertile, but they can be troublesome when used for dam walls and building foundations.
Experiment
Use a colour chart to map the colour of soils in your area by digging a hole down to the C horizon or by vertically scraping the side of a hill or gully.
6.4.2 Soil components
Soil is weathered rock material containing minerals plus water, air, organic matter and living organisms.
Soil forms a thin layer on the earth's surface.
It is the growing medium for all terrestrial plants including forests and agricultural crops.
The approximate major components of a soil by volume are as follows: minerals 45%, air 25%, water 25% (after rain, not air dry),
The five main components of soil:
1. Mineral particles, the inorganic fraction derived originally from rocks by weathering.
The main particle sizes are sand, silt and clay.
2. Organic matter, the dead and decaying plants and animals, and animal products.
3. Water, the soil solution in which nutrients for plants are dissolved.
4. Air, atmosphere that fills the spaces (pores), between the soil particles not filled by water.
5. Organisms, ranging in size from small animals to bacteria.
Experiment
See diagram 6.02: Beaker of soil under water
1. Prepare clean dry samples of sand, clay and local garden topsoil.
For sand, use beach sand or builder's sand (paver's sand),
For clay, find some almost pure clay in a road cutting or near a river.
Mix each sample with water and take out any large particles and large bits of plants and animals, but leave the black humus in the topsoil sample.
Leave the samples to dry in the sun then break any lumps into particles by rubbing the samples between your thumb and first finger.
Pour a small area of each sample onto a microscope slide and examine the samples with a magnifying glass.
Record the size, shape and colour of the particles.
Feel the particles between your thumb and first finger.
Note which sample feels the roughest, fools the smoothest.
2. Dig a 3 cm depth sample of topsoil from the garden, put it in a beaker and leave it to dry in the sun.
Put the air-dried sample into a test-tube.
Heat the test-tube and record what happens to the soil sample after a short time and after a long time of heating when the sample no longer contains any air, water, organic matter, or living organisms.
Repeat the experiment with the clean dried samples of sand and clay.
3. Fill a 50 mL beaker with garden topsoil.
Tap on the surface of the soil in the beaker with a rubber stopper, then add more topsoil until the beaker is completely full of topsoil.
Put the 50 mL beaker and soil into a larger beaker full of water so that the 50 mL beaker is completely under the water.
Observe the air coming out of the 50 ml beaker.
The air comes from the spaces still between the soil particles after tapping down with the rubber stopper.
6.4.3 Soil formation
See diagram 6.03: Soil formation:
1. Rainfall, dilute carbonic acid
2. litter build up
3. chemical weathering
4. soil forms
5. rocks split by growing roots
6. weathering by water, temperature, frost
7. transport by falling weathered material
8. soil from fallen particles
9. rocks dissolved by organic acids
10. transported soil
11. alluvial soil from upstream
12. more breakdown of rocks and soil particles downstream
Soils are forming all the time, but this is usually slower than the loss of soil through human use.
The soil we cultivate in farms is also still undergoing slow change.
Topsoil takes hundreds or even thousands of years to form.
Loss of soil results in reduced food and animal production.
Soil forms when rock is weathered (broken down), into smaller particles and moved around the landscape, a slow process.
In the two hundred years, only a few millimetres of rock have been turned into the mineral particles of soil.
The type of soil depends on its parent material.
A sandstone rock provides the material for a sandy soil.
Basalt, a volcanic rock, provides the material for a clay soil.
Soil never looks like bits of weathered rock, because a lot more has happened to it than weathering from the parent material.
It can be moved about by surface water and wind.
It can be washed down a hill slope into the valley.
It can have organic matter added to it.
As water moves through it, the small clay particles are moved deeper into the soil.
Burrowing animals, plant roots and cultivation move the soil around.
Plant nutrients are taken out by plants or crops and can be added by farmers.
Plants and animals contribute organic matter to the soil.
Plants grow, mature and die in the soil so their leaves and roots are added to the soil.
Animals grow, excrete wastes and die.
Bacteria and fungi breakdown plant litter and animal remains, which eventually become organic matter so adding nutrients to the soil.
Soil forms from rocks by heating and mechanical action.
Soil forms from rocks by acids produced by plant roots and decaying plant material, humus.
Roots can penetrate between layers of rooks and push them apart.
Experiment
1. Heat rocks then pour cold water on them.
The rocks often break up both when being heated and when being rapidly cooled.
2. Find some soft rocks such as shale or weathered limestone in your locality.
Crush and grind them into small particles.
2. Soil forms from rocks by acids produced by plant roots and decaying plant material, humus
3. Find soils with a deep layer of humus and test the pH.
6.4.4 Soil horizons of a soil profile
See diagram 6.26: Soil horizons
Mature soils usually show a well-marked profile consisting of layers called horizons.
Mature soils usually show a well-marked profile consisting of three main layers or horizons designated A, B, and C.
These differ in colour, texture, and structure and vary in thickness.
The A horizon is called the topsoil.
Soluble materials are removed by percolating water.
Topsoil is usually rich in organic matter and in soil organisms.
The, B horizon is called the subsoil.
This horizon accumulates clay washed out of the topsoil above.
Iron minerals are usually present and most likely they will be oxidized.
The, C horizon is the unconsolidated, weathered parent material.
O
The O horizon is the decaying plant and animal material on the soil surface.
Horizon depth is important for the growth of plant roots.
A forest soil may have three distinct organic layers:
1. leaves and twigs
2. partially decomposed organic material
3. dark layer of decomposed humus.
This layer has a greasy feel, light weight when dry, and a high fibre content and only a small amount of mineral matter by volume.
This layer is black or dark brown in colour due to humus material.
A
The A horizon, topsoil contains most of the nutrients and water needed for plant growth.
Soils with deep A horizons are the most usable.
It is a mineral layer of soil enriched in organic matter that accumulates from the decomposition of plant material.
It is an accumulation of humus organic matter intimately mixed with the mineral fraction.
It may have properties resulting from cultivation, pasturing, or other disturbances.
E
The E horizon is a mineral horizon in the upper part of the soil below an O or A horizon and above a B horizon.
It is a light coloured concentration of quartz and sand grains or these particles with coatings of iron oxides, formed by leaching.
B
The B horizon, the subsoil has a different colour, structure and texture.
It is less fertile than the A horizon.
It is usually a zone of accumulation of alluvial silicate clay, gypsum, carbonates, humus and oxides and hydroxides of iron. aluminium, and manganese where rain water percolating through the soil has leached material from above and it has precipitated within the B horizons or the material that weathered in the B horizon.
Well-drained soils usually have the brightest yellow brown to strong brown colours within the B horizon.
C
The C horizon, the "substratum" is broken up parent material from which the soil is developed.
The C horizon is little affected by soil forming processes.
The colour is that of the unweathered geologic material.
Most horizons are mineral layers.
Accumulations of silica, carbonates, gypsum, or more soluble salts are included in C horizons if not from above horizons.
R
The R layer is the hard bedrock, e.g. granite, basalt, quartzite, limestone, sandstone.
Usually it can be ripped only with heavy power equipment.
Any cracks are too small to allow roots to penetrate and they may be coated or filled with clay or other minerals.
Horizons and plant roots
Plants with shallow roots, e.g. wheat and pasture grasses, have most of their roots in the A horizon, although they may get water from the B horizon during dry periods.
In a shallow soil, the depth of the A horizon + B horizon may be = 15 cm and be hard to farm.
It may have stones coming to the surface, be hard to plough and be not be deep enough for plants to grow properly.
Shallow soils occur on hilltops and in mountain areas.
In river valleys, the depth of the A horizon + B horizon may be > 3 m and be easy to farm.
If water carries clay particles down into the B horizon, the roots cannot penetrate it, because a horizon rich in clay has small pore spaces.
When soil is lost by erosion, the O horizon and most of the A horizon may be lost.
Specific features
Soil horizons may have specific feature that affect plant growth.
* Accumulation of calcium carbonate, gypsum or salts more soluble than gypsum, silicate clay, sodium, sesquioxides.
* Concretions or nodules of dolomite, calcite or more soluble salts that contain iron, aluminium, manganese, or titanium.
* Frozen soil where a horizon contains permanent ice.
* Highly decomposed organic material with low fibre content.
* Alluvial accumulation of organic matter sesquioxide complexes.
* Physical root restriction caused by dense base till, plough pans, and other mechanically compacted zones.
* Root restrictive cementation by carbonates, silica; iron, gypsum, lime, and other salts.
* Slickensides from swelling of clay minerals and shear failure along a fault.
* Strong gleying showing iron reduced to ferrous form caused by saturation with stagnant water.
* Tillage disturbance.
Experiments
1. Make models of soil horizons from various places and compare the depths of the A and B horizons.
2. Dig a hole with a flat spade one metre deep in the soil.
Make one side of the hole flat to see the different layers or parts of the soil.
Soils may be dark or light at the surface, deep or shallow depth to bed rock, stony or sandy.
The roots of plants may go deep or are all near the surface.
3. Examine soil horizons in fresh road cuts or in gullies.
4. Use a flat spade to make a clean vertical cut to expose the horizons.
Let the material dry.
Smear glue on one side of a board and press the board against the cut soil so that particles from each horizon stick to the board.
Compare different soil profiles, noting the depths of the horizons and the composition of the material at each horizon.
6.4.5 Soil particle size
The size of soil particles, and their respective ratio is important in the economic management and productivity of soil.
Experiment
1. Fill a 100 mL graduated cylinder to the 30 mL mark with the finely divided soil sample, with stones and plant debris already removed.
Add water to the 100 mL mark.
Close the graduated cylinder with a rubber stopper.
Shake vigorously several times until the whole of the soil sample is suspended.
Allow the graduated cylinder to stand while the soil particles settle.
The soil particles settle out according to their rate of sedimentation, in the following sequence, from the bottom to the top:
coarse sand 2.0 mm to 0.2 mm,
fine sand 0.2 mm to 0.02 mm,
silt or dust 0.02 mm to 0.002 mm,
clay < 0.002 mm.
The clay particle may remain in suspension for a long time.
Examine a small sample from each layer with a magnifying glass.
2. Use a glass jar that holds about 2 litres.
Place several handfuls of soil in the jar.
Fill the jar with water, and then thoroughly shake up the soil in the water.
Let the jar stand for several hours.
The size, shape and density of soil particles determine the order in which they will settle.
The largest, more angular, and densest particles will settle out first, and will be on the bottom.
The layers in the jar after settling will show decreasing size, angularity, and density from bottom to top.
Examine a small sample from each of the layers with a magnifying glass.
6.4.6 Soil temperature
Plant growth is slower at low temperatures, but at very high temperatures plants can wilt, because water evaporates from the soil.
Soil temperature levels determine:
1. the number, type and activity of soil organisms and so the rate of breakdown of organic matter
2. the time taken for seeds to germinate, e.g. winter and summer crops.
3. the rate of crop growth and development, e.g. tuber formation in potatoes will not occur at temperatures exceeding 30oC.
Factors affecting soil temperature
1. Moisture content: Water has a high specific heat so soils that hold a lot of water, e.g. clay soils, tend to be cool soils, while those that hold less water are often warm soils, e.g. sandy soils.
2. Colour: Dark soils absorb heat while light-coloured soils reflect heat.
3. Slope / aspect: In the Southern hemisphere, fields facing the north receive more heat in winter.
Soils lying on hilltops tend to be warmer than soils on valley floors.
Hilltop soils are better drained and drier than valley soils and so are warmer.
Cold air is denser than warm air so it tends to moves downwards at night to cool valley soils.
4. Cultivated soils tend to be warmer than those not cultivated soils because the cultivated soil surface dries out and becomes readily warmed by the sun.
Soil temperature is usually measured with bent stem thermometers or straight stainless steel sensors for measuring soil temperature of shallow layers.
They are mercury-in-glass thermometer with the stems bent at right angle below the lowest graduation.
The thermometer bulb is sunk into the ground to the required depth, e.g. 5, 10, 15 20 cm.
Experiment
Use thermometers to observe the temperature of the soil
1. at different depths
2. under different ground cover, e.g. bar ground, grassy ground, forest floor, mulched soil.
6.4.7 Soil texture
Soil texture is the feel of the soil caused by the size of the soil particles.
2.1.1 Sandy soil has a coarse texture.
The sand particles are too large.
Sandy soils drain too quickly.
Sandy soils feel rough.
2.1.2 Clay soil has a fine texture.
The clay particles are too small.
Clay soils may not drain at all and the soil becomes waterlogged.
Clay soils feel smooth.
A good soil has a mixture of sand, clay and other particles between these sizes.
You can improve soil texture by mixing soils with different-sized particles, i.e. add sandy soils to clay soils add clay soils to sandy soils.
Use samples of different soils, e.g. clay, silt, sand and a small quantity of water.
The texture of a soil affects soil characteristics such as ease of cultivation, its ability to accept water and how much water it can hold for plants and other organisms to use.
A quick and simple technique to measure soil texture is useful when deciding agricultural activities such as ploughing.
Soil texture reference information:
1. Sands have grains that do not stick together.
Sands cannot be moulded.
Single sand grains stick to the fingers.
2. Loam sands form fragile shapes that just stand handling, give short ribbons that break easily, usually discolour the fingers.
3. Fine sandy loam forms shapes that will just stand handling.
Fine sand grains may be felt, but otherwise they feel smooth and may feel greasy if much organic matter is present.
Fine sandy loam may be formed into ribbons 15 to 25 mm long.
4. Silt loam will stick together, but will crumble.
Silt loam feels very smooth and silky and may be formed into ribbons 25 mm long.
5. Clay loam will stick together to form shapes.
Clay loam has a spongy feel and is plastic when squeezed between the thumb and forefinger.
It is smooth to manipulate and may be formed into ribbons 40-80 mm long.
6, Clays form smooth, plastics casts and show some resistance to manipulation, i.e. toughness.
They may be formed into ribbons at least 80 mm long, depending on the heaviness of the clay.
Sand grains can be felt in sandy clays that may be formed into ribbons 40-50 mm long.
Experiment
1. Take a small amount of soil enough to fit in the palm of the hand.
Discard obvious pieces of gravel.
2. Moisten the soil with water, a little at a time and knead until there is no apparent change in the feel of the ball.
The moisture constant should be such that the ball just fails to stick to the fingers.
3. Inspect the sample to see if sand is visible, if not, it may still be felt and kneaded as the sample is worked.
5. Finally, squeeze it out between the thumb and forefinger with a sliding motion and note the length of self supporting ribbon that can be formed.
6. Give the students some pure sand, silt and clay to test.
Then test soil from: a garden, a forest, bank of a river, top of a slope, bottom of a slope.
7. Make a soil texture museum.
Try to collect samples of soil with the six types of texture.
Keep samples in glass jars for reference.
6.4.8 Soil texture by mechanical analysis, texture triangle
See diagram 6.6.2a: Soil texture triangle
See diagram 6.6.2b: Using the soil texture triangle
Mechanical analysis of soil texture.
An ideal soil for crop growth is a balanced mixture of sand, silt and clay.
Only specialized plants can grow in unbalanced soils.
The large particles give the soil, good drainage, good aeration, and ease of cultivation.
The smaller particles give the soil, nutrient retention, water retention and good structure with variety of pore space.
The ideal texture in a garden is a loam with > 25% clay in the A and B horizons.
However, where rainfall comes in summer, the ideal soil can store the water and not let it evaporate, so these soils should have 30-50% clay in the A and B horizons.
Experiment
Put a small amount of soil and water into a measuring cylinder and shake it.
Let the mixture to settle for two days.
Calculate the proportions of sand, silt and clay.
Express the proportions as a percentage of the total soil content.
Find the texture with the texture triangle.
See the map of soil textures on the soil texture triangle
If the percentage of sand, silty and clay is known, use a soil texture triangle to describe the texture of the soil.
If a soil, sample contains 35% sand, 25% silt and 40% clay.
Find the position of 40% clay on the left side of the triangle and draw a line parallel to the base of the triangle.
Find the position of 25% silt on the right side of the triangle and draw a line parallel to the left side of the triangle.
Find the position of 35% sand on the base side of the triangle and draw a line parallel to the right side of the triangle.
The intersection of the three lines on the map of the soil texture triangle shows that textual class is a clay.
Similarly, if a soil, sample contains 20% sand, 40% silt and 40% clay, the textual class is silty clay.
Also, if a soil, sample contains 60% silt and 30% clay, the % sand is 10% and the textual class is silty loam.
6.4.9 Soil texture, sand type, loam type, clay type
Experiment
1. Sand type: It will not mould, feels very gritty.
It has poor nutrient carrying capacity.
Drainage test: Dig a hole and fill with 30 cm of water.
If water is drained < 1 hour, it has poor water holding capacity.
Treatment: Mix compost into top 30 cm then add 75 mm of organic much to the soil surface.
2. Loam type: It can be moulded into short rolls and feels slightly gritty and crumbly.
Drainage test: Dig a hole and fill with 30 cm of water.
If water is drained between 2 - 5 hours, it has a good balance of drainage and water holding capacity.
Treatment: Maintain the texture with mulching and compost.
3. Clay type: It can be moulded into long, thin rolls and feels smooth and pliable.
It has high nutrient carrying capacity.
Drainage test: Dig a hole and fill with 30 cm of water.
If water is drained > 10 hours, it has poor drainage.
Treatment: Add gypsum, cultivate to 30 cm with a rotary hoe and add organic mulch to the soil surface.
Experiment
Soil texture depends on feel, i.e. the proportion of sand, silt, and clay particles is in each horizon.
An ideal soil for crop growth is a balanced mixture of sand, silt, clay and humus.
Only specialized plants can grow in unbalanced soils.
The large particles give the soil, good drainage, good aeration, and ease of cultivation.
The smaller particles give the soil, nutrient retention, water retention and good structure with variety of pore space.
The ideal texture in a garden is a loam with > 25% clay in the A and B horizons.
However, where rainfall comes in summer, the ideal soil can store the water and not let it evaporate, so these soils should have 30-50% clay in the A and B horizons.
Experiment
Table 6.5.1 Soil texture
-
 Soil high in sand
 Soil high in clay
Water entry quick entry slow entry
Drainage drains quickly drains slowly
Water retention not hold enough water holds enough water
Air content air moves easily, if good structure
 big pore space, if good structure
Nutrient retention cannot hold nutrients, fertilizer lost can holds nutrients, needs less fertilizer
Ease of cultivation easy to plough, not good seed bed hard to plough if wet

6.4.10 Soil texture measured by squeezing the sample
Use samples of different soils e.g. clay, silt, sand and a small quantity of water.
The texture of a soil has an important effect on soil characteristics such as ease of cultivation, its ability to accept water and the amount of water it is able to hold for plants and other organisms to use.
A quick and simple technique to determine soil texture is useful when deciding on agricultural activities such as ploughing.
Soil texture classification
1. Sands, sand grains do not stick together, cannot be moulded, single grains stick to fingers.
2. Loamy sands, form fragile shapes that just bear handling, give short ribbons that break easily, discolour fingers.
3. Fine sandy-loams, form shapes that will just stand handling, fine sand can be felt in some, otherwise it feels smooth, may feel greasy if much organic matter is present, will form ribbons 15 to 25 mm long.
4. Silty loams, will stick together, but will crumble, very smooth and silky, will form ribbons 25 mm long.
5. Clay loams, sticks together to form shapes, with a rather spongy feel, plastic when squeezed between thumb and forefinger, smooth to manipulate, will form ribbons 40-80 mm long.
6. Clays, smooth, plastics casts, some resistance to manipulation (toughness), form ribbons at least 80 mm long depending on heaviness of the clay.
Sand grains can be felt in sandy clays which form ribbons 40-50 mm long.
Experiment
Give the children some pure sand, silt and clay to test.
Then test soil from a garden, a forest, bank of a river, top of a slope.
1. Measure texture by squeezing the sample
Take a sample of soil sufficient to fit in the palm of the hand.
Discard obvious pieces of gravel.
Moisten the sample, knead it into a ball, squeeze it and push it out between the thumb and forefinger.
Again moisten the soil with water, a little at a time, and knead until there is no apparent change in the feel of the ball.
The moisture constant should be such that the ball just fails to stick to the fingers.
Inspect the sample to see if sand is visible, if not, it may still be felt and kneaded as the sample is worked.
Finally, squeeze it out between the thumb and forefinger with a sliding motion and note the length of self-supporting ribbon that can be formed.
* If ribbon forms:
Ribbon can be bent without breaking, (clay soil)
Ribbon cannot be bent without breaking, (clay loam soil)
* If ribbon does not form:
Soil feels gritty, (sandy loam soil)
Soil feels smooth, (silty loam soil).
2. Measure texture by feeling the sample
2.0 To estimate the texture of a soil, moisten a handful of soil with water, knead the soil a little at a time and continue to moisten it until you have a ball of soil moist all the way through.
Use the following key to estimate the texture of the soil:
2.1. Soil will not roll into a ribbon. Go to 2.2
2.1 Soil will roll out into a ribbon about 8 cm × 0.5 cm, but cannot be turned into a ring without cracking. Go to 2.4
2.1 Soil rolls easily into a ribbon and can be turned into a ring.
No sand can be felt. Go to 2.7
2.2 Soil feels gritty. Go to 2.3
2.2 Soil feels silky, it is a (silty loam).
2.2 Soil feels neither gritty nor silky, it is a (loam).
2.3 Soil will make a firm ball, it is a (sandy loam).
2.3 Soil does not make a firm ball, but colours your fingers, it is a (loamy sand).
2.3 Soil neither makes a firm ball nor colours your fingers, it is a (sand).
2.4 Soil feels gritty. Go to 2.5
2.4 Soil feels silky. Go to 2.6
2.4 Soil feels neither gritty nor silky, it is a (clay loam).
2.5 Soil feels like gritty Plasticine to mould, it is a (sandy clay).
2.5 Soil feels earthy, it is a (sandy clay loam).
2.6 Soil feels like Plasticine to mould, it is a (silty clay).
2.6 Soil feels silky, but more earthy, it is a (silty clay loam).
2.7 Soil is easy to mould, it is a (light clay).
2.7 Soil is fairly stiff to mould, it is a (medium clay).
2.7 Soil is very stiff to mould, it is a (heavy clay).
3. Texture museum.
Try to collect samples of soil with the six types of texture.
Keep them in glass jars for reference.
6.4.11 Soils contain air
See diagram 6.8.2: Air in soil
Experiment
Put soil in a container and slowly add water.
Observe the air bubbles that rise through the water from the soil.
6.4.12 Surface / volume ratio of particles
See diagram 6.6.3 Surface / volume ratio
1. Surface / Volume Ratio, 2. Surface area of one block or 18 particles 3. In the diagram:
The diameter of 1 fine grain = 0.2 mm, so the surface area of 1 fine grain = 0.2 × 0.2 × 6 = 0.24 mm2
If the diameter of 1 coarse grain = 2.0 mm, so the surface area of 1 coarse grain = 2.0 × 2.0 × 6 = 24.0 mm2
The surface area of 1000 fine grains, together equal in volume to 1 coarse grain = 0.24 × 1000 = 240 mm2
The chemical activity of a particle is proportional to the surface area.
So the chemical activity of 1000 fine grains is 10 × the chemical activity of 1 coarse grain.
The volume of 1 coarse grain = 0.2 × 0.2 × 0.2 = 0.008 mm3. The volume of 1 coarse grain = 2.0 × 2.0 × 2.0 = 8 mm3
Table 6.5.3 Surface / volume ratio of particles
One grain of:
 Fine grain
 Coarse grain
Surface area
 0.24 mm2
 24.0 mm2
Volume
 0.008 mm3
 8.0 mm3
Surface area / volume
 30
 3

So, the surface / volume ratio of 1 fine grain is 10 × the surface / volume ratio of coarse grain.
2. Surface area of one block or 18 particles, pore spaces
Estimate the surface area of the one block
Estimate the surface area of one of the 18 particles.
Estimate the surface area of the 18 particles if separated.
The block has no pore space.
Between each 4 particles in the block is pore space.
When all the 18 particles are together there are 10 pore spaces.
The pore spaces can hold water.
6.4.13 Wind deposits
Experiment
Use three large trays filled with moist sand, dry sand, and flour.
Place the tins near an electric fan directed to blow towards the tins.
Move each tray towards the blowing fan until a slight movement occurs in the pile of material.
Note the distance at that each showed movement, because of the artificial wind.
Observe any pattern in the way the materials have blown.
The lightest material is the farthest away and the heaviest is the closest.
Note how larger particles are sorted.
This sorting mechanism occurs frequently in nature.
6.4.14 Nutrition from the soil
The rate of plant growth reflects the ability of plants to extract nutrients from rocks.
Good agricultural soils have low levels of "exchangeable" sodium.
With high exchangeable sodium, aggregates break down to form a dispersed layer causing waterlogging and later particles dry to form hard clay
Experiment
1. Grind samples of quartzite, schist, basalt, limestone.
Plant radish seeds in each sample and note rates of plant growth.
2. Use swelling clay from a dry clay pan, e.g. montmorillonite.
Pack clay into two 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.
6.5.1 Good management practices
Using land wisely - land capability
Decisions on how to make the best use of land can be complex as climate, landscape position, the soil itself and the proposed use need to be taken into account.
Much cropping and grazing land needs to be protected from soil erosion.
1. Maintaining vegetation.
Maintain stubble or trash after the harvesting of a crop.
Maintain natural vegetation by avoiding overgrazing of pastures.
2. Maintain trees.
While the trees are growing on the slope, the roots hold the soil and they take up water keeping the water table from rising.
If the trees are cleared, the soil may easily erode and the water table may rise waterlogging the root zone and perhaps bringing salts to the root zone as well.
Tree roots hold soil together and take up water.
3. Maintain good soil structure.
This is a combination of cultivation practices, ploughing and retaining crop residues and putting a beneficial crop rotation system into place.
4. Maintain proper cultivation techniques.
Plough around contour lines, not up and down a slope.
Contour ploughing slows the water down as it has to fill the furrow before it spills over.
On a long or steeper slope, it would be better to build contour banks and grassed waterways.
Ploughing direction can affect soil loss.
Use minimum tillage (which cuts down on the amount of cultivation), by planting directly into the soil through the retained crop residues.
5. Using land conservation techniques.
There are a number of land conservation techniques which can be used primarily to reduce the rate of runoff water.
These can be structural (e.g. introduce contour banks on sloping land, or grassed waterways), or agronomic (e.g. strip cropping),
Strip cropping also helps reduce wind erosion in the same way as windbreaks of trees.
Experiments
1. Visit a locality where running water has caused damage by cutting gullies.
Note the damage and examine any ways to prevent it.
2. Fill two trays with loose soil, and tilt both trays to the same angle.
Make furrows with a stick as follows: 2.1 running up and down the hill in one tray, and 2.2 running across the hill in the other tray.
Sprinkle each tray with the same amount of water.
Observe the erosion in each case, and the runoff water.
In light rainfall furrows across the hill prevent soil erosion and increase soil moisture.
In very heavy rainfall, e.g. in tropical areas, furrows across the hill make cause greater loss of soil as water pushed on the walls of the furrows.
3. Fill the trays with loose soil then add water until the water forms gullies.
Block the gullies at intervals with small stones.
Add water again and observe the effect of blocking the gullies.
6.5.2 Plants can prevent soil erosion
Experiment
1. Observe an area where the soil has eroded, because of lack of plant cover.
Discuss why the area looks as it does.
Show how plants may stop the soil from blowing or washing away.
2. Plant grass seed in a section of sandy soil, in an erosion table.
Run water down the erosion table when the grass has developed a network of roots.
Note how the roots have a holding effect on the soil.
Pull out some sprouting grass, run water on this area and note any new soil erosion.
6.5.3 Raindrops affect soils
See diagram 6.52: Splash sticks and soil particles
Raindrops that have just fallen from a cloud have a radius < 1 mm and are spherical due to the weak hydrogen bonds of surface tension of water.
Larger raindrops, 2-3 mm, have a more flattened or even indented base as the fall velocity increases and the pressure up on the base increases.
When larger than 4.5 mm radius they become more indented then break into smaller raindrops.
So a raindrop is never tear-shaped as shown in popular illustrations.
Raindrops splash on bare soil, but plant cover protects the soil from raindrop splash erosion.
Falling raindrops hit the ground at high speed, up to 60 km / h.
When the ground surface is bare, the energy of raindrops shatters soil particles into tiny fragments and splashes them in all directions.
The soil particles can block the natural air spaces (soil pores), and seal the soil surface so less water soaks into the soil and water builds up on the surface.
Soil particles that are suspended in this water are lost from that area as the water runs off.
Raindrops hitting this runoff water cause more turbulence, keeping the soil particles in suspension and these wash away.
Plant cover will protect the soil surface from raindrop impact, allowing water to enter the soil.
Old root systems help to bind the soil together and provide channels for better water entry so runoff and soil loss are greatly reduced.
Experiment
Clip a sheet of white paper to a board and lay it flat on the floor.
Use a medicine dropper to let drops of coloured water fall on the white paper.
Note the size and shape of the splashes.
Repeat the experiment with the white paper at different angles to the horizontal.
Repeat the experiment with the medicine dropper at different heights.
Keep the sheets of paper as a record of the different "raindrops".
6.5.4 Running water changes soils
See diagram 6.50: Erosion and runoff test
Experiment
1. After a heavy rainfall, take samples of running muddy water.
Leave the samples to stand for several hours then observe the settled sediment.
2. Fill a tray with loose soil and fill another tray with firmly packed soil.
Tilt both trays to the same angle.
Place two containers to collect runoff water.
Use a watering can to sprinkle the same volume of water on each tray.
Observe which soil is carried down faster and measure the runoff water.
3. Fill both trays with soil, but cover one tray with grass sod from a grass farm.
Water equally as before and observe both the erosion and the runoff water.
5. Fill both trays with soil, but give one tray more slope than the other.
Water equally as before and observe both the erosion and the runoff water.
6.5.5 Soil erosion
All land has a limit to how intensively it can be used.
If a soil is managed poorly either used for a purpose to which it is not suited, or used too intensively, it will become degraded.
Land conservation is a way of managing soils so they do not degrade.
Land degradation includes soil erosion, salinization, soil fertility decline, soil structure decline, soil acidification, woody weed invasion tree die back and urban encroachment.
Soil erosion, the process of soil being washed away by water or blown away by wind, is the most common and most devastating form of land degradation in Queensland.
The greatest amount of soil erosion is caused by water runoff.
High intensity summer storms, combined with the farming of sloping land and clearing of natural vegetation, have led to major soil erosion problems.
Clearing the natural vegetation and ploughing the soil into a fine seedbed or letting stock overgraze the land, leaves the soil unprotected and allows it to be washed away when heavy rains fall, or blown away when there are strong winds.
The cost of soil erosion: One millimetre of topsoil washed off a hectare of a paddock amounts to 10 tonnes of soil.
The productivity of the washed off soil is then lost forever.
Experiment
1. Visit a locality where running water has caused damage by cutting gullies.
Note the damage and examine any ways to prevent it.
2. Fill two trays with loose soil, and tilt both trays to the same angle.
Make furrows with a stick (a), running up and down the hill in one tray, and (b), running across the hill in the other tray.
Sprinkle each tray with the same amount of water.
Observe the erosion in each case, and the runoff water.
In light rainfall furrows across the hill prevent soil erosion and increase soil moisture.
In very heavy rainfall, e.g. in tropical areas, furrows across the hill make cause greater loss of soil as water pushed on the walls of the furrows.
3. Fill the trays with loose soil then add water until the water forms gullies.
Block the gullies at intervals with small stones.
Add water again and observe the effect of blocking the gullies.
6.5.6 Splash sticks
See diagram 6.52 : Splash sticks and soil particles
Experiments
1. Clip a sheet of white paper to a board and lay it flat on the floor.
Use a medicine dropper to let drops of coloured water fall on the white paper.
Note the size and shape of the splashes.
Repeat the experiment with the white paper at different angles to the horizontal.
Repeat the experiment with the medicine dropper at different heights.
Keep the sheets of paper as a record of the different "raindrops".
2. Paint metre sticks white on one side to show mud splashes.
Push the white sticks vertically down into the soil.
Use the same force of water from a hose to make mud splashes on the splash sticks.
After a rainstorm, note the height to that mud has been splashed on each splash stick.
6.5.7 Water erosion
On sloping land, any excess water on the soil surface runs down the slope.
This overland flow of water has the potential to pick up and carry large quantities of soil.
The steeper the land, the faster the water flows and the more soil it can carry away.
The soil is deposited along fence lines, over roads and railway lines and can silt up watercourses, dams and estuaries.
Easily recognized consequences of water flowing down slopes are sheet erosion, rill erosion, gully erosion, tunnel erosion and sometimes landslip.
Uncontrolled water flow in streams can cause stream bank erosion.
Experiment
To prevent soil erosion by water, divert overland flow onto grassed waterways before it reaches the area at risk, on the area at risk make the slope shorter to slow the water by contour banks, protect the soil surface with vegetation cover and maintain good soil
surface structure and topsoil depth. For stream banks, retain natural vegetation along streams and rivers.
Contour banks shorten the length of slope over which runoff travels and carry excess runoff water away from cultivated land to stable well-grassed waterways.
6.5.8 Wind erosion
Wind erosion can damage bare or unprotected soil such as cultivated paddocks or paddocks which have been overgrazed.
Soil particles that are picked up by the wind are sometimes dropped within a short distance, but if the wind is strong the smaller
particles, e.g. clay, can be carried long distances away from the area.
Experiment
To cut down soil loss by wind erosion, protect the soil surface by maintaining plant cover and slow down the wind by planting windbreaks across the path of the wind.
6.7.1 Soil profiles
See diagram 6.26: Soil profiles
Experiment
To make a soil profile with a spade, dig a hole 1 metre deep in the soil.
Make one side of the hole flat to see the different layers or parts of the soil.
Not all soil profiles are the same.
Soils may be dark or light at the surface, deep or shallow (depth to bed rock), stony or sandy.
In some soils the roots of plants go deep.
In other soils the roots are all near the surface.
1. Dig profiles in different places then record what you see.
2. Dig down until the soil profile does not change.
The holes should not be more than 1 metre deep.
Fill the holes after each lesson.
3. Dig a profile in the most common kind of soil.
Record what you see. [The top part of the soil is the darkest.
The soil is in layers.]
The dark soil at the top of the profile is the topsoil.
This is the richest part of the soil.
You can see the dark colour where there are rotting plant leaves and roots.
The black colour comes from organic carbon compounds left when the leaves and roots rot completely.
The lower part of the soil is the subsoil.
Below the subsoil is the bed rock or coral reef.
6. Dig other profiles and record what you see.
Then fill in the hole.
5. Dig a hole in a place where the topsoil is very dark.
Collect samples of black soil near the top of the profile and light sandy soil down near the bottom of the profile.
Put these samples into two separate tins with small holes in the bottom of the tins.
Sow several different kinds of seeds in both soils.
Water each soil.
Place tins in a shady place where they will only get morning sun.
After the seeds have germinated and grown, which soil made the plants grow best? [The dark topsoil is better than the subsoil for growing plants.]
6.7.2 Soil changes with depth
See diagram 6.7: Soil auger
Experiment
1. Make a soil auger from pipe.
Turn the auger down into the soil and pull out samples from different places and at different depths.
Keep the soil depth samples in plastic boxes.
An alternative to using a soil auger is to examine soils exposed in roadside cuttings.
2. Construct a soil auger from a carpenter's wood drill bit.
This should be welded to a steel shank about 2 cm in diameter and about 50 cm in length.
A cross member welded to the shank will provide leverage to rotate the auger when it is drilled into the ground.
By simply turning the auger down into the soil and pulling it out of the ground at intervals, samples which stick to the bit can be extracted from various depths.
A grid for a specific area of the land can be made, and soil depth samples taken and compared to give a picture of subsurface conditions for that area.
Afterwards, these individual soil samples can be mounted as models, or the simple observations can be recorded.
6.8.1 Angles between mud cracks
Experiment
Use trays to collect samples of different types of clay and fine silt.
Add just enough water to cover the samples.
Put the collection trays in direct sunlight and note how mud cracks form.
Compare the number of formed mud cracks from tray to tray.
Note the angles formed as the mud cracks appear.
The pieces of dried mud have a roughly hexagonal shape.
6.8.2 Soil contains water
See diagram 35.6.2: Water from dry soil
Soil water refers to any water that enters the soil either by rainfall or irrigation.
Some of this water evaporates back into the atmosphere
some water drains away and some water is used by the plants growing in the soil.
The lost water is replaced by air.
Soil water is essential for plant growth, because plants need water.
Soil water carries in solution many nutrients needed for plant growth.
Types of soil water:
1. Capillary water exists in the pores between the soil particles or between tightly packed soil aggregates.
This water is attracted to the soil particles forming a thin film over the their surface.
This water is taken up by the roots of the plants.
2. Gravitational water moves down through a soil under gravity, after rainfall or irrigation.
This water carries nutrients and minerals causing leaching.
The water may collect deep in the soil profile.
The top of this stored water is the water table.
When this water collects too far below root level, it is not available to plants.
3. Hygroscopic water (chemically bound water), is held strongly by clay colloids and is not available to plants.
Other soil properties affect soil water
A soil with poor structure does not let water move easily into it or through it and cannot store water.
3.1 Infiltration rate is the rate (how quickly), water can enter into asoil.
3.2 Permeability measures how easily water can move through a soil.
3.3 Porosity is the amount of space around soil particles and soil aggregates that is filled by water and / or air.
Some soils have structure, but the soil aggregates are weakly held together.
If the soil aggregates are broken down, the clay particles in them can scatter (disperse),
The fresh clay particles then clog the soil pores.
Soils which have a high sodium content are more likely to disperse.
If there is dispersion in the B horizon, then water and air cannot permeate through it.
The topsoil can become waterlogged if there is a lot of water and the subsoil cannot store water, because the water cannot move into the subsoil.
If dispersion happens on the surface of the A horizon, a crust can form and water cannot infiltrate, so it runs off, causing soil erosion.
The addition of gypsum or organic matter to soils may open the soil surface for better water and air entry.
The soil water available to plants is measured by the field capacity, the amount of water left in soil after being saturated for 2-3 days, i.e. the total amount of water this soil can hold.
This water held is available for plant growth, except for the hygroscopic water, and plants would usually take up most of this water through their roots.
However, if the plants cannot take up water as quickly as it is lost by evaporation through the leaves, wilting begins (wilting point),
If the plant continues to get not enough water from the soil, it will die (permanent wilting point),
Experiment
Put soil on a piece of tin or in an evaporating dish and heat it slowly.
Cover the soil with a glass cup or filter funnel and note water condensing on the cool sides.
6.8.3 Soil permeability
Experiment
Cut off the ends of metal beverage cans to make cylinders.
Attach cloth as a fine screen material over the bottom ends of the cylinders and fasten the cloth with wire.
Put filter paper inside the beverage can on top of the fine screen material to prevent fine particles of soil from passing through the screen.
Collect samples of coarse, medium, and fine soil.
Heat the samples in an oven at 105oC until they are dry.
Put equal amounts of the three soils in the beverage cans.
Put the three beverage cans on stands so that water can be poured into them and collected underneath.
Pour equal volumes of water into each container.
Record the time taken for the water to stop filtering through the soil.
Record the volumes of water collected under each beverage can.
Note that soils are the most permeable and that soils retain the most water.
6.8.4 Soil water-holding capacity
After rain, soil contains much water, but usually most of the water goes straight down through the soil and into the water table.
Experiment
1. To show that water goes through the soil, make a funnel by bending some cardboard round and pin the sides together.
Fill the funnel with soil.
Pour water onto the top and see how much water goes straight through the soil.
2. Cut off the ends of plastic drink bottles to make cylinders.
Attach cloth over the bottom ends of the cylinders and fasten the cloth with wire.
Use a curved trowel to dig out cylinders of soil with diameters equal to the internal diameters of the drink bottles.
The aim of this procedure is to reduce any change in the natural compaction of the soil sample.
Put equal volume soil samples in the cylinders.
Under each cylinder, place a dish to hold surplus water.
Pour measured volumes of water into each cylinder until the water begins to run through the cloth at the bottom.
Note the water holding capacities of different soils.
6.8.5 Soil water
See diagram 35.6.2: Water from dry soil
Water makes a very thin coating on the outside of each grain of sand.
However, coral sand grains also have little holes in them, so the water also goes inside the grains.
You cannot see this water, but you can show that it is there.
Experiment
Dig down about 15 cm into the soil and take a match box full as a sample.
Put some of this soil onto a piece of tin.
Cover it with a glass or glass cup.
Heat the underside of the tin with a lamp or a candle.
Small drops of water will form on the inside the glass showing that water is in the soil.
6.8.6 Soil water moves down through soil
See diagram 35.6.3: Water through soil
Water may not stay in the soil.
After rain, soil contains much water.
Most of the water goes straight down through the soil and into the water table.
Experiment
To show that water goes through the soil, make a funnel by bending some cardboard round and pin the sides together.
Fill the funnel with soil.
Pour water onto the top and see how much water goes straight through the soil.
6.8.7 Soil water moves up in three ways
See diagram 35.6.1: Water climbs up
Water moves up in three ways:
1. The roots of plants take water from the soil.
2. The hot sun evaporates water out of the soil near the surface.
It goes away as water vapour.
3. Water can rise in the soil by capillarity, a phenomenon linked to surface tension that causes liquids to rise up fine tubes, capillary tubes, and be absorbed by the spaces in absorbent paper.
Soil does not contain capillary tubes, but the spaces within soils have large enough relative surface areas for capillarity to occur.
Experiment
1. Dip the end of a piece of chalk into coloured water.
The water rises up the chalk, because it has many very small holes.
Repeat the observation with dry newspaper or cloth.
Dip the end in water and the water slowly rises up the paper or cloth.
A wick of a candle works in this way.
2. Cut off the ends of plastic drink bottles to make cylinders.
Attach cloth over the bottom ends of the cylinders and fasten the cloth with wire.
Put 15 cm depths of soil in the cylinders, e.g. sand, loam, gravel, clay.
Stand the cylinders in a pan containing 3 cm depth of water.
Note the rise of water in the soil, because of capillarity.
6.8.8 Soil water content
Experiment
Collect samples of soil in metal beverage cans.
Weigh the beverage cans and adjust the soil content so that the weight of each beverage can and soil is the same.
Heat the beverage cans and soil in an oven at 105oC until all the soil is dry.
Calculate the water content of each soil sample.
Compare soil samples sheltered from rainfall with unsheltered soil samples.
Compare the absorption of these samples with daily rain gauge data.
For example, compare how 25 mm of rainfall affect the absorption of exposed soil, contrasted with an unexposed control sample.
6.8.9 Capillary action in soil and deposition by groundwater
See diagram 6.21: Solution and deposition: A heat lamp, B sand and salt, C clean sand
Experiment
Put a 2 cm depth of a mixture of sodium chloride and fine dry sand in a big plastic container.
Cover this layer with 5 cm of clean sand with no salt in it.
At one end of the container insert a long stem funnel vertically into the sand so that the end of the stem reaches the sand and salt layer.
At the other end of the container fix a heat lamp or put only that end in direct sunlight.
Pour enough water into the funnel to wet a layer about 2 cm deep along the bottom of the container.
Agitate the stem of the funnel to help the water to move down.
Turn on the heat lamp or leave that end of the container in the sunlight for 2 hours.
Observe through the side of the container so that you can see water moving through the sand.
Near the lamp or sunlight, water rises through the sand by capillary action, bringing the salt in solution up with it.
The heat causes the water to evaporate, and the salt forms deposits near and at the surface of the sand.
6.8.10 Infiltration and capillary action by groundwater
See diagram 6.22: Infiltration and capillary action by groundwater
Experiment
Half fill two glass tubes, 2 cm in diameter and 30 cm long with dry, fine sand.
Support the tubes vertically with clamp stands so that the lower end rest in a dish.
Pour water into one glass tube.
Observe the water moving down through the pore spaces of the sand, then moving into the dish, then starting to move up the other tube by capillary action.
6.8.11 Water from leaves
See diagram 9.194.1: Water comes out of leaves | See diagram 9.195: Plants lose water
Experiment
Use a mirror and a tree with leaves that you can push inside a clear plastic bottle or plastic bag.
Water is in all parts of the plant - in the stems, leaves, flowers and roots.
Also, coconuts have nut water in the fruit.
Plants lose water from their leaves.
You cannot see this water, because it is water vapour, but you can show that it is there.
You can do simple experiments with water vapour.
Take a piece of glass and breathe on it.
You cannot see the water in your breath, but you can see the water that collects on the glass.
Push some live leaves from a growing plant inside a clear plastic bottle or plastic bag.
Do not break the leaves off the tree.
Close the mouth of the bottle or plastic bag.
After 20 minutes go out and look at the bottle or plastic bag.
Small drops of water form inside it. This water has come out of the leaves.
The movement of water through the plant is called transpiration.
6.8.12 Freezing water expands
Water has a maximum density at 4oC.
When water cools from room temperature to 4oC, it is contracting in volume.
When water is cooled from 4oC to 0oC, it is expanding.
At 4oC the density of water is 1000 kg m-3.
At 0oC the density of water is 999.87 kg m-3 and the density of ice is 918 kg m-3.
The temperature of maximum density decreases with salinity.
Experiment
Fill a glass bottle with water and put the top on securely.
Wrap the bottle in a cloth, to prevent the shattered glass from falling.
Put the bottle into the freezing compartment of a refrigerator.
After 24 hours, carefully remove the bottle and examine it.
The glass is cracked, because the water increased in volume during freezing.
Water has a maximum density at 4oC.
When water cools from room temperature to 4oC, it is contracting in volume.
When water is cooled from 4oC to 0oC, it is expanding.
At 4oC the density of water is 1000 kg m-3.
At 0oC the density of water is 999.87 kg m-3 and the density of ice is 918 kg m-3.
The temperature of maximum density decreases with salinity.