School Science Lessons
Soils
Updated: 2009-09-01
Please send comments to: J.Elfick@uq.edu.au
See: Interesting websites
See: Soil classification, NRCS, USA (website)
See: Soil, Queensland Department of Agriculture (website)
Table of Contents
S1. Study soils
S2. Types of soils
S3. Soil nutrients (plant nutrients) fertilizers
S3a. Compost
S4. Soil particles, soil texture
S5. Soil profile
S6. Soil air
S7. Soil water, capillary water
S7a. Mulch
S8. Soil structure
S9. Soil acidity, soil pH
S10. Soil colour
S11. Soil temperature
S12. Soil conservation
S13. Soil minerals
S14. Soil life
S15. Coral soils
S1. Study soils
1.33 Plants need soil (Primary)
6.01 Plants need soil
6.02 Soil components
6.03 Soil formation
6.03.1 Soil forms from rocks by heating and mechanical action
6.03.2 Soil forms from rocks by acids produced by plant roots and decaying plant material, humus
6.04 Weathering
S2. Types of soils
2.37 Collect soils (Primary)
3.35 Soil contents (Primary)
5.39 Make clay pots (Primary)
6.31 Describe soils (Primary)
S3. Soil nutrients (plant nutrients) fertilizers, compost
4.35 Natural fertilizers (Primary)
5.18 Fertilizing the soil
5.25 Plant nutrients from plant ash
5.35 Fertilizer trial (Primary)
5.36 Cover crops (Primary)
6.4 Fertility of different soils
6.4.1 Soil fertility decline
6.9 Compare fertility of subsoil and topsoil
6.9.6 Preparing ground
6.9.7 Improving soil
6.9.14.1 Humus
6.9.15 Fertilizing the soil
6.9.17.0 Chemical fertilizers
6.33 Fertilizing soil (Primary)
6.34a Chemical fertilizers (Primary)
6.37 Fertilizing the soil
6.37.1 Nutrition from the soil
6.38 Plant foods
6.39 Plants need nitrogen, nitrogen cycle
6.40 Legumes
6.65.8.3 Cover crops and green manure (Primary)
6.42 Artificial fertilizers
6.44 Nutrient cycles
6.46 Crop rotation
6.65.8.1 Natural fertilizers (Primary)
6.65.8.4 Fertilizer trial (Primary)
9.11.3.1 Natural fertilizers
S3a. Compost
6.41 Make compost
6.9.14.0 Composting
6.9.14.2 Organic materials for composting
6.9.14.3 Composting, carbon / nitrogen ratio
6.9.14.4 The 3 methods of composting
6.9.14.5 Compost inspection
6.9.14.6 Starting composting for the school garden
S4. Soil particles, soil texture
1.34 Good soil and bad soil (Primary)
1.35 Feel good soil (Primary)
1.37 Soil with a magnifier (Primary)
2.38 Shake soil in water (Primary)
6.6 Particle sizes of soils
6.6.1 Measure soil texture
6.6.01 Measure texture by feel and drainage, treatment
6.6.1.1 Measure texture by feel
6.6.2 Measure soil texture by mechanical analysis, texture triangle
6.6.3 Surface / volume ratio of soil particles
6.29 Wind deposits
6.32 Test soil texture (Primary)
S5. Soil profile
5.34 Soil profiles (Primary)
6.26 Soil horizons of a soil profile
6.30 Soil profiles
6.31 Soils change with depth
S6. Soil air
2.40 Air in soil (Primary)
3.34 Soil air (Primary)
6.8 Soils contain air
S7. Soil water, capillary water, mulch
3.33 Soil water (Primary)
6.10 Soils contain water
6.11 Water content of soils
6.13 Water rises in soils
6.14 Water-holding capacity of soils
6.25 Angles between mud cracks
6.32 Water from leaves
6.33 Soil water
6.34 Water through soil
6.35 Water rises up soil
6.49.3 Soil water (Primary)
4.37 Soil water bottle (Primary)
6.49.5 Soil water bottle (Primary)
2.39 Water through soil (Primary)
3.36 Waterlogged soil (Primary)
4.36 Water climbs up soil (Primary)
6.19 Soil permeability
6.21 Capillary action in soil and deposition by groundwater
6.22 Infiltration and capillary action by groundwater
S7a. Mulch
6.36 Mulch saves water
6.9.13 Mulching the soil
6.65.8.2 Mulch garden soil (Primary)
5.38 Mulch garden soil (Primary)
S9. Soil acidity, soil pH
5.19 Acid soils and alkaline soils
6.12 Test soil pH, acid soils and alkaline soils
6.12.1 Use of a commercial soil pH test kit
6.12.2 Preferred pH ranges
6.12.3 Change the pH of potting mix
6.12.4 Raise soil pH with agricultural lime / dolomite
6.12.5 Lower soil pH with agricultural sulfur
S12. Soil conservation
1.36 Protect topsoil (Primary)
4.34 Protect soils (Primary)
5.37 Rain on slopes (Primary)
6.15 Running water changes soils
6.16 Raindrops affect soils
6.17 Splash sticks
6.18 Soil erosion
6.18a Water erosion
6.18b Wind erosion
6.18.1 Good management practices
6.27 Plants can prevent soil erosion
6.30 Protect our soils (Primary)
6.31 Describe soils (Primary)
6.62.1 Protect our soils (Primary)
6.62.3 Protect topsoil (Primary)
S13. Soil minerals
6.20 Soil salts, soil minerals in solution
6.20.01 NPK grade formula
6.20.1 Salinisation
6.23 Oxidation of iron
6.24 Freezing water expands
6.43 Chalk (lime) content of the soil
9.11.3.2 Make potash from ash
9.9.18 Hydroponics
19.1.20.6 Test for nitrate / nitrite with dipsticks
35.14.0 The main rock-forming minerals
35.20.0 Other minerals and ores
35.21.0 Major groups of rocks
S14. Soil life
3.32 Soil animals (Primary)
6.28 Life in the soil
6.63.1 Soil animals
S15. Coral soils
6.29 Protect our coral reefs (Primary)
6.47 Water lens in atolls.
6.48 How soils form in atolls
6.49 How atoll soils change
6.01 Plants need soil
Plants need soil for the following reasons:
1. Soil holds up the plants. It supports them and holds them firmly.
2. Soil holds water for the plants. Plants need water to grow.
3. Soil holds foods for the plants. Plants must have these plant foods or they cannot grow.
6.02 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 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.
The approximate major components of a soil by volume are as follows: minerals 45%, air 25%, water 25% (after rain, not air-dry).
Soil components 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,
Soil components 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.
Soil components 3.
See diagram 6.02: Beaker of soil under water
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.
6.03 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.
6.03.1 Soil forms from rocks by heating and mechanical action
Heat rocks then pour cold water on them. The rocks often break up both when being heated and when being rapidly cooled. Roots can penetrate between layers of rooks and push them apart. Find some soft rocks such as shale or weathered limestone in your locality. Crush and grind them into small particles.
6.03.2 Soil forms from rocks by acids produced by plant roots and decaying plant material, humus
Find soils with a deep layer of humus and test the pH.
6.04 Weathering
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.
1.1 Frost action occurs when water in cracks and crevices may break the rock as it freezes and expands
1.2 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.
1.3 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 causing 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.
2.1 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 you should 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.
6.1 Types of soils
Plants need soil to support them and hold them firmly, provide water, provide mineral salts. 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.
6.4 Fertility of different soils
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. 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 mu m, (2 X 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.
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 - 6.7. 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
6.4.1 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.
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.
6.6 Particle sizes of soils
1. The size of soil particles, and their respective ratio is important in the economic management and productivity of soil. 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. Obtain 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.
6.6.1 Measure soil texture
Soil texture depends on feel, i.e. the proportion of sand, silt, and clay particles is in each horizon.
-
 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
for plant growth		 holds enough water,
may be waterlogged
Air content air moves easily,
if bad structure
small pore space
-
 if good structure big
pore space
if bad structure
small pore space
Nutrient retention cannot hold nutrients,
fertilizer washed away		 can holds nutrients,
needs less fertilizer
Ease of cultivation easy to plough, not
good seed bed		 hard to plough if wet
-
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.
6.6.01 Measure texture by feel and drainage, treatment
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.
6.6.1.1 Measure texture by feel
Moisten a small amount of soil, knead it into a ball, squeeze it and push it out between the thumb and forefinger.
1.1 If ribbon forms:
1.1.1 Ribbon can be bent without breaking, clay soil
1.1.2 Ribbon cannot be bent without breaking, clay loam soil
1.2 If ribbon does not form:
1.2.1 Soil feels gritty, sandy loam soil
1.2.2 Soil feels smooth, silty loam soil
2. Measure texture by feel
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.
The soil will not roll into a ribbon, go to 2.2
2.1.1 The soil will roll out into a ribbon about 8 cm long and 0.5 cm thick but it cannot be turned into a ring without cracking, go to 2.4
2.1.2 The soil rolls easily into a ribbon and can be turned into a ring. No sand can be felt, go to 2.7
2.2.
The soil feels gritty, go to 2.3
The soil feels silky, the soil is a silty loam.
The soil feels neither gritty nor silky, the soil is a loam.
2.3
The soil will make a firm ball, the soil is a sandy loam.
The soil does not make a firm ball but colours your fingers, the soil is a loamy sand.
The soil neither makes a firm ball nor colours your fingers, the soil is a sand.
2.4
The soil feels gritty, go to 2.5
The soil feels silky, go to 2.6
The soil feels neither gritty nor silky, the soil is a clay loam.
2.5
The soil feels like gritty Plasticine to mould, the soil is a sandy clay.
The soil feels earthy, the soil is a sandy clay loam.
2.6
The soil feels like Plasticine to mould, the soil is a silty clay.
The soil feels silky but more earthy, the soil is a silty clay loam.
2.7
The soil is easy to mould, the soil is a light clay.
The soil is fairly stiff to mould, the soil is a medium clay.
The soil is very stiff to mould, the soil is a heavy clay.
6.6.2 Measure 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. 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.
6.6.3 Surface / volume ratio of soil particles
See diagram 6.6.3: 1. Surface / Volume Ratio, 2. Surface area of one block or 18 particles
1. In the diagram:
The diameter of 1 grain of fine sand = 0.2 mm, so the surface are of 1 grain of fine sand = 0.2 X 0.2 X 6 = 0.24 mm2
If the diameter of 1 grain of coarse sand = 2.0 mm, sp the surface are of 1 grain of coarse sand = 2.0 X 2.0 X 6 = 24.0 mm2
The surface area of 1000 grains of fine sand, together equal in volume to 1 grain of coarse sand = 0.24 X 1000 = 240 mm2
The chemical activity of a particle is proportional to the surface area, so the chemical activity of 1000 grains of fine sand is 10 X the chemical activity of 1 grain of coarse sand.
The volume of 1 grain of fine sand = 0.2 X 0.2 X 0.2 = 0.008 mm3
The volume of 1 grain of coarse sand = 2.0 X 2.0 X 2.0 = 8 mm3
One grain of:
 Fine sand
 Coarse sand
Surface area
 0.24 mm2
 24.0 mm2
Volume
 0.008 mm3
 8.0 mm3
Surface area / volume
 30
 3
The surface / volume ratio of 1 grain of fine sand is 10 X the surface / volume ratio of coarse sand.
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.
S8. Soil structure
See diagram 6.8.0: Good structure, a soil aggregate, capillary water | See diagram 6.8.1: Plough pan
1. The soil structure refers to the way soil particles (sand, silt and clay) and organic matter are grouped together to form soil aggregates.
2. Soils without structure include:
2.1 Soils in sand dunes consisting of single particles of sand that do not cling together.
2.2 Soils consisting of particles that stick together in one solid mass (massive soils).
3. Soils with good structure break up easily into soil aggregates with definite shapes and sizes. 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. 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. A soil with good structure will easily take in water through the spaces between the soil aggregates.
4. Water retention of the soil (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.
5. Soil aeration is a measure of the space formed when excess water drains through the soil spaces to be replaced with air.
6. A soil with good structure loses less soil particles on the surface from being washed away by water or blown away by wind.
Porosity of the soil is the space between the soil aggregates + the space within the soil aggregates.
7. Soil structure is destroyed by:
7.1 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
7.2 Compaction either by creating a plough pan or from the pressure of the tractor tyres. Plough pans reduce water and root penetration.
Excessive cultivation, or tillage under the wrong conditions, produces 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.
7.3 Not maintaining the organic matter content of the soil.
7.4 Cattle and sheep trampling, especially around watering points.
8. Methods for improving soil structure
Soil structure can be maintained by changing management practices:
8.1 Limit the frequency of cultivation
8.2 Cultivate only when the soil is not too wet
8.3 Limit tillage traffic.
9. Lost soil structure can be regained by:
9.1 Using an implement for deep ripping to destroy a plough pan, an expensive procedure.
9.2 Adding organic matter, by growing a green manure crop, or not burning off stubble then ploughing it in
9.3 Using a pasture phase for 4-5 years as part of a crop rotation plan
9.4 Using runoff control structures, e.g. contour banks on sloping or steep land.
10. 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 and this 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.
6.8 Soils contain air
See diagram 6.8.2: Air in soil
Put soil in a container and slowly add water. Observe the air bubbles that rise through the water from the soil.
6.9 Compare fertility of subsoil and topsoil
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 Soils contain water
See diagram 6.10: 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.
1. Infiltration rate is the rate (how quickly) water can enter into a soil.
2. Permeability measures how easily water can move through a soil.
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).
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.
6.11 Water content of soils
Collect samples of soil in metal drink-cans. Weigh the drink-cans and adjust the soil content so that the weight of each drink-can and soil is the same. Heat the drink-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.
6.12 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.
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.
6. 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.
6.12.1 Use of a commercial soil pH test kit
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.
6.12.2 Preferred pH 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 6.5. 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 6.5. 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.
6.12.3 Change the pH of potting mix
The mix must be moist enough to use for potting. Raise pH with dolomite. Add 1 to 1.5 g/L of mix to raise pH by about one unit.
Lower pH with sulfur. Add 0.3 g/L to lower pH by about one unit. 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. 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.) You should pot plants again if the pH of the mix is below 6.5. 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.
6.12.4 Raise soil pH with agricultural lime / dolomite
Raise soil pH by adding agricultural lime or dolomite. A 1:1 mixture of the two often gives best results.
6.12.5 Lower soil pH with agricultural sulfur
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.
S10. 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.
S11. 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 the time taken for seeds to germinate, e.g. winter and summer crops, rate of crop growth and development e.g. tuber formation in potatoes will not occur at temperatures exceeding 30oC, number, type and activity of soil organisms and so the rate of breakdown of organic matter.
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.
6.13 Water rises in soils
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.
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.
6.14 Water-holding capacity of soils
1. After rain, soil contains much water but usually most of the water goes straight down through the soil and into the water table. 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.
6.15 Running water changes soils
See diagram 6.15: Water running through tilted trays
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.
6.16 Raindrops affect soils
See diagram 6.7: Raindrop shapes and radius
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.
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".
6.17 Splash sticks
See diagram 6.7: Splash stick
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.
6.18 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, salinisation, 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.
6.18a 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. 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.
6.18b 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. 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.
6.18.1 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.
Maintaining vegetation
Maintain stubble or trash after the harvesting of a crop
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.
Maintain natural vegetation by avoiding overgrazing of pastures
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.
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.
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.
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.
6.19 Soil permeability
Cut off the ends of metal drink-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 drink-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 drink-cans. Put the three drink-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 drink-can. Note that soils are the most permeable and that soils retain the most water.
6.20 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.
1. The soil contains salts to be taken up by plants and used as nutrients. 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.
6.20.01 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 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).
6.20.1 Salinisation
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.
6.21 Capillary action in soil and deposition by groundwater
See diagram 6.21: Solution and deposition
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.
6.22 Infiltration and capillary action by groundwater
See diagram 6.22: Capillary action through soil
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.
6.23 Oxidation of iron
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.
6.24 Freezing water expands
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.
6.25 Angles between mud cracks
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.
6.26 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.
O horizon is the decaying plant and animal material on the soil surface. Horizon depth is important for the growth of plant roots.
are soil layers with a high percentage of organic matter. 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 humic material.
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 humic organic matter intimately mixed with the mineral fraction. It may have properties resulting from cultivation, pasturing, or other disturbances.
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 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 illuvial 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 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 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.
Soil horizons may have specific feature that affect plant growth.
1. Accumulation of calcium carbonate, gypsum or salts more soluble than gypsum, silicate clay, sodium, sesquioxides.
2. Concretions or nodules of dolomite, calcite or more soluble salts that contain iron, aluminium, manganese, or titanium.
3. Frozen soil where a horizon contains permanent ice.
4. Highly decomposed organic material with low fibre content.
5. Illuvial accumulation of organic matter sesquioxide complexes.
6. Physical root restriction caused by dense basal till, plough pans, and other mechanically compacted zones.
7. Root-restrictive cementation by carbonates, silica; iron, gypsum, lime, and other salts.
8. Slickensides from swelling of clay minerals and shear failure along a fault.
9. Strong gleying showing iron reduced to ferrous form caused by saturation with stagnant water.
10. Tillage disturbance.
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.
6.27 Plants can prevent soil erosion
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.
6.28 Life in the soil
See diagram 6.28: Berlese funnel
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.
6.29 Wind deposits
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.
6.30 Soil profiles
See diagram 6.26: Soil profiles
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 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.]
6.31 Soils change with depth
See diagram 6.7: Soil auger
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.
6.32 Water from leaves
See diagram 9.194.1: Water comes out of leaves | See diagram 9.195: Plants lose water
You need 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.
6.33 Soil water
See diagram 35.6.7: Water from dry soil
6.34 Water moves down through soil
See diagram 35.6.7: 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. 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.
6.35 Water rises up soil by capillarity
See diagram 35.6.7: Water moves up
6.36 Mulch saves water
See diagram 6.65.3: Coconut mulch
1. You can save soil water in two ways:
1.1 Cut down weeds and bushes that grow near your coconuts, breadfruit, vegetables or bananas. Then the roots of these plants cannot steal the soil water.
1.2 Cover the surface of the soil around your plants with a layer of dead leaves, grass or other material. This layer is called a mulch. The mulch stops the hot sun from making the soil surface dry.
2. Weeds and bushes take soil water from the coconuts, breadfruit and other useful plants. Cut down but do not burn these weeds because you can use them for mulch.
3. Mulch keeps the soil moist around plants. Mulch is any light, loose covering on the soil. Old dead grass, coconut leaves and breadfruit leaves make good mulch.
4. Some ways to save soil water:
4.1 Cut down weeds that steal soil water from trees.
4.2 Put a mulch of dead leaves around young trees or vegetables.
4.3 Make mulch. Gather dead leaves or grass and make a mulch around some young tree such as a coconut seedling or a young breadfruit. The next week, lift up the mulch to see that the soil under it is cool and moist.
History
These lessons were originally written and illustrated by Mr J. A. Sutherland, Faculty of Education, University of New England, Armidale, Australia and later edited by Dr J. Elfick, School of Education, University of Queensland, Brisbane, Australia or made available to UNESCO by PHYWE SYSTEME GMBH, Robert-Bosch-Breite 10, D-37070, Gottingen, Germany and edited by Dr J. Elfick, School of Education, University of Queensland, Brisbane, Australia, or are based on the lessons in the New UNESCO source book for science teaching, Third impression 1979, ISBN 92-3-101058-1, and edited by Dr J. Elfick, School of Education, University of Queensland, Brisbane, Australia, working under UNESCO Contract No. 8347201, 2001-12-15. Experiments 32 to 40 were written by Dr J. Elfick, School of Education, University of Queensland, Brisbane, Australia. The experiments in this file were reviewed and edited by soil scientist Dr R. C. Bruce in July, 2005.