Soil 101: Essential Information on Soil Types, Composition and Functions

Objectives

This blog post provides readers with the following objectives. The reader will be able to:

o   Identify the components and different soil types.
o Determine the presence of living things in soil.
o   Determine the percentage organic matter, soil water and soil air content in soil samples
o   Explain how soil loses its fertility
o   Explain conservation, maintenance, renewal of soil fertility and soil reclamation.

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SOIL

Soil is mixture of organic and inorganic materials, which forms an ecosystem of living organisms and provides minerals for plant growth. Soil provides a substrate for plants (roots anchor in soil), a source of food for plants, and a home for many animals (insects, spiders, centipedes, worms, bacteria, and many others).

Types of Soil

The main types of soil are Sandy, Clayey and Loamy

Sandy Soil

o The particles are of different sizes and irregularly shaped

o It has large particles, feels rough and gritty

o It is very porous and well aerated.

o It has very poor water holding or retaining capacity.

o It is often called light soil because it is loose, easy to hoe dig or plough.

o It cannot be molded even when wet.

Its usefulness for plant production can be improved by adding humus. Humus binds the soil particles together and reduces the water drainage ability.

Clay Soil

o It has very small particle size

o   The particles are tightly packed
o   It has small air space
o   It feels smooth and soft when dry, it is sticky and heavy when wet
o   Its holds or retain large amounts of water
o   It easily become water-logged. Water-logged soil cause microbes to respire anaerobically which increase acidity of the soil.

Fertility of clay soil can be improved by adding lime (calcium hydroxide) and humus. Humus makes the soil lighter, improve drainage and air flow. Lime improves the soil texture and neutralizes acidity of the soil.

Loam Soil

□ It has the mixture of particles of different sizes of sand, silt and clay

□       It has a suitable proportion of humus
□       It is not rough or sticky
□       It is fertile, well-drained, intermediate water retention, and good aeration

□ It provides adequate conditions for plant growth

Physical Properties of the Types of Soils

Property	Sand	Clay	Loam
Texture	Large particles, coarse, when wet feels gritty	Small particles, fine, feels sticky when wet	Medium, particles made up of combined sand and clay
Capillarity	Initially faster but rises very low	Initially very slow but rises very high	Medium capillarity
Water retaining ability	Low water holding capacity	High water holding capacity, may become water logged	Hold water very well but does not become water logged
Porosity or Permeability	Very porous	Less porous	Medium porosity
Air content	High (large air spaces)	Low (small air spaces)	Air spaces vary in sizes

Experiments on Soil

1. Sedimentation Experiment

Aim: To separate soil particles and humus by sedimentation

Materials: Measuring cylinder or glass jar, sodium carbonate, water, soil sample

Procedure

1. Place a 1/2 cup of soil sample into glass jar.

2. Fill the jar with water until it is almost full.
3. Cap the jar, and shake for 5 minutes.
4. Add dispersing agent e.g., sodium carbonate to aid in the dispersion of the particles.

5. Leave the jar on the desk. Allow it to settle for about an hour.

Observation: the various constituents of the soil separate out. When view from the bottom, the layers occur in this sequence: gravel, coarse sand, fine sand, silt, clay particles, particles of clay suspended in water and humus.

Conclusion: soil consists of different sizes of particles such as gravel, sand, clay, and silt.

separating soil particles and humus by sedimentation

2. Comparing the Water holding Ability and Permeability (porosity) of Sand, Clay and loam

Aim: To compare the permeability and water-retaining abilities of sand, clay and loam.

Materials: Measuring cylinder, water, cotton wool, dry sand, clay and loamy soil, stop clock.

Method

1. Equal weight of dry sandy, dry clayey and dry loamy soils is placed in separate funnels plugged with cotton wool.

2. Stand each funnel in the open end of a measuring cylinder.
3.· Pour 50cm3 of water into each soil sample and start the stop clock.

4. Note and record the time taken for water in each set-up to drain.

Observation: Water drains slowly in clay. Clay retains large amounts of water. Water drains faster through sand. Sand retains small amount of water. Sandy soil is more porous, i.e. water passes through it with ease. Loam holds water well but does not become water logged.

Water holding capacity or retaining of Soil

Conclusion: The results show that clay soil has higher retaining ability, followed by loamy soil, whiles the sandy soil has the least water-holding ability. In terms of permeability or porosity, sandy soil is very porous (high porosity), followed by loamy soil, with the clay soil having the least porosity.

3. Experiment to Determine Capillarity (capillary action) in Sandy, Clayey and Loamy soil

Aim: To demonstrate capillary action in sandy, clayey and loamy soils.

Materials: Three long glass tubes (open at both ends), cotton wool, dry sand, dry clay, dry loam, trough, stop clock, water.

Method

1. Plug one end of the three glass tubes with cotton wool

2.      Fill each of the 3 glass tubes tightly with dry sandy, clayey and loamy soil separately; clearly label each tube.
3.   Fill the trough with water.
4.      Immerse the tubes vertically in the beaker with the plugged end towards its base.
5.      Allow the experimental set up to remain for several hours (18 – 24 hours)

6. Make note of the levels of the water as it rises in the glass tubes containing each type of soil.

Observation: Initially the water rises fastest in the sand, followed by the loamy soil, and clayey soil. However, after a day, the water fails to rise any higher in the tube containing sand, whereas in those containing clay and loam continue to rise until it reaches the top of the tube.

Conclusion: Clayey soil has the highest capillarity, followed by loamy and sandy soil.

Components of Soil

The components of soil are

1.   Organic Components
a.  Organic matter (humus)
b.  Living organisms
2.   Inorganic Components
a. Soil minerals
b. Soil water
c. Soil air

Soil Organic Matter

Organic matter consists of dead plant parts and animal and microbial waste products in various stages of decomposition. Eventually, these things break down into humus, which is relatively stable in the soil. It is dark brown in color. Organic matter is an extremely important part of soil because it influences the plant, animal, and microbial life in the soil. It acts like glue that helps hold soil aggregates together. Organic matter enhances water holding capacity of the soil as well as the rate of water loss through percolation. In addition, organic matter is an important reserve for nutrients, especially, nitrogen, phosphorus, and Sulphur.

Experiment to Determine of Percentage of Organic Matter (Humus) in a Soil

Aim: To determine of percentage of humus in a soil

Procedure

1. Pre-heat a crucible, allow it to cool in a desiccator to room temperature and find its mass. Record the mass as M₁

2. Fill the crucible half full of oven dried soil, and find the mass. Record as the mass of the crucible and soil sample before heating (M₂).

3. Place the crucible and contents on the triangle and heat with a moderate flame, until a red glow is seen within the crucible. Maintain the red glow for 30 minutes. After the 30 minutes, remove the crucible from the flame and allow to cool in a desiccator to room temperature.

4. When cool, find the mass and record as the mass of the crucible and soil sample after heating (M₃).

Mass of soil sample before heating = (M₂-M₁) g

Mass of soil sample after heating = (M₃-M₁) g

Mass of organic matter = (M₂-M₁) - (M₃-M₁) g

% Organic matter = mass of organic matter x 100

mass of soil sample before heating

% Organic matter = (M₂-M₁) - (M₃-M₁) g x 100

(M₂-M₁) g

Soil Organism

Among the numerous living organisms in the soil are, microscopic bacteria, protoctists, algae and fungi. Some of these bacteria cause decay and help in humus formation. Others enrich the soil with nitrates by nitrogen fixation. Examples of some of the organisms in the soil include termites, roundworms, ants, millipedes, beetles, earthworms, etc.

Experiment to Show the Presence of Living Organism in the Soil

Aim: To show the presence of living organisms in the soil.

Materials: Lime water, Garden soil, conical flask, Rubber cork, piece of cloth, string.

Method

1.   Place some garden soil in a piece of cloth and tie the tip up with a string
2.   Place it inside a conical flask containing lime water
3.   The other end of the flask is corked to make sure that no gases or air enters the flask.

4. Allow the set up to remain for 4-6 hours.

Observation: The lime water turns milky which shows that carbon dioxide is released from the soil sample into the flask through respiration by living organisms in the soil sample.

Conclusion: The soil also contains living organisms

Soil Minerals

Common minerals elements in the soil include silicon, iron, aluminum, calcium, magnesium and sodium. They usually exist in solution as a film around soil particles but they can also be part of soil particles. Plants depend on these mineral components for their survival and productivity. The regular availability of minerals in soil forms the link between soil fertility and food production. The amount of organic and inorganic components in the soil also contributes to the pH (acidity and alkalinity) of the soil.

Experiment to Test for the Presence of Minerals Salts in Soil

Aim: to test for the presence of minerals salts in soil

Materials: Conical flask, filter paper, test tube, barium chlorides, sodium hydroxide solution, concentrated sulphuric acid, dilute hydrochloric acid, freshly prepared ferrous sulphate.

Procedure

1. A conical flask is filled with fresh soil

2. Distilled water is added until the flask is about half full

3. The solution is then shaken vigorously

4. The solution above the soil is filtered several times until the filtrate is clear

5. Test are performed to identify the ions contained in the filtrate as follows

Chemical test for minerals in soil

Test
Observation
Inference
Filtrate + dilute sodium hydroxide in drops
White precipitate of calcium hydroxide
Ca²⁺
Filtrate + dilute sodium hydroxide in drops
Green gelatinous precipitate
Fe²⁺
Filtrate + dilute sodium hydroxide in drops
Brick red precipitate
Fe³⁺
Filtrate + barium chloride + dilute HCl
White precipitate of barium sulphate
SO₄^2-
Filtrate + freshly prepared ferrous sulphate and concentrated sulphuric acid slowly down the side of the test tube
Brown ring showing the boundary between the two solutions
NO₃^-
Filtrate + dilute sodium hydroxide
White precipitate which later dissolves as more sodium hydroxide is added
Zn²⁺

Soil Air

Circulation of air in the soil is called aeration. Air is found between soil particles. Some soils have large air spaces. Such soils are described as porous soils. Clayey soils have fewer air spaces and therefore have less air. Sandy soils have large air spaces and so have more air.

Soil air is mostly made up of oxygen and nitrogen. The oxygen is required by plant roots, microorganisms and macro-organisms in the soil for respiration. Soil aeration influences the availability of many nutrients. Particularly, soil air is needed by many of the microorganisms that release plant nutrients to the soil. Oxygen is use in decomposition of organic matter in the formation of humus. Nitrogen fixing bacteria in the soil also converts the nitrogen into nitrates which are used by plants.

Experiment to Determine the Percentage of Air in Soil Sample

Aim: To find the percentage of air in the soil.

Apparatus: Garden soil, milk tin, water, measuring cylinder.

Method

1. Cut one end of a milk cup open

2. To measure its volume, the tin is filled with water, which is then poured into a measuring cylinder.

3. Press the open end of the tin carefully into the soil making sure that no empty spaces exist in the tin.

4. The soil around the sides of the tin is dug away and the tin is carefully removed with the palm tightly covering the open end to prevent soil from pouring.

5. A measuring cylinder is taken and half filled with about 200cm³ water.

6. The soil in the tin is then poured into cylinder containing water and stirred until no bubbles of air escape again.

7. The new volume of the mixture of the sand and water solution is measured

Result and calculation

The volume of the tin = volume of air + volume soil

Volume of soil = final volume of water – 200cm³

Volume of air = volume of thin – volume of soil

% of air in soil = volume of air x 100

volume of tin

Soil Water

The thin film of water around soil particles and root hairs is called capillary water. Capillary water is available to plants roots and the inhabitants of the soil. This water contains the dissolved soil minerals. These are carried into the xylem and up the plant stem to the leaf mesophyll. Water is a carrier for the mineral nutrients.

Rain-water soak downwards and fill all the spaces between the soil particles. In well-drained soil, gravity causes water to sink through the soil. The amount of water retained after gravitational water has drained is the field capacity of the soil. The ability of soil to hold or retain water draining through it depends on its structure and organic matter content. Different types of soils have different water-retaining capacity. Sandy soil keeps very little water as they have large spaces between large sand particles. The water retaining capacities of loamy and clay soils are high because they have smaller spaces between smaller particles. The water holding capacity of sandy soil can be improved by adding organic matter.

Experiment to Determine the Percentage of Water in a Sample of Soil

Aim: To determine the percentage of water in soil

Materials: Evaporating dish, chemical balance, soil sample, oven, desiccator, stirring rod.

Procedure

1.   Weigh the evaporating dish and record its weight.
2.   Place soil sample into the evaporating dish, reweigh it and record the weight.
3.   Heat the soil in an oven set at 105^oC for about 30 minutes.
4.   Cool the soil in desiccator.

5. Reweigh the dish filled with the dried soil, and record weight

Calculations:

Percent soil water by weight (%w) = wet weight - oven dry weight x 100

weight of fresh or wet soil

Soil Fertility

Soil fertility is the capacity of soil to support plant growth and the many beneficial processes that occur in soil.

Characteristics of fertile soil

1.   Should be well aerated
2.         Good water retention capacity
3.         Adequate humus or organic matter
4.         Adequate minerals nutrients
5.         Correct proportion of soil particles

6. Good drainage

Ways which Soil may lose its Fertility

1. Over Cropping: Over cropping resulting from continuous cropping over a long period, results in the depletion of nutrients in the soil.

2.    Leaching: The washing down of important nutrients to lower levels (leaching) below the reach of crops which are shallow rooted by rain.
3.   Burning of Bush: Burning of vegetation before planting destroys nutrients as well as microorganism.
4.   Surface Compaction: As people and livestock continually walk over the soil, it become compact affecting nutrient uptakes by roots.
5.    Soil Erosion: Soil erosion involving the removal of the top soil together with its stored nutrient by water or wind.

6. Overgrazing: The soil surface is exposed to erosion when there is constant grazing by animals. Overgrazing may also pave the way for surface compacting.

Ways of Maintaining/Renewing Soil Fertility or Conserving Soil

1. Shifting Cultivation or Bush Fallowing: lands are cleared by cutting natural vegetation’s. Crops are planted for some years (2-4 years). Due to decreasing soil fertility, the farmer leaves the plots for some years, during which time the fertility of the land is restored.

2. Manuring/Fertilizers Application: this is addition of organic material or green manure or farm manure or inorganic material to enrich the soil with valuable plant materials to renew the humus content which improves soil fertility or texture, aeration and water holding capacity of the soil.

3. Crop Rotation: divide a piece of land into several plots. Plant different crops with different crops nutrients requirements in successive seasons in a definite order i.e. deep rooted crops are planted alternatively with shallow rooted crops to ensure removal of nutrients from different levels of soil. Legumes are cultivated to add nitrates to the soil.

4. Cover Cropping: is the growing of certain crops to cover the soil so that their roots help in holding the soil particles together and thereby reducing erosion by water. Rainfall is prevented from hitting the soil surface directly because of the leaves and thereby not losing the soil.

5. Strip Cropping: is the planting of crops along the contours shape to check erosion.

6. Irrigation: add water regularly by artificial means to improve plant growth. In area with inadequate water supply, water must be supplied in proper quantities to avoid water logging.

7. Terracing: erosion is controlled by the construction of terraces which are barrier built along the contours of the land and prevent rapid flow of water down the slope.

8. Mulching: materials left after weeding or harvest are left to lie on the soil protecting the soil against agents of erosion

9. Afforestation: growing trees and shrubs to cover and protect the soil.

10 Contour ploughing: ridges are made along the contour of a slopping land to prevent water running down the slope and washing away soil.

ways of Maintaining, Renewing Soil Fertility or Conserving Soil

Reference on soil notes

USDA Natural Resources Conservation Service - Soil Health
- USDA NRCS: Soil Health
National Geographic - Soil
- National Geographic: Soil
BBC Bitesize - Soil and Soil Formation
- BBC Bitesize: Soil and Soil Formation
Encyclopedia Britannica - Soil
- Encyclopedia Britannica: Soil
Soil Science Society of America - Soil Basics
- Soil Science Society of America: Soil Basics
University of California Agriculture and Natural Resources - Soil Properties
- UC Agriculture and Natural Resources: Soil Properties
The Nature Education Knowledge Project - Soil Formation
- Nature Education Knowledge Project: Soil Formation
Soil Health Institute - Soil Health and Management
- Soil Health Institute: Soil Health and Management

Soil 101: Essential Information on Soil Types, Composition and Functions