Soil (O-level biology)

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Soil

The term soil is applied to a layer of material overlying the rocks of earth crust. Suitable nutrient contents and soil particle structure are essential for successful growth and production in ecosystem.

Components

Soil contains mainly

Component	Percentage of the soil
mineral salt (inorganic particle)	50 -60%
Organic matter	Up to 10%
Air	15 -25%
water	-35%

Inorganic particles.

They are usually classified by the average size of particular shown below

Particle diameter (µm)	Particular name
<2	clay
2    –   20	silt
20 -200	fine sand
200- 2000	coarse sand
2 – 20 mm	gravel

Soil that contains a mixture of sand, clay, silt is the loam soil. Loams soil is a mixture sand and clay soil in equal proportional with humus is the best soil for plant growth.

Physical properties of sand, clay soils are summarized below.

Properties	sand	clay
Texture	coarse	fine
Structure	light	heavily
Aeration	good	poor
Drainage	fast	slow
Water retention	poor	good
Nutrients	poor	good

Air and water

Water and air occupy the same space therefore the soil that contain more air will contain less water and vice versa. Aeration is the term used to describe the amount of air in the soil. Drainage is the ability of water to go through the soil. Oxygen is particularly required for respiration of micro organism and plant roots; decomposition of organic matter, germination of seed, root hair formation and growth and water absorption.

Soil water is important

• As a source of water and dissolved mineral salt in plant.
• Promotes seed germination
• Excess water (water logging) slows down water absorption (by lowering the respiration of plant roots since it displaced air from the soil) and decay.
• Water softens soil for easy root penetration

Importance of soil air

• Oxygen is used to breakdown soil organic residue to less poisonous products such as carbon dioxide. In absence of oxygen organic residues are broken to pollutants like methane and organic acids
• Oxygen is used by microorganisms to oxidize nitrogen and sulphur into usable forms.
• Soil oxygen is used by plant roots to respire.
• Soil oxygen is used for respiration of microorganisms leading to oxidation of nitrogen and sulphur and recycling of nutrient,
• Poor aeration results in the development of toxins and other injurious substances such as ferrous oxide, H2S gas, CO2 gas, ethylene, organic acids, etc., in the soil.
• A deficiency of oxygen has been found to check the nutrient and water absorption by the plants
• Insufficient aeration of soil leads to the development of some diseases like, wilt of gram and dieback of citrus and peach.

Experiment to determine the percentage of water in a soil sample

A sample of a known mass of soil is dried in an oven at 1050C to a constant mass. The difference in the initial and final mass is the mass of water in the soil

Results

Mass of crucible                                                                                              = a g

Mass of crucible + soil                                                                                    = b g

Mass of crucible + soil after drying to constant mass at 100⁰C in an oven    = c g

Mass of  soil                = (b – a)

Mass of dry soil          = (c – a)

Mass of water              = (b – a) – (c – a) = (b – c)

 

Experiment to determine the percentage of air in the soil

A given volume of soil is placed in a measuring cylinder followed by a given volume of water. The mixture is stirred to a constant volume.

Result

Volume of the soil added to measuring cylinder        = a cm³

Volume of water added to measuring cylinder          = b cm³

Volume of soil and water after stirring                       = c cm³

Volume of air                                                              = ((a + b) – c)

  The percentage of air in the soil                              

Organic matter

Dead organic matter is derived from soil organisms and from organism that live above the soil surface. Fungi and bacteria in the soil decompose organic materials to humus. Humus improves the water retention in the soil and therefore the soils containing humus will resist leaching.

Experiment to determine the percentage of humus in the soil

A mass of a dry soil sample is heated to redness to a constant mass. The difference in the masses is the mass of humus.

Results

Mass of the dish                                                                      = a g

Mass of dish + dry soil                                                           = b g

Mass of dish + soil after heating to redness =                        = c g

Mass of  dry soil                                             = (b – a)

Mass of dry soil without humus                     = (c – a)

Mass of humus                                                = (b – a) – (c – a) = (b – c)

 

 Soil organism.

The soil organism range in size from bacteria and protoctist to fungi, Nematode, insect, worms and a few mammals such as screw:

Role of soil organism

• Microorganisms such as bacteria and fungi.
• Promote germination by breaking seed coats
• Fix nitrogen in the soil
• Decompose and recycle or organic matter
• Macro organism such as rodent, termite and worms turn the soil and improve aeration and drainage
• Promote decay by breaking big pieces into small pieces
• When they die they decay and add humus to the soil

Dissolved minerals

The nature of dissolved minerals in the soil depends on the parent rock, organism growing in and above the soil and whether aerobic conditions prevail chalk soil, for example, are high in calcium carbonate. However, are low in phosphate, nitrogen and iron, again as is the case with oligohophic lakes, species diversity is high. When soils are waterlogged, ions capable of existing in either oxidized or reduced state are found in their reduced form. Fe ²⁺ for instance occurs instead of Fe³⁺ some plants are very sensitive to Fe²⁺, so the vegetation found on waterlogged soil differs from vegetation in well drain condition

Soil pH

is the acidity or alkalinity of the soil. It usually lies between 3 and 8.

Effect of soil pH

• Acidic soil with pH of less than 4.5 reduce the availability of nitrogen and phosphorus to plant on the other hand,
• the concentration of toxic Al³⁺   ions increase as the pH falls. These two factors enable a few plants to grow in acid bogs. Those that do are often carnivorous and this supplies them with extra nutrients.
• Affect the activity of microorganism which may increase of reduce the rate of decay and nutrient recycling

Soil Structure

This is the arrangement of soil particles, it refers to the looseness or compactness of the soil particle.

Its effects are

• Drainage for loose soil is faster than that of compact soil
• Erosion for loose soil is faster
• Aeration is higher in loose soil

Soil temperature.

High soil temperature increases the rate germination, decay, absorption of water

Soil erosion

is the washing away of top soil

Types of soil erosion

1. Sheet erosion: the surface of the soil is removed evenly over the whole surface of a slope
2. Rill erosion: top soil is removed to an extent that small channels in which the run-off is concentrated appear
3. Gully erosion: top soil is removed to an extent that deep valleys are formed by water run-off.

Agents of soil erosion

• Wind
• Water
• Animals

Man’s activities that cause soil erosion include

• Overgrazing
• Clearing land for agriculture
• Deforestation
• Road construction

Methods to reduce soils erosion are

• Afforestation
• Planting grass cover
• Mulching
• Strip cropping
• Terracing
• Contour cropping

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Soil

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Compiled by Dr. Bbosa Science