Factors that determine amount of air in the soil

Factors that determine amount of air

The amount of water in the soil: The more the water in the soil, the less the air in the soil.

Soil structure: Granular and crumb structure encourage better aeration than a platy structure.

Type of soil and texture: Sand soil is better aerated than clay soil.

Organic matter content of the soil: Organic matter improves the soil structure and aeration.

Soil depth: the deeper you go into the soil profile, the less the amount of air present in the soil.

Components of soil air

Oxygen: for respiration of roots and microbes.

Carbon dioxide

Nitrogen for protein formation by microbes.

Water vapor: helps to prevent desiccation of plant roots and microbes.

Importance of soil air

Necessary for respiration of plant roots and microbes.

Necessary for seed germination

Good air circulation dissolves C02 which may be toxic to plants and other microbes.

Expansion and development of plant roots.

Supports existence of soil microbes.

Some soil air is converted into plant nutrients that support the growth of plants e.g. nitrogen

Effects of poor aeration

Reduced microbial activities

Reduced root growth

Wilting and yellowing of plants due to reduced energy absorption by plants

Toxic substances are formed from organic substances

Anaerobic conditions leading to formation of harmful acids.

Increased denitrification

Differences between soil air and atmospheric air

Soil airAtmospheric air
Not uniformly distributedUniformly distributed
Varies in composition from place to placeComposition is the same
Has higher moisture contentHas low moisture content
Has high carbon dioxide contentHas low carbon dioxide content
Has low oxygen contentHas high oxygen content

Experiment to determine volume of air in the soil


Measuring cylinder


Stirring rod

Soil sample


Place a measured amount of soil in a measuring cylinder.

Add a measured amount of water to the soil in the measuring cylinder.

Stir the contents well and leave to settle.


Bubbles are seen escaping from the soil indicating presence of air.

After settling, the total volume of the two contents is less than the expected volume of the mixture.

To calculate the volume and percentage of air,

Volume of air = Expected volume – Actual volume

Percentage of air = volume of air in sample     x 100

                             Volume of soil


Calculate the volume and percentage of air in the soil using the following:

Amount of soil               100 cm3

Amount of water           100 cm3

Actual volume of the mixture 195 cm3

Soil living organisms

These are creatures that live or exist in the soil. They include the flora and fauna.

Types of soil living organisms

Micro organisms: these are tinny organisms that can only be seen by a microscope e.g. viruses, fungi, bacteria, protozoan.

Macro organisms: these are bigger in size and can be seen easily with naked eyes. E.g. termites, earthworms, millipedes, centipedes, rodents, etc.

The contribution of living organisms depends on their feeding habits i.e.

Symbiosis: This occurs when two organisms live together and each benefits from the association. E.g. legumes and the nitrogen fixing bacteria in the root nodules of the legumes.

Parasitism: occurs when an organism (parasite) feeds or lives on or in another (host) without the host benefiting from association. E.g. eel worms in roots of tomato plants.

Saprophytes: these are organisms that feed on dead and decayed organic material.