Soil and the Nitrogen Cycle

The soil is the topmost layer of the Earth’s crust.
It consists of water, air, inorganic minerals (rock, sand, clay, and silt), and organic matter (dead plants and animals).
It provides shelter for many animals such as insects, centipedes, burrowing animals, and microorganisms.
It is often called the ‘Skin of the Earth’.
Soil is a mixture of:
- Broken up bits of rock (formed by weathering, can vary in size from large rocks to smaller pieces of gravel, sand, or clay).
- The decaying remains of plants and animals (called humus; the more humus the soil contains the more fertile the soil is).
- Air and water.
- Dissolved chemicals.
- Different living things, both plants and animals.

This layer is the uppermost layer of the soil, rich in organic matter, such as the remains of plants and dead animals.
Due to high organic content, this layer is typically black or brown.
The O horizon is thin in some soil and thick in some others.

Found below the O horizon, it has a dark brown color as it contains the maximum organic matter of the soil.
The A horizon is thus also called the humus layer.
The topsoil is the region of intense biological activity and has the most nutrients.
Insects and earthworms, centipedes and bacteria and fungi are found inside this layer.
The humus makes the topsoil highly porous, allowing it to hold air and moisture necessary for seed germination.
Here the plants stretch their roots deep down, allowing the top soil to be held together.
In this layer, minerals and clay particles may dissolve in the fresh water and get carried to the lower layers as water moves down the soil.

This layer consists of nutrients leached from O and A horizons.
Leaching of clay, minerals, and organic matter leaves this layer with a high concentration of sand, silt particles, and quartz and other resistant materials.
E horizon is not found in all soils but is most common in forested areas.

This is also known as the regolith.
It lies below the subsoil.
It is called the parent rock because all of the upper layers developed from this layer.
It has no organic matter and is made from broken up bedrocks.

The formation of soil is a continuous process occurring still today from the time of the earth’s beginning.
The process starts when big rocks are broken down into smaller ones by wind and rain. This process is known as weathering.
The two types of weathering processes are physical and chemical weathering.
These rocks get further broken down into finer particles such as sand, silt, and gravel, and the process continues.
This process continues for thousands of years to form just a 1 cm layer of soil.
These fine particles ultimately form the topmost layer of the soil.

Some plants will grow better in acidic soils, while others will prefer neutral or alkaline soils.
Cabbages grow best in alkaline soils.
Potatoes grow best in acidic soils.
Hydrangea color varies with acidity of the soil.

Garden rubbish is often used to make compost, to improve the soil.
Household rubbish is also useful.
Eggshells, tea bags, paper, potato peelings, sawdust, ashes and even hair cuttings – all make good compost!
Compost can add nitrates to the soil which is important for plants.

Nitrogen is an element which is essential to living organisms because it is needed to make proteins which are essential for growth and repair.
Most nitrogen is found in the air as a gas.
Plants obtain nitrogen in the form of nitrates from the soil and use them to produce proteins.
Animals obtain nitrogen as part of the proteins they consume.
The movement of nitrogen between the air, soil compounds and compounds in the bodies of living organisms is called the nitrogen cycle.
Different groups of bacteria are very important for keeping the cycle going so that nitrogen is always available in a form that living organisms can use.