Soil makes up the top layer of the Earth’s crust. Soil provides nutrients, water and minerals to plants and trees and is home to billions of insects, small animals, bacteria and many micro-organisms. Soil is where life begins. Composed of minerals, water, air and organic matter, soil provides nutrients for plants and animal life and acts as a basis for food. Soil can filter and store water before it reaches the bedrock. Soil is vital for agriculture since plants take nutrients from it so that they may grow and produce food both for people and for farm animals. It is a non-renewable resource – its loss is not recoverable within a human lifespan. It can take hundreds to thousands of years to form one centimetre of soil from parent rock, but that centimetre of soil can be lost in a single year through erosion. We will explain how this happens in the next lesson.

One may say that new soil is being formed each day. I is formed by the breaking up and weathering of rocks. Rocks are cracked and broken up into smaller fragments by the forces rain, dew, sunshine and changes in temperature. Even plants help in weathering rocks into soil. This can be noticed in the garigue areas. Here plants grow in cracks and potholes for shelter. They use their roots which enter the narrowest crack in order to find water. After a period of time the plants’ roots grow wider, expanding the cracks in the rocks which break up further. During this process, the plants deposit remains of leaves and roots which mix with the dust of the broken rocks by the action of microorganisms, worms and fungi. This process takes thousands of years to complete and this is the reason why in most countries soil is protected by law.

There are a number of factors which affect the formation of soil. These include:

Climate: This is the single most factor in the formation of soil. Climate influences the rate of weathering which in turn controls the depth of the soil. It also effects the rate of the breakdown of organic content into humus (decayed plant and animal material). The amount of precipitation in an area also affects soil formation and fertility as water can wash downwards important minerals.

Parent Material: This will determine the type and amount of minerals present in the soil. These minerals such as calcium and potassium provide nutrients and food to plants. The parent material also controls the texture, colour and soil depth.

Flora and Fauna: Soil organisms such as bacteria, fungi and earthworms are important in keeping the soil fertile. Creatures found in the soil perform important functions. It is calculated that 600 million bacteria are present in every gram of soil. Good soils also host many earthworms which carry the leaves and other organic matter to the subsoil, eat them and later deposit them as waste in the topsoil. Organic matter includes the remains of once living plants and creatures which died and rotted, like leaves, branches, roots, animal waste and remains of creatures themselves. Soil cannot remain fertile unless this process of humus formation from organic matter keeps going on. Humus fills the soil with nutrients and hence plants and trees may live and thrive there.

Time: As we said earlier it takes hundreds and sometimes thousands of years for 1 cm of soil to form. It takes time for soil to develop fully in response to the processes that occur due to climate, relief, parent material and living things.

As the diagram below shows soil contains four main ingredients. It is important to point out that the proportions of each vary greatly from one soil type to another. These are:

Mineral particles: such as sand, silt and clay obtained from the breakdown or weathering of rocks.

Organic matter: living organisms such as earthworms and bacteria and humus which includes decayed plant and animal remains.

Water: found in the spaces of the soil particles.

Air: in the pore spaces.

After thousands of years, soil forms a profile with different layers as can be seen in the picture below. A soil profile is a section which looks at the soil from the earth’s surface to the bedrock below. Most profiles have three main layers called Horizon A, B and C.

Horizon A is the topsoil or active soil and is made up of fine dark material. It is a fertile layer where one finds most living organisms and organic material. Thus this Horizon is rich in humus. The humus gets mixed with the mineral by the action of earthworms and burrowing animals. Humus is the decayed plant and animal material.

Horizon B is also known as the subsoil. This layer is generally coarser and much less fertile than the layer on top since it has little organic material.

Horizon C is the parent material or the bedrock.

In areas where there is excessive rainfall nutrients may be lost from soils. This process is known as leaching. In this way nutrients are washed into the lower layers of the soil, out of reach of plants. A certain amount of leaching is need to wash humus into the soil. However excessive leaching can rob the upper parts of the soil of the nutrients that plants need to grow. This soil becomes infertile. On the other hand in areas where evaporation is much higher than precipitation a process known as salinisation occurs. This happens when mineral salts move up through capillary action instead of down into the soil. Evaporation causes salts in ground water to rise through the soil and collect in the upper layers forming a white crust.

Now you can work out the worksheet entitled Soils 1 Worksheet available in the resources section of this lesson.