Soil is a natural body consisting of layers (soil horizons) of mineral constituents of variable thicknesses, which differ from the parent materials in their physical, and chemical, characteristics.

• What is the soil composed of?

• Soil is made up of two main components:

• Tiny bits of mineral particles, which includes salts of iron, calcium, potassium, magnesium and phosphorus, etc.

• The other components is humus, which is dark brown in color & consists of decaying remains of plants & animals.

• Bacteria convert nitrogen of the air into nitrogen compounds , which can be used by plants .This is known as Fixing of nitrogen

Fixing of nitrogen

• Was soil present on earth at the time of its formation?

• No, long ago, the ground was mostly solid rock. Over millions of years , this rock got partially changed into soil by the process known as weathering.

• The surface rocks break down into smaller pieces through a process of weathering and is then mixed with moss and organic matter. Over time this creates a thin layer of soil. Plants help the development of the soil. How? The plants attract animals, and when the animals die, their bodies decay. Decaying matter makes the soil thick and rich. This continues until the soil is fully formed. The soil then supports many different plants

Weathering is the process of the breaking down rocks. There are two different types of weathering.

Physical weathering and chemical weathering.

In physical weathering it breaks down the rocks, but what it's made of stays the same.

In chemical weathering it still breaks down the rocks, but it may change what it's made of. For instance, a hard material may change to a soft material after chemical weathering.

Weathering in rocks.

• Have you ever seen a deep hole in the ground, such as that dug up for laying the foundation of building?

• You may have noticed that soil changes colour as one goes deeper. The size of the bits of rocks changes to. This are the layers on soil.

• The main layers of soil are Topsoil, Subsoil & the parent rock.

• Topsoil ( A-horizon)• Humus and the smallest

particles of rock form the upper layer of soil called topsoil or A-horizon

• The presence of humus gives it a dark colour.

• It is the layer containing the most nutrients for plants.

• Humus makes this layer porous and fertile.

Subsoil (B-horizon)• This is the layer below the

topsoil. • Subsoil does not ave much

humus and is,• therefore, lighter in colour

then topsoil.• It is made up mostly of rock

bits and some nutrients, such as soluble minerals and iron oxides.

• Rainwater collects in this layer.

• Parent rock (C-horizon)• Below the subsoil is the

parent rock or C-horizon, which consists of small pieces of rocks with cracks & crevices.

• The particles of rocks found in soil come from weathered rock in the C-horizon.

• Below this is solid rock called bedrock.

• It is non porous.• Rainwater gathers over it

to form the water table.

• Soil is not similar everywhere. Soil is classified into various types based on the appearance and proportion of particles.

• Sandy soil• Clay soil• Loamy soil

Properties of sandy soil:

Properties of clay soil:

Properties of loamy soil:

If the soil is made of a greater proportion of big particles, it is called sandy soil.The sand particles are relatively large. They cannot fit closely together.There are large spaces between them. These places are filled with air.Therefore, in this type of soil, water absorption is very high as the water passes quickly through these spaces.These soils are light, well aerated and dry

It is made of a relatively high proportion of fine particles.These soils have very less space between particles. Because the particles are smaller in size, water can be trapped in the tiny gaps between them.Clay soils are heavy and hold more water

It is a mixture of sand, soil and silt.Silt particles are present between the particles of sand and soil.It also contains humus, and is, therefore, considered the best for the growth of plants

• If you spill water on the cement floor of your house, it stays there for a long time. however, if you spill water on the ground outside the house, it quickly gets absorbed in the soil. This is because water can percolate into the soil, but not into cement.

• Rainwater that percolates through the soil collects above bedrock.

• This is natural level of groundwater is called the water table.

• Now let us see which kind of soil absorbs more water.

• Sandy soil absorbs less water and allows more water to percolate.

• Clayey soil absorbs more than sandy soil , and allows less water to percolate.

• We have already seen that sandy soil cannot absorbs much water because of the large spaces between the soil particles . On the other hand , clayey soil absorbs and retains lot of water.

• Climatic factors as well as the components of the soil determine the types of vegetation and crops that grow in a particular area.

• Cereals like wheat and gram are grown in clay and loamy soils, because these soils have better water retention capacity.

• Soils rich in clay and organic matter with good water retention capacity are ideal for paddy.

• Loamy soils, which drain water easily, are suitable for lentils and other pulses.

• For cotton, loamy or sandy soils are more suitable, because of their water draining and air logging capacity.

• Clay soils are rich in humus and fertile. These soils are very good for wheat.

• A natural is anything that comes from the earth and is used by us. Soil is our most important natural resource. It is useful to us in many ways:

• We depend on soil for food, clothing and shelter.

• We depend on the soil for minerals, such as coal, petroleum and natural gas.

• We depend on the soil for water.

• Soil is made of mineral particles , humus, air , water and living organisms.

• Soil is formed by weathering of rocks by the action of water, wind, glaciers, temperature, variations, roots of plants, etc.

• Soil has three main layers-• Topsoil (A-horizon)• Subsoil (B-horizon)• Parent rock (C-horizon)• Soil is very important natural resource.

Made by: Rabiya

Class: 7th .

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learning

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