What is soil

Back to: Senior six Agriculture

What is soil

Soil is the top most layer of earth’s surface from which plants grow and obtain their nutrients. It consists of;

• Living organisms.
• Inorganic or mineral matter.
• Organic matter or humus, ü Air 
• Water.

Soil formation.

Soil is formed from parent rocks through a process called Weathering. Weathering is the process of gradually breaking down hard rocks into smaller particles to form soil. The rate of soil formation depends on the;

1. nature of the parent rock.
2. type of weathering process.

Although there are many types of parent rocks, three major forms occur, namely; igneous rocks, sedimentary rocks and metamorphic rocks.

Igneous rocks are the hard volcanic rocks formed when molten lava from a volcano or crack in the earth’s crust pours out over the land. The molten rock is called magama and as it cools it becomes very hard and solid.

Igneous rocks are hard to break down. They give rise to granite and basalts which are rich in potassium minerals.

Sedimentary rocks are formed as a result of deposition of particles like sand, mud by running water in water bodies. Over a long period of time, the sediment becomes compressed by earth movements and the particles become stuck together or cemented. Sediment rocks are ease to break down and give rise to sandstones and limestone which are rich in calcium.

Sedimentary rock is often found in the sea beds and beneath lakes and low-lying river valleys.

Metamorphic rocks are formed from either volcanic or sedimentary rocks which have undergone changes in formation beneath earth’s surface by means of intense heat and pressure. The rock material is squeezed and stretched so that it becomes flattened. 

Types of weathering.

Include;

• Chemical weathering. ü Physical weathering.
• Biological weathering

Physical weathering involves action of heat, frost, water, etc, on the parent rock which eventually break down. 

Chemical weathering is due to action of chemicals such acids which weaken the rocks and cause them to eventually breakdown. Oxygen in the air may also cause oxidation of the iron in the rocks. 

Biological weathering is due to the action of organisms on the rocks. E.g. plant roots, etc. As the plant roots penetrate and expand in the soil, they cause the rocks to split. Animals dig and burrow in the ground and so cause breakdown of rocks.

Importance/functions of soil components.

Air.

• Oxygen is used for tissue respiration of the roots and soil organisms, germination.
• Carbon dioxide increases soil acidity hence determining the distribution of soil organisms.
• Nitrogen is used in making proteins, amino acids, etc which are important for plant growth.      Water.
• It is a medium in which soil nutrients dissolve.
• Medium of transport in plants.
• Necessary for germination of seeds.
• Used in translocation of plant nutrients.
• Washes away accumulated salts and other toxic substances from the soil.
• Cools the plant when it evaporates from plant surfaces during transpiration.
• Is raw material for photosynthesis.
• Provides support in plant cells through turgidity.    

 Inorganic matter.

• Forms the frame work of the soil.
• Provides firm anchorage to plant roots hence support.
• Provides mineral salts to the plants.
• Have pores in between them which are occupied by air and water.

Organic matter.

• Is source of food for soil organisms.
• Is source of mineral salts when they decompose.
• is spongy hence allows water to percolate or drain through the soil.
• Forms a sticky coat around particles hence binding them together i.e. improves soil structure.
• Improves soil water holding capacity.
• Insulates the soil against extreme heat and cold i.e. moderate soil temperature.

Soil organisms.

• Improve soil aeration and drainage by construction of tunnels that are filled with air and allow water to pass through, e.g. tunnels of termites.
• Mix top soil with subsoil, e.g. termites when building their nests, earthworms, etc.
• Decompose organic matter.
• Weather parent rock particles to form soil.
• Some cause diseases to plants and animals, e.g. bacteria and fungi, etc.