Soil Formation

Soils vary in texture, structure, porosity, and colour:

  1. Texture - refers to the feel, appearance, and consistency of soil.

  2. Structure - refers to the clumping of the textural components.

  3. Porosity - is a measure of the porous space in a material.

  4. Colour - refers to the different colour of the soil horizons

  • Soil is made up of rocks, humus, air, and water. Humus is the part of the soil made up of dead plants and animals

  • The three main kinds of rock pieces often found in soil are sand, silt, and clay. Sand is loose and does not hold water well. Silt holds water, but is very light and can be blown away easily. Clay holds water, but is heavy and may not be best for growing.

  • A mixture of all three types of rock is called loam. Loam would be the best rock mixture found in soil for growing.

  • Soil - unconsolidated regolith modified over time by water, air, and organic material.

  • Fertile soil - a mixture of minerals, air, water, and organic matter in the correct proportions


Soil Science Divisions:

  1. Pedology – The study of soil as a geological entity, focusing on its morphology, classification, chemistry, and other characteristics.

  2. Edaphology – The study of soil as a habitat for living organisms


Soil Formation

  • Soil formation comes in 4 distinct steps:

  1. Disintegrating Rock: The first stage shows bare rock being broken down by physical (rainfall, temperature fluctuations) and chemical weathering.

  2. Early Soil Development (C Horizon): As weathering continues, the disintegrated rock forms the C Horizon, which consists of partially weathered parent material

  3. Formation of A Horizon (Topsoil): Over time, more organic matter accumulates on the surface, and biological activity (from plants, microbes, and other organisms) increases. This leads to the development of the A Horizon.

  4. Development of B Horizon (Subsoil): As the soil matures, materials such as clay, iron, and organic compounds leach from the A Horizon and accumulate in the B Horizon (subsoil). 

Factors affecting soil formation:

  • Parent Material: The type of rock being eroded.

  • Time: Affects soil quality.

  • Climate: Affects the rates of weathering.

  • Organisms: Their presence and activities contribute to soil formation.

  • Topography: Refers to the land surface and its effects on formation.

  • Weathering: different agents affecting soil formation

Soil Horizons (Layers):

  1. O Horizon (Organic Layer) – Mostly composed of organic material (humus), often absent in some soils.

  2. A Horizon (Topsoil) – Characterized by leaching of elements like potassium (K), magnesium (Mg), sodium (Na), and clay.

  • The topsoil is the most fertile layer and is vital for plant life

  • The organic material mixed with minerals gives this horizon a dark color

  1. B Horizon (Subsoil) – Accumulation of materials from above, absent in younger soils, but distinct in older soils. Contains aluminum (Al), iron (Fe), clay in moist conditions, and silicon (Si), calcium (Ca) in arid conditions.

  • It is more compact and has less organic material than the A horizon, often with a higher clay content.

  1. C Horizon – Composed of parent material.

  • This is the lowest layer, made up of weathered parent material from which the soil is derived.

Soil Texture:

  • Soil texture is classified based on the proportion of sand, silt, and clay.

Soil Textural Triangle:

  • A diagram used to classify the texture of soil based on the percentages of sand, silt, and clay.

  • This helps in understanding the soil’s water-holding capacity, drainage characteristics, and suitability for different types of crops or plants.