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In this article, we would discuss soil horizons and processes for the formation of soil.

Soil formation is a result of various processes that tend to affect the soil profile (parallel layers of soil each with distinct physical characteristics). Following are the processes that result in soil formation:

1. Eluviation: It is the mechanical translocation of clay or other fine particles from upper layers to lower layers due to downward precipitation. The particles could also translocate in dissolved form.

2. Illuviation: It is the accumulation of the washed off material i.e. eluviated material in the lower layers of the soil profile. Water percolating the soil leaches out soluble salts and mineral particles, thereby adding them to the lower portions.

3. Leaching: It is the loss of water soluble material like calcium due to percolation of water down the soil profile.

4. Cheluviation: It is a combination of weathering of minerals by organic acids and translocation of organic matter through the layers of soil profile. It is similar to leaching but under the influence of organic complex compounds.

Soil Horizons

1. Layers of soil parallel to each other whose physical characteristics are different from one another.

2. They are separate from one another by boundaries which range from indistinct to abrupt to clear.

3. Horizons are formed due to differences in depth, amounts of humus accumulated, translocation or loss of colloids.

4. Capital letters A, B, C, E, O and R are used to define master horizons.

5. Lowercase letters are used to indicate specific characteristics of the master horizon.

6. Study of the soil profile gives important information about the fertility of soil. Its composition helps to establish the process of its formation and is an important tool for agriculture.

Soil fertility and plant nutrition

1. Soil fertility depends on the amount of essential elements and organic matter present in it.

2. Organic matter like the remains of dead plants and animals (decomposed by microorganisms) in the soil act as a reservoir for nutrients.

3. Absorption and utilisation of essential elements that are necessary for plant growth and their metabolism ensures good plant nutrition.

4. Complex interactions like decomposition of organic matter, weathering of rock and microbial activity produces inorganic minerals in soil.

5. Plants need at least 16 essential elements, namely carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulphur, calcium, iron, magnesium, boron, manganese, copper, zinc, molybdenum and chlorine.

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