Soil Profile and Horizons

Soil represents the uppermost layer covering the Earth's crust, comprising a blend of rock particles combined with organic matter. 

• Pedology serves as the scientific study of soils within their natural settings, focusing on their properties, distribution, and formation processes.
• Pedogenesis encompasses the natural course of soil creation, influenced by an array of actions including weathering, leaching, calcification, and more.
• The formation of soil is intricately linked to factors such as the original rock material, surface topography, climate conditions, and the natural flora present in a particular region. 
• Soil evolves through the breakdown of rocks under the influence of environmental elements like wind, water, and climatic conditions.

SOIL TYPE- SANDY- CLAYEY-LOAMY

Soil classification is based on the proportion of various particles' sizes. Different soils exhibit unique properties based on these proportions:

• Sandy Soil: A soil type characterised by a higher proportion of larger particles, allowing water to drain quickly through spaces between particles. Sandy soils tend to be light, well-aerated, and dry.
• Clayey Soil: Comprising relatively smaller particles, clay soils have particles that pack closely together, offering little air space. These soils retain water within the tiny gaps between the particles, making them heavier and holding more water than sandy soils.
• Loamy Soil: Known as the best topsoil for plant growth, loamy soil is a mixture of sand, clay, and silt particles. It contains organic matter known as humus and possesses the ideal water-holding capacity for plant growth.

Soil types cater differently to various crops:

• Cereals like wheat and gram grow well in clayey and loamy soils due to their water retention properties.
• Paddy cultivation thrives in clay-rich soils with organic matter, showcasing good water retention capacities.
• For lentils (masoor) and pulses, loamy soils that drain water effectively are preferred.
• Cotton crops flourish in sandy loam or loamy soils that facilitate water drainage and air retention.

SOIL PROFILE

• A soil profile, a vertical view of the soil layers, showcases various layers from the surface to the underlying parent material.
• The succession of these layers, referred to as horizons, collectively forms the soil profile, distinct in characteristics based on colour and particle size.
• Fundamentally, the soil profile comprises three primary layers: the Parent Rock, topsoil, and subsoil, encompassing five master horizons. 
• However, not all soil profiles exhibit each of these horizons, leading to diversity across locations.
• The depth, colour, texture, and chemical composition of each layer vary, with every stratum, called a horizon, having a designated designation.
• The development of horizons occurs due to processes like leaching and capillary movements, influencing the upward flow of water and materials in the soil.
• Additionally, the soil profile aids in ascertaining the soil's nature and function, facilitating the distinction of one soil sample from another based on features such as colour, texture, structure, thickness, and chemical composition.

LAYERS OF SOIL

• The soil profile model, employing capital letters O, A, B, C, and E, is an approach used by soil scientists (pedologists) to identify distinct layers within the soil structure.
• Each of these layers holds unique characteristics that differentiate them from one another.
• These layers can be impacted by various environmental factors, such as heavy rain or wind, leading to the erosion of the upper layer and the removal of soil minerals.
• Within the soil profile diagram, the primary surfaces—surface horizon, subsoil, and substratum—are represented by A, B, and C.
• The surface horizon holds an organic mineral cover (O), through which the soil acquires its natural minerals.

O HORIZON

• The O-Horizon of soil comprises decomposable organic matter like dried leaves, small rocks, fallen trees, and other organic debris. 
• This layer typically consists of approximately 20 to 30 percent organic materials. The colour of this horizon tends to be dark brown or black due to the presence of this organic matter. 
• It is particularly prominent in forested regions where there's an accumulation of tree debris.

A HORIZON

• The A-Horizon, often termed the Humus Layer, predominantly comprises mineral particles (sand, silt, and clay) with minimal organic content.
• It constitutes the topmost layer of soil enriched with dark decomposed material known as humus. 
• Humus is vital in nourishing the soil, enhancing its fertility, and maintaining its biological activity.
• This layer is commonly found on the surface of many grasslands and agricultural soils.
• Its soft texture allows it to retain more water than other layers, significantly increasing the soil's water-holding capacity. 
• Furthermore, this layer supports various living organisms such as beetles and worms, fostering the soil's ecosystem.

E HORIZON

• The E horizon, representing a master horizon underlying other soil layers, is a subsurface horizon characterised by severe leaching.
• Leaching is the process by which soluble nutrients are lost from the soil due to factors like precipitation or irrigation.
• This horizon is typically found in wooded areas, identifiable by its pale coloration. 
• The E symbolises the eluviation process, where dissolved particles from the A-Horizon migrate to this layer.

B HORIZON

• The B-Horizon, referred to as the subsoil, resides beneath the topsoil and serves as a reservoir for water that has filtered through the top layer. 
• It accumulates rainwater, storing it for future use.
• While it contains humus, the amount is generally less compared to the A-Horizon, along with organic materials and soluble minerals. 
• Additionally, the B-Horizon acts as a deposition site for minerals that have leached from the layers above.
• This layer is frequently hard and compact due to its higher mineral content.

C-HORIZON

• The C-Horizon, located beneath the B-Horizon, is primarily composed of relatively less weathered or unaltered rocks, known as parent material. 
• This horizon lacks any organic material and is considered the least weathered layer in the soil profile. 
• It consists of loose, unconsolidated materials and is often referred to as saprolite.

R HORIZON

• The R-Horizon represents a layer comprising partially weathered or unweathered rocks, often considered as the foundation or base layer of the soil profile.
•  Within this layer, one might find elements such as manganese, granite, limestone, and basalt.

FAQs on Soil Profile, and Soil Layers

1. What defines sandy, clayey, and loamy soils?

Ans. Sandy soil contains larger particles, drains water quickly, and is well-aerated, while clayey soil, with smaller particles, retains water but has less air space. Loamy soil, a mixture of sand, clay, and silt, is ideal for plant growth, offering balanced water-holding capacity.

2. What types of crops thrive in different soil types?

Ans. Cereals like wheat and gram grow well in clayey and loamy soils due to their water retention properties. Paddy cultivation prefers clay-rich soils with good water retention. For lentils and pulses, loamy soils that drain water effectively are preferred. Cotton crops flourish in sandy loam or loamy soils that facilitate water drainage and air retention.

3. What is a soil profile, and what are its constituents?

Ans. A soil profile is a vertical section illustrating the layers from the surface to the underlying parent material. It comprises multiple layers, or horizons, such as O, A, B, C, E, R, each with unique characteristics based on colour, texture, and chemical composition.

4. What is the role of each layer in the soil profile?

Ans. The O-Horizon contains organic debris like leaves and tree matter. The A-Horizon, rich in humus, supports plant growth and water retention. The E-Horizon is leached of minerals. The B-Horizon acts as a water reservoir. The C-Horizon is the least weathered layer, composed of parent material. The R-Horizon includes unweathered rocks.

5. How do these soil layers influence plant growth?

Ans. The top layers (O and A-Horizons) contribute to plant nutrients, water retention, and root development. The deeper layers (B, C, and R-Horizons) impact water storage and support the soil's overall foundation.