Soil Profiles and Horizons

Summary

In this lecture, the importance of soil profiles and horizons is discussed, especially in relation to the licensure exam for agriculturists. The soil profile is essentially a cross-section of soil that reveals various layers or horizons. The key horizons include the organic layer (O), topsoil (A), eluviation (E), subsoil (B), and parent material (C), with the bedrock (R) being the deepest layer. The lecture also explains how soil age can be determined by the number, thickness, and distinctiveness of these horizons, providing insights into soil development and characteristics.

Takeaways

• 😀 Soil profile refers to a vertical cross-section of the soil that shows different layers called horizons.
• 😀 The horizons in a soil profile differ in their physical, chemical, and biological properties.
• 😀 The O horizon (organic layer) is the thinnest layer, made mostly of humus or decomposed organic matter.
• 😀 The A horizon (topsoil) consists of both organic matter and minerals, making it lighter than the O horizon.
• 😀 The E horizon (eluviation layer) is a lighter-colored layer where minerals and organic matter are leached out.
• 😀 The B horizon (subsoil) is where leached materials from the E horizon accumulate and is typically darker due to mineral salts.
• 😀 The C horizon (parent material) consists of partially weathered rock or material that will eventually form the upper soil horizons.
• 😀 The R horizon (bedrock) is the unweathered base material beneath all the other soil layers.
• 😀 Older soils have more distinct, thicker horizons, while younger soils have fewer and thinner horizons.
• 😀 Regolith refers to any partially weathered material above the bedrock, including the O, A, E, B, and C horizons.
• 😀 The study of soil profiles helps in estimating the age of the soil and understanding its fertility and suitability for plant growth.

Q & A

What is a soil profile?

A soil profile is a vertical cross-section of the soil that reveals its various layers, known as soil horizons, each with unique physical, chemical, and biological properties.

What are soil horizons?

Soil horizons are distinct layers found within the soil profile. Each horizon differs in properties such as color, mineral content, organic material, and texture.

What is the O Horizon, and what does it consist of?

The O Horizon is the topmost layer of soil, primarily composed of organic matter such as humus, which is partially decomposed organic material. It is the darkest layer due to the high concentration of organic material.

What makes the A Horizon different from the O Horizon?

The A Horizon, or topsoil, consists of both organic matter and minerals, making it lighter in color compared to the O Horizon, which is composed mainly of organic material.

What is the E Horizon, and why is it important?

The E Horizon is also known as the eluviation layer, characterized by its light color. It is the zone where minerals and organic material are leached away due to water movement, making the soil appear 'washed-out'.

What does the B Horizon represent, and what is its composition?

The B Horizon, or subsoil, receives leached minerals and organic matter from the E Horizon. It is typically darker due to the accumulation of these minerals and salts.

What is the C Horizon, and how does it contribute to soil formation?

The C Horizon consists of partially weathered rock, which is the source of the soil's mineral components. Over time, weathering of this rock contributes to the development of the higher soil horizons.

What is the R Horizon, and how does it relate to soil formation?

The R Horizon is the bedrock, which is the deepest and unweathered layer. It forms the base of the soil profile, and the weathering of this rock eventually creates the soil horizons above it.

What is the difference between Solum and Regolith?

The Solum includes the O, A, E, and B Horizons, which are the active layers of soil where most soil-forming processes occur. The Regolith, on the other hand, includes all material above the bedrock (R Horizon), including the O, A, E, B, and C Horizons.

How can we estimate the age of soil based on its profile?

Soil age can be estimated by the number and thickness of its horizons. Older soils tend to have more distinct, thicker horizons, while younger soils have fewer, thinner horizons with less pronounced differences between layers.