There are 5 types of soil profile in existence, namely; the O Horizon, the A Horizon, the B Horizon, also known as sub-soil, the C Horizon, also referred to as the soil-base, and the E Horizon.
Overview of soil layers and content
Below is an in-depth analysis of each of them.
- The O Horizon
The O horizon is the surface horizon or topmost layer of the soil. It is comprised of 20% organic material at various stages of decomposition. It is also referred to as the litter zone.
This layer is commonly found in forest areas where large debris of fallen leaves and twigs exist. It is also found in areas containing other vegetative covers such as lichens and moss and is often black or dark-brown in color due to its rich organic content.
Roots of small grasses are found in this layer. However, this layer does not exist in deserts and grasslands.
- The A Horizon
The A horizon is the humus layer of the soil and is also known as the root zone. It is rich in organic matter and also forms part of the top soil together with the O horizon.
In this layer, organic matter is mixed with other decomposing materials or mineral matter. The layer therefore contains the most soil life and the greatest accumulation of both organic and mineral matter such as clay, silt and sand.
It is porous enough, allowing it to hold enough air and water for seed germination and the growth of plants. It is rich in microorganisms such as fungi bacteria and earthworms, and is also rich in soil minerals such as iron, calcium, magnesium and aluminum.
This layer is also a predominant surface layer of most agricultural lands as well as grasslands and is also the most vulnerable to wind and water erosion that is caused by heavy rainfall.
- The E Horizon
When the A horizon gets depleted of minerals, microorganisms and other organic matter, it leads to the next layer of subsurface soil known as the E Horizon. This layer is lighter in color compared to the A horizon.
It is also significantly leached of iron, clay and aluminum oxides, leaving it with a concentration of resistant minerals such as quartz in the size of silt and sand.
The clay concentration in this layer is much lower than that found in the A horizon. This horizon is common in forested areas. Usually, it is composed of nutrients leached from the O and A horizons.
- The B Horizon
The B horizon comes after the E horizon. It is harder and more compact than the top soil found in O and A horizon.
It contains all the leached minerals from the A and E horizons, leading to accumulation of clay, iron, aluminum and organic compounds, a process known as illuviation.
Illuviation is the opposite of eluviation, the depletion of minerals found in the A and E horizons. This horizon holds more water than the top soil. Roots of gigantic trees end up in this layer.
- The C Horizon
The C horizon, also known as saprolite, comes immediately after the B horizon. It is made up of broken bedrock or large unbroken rocks and does not contain any organic matter.
All weathered or partially altered parent material accumulates in this layer and is usually in form of sedimentary deposits. It is therefore course-grained.
It is the layer that contains geological material. It also contains the more soluble compounds or inorganic material. The layer therefore lacks all the components of the O, A, E and B horizons.
- The bedrock
The bedrock is the layer that contains continuous masses of hard rocks, granite, limestone and basalt. It is below 50 feet of all the soil profiles.
In actual fact it is also referred to as the R horizon by scientists in some parts of the world although it is not soil per se.
It is a compacted and cemented layer of partially weathered bedrock at the bottom of the soil profile. The compacting is caused by the sheer weight of the overlying horizons. It is un-weathered parent material.
- Regolith and Saprolite
Regolith is the total sum of all weathered material found within a soil profile. It has two components, namely; solum and saprolite.
Solum refers to the upper horizons of the soil containing the most weathered part of a soil profile, while saprolite refers to the lower horizons of the soil containing the least weathered part of a soil profile.
What is a soil profile and why is it important?
A soil profile is the vertical section of the soil from the top surface going all the way down to the bedrock. It is a cross-section of the soil and is made up of layers that run parallel to the surface.
It is these layers that depict what is known or referred to as soil horizons, starting from the O horizon going all the way down to the C horizon just before the bedrock or R horizon.
Importance of soil profile
Soil profile is important because it helps in the management of soil nutrients. This is because soil is a mixture of organic matter, minerals, air and water.
Each type of soil forms differently. When a soil profile is studied, it provides valuable and vital information about the fertility of the soil, also known as soil chemistry, which includes an analysis of soil pH.
A farmer is therefore able to know the needs of the soil and take necessary measures based on what the farmer desires to grow.
It also helps in determining the state of the soil and differentiate between the different soil samples based on color, structure, texture, chemical composition and thickness.
Changes in soil profile
Soil profile however changes over time as organic matter decomposes. For instance, poor soil usually has a light-colored layer in the subsurface of the soil due to leaching, while a highly fertile soil often has a deep surface layer containing large quantities of organic matter.
A soil profile therefore gives information on how the soil is likely to perform under certain nutrient management conditions.
Role of subsoil in the soil profile
It also forms a firm foundation for the roots of trees and helps vegetation to withstand drought.
4 components of soil profile
Soil profile consists of 4 major components. These components are; 1) inorganic mineral matter, 2) organic matter, 3) water and air, and 4) living matter.
Usually, the composition of the soil is 50% solid and 50% pore space. Inorganic mineral material makes up 45%, organic matter 5%, water 25%, and air 25% of soil volume.
The quantities of these 4 components depend on the amount of vegetation, water and air that is present in the soil and also how compact the soil is.
Good healthy soil contains sufficient water, air, minerals and organic matter to promote healthy plant growth and life.
Composition of organic matter
Organic matter or material is made up of humus. Humus improves soil structure and provides water and minerals to growing plants, while inorganic mineral matter consists of rock that has been broken down slowly over time into small particles that vary in size from gravel to sand, silt and finally loam.
Layer of the soil that is the most fertile
The top soil is the most fertile layer of the soil. It contains the most nutrients necessary for healthy plant growth and production.
Any farmer with poor soil will benefit from acquisition of top soil as it adds to the soil all the nutrients necessary for healthy plant life.
Formation process of top soil
Top soil is formed out of introduction of organic material from both decaying plants, leaves and twigs, and decomposing animals such as worms and insects into the soil.
The decaying process returns the previously living matter to the soil, thus keeping it fertile. It is also formed from the weathering of rocks. Good and bad insects, and organisms such as earthworms, live in the top soil.
These worms also enrich top soil by feeding on organic material and converting it into plant nutrients. They also improve soil aeration as they move through the earth.
Nutrients found in top soil
Top soil contains nutrients such as nitrogen, potassium, magnesium and phosphorus, all of which play critical roles in the fertility of the soil. They enrich the soil and vary in both quantities and quality.
Nitrogen promotes growth as well as healthy leaves and stems. Phosphorus creates the cells and DNA of plants, while magnesium and potassium help with photosynthesis, resulting in lush green leaves.
Compacting of top soil
Top soil can become compacted due to use of machinery and foot traffic. This therefore necessitates tillage during the planting season, sieving of garden soil and the adding of compost when tilling to ensure the soil is kept as rich and as loose as possible to guarantee adequate aeration.
How deep should top soil be?
Top soil is the soil that is naturally resident on the surface of the earth. It is usually between 2 to 8 inches deep, but typically between 5 to 8 centimeters in depth. In some regions, top soil may have a depth of 0.5 to 1 meter.
In order to fill a garden bed, you would need at least 8 inches depth of topsoil. New lawns require 3 to 6 inches of top soil before replanting.
Purchase enough topsoil for your garden bed to ensure it has good depth. Buy it in bulk and spread it directly on top of existing soil. Top soil is usually found in three grades: premium, general-purpose an economy grade.
Depletion of top soil
World over, top soil is declining much faster than it is able to form. This rate of depletion is likely to become a world environmental catastrophe because crops cannot grow optimally without it.
The depletion of top soil is largely caused by reckless deforestation and lack of afforestation. Crops draw nutrients from the top soil continually. Soil nutrients must therefore be replenished every growing season to guarantee healthy and bumper harvest.