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What Is a Soil Profile?

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  • Written By: Christina Edwards
  • Edited By: W. Everett
  • Last Modified Date: 30 March 2014
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Soil typically consists of layers of material, called horizons, which differ in both texture and appearance. A soil profile is a cross section of these layers, and it measures the different characteristics of each layer. Although every soil from around the world has a different soil profile, most soils consist of three or more layers, including the topsoil, subsoil, and bedrock. The top layer is generally finer and contains less rocks than the deeper layers.

Topsoil is the uppermost part of a soil profile, and it is the ground on which people and animals walk. Plants will also typically lay the majority of their roots in the topsoil. It can be as thin as two inches (5.1 cm) or as thick as 5 feet (1.5 m), and it is often a dark color, sometimes even black.

In uncultivated areas, it may be littered with such organic matter as leaves, twigs, or dead animals that serve to help prevent erosion, hold moisture, and produce nutrient-rich soil. When organic matter decays, it is often referred to as humus, and it contains vital nutrients. It is this layer of the soil profile from which plants get most of their nutrients.

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The subsoil is the layer of the soil profile that lies directly beneath the topsoil. There is usually no litter or debris present in this soil layer, and it is often lighter in color. Subsoil often consists of clay, silt, pebbles, and sand, depending on the area, and it generally contains an abundance of minerals that have leached down from the upper layers of the soil.

As a person digs deeper and deeper into the soil, he will find that it gets rockier and rockier. Some scientists consider the next layer of the soil profile, called the regolith, to be part of the subsoil, while others consider it to be a completely separate layer. This layer almost never contains plant roots or other organic matter, but is made up primarily of soil and small, weathered rocks.

The bedrock layer is present in just about every different type of soil profile. This layer is made of hard, solid rock, which is eroded and weathered to produce most of the soil above it. Bedrock can be as little as 5 feet (1.5 m) below the surface, or it can even be exposed in some areas. In situations where much of the upper soil has been deposited from somewhere else, however, the bedrock can lay hundreds of feet beneath the surface.

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highlighter
Post 3

How do we improve the profile of our soils? What are some of the ways that we can re-engineer soil so that we are not using it faster than geological processes create it?

istria
Post 2

@ Parmnparsley- Believe it or not, for the most part your friend is right. A mature soil profile takes hundreds to thousands of years to develop. Soil is broken up into three soil horizons above the underlying bed rock, and for these three layers to mature takes a number of different geologic and biologic cycles. Weathering and erosion must break rock into smaller particles, thus releasing minerals. These minerals need to be small enough for plants to absorb. These minerals are then deposited on top of other larger particles forming the subsoil layers. The topsoil layer is the next layer that is formed from small particles in the subsoil layer mixing with the larger particles from the very top layer, which is the layer of humus and decaying matter. Insects and plants help to mix these layers and break down nutrients to be re-inserted back into the soil.

The reason that soil is a finite resource is because humans are degrading soil faster than it can be naturally replenished. There are soil re-engineering techniques that are in use today, but nothing on a scale to make soil a renewable resource.

parmnparsley
Post 1

How long does it take for a mature soil profile to form? Someone told me the other day that soil is a limited resource and we may one day run out of soil, but I didn't believe that person. Now that I am thinking about it, soil creation must be a painfully slow process. Can anyone give me a better understanding of what it takes to create a mature soil profile?

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