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What is Topsoil? - Definition, Composition & Uses

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  • 0:01 Definition and…
  • 1:20 What Does Topsoil Do?
  • 2:17 Biological Cycles in Topsoil
  • 3:25 Lesson Summary
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Lesson Transcript
Instructor: Brekke Peterson Munks
Have you every heard the word topsoil and wondered what was so special about topsoil? In this lesson you will learn how to determine topsoil from other types of soil, why topsoil is important and what topsoil is used for!

Definition and Composition of Topsoil

Have you ever been on a river bank and noticed layers of soil? You might have observed that the top layer was dark colored and lush. This is topsoil, often referred to as the A or O horizon in soil. Take a closer look at the image on screen. Notice the dark layer of topsoil where the plants root themselves.

Topsoil is defined by the top two to seven inches of soil that has the greatest organic matter content and microorganisms. Organic matter is what is left after plant roots, stems and leaves decompose. Topsoil is made up of carbon and nitrogen, microbes and larger creatures such as worms, beetles and other insects. A fertile topsoil also has concentrations of nutrients including potassium, phosphorus and iron. Topsoil varies depending on geographic region and climate, as well as human activity.

Often, the blacker the topsoil is, the richer it is in nutrients, and the stronger its earthy smell. Rich, nutritive topsoil is able to hold onto more water. Consequently, greater nutrients and water availability allow topsoil to be more productive in terms of plant growth. Farmer's try to manage topsoil to allow for a reduction in dependence of chemical fertilizer and additives to prevent disease.

What Does Topsoil Do?

Most importantly, topsoil provides nutrients to growing plants. This is because this region of soil does the main biological nutrient cycling, providing needed carbon and nitrogen molecules for plants to grow. We'll discuss these cycles more next.

Topsoil is the home to a diverse community of microbes that do this cycling. More importantly, the more diverse a microbial population is, the less soil diseases or plant diseases occur, reducing a farmer's reliance on herbicide or pesticide because the soil community ensures healthy crops.

Topsoil is required to plant lawns and gardens. When building a new home, people search for topsoil to ensure a healthy lawn. Gardeners tend to till in topsoil into their gardens to increase nutrient cycling, organic matter and water holding capacity. This makes sure that their time and effort in gardening is well worth it because the soil is healthy to provide a productive, bountiful garden.

Biological Cycles in the Topsoil

Topsoil is the part of the soil where all of the biological activity occurs. Biological activity is the work associated with decomposition of biological biomass of plants and microbes. Specifically, the carbon cycle and the nitrogen cycle take place in the topsoil.

The carbon cycle is a set of carbon transformations that removes carbon from the atmosphere through plant uptake. Plants then make simple sugars and these sugars are released into the topsoil if the plant has excess. These sugars are then cycled through microbes and taken back up by other plants. This carbon is then recycled in a sense and released back to the atmosphere as carbon dioxide or methane, both carbon containing molecules.

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