Fourth Grade
Worksheets
Soil Horizons
Soil covers Earth's land. It is a vital natural resource. Soil is a thin layer in which plants can grow.
Soil is not all the same. It has different layers. The layer on top is made of humus (HYOO-muss). Humus is dark brown or black. It is made of the remains of plants and animals that were once alive. As they rot, they become part of soil. This top layer of soil is called the O horizon. O stands for organic. Organic matter came from living things. This layer may be only about one or two inches deep. The O horizon is "O" so important for having healthy soil.
The second layer from the top is called the A horizon. It is a mixture of humus and small bits of minerals. It is dark colored, too, but not as black as humus. People often call this layer "topsoil." In the A horizon, you will find plant roots. Living things like earthworms, ants, and beetles make their homes here. Microorganisms like bacteria and fungi are found here, too. Microorganisms are too small to see without a microscope. They break down, or decompose, organic materials. They are important to healthy soil. This layer can be eight to twelve inches thick. The A horizon is "A" home to living things.
