what are the three layers of soil?
O Horizon - The top, organic layer of soil, made up mostly of leaf litter and humus (decomposed organic matter).
A Horizon - The layer called topsoil; it is found below the O horizon and above the E horizon. Seeds germinate and plant roots grow in this dark-colored layer. It is made up of humus (decomposed organic matter) mixed with mineral particles.
E Horizon - This eluviation (leaching) layer is light in color; this layer is beneath the A Horizon and above the B Horizon. It is made up mostly of sand and silt, having lost most of its minerals and clay as water drips through the soil (in the process of eluviation).
B Horizon - Also called the subsoil - this layer is beneath the E Horizon and above the C Horizon. It contains clay and mineral deposits (like iron, aluminum oxides, and calcium carbonate) that it receives from layers above it when mineralized water drips from the soil above.
C Horizon - Also called regolith: the layer beneath the B Horizon and above the R Horizon. It consists of slightly broken-up bedrock. Plant roots do not penetrate into this layer; very little organic material is found in this layer.
R Horizon - The unweathered rock (bedrock) layer that is beneath all the other layers.
Most soils have four layers.
The top layer is called the organic layer. This layer is about an inch thick and takes from 100 to 600 years to form. Within this layer, living things carry on with their life activities. Also in this layer are millions of dead plant and animal organisms that are slowly decomposing, or rotting, away. As these once-living things decay, the organic layer becomes rich in nutrients. If you dig more than an inch or two deep, you might be past this layer already!
Section A is called the upper soil layer. This is where you will find many plant roots, different types of fungus, and other very tiny living things. This soil is dark in color because there is are so many chemical reactions taking place as living things grow and die. A great deal of bacteria is found in this layer. The bacteria help make chemical reactions happen so that materials of the earth can be recycled. This layer is usually about a foot deep.
Section B is the middle soil layer. It has less living and once-living things and less of the darker topsoil. The soil here has less air, too. Because of these characteristics, plants do not grow well here. You will find fewer roots and fewer signs of life. At about two to three feet deep, you are digging into the subsoil. Often, you might find signs of human activity in the upper soil layer and the subsoil. A broken piece of pottery, or an arrowhead may have been buried this deeply.
The last layer, section C, is lowest layer. In this layer you will find that the soil may have an orange or yellowish color. It may be more sandy or have more gray clay. In this layer you will see that there are many pebbles and rocks. This layer has the least amount of living and once-living things. If you are digging a hole and you get more than two or three feet deep, your shovel may begin to hit many rocks. If this happens, then you are probably in the lowest soil layer.