Soil Formation

What soil is made of

Soil is made of four things.

  1. It has rock pieces and the rocks are made of minerals.
  2. It has dead and living things.
  3. It has water.
  4. It has air.

Soil has rock pieces that are made small by wind, rain, sun, and snow. The rocks are made of minerals and the minerals dissolve in water. Some of the minerals dissolved in water can be used as food by plants. Soil also has dead and living things (organic matter) in it. When a plant dies, the dead leaves are eaten by bacteria in the soil. When the bacteria are done eating, what is left behind is called humus. When the bacteria die they put the plant food (minerals) back into the soil. A food for plants is called a “plant nutrient.” There are many kinds of plant nutrients. Soil has many empty spaces. Half of the soil is space. The spaces are filled with water and air. Water can enter the soil because there are spaces in the soil. Plants drink the water and the minerals in the water. Plant roots need air to live. If plant roots do not have air, the roots will die. If the plant roots die, the plant will die. A plant that dies is eaten by bacteria and becomes plant nutrients again.

There are many kinds of soil. Each kind of soil has big and small rocks in it and some humus. If rocks in a soil are as big as your fingers, we call those rocks gravel. Smaller rocks are called sand. Very small rocks are called silt. Very, very small rocks are called clay. You can see sand with your eye. Sand feels rough between your toes. Silt rock is very small and you must use a lens to see silt. Silt rock is smooth between your toes. Clay rock is too small to see with a lens. To see the smallest things you must use a big microscope.

Another important fact about soil is that it differs greatly from place to place. This is mainly because the climate varies, and the underlying rocks may also vary. Weather changes the soil. It is cold on the Earth near the north and south poles. It is hot near the equator of the Earth. Some places on Earth get a lot of rain and some places get no rain. Hot, wet weather makes one kind of soil. Cold, dry weather makes another kind of soil. Rain water makes small things in the soil move down with the water. When the things in the water get stuck in the soil, those things make a layer in the soil. If you dig down into the soil you may find many layers in the soil. The layers may have different colors. The layers may have different “soil textures.” The top part of the soil may have a lot of humus and sand. Below that layer there may be a layer of silt. Below that layer there may be a layer of clay. The sand stays on the top because it is large. The silt goes down a little with the water and makes a layer because it is small. The silt is smaller than some of the spaces in the soil. The clay can go down even lower with the water because it is the smallest. The sand will make one layer, the silt will make another layer, and the clay will make another layer. The humus can move down with the water and make a layer too. The silt and clay and humus can move down because of the spaces in the soil. But the silt, clay, and humus will fill the spaces in the soil. When the spaces in the soil are closed, it makes it hard for air to go into the soil. Plant roots do not go where there is no air. When we dig down we find the layers that are made.

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