Soil

Soil is loose material which lies on top of the land. It has many things in it, like tiny grains of rock, minerals, water and air. Soil also has living things and dead things in it: "organic matter". Soil is important for life on Earth.

Because soil holds water and nutrients, it is an ideal place for plants to grow. Soil holds the roots, and lets plants stand above the ground to collect the light they need to live. Almost as important are the fungi. There are fungi which grow into plant roots, a symbiosis known as mycorrhiza. This helps trees grow. There are many other fungi that live by breaking down dead organic material: the remains of other living things. The broken down material is a main source of plant nutrients.

Many animals dig into the soil and make the soil their home. The large animals use soil to make dens for sleeping and giving birth. The small animals live most of their life in the soil. Earthworms are famous for improving soil. This is because the holes they make let air go into the soil. The holes also let water go through.

The soil also has many microorganisms living in it. Many of them eat the organic material in the soil. They use oxygen and release carbon dioxide. They also release mineral nutrients into the soil.

Soil is different in different places on Earth. This is because the climate and rocks in the Earth are different in different places on Earth. Soils are usually thicker in places where ice sheets covered the ground during the Pleistocene ice ages. This is because the ice sheets ground the rock into powder as they slowly moved over the surface.

What soil is made of
Soil is made of four things.
It has rock pieces and the rocks are made of minerals,
It has dead and living things (organic matter, humus),
It has water.
It has air.

Soil has rock pieces made small by wind and rain and sun and snow.  The rocks are made of minerals and some of 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, it is eaten by animals including bacteria in the soil. When the bacteria are done eating, what is left behind is called humus. When the bacteria die the plant food (minerals) goes 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 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. You must use a big microscope to see clay too. Clay rock feels slippery between your toes. Most soils contain all kinds of small rocks. The three best rocks for making soil are sand, silt, and clay.

Soil texture 
Every soil has different amounts of sand and silt and clay. The mix of sand and silt and clay is the "texture" of the soil. We can also say the mix has a "soil texture". A soil with a lot of sand is called a "sandy soil texture". A soil with a lot of silt is called a "silt soil texture". A soil with a lot of clay is called a "clay soil texture". Farmers like to grow food in the best soil. The best soil is half sand and some silt and a little clay. The organic matter found in the soil is not counted in the soil texture. Only the rocks are counted when we discover the soil texture. Soil texture is very important.

Clay and humus are special parts of the soil. They help keep water and plant food (plant nutrients) in the soil. Water and plant nutrients stick to clay and humus. Water sticks to all of the rock in the soil. But, water sticks best to clay. Water is taken into (absorbed) into humus like a sponge absorbs water. Humus holds a lot of water and plant nutrients. Clay and humus keep water and plant nutrients in the soil. Sand will hold only a little water in the soil. If the soil has too much sand, the water will flow down into the earth. The water that flows down takes plant nutrients down too. The plant roots can not get at the water and plant nutrients if they go too deep. It is best for soil to have a little bit of clay and humus in it for growing plants.

Soil structure (clumps)
The smallest parts of soil are sand and silt and clay. Those small parts join to make larger parts we call "clumps" or "aggregates". The clumps are made when sand and silt and clay stick together. The humus and clay and  minerals in the soil are like glue. The glue sticks the sand and silt and clay together and makes clumps. The clumps make shapes by themselves. Some soils have small round clumps. Other soils have large, hard and flat clumps. The soil with small round clumps is best because it lets in air and water. A little glue is best. If the soil has only a little glue there will be space for water and air and the soil will be soft. If the soil has too much glue the soil will be hard. If the soil has no glue, there will be no space in the soil for air and water. A soil with no spaces is not healthy. Worms in the soil make a slippery glue. When worms make holes in the soil they leave some glue in the soil. Plant roots also make spaces in the soil. When the roots die they leave holes in the soil.

Soil horizons (layers) 

A soil has a "soil texture" (sand and silt and clay) and it has organic matter mixed in it. But 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 and wet weather make one kind of soil. Cold and dry weather make 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 layers in the soil.

We call those layers "soil horizons". The top horizon may be an inch (25mm) thick. We call that layer the "O" horizon or sometimes "topsoil". The next layer (horizon) is the "B" horizon.  The next lower layer of soil is the "C" horizon. The bottom layer has many rocks and may be called the bedrock or "R" horizon for "rock".   Deep down, there is always bedrock.   But you may have to dig down a mile (a km) or more. When the soil dries, the soil may shrink and cracks will form in the soil. The soil in the top layer may fall down into the cracks. This causes the layers of soil to be changed because they are mixed. There may be many types soil where you live or only one type of soil. Different rocks make different soil textures. Different weather makes the different soil textures different. And so, there are different soils all over the world.

Related pages 
 Agronomy
 Soil fertility

References

Other websites
 short video about soil from The Guardian newspaper

Ecology
Geology
Natural resources
Soil