ImageHave you ever run barefoot through the grass? Have you ever looked at a field of corn? Have you ever planted a flower seed? Grass, corn, flowers, and all other plants need soil in order to grow. Without soil there would be no crops for food, no forests, flowers, or grasslands. You could say that life on Earth depends on soil.

Soil is everywhere around you. Soil covers much of the surface of Earth. The soil in the Midwestern United States is black. The soil in some Southern states is a reddish color. Near deserts, lakes, and oceans, the soil can be gritty and sandy. What a soil looks and feels like depends on how it is formed. Pick up some soil from your yard or a park. Look at its color. Feel if it is gritty or smooth.


Soil is made of minerals, air, water, and plant and animal material. The minerals in soil are tiny particles. The soil particles come in three main size groups. From largest to smallest, these groups are sand, silt, and clay. What a soil feels like depends on the size of its particles. Sandy soil is loose and dry. Clays are thick and sticky. Soils made mostly of sand tend to hold less water than soils made mostly of clay.

Soil has lots of tiny holes called pores. Pores let air and water into the soil. Roots grow into pores. Tiny bugs crawl through pores.


Look closely at a pile of soil. It is full of living creatures. You may see earthworms, mites, millipedes, centipedes, grubs, termites, and other animals large and small. Soil is also full of things you can only see under a microscope.

In this small world of soil a lot is going on. Worms and other fairly large creatures eat decaying parts of plants. Bigger animals eat smaller ones. They expel waste into the soil. Microscopic life forms called bacteria and fungi feed on the bodies of dead animals and reduce them to simpler materials that plants can use for food. Decayed plant and animal parts create a rich, dark-colored soil called humus.


Most soils begin to form when big rocks break up. The breaking up of rocks is called weathering. Weathering makes pieces of rock smaller and smaller. There are two kinds of weathering, physical weathering and chemical weathering. After weathering breaks up rocks, a process called erosion spreads the broken bits about.


Most physical weathering is caused by ice. Ice is frozen water, and water expands when it freezes. Freezing water makes a powerful force. When water seeps into cracks in rocks and freezes, it can split the rock apart. Strong winds and growing tree roots can also break up rocks.

Water causes most chemical weathering. Chemical weathering changes the materials that make up rocks. Rain pours down on rocks, rivers flow over rocks, and waves pound rocks along beaches. The water takes certain minerals out of rocks. For example, grains of sand form after water takes a mineral called feldspar out of granite rock.


Erosion also makes soil. Erosion can help break up rocks, but it mainly moves weathered rock. Water, wind, and glaciers cause erosion. Wind or water can wear away rock on a hillside. Water moves the eroded rock down the hill. Wind blows dust away. Glaciers are big sheets of ice that move over land. The moving ice grinds up and carries the rocks below it.


Most soils form very slowly. It can take as long as a million years for weathering to break down some rocks.

Chemical weathering works faster in warm, wet climates than in cool, dry climates. Also, plant and animal parts decay and make humus faster. Soils that form in warm, wet climates are usually better for growing plants. Physical weathering is the main type of weathering in cool, dry climates.


Although erosion helps form soil, it is also a major danger to soil. Soil can erode when forests are cut down because tree roots help hold soil in place. Erosion can wash soil away from farmland. Planting trees along fields helps prevent erosion. Farmers also plow their fields in ways that help prevent erosion.

Soil can become polluted. Pesticides (bug killers) can pollute soil. Spilled chemicals can pollute soil. Acid rain can change soil. Acid rain is caused when smoke from factories goes into the air and mixes with water droplets in clouds. Acids in these droplets then fall to the ground in rain.

Because it takes so long for soil to form, people now understand that they must protect this important resource.

Source: Microsoft ® Encarta