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United States Department of Agriculture

Agricultural Research Service

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All About Soil
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Dig that Stuff
All about Soil

Who Needs Dirt Anyway?

Dirt! Without it, we would be naked, homeless, and starving. Plants, which rely on dirt for survival, fill some of our most basic human needs. Shelter and clothing, whether made from natural or synthetic materials, are plant based. Our food, even if it's animal, can be traced to a reliance on plants. Dirt isn't so dumb after all.

What Is It and Where Does It Come From?

Dirt, or soil, is made from rocks that break apart or wear away over many years. This is referred to as weathering. It may take 100 to 1000 years for 1 cm of soil to form through weathering. Soil also contains air, water, and humus, the decayed remains of dead animals and plants. Soil can actually be separated into 5 main parts: humus, clay, silt, sand, and gravel.

Humus: A dark, moist soil composed of bits of dead, rotting insects, animals, leaves, roots, sticks, and food. Humus adds nutrients to the soil which plants need to grow and live.

Clay:A soil that holds water. When wet, clay feels slippery and slimy. It is made of particles that are smaller than 0.004mm.

Silt:Soil that looks like fine grains or tiny pieces of rock. Particles classified as silt measure from 0.004mm to 0.006mm.

Sand: Soil that is coarse and drains quickly. Sand particles measure from about 0.06mm to 2.00mm.

Gravel:Visible rock particles, sometimes referred to as pebbles.

The smaller the particles in the soil, the smaller the air spaces. This makes it hard for plant roots to push through, but this type of soil retains moisture longer than soil with larger particles. A plant that needs a dry environment, like the cactus, does well in soil that is coarse, like sand, which does not retain a lot of water. Plants that require more moisture, like vegetable plants, do better in a soil that has a balance of humus, clay, and sand. Nature does not always cooperate by providing the best type of soil for the crops farmers want to grow. Therefore, they plow, add fertilizers, and irrigate to create a healthy growing environment for their crops.

Soil, Science, and Technology

Today scientists are developing new technology to aid farmers in providing healthy growing environments for their crops. It is referred to as site-specific farming or precision agriculture. In almost any field, you can find a wide variety of soil types. As a result, different portions of the same field may require different amounts of water and fertilizers. Global Positioning Systems (GPS), receivers and satellites, are used to help map and analyze soil. With this information, computers are used to help regulate the amount of water or fertilizer being applied to different parts of the same field through sprinkler or surface drip irrigation systems and subsurface irrigation systems. By regulating distribution, water is being conserved, fertilizer is distributed only as needed, and the crop yield is at its maximum. For more information on new farming technology check out Precision Agriculture or Site-specific Farming.
Image of GPS receiverdrawing of satellites orbiting the earth
GPS receiver and satellite
Aerial view of the site-specific Center Pivot Irrigation
Site-specific Center Pivot Irrigation System
(aerial view)
side view of the site-specific Center Pivot Irrigation SystemSite-specific Center Pivot Irrigation System sprinkler view
Site-specific Center Pivot Irrigation System
(side view)
Site-specific Center Pivot Irrigation System
(view along sprinkler)




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Last Modified: 8/12/2016
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