Soil Composition and Horizons

• Soil is a resource that supports plant life, made of nonliving and once-living things.

What is soil?

• Soil forms as rocks weather over time, with microscopic organisms growing among the rock bits.
• These organisms break down rocks into chemicals that nourish plants.
• Grasses, bushes, and trees grow as rock weathers, followed by animals that feed on the plants.
• When plants and animals die, their bodies add organic nutrients back to the soil.
• "Organic" means having to do with or coming from living things.
• Soil is a mixture of rock bits and once-living parts of plants and animals, covering most of Earth's landmasses.
• Without soil, plants and animals would not be able to live on land
• Soil forms layers as rocks weather.

Soil Horizons

• Each layer of soil is called a soil horizon.
• The A horizon contains the most nutrients and humus.
• Humus is made of decayed organic materials from dead plants and animals decayed by microscopic organisms.
• Humus contains nutrients that feed plants, and soaks up and holds water more easily than bits of rock.
• The soil in the A horizon is called topsoil, where most plant roots grow and absorb nutrients and water from humus.
• The B horizon is called subsoil.
• Subsoil contains less humus and many fine particles of rock, such as clay.
• The C horizon mostly consists of larger pieces of weathered rock.
• Soil horizons rest on solid, unweathered bedrock.
• Different areas have different depths of soil horizons, or may not have all soil horizons.

How is soil used?

• Soils in different places have different properties.
• Each type of soil supports different plant and animal life.
• Most of the United States is covered by forest soil, desert soil, and grassland and prairie soil.
• Forest soil has a thin layer of topsoil with little humus, where frequent heavy rainfall carries minerals deep down, so shallow roots cannot reach them.
• Most of the forest soil in the United States is in the eastern third of the country.
• Desert soil is sandy, doesn't hold much humus, and desert soil is rich in minerals.
• Animals can be raised on desert soil, and crops can be grown here if provided water.
• Grasslands and prairies in the United States are between the Rocky Mountains and eastern forests.
• The soil in grasslands and prairies is rich in humus, providing nutrients for crops like corn, wheat, and rye grown from Texas to North Dakota.
• Soil is a resource that can be used up, wasted, or spoiled, and can be eroded by flowing water and wind if plant roots are removed.
• Removing plants can change the type of plants that can grow in an area or make it difficult for any plants to grow.
• Nutrients in soil are naturally removed by plants, they are normally replaced when plants die and decay.
• When a farmer removes all crops, the land becomes less able to support new crops.

Pollution

• Pollution is the addition of harmful materials to soil, air, or water.
• Soil can be polluted by chemicals placed in the ground, used to kill insects and weeds, or by dumping garbage on the ground.

How is soil conserved?

• Conservation is the preservation or protection of natural resources, including soil.
• Fertilization is the addition of fertilizers containing nutrients to replace those used by previous crops.
• Crop rotation is planting different crops on the same land in different years to replace removed nutrients.
• Strip farming involves planting grasses between rows of other crops, since plant roots help prevent soil from being washed or blown away.
• Contour plowing helps slow the speed of water flowing down a hill, instead of up and down the slope, farmers plow furrows across the slope.
• Terracing involves cutting terraces into a hillside where crops are planted along each terrace to slow the speed of water flowing down a slope.
• Planting tall trees along the edges of farmland slows the speed of wind across the ground, lessening the chances of blowing away the topsoil.
• Governments may pass laws to stop the pollution of soil.
• People can avoid polluting soil with trash and help clean up land that is polluted.
• Education can advise people of soil value and how to conserve it.