Environmental Science: Soil

R horizon

- Parent material beneath the soil - sometimes called rigolith

Soil

- an important natural resource - food is grown in it - the thin upper layer of Earth's crust that supports plant life - holds nutrients and water that plants need to live - home to thousands of different animals - earthworm - gopher - microscopic living things play an important role such as decomposing dead organisms - filter harmful materials out of the soil. - provides structural support. homes are built on it. - keep Carbon levels in balance. - plays a role in Global Nutrient Cycling. process of decomposition goes on in the soil.

C horizon

- loose material recently weathered away from its parent material

B horizon

- usually lighter in color than the A horizon - accumulation of clay that gets washed down by the soil above it

PHYSICAL PROPERTIES OF SOILS

By now, you have seen that soils are different all over the world. Soils have different physical and chemical properties. Some physical properties of soil include color, water content, and texture. Soil texture is a measure of how the soil feels. It is determined by the sizes of particles in the soil. The largest particles are sand-sized. Soils that are composed mainly of particles in this size group feel gritty in texture. The next smaller particles are called silt. Soils that are composed mainly of silt feel smooth like flour. The smallest soil particles are called clay. Clay soils feel sticky and are often thick and hard when dry. A soil that is a mixture of all particle sizes is a loam.

OBJECTIVES

Define soil and discuss the composition and creation of soil. Explain why soil is an important natural resource and list functions of soil. Explain why soils vary throughout the world. Illustrate and describe the main horizons of a soil profile. Describe the physical and chemical properties of soil. List the biotic components of soil.

LET'S REVIEW!

In this lesson, you have covered the following points: - Soil is the thin layer of materials at Earth's surface that supports plant life. - Soil is an important natural resource that serves various functions, including supporting crop production, filtering water, providing structural support to buildings, and providing habitats for living things. - Soil is made of organic and inorganic components. Soils vary throughout the world, depending on the climate, topography, vegetation, and parent material. - Some physical properties of soils include color, water content, and texture. Soil texture describes the relative percentages of sand, silt, and clay-size particles in soil. - Some chemical properties of soils include fertility and pH.

Visualizing Soil Profiles

Now it is your turn to find out about the soil profiles of your home area. Visit State Soils to find the soil profile for any state throughout the United States. Just select the state that you live in, and you will be able to see pictures and interesting information about your local soils. If you have a suitable safe place to do so, and if you have permission from a parent or teacher, you may even go outside and start digging to explore your own local soil. See if you can identify soil horizons in your local area.

WHAT IS SOIL?

Scientists define soil as the top layer of the surface of the Earth in which plants can grow. Throughout this lesson, you will learn about the functions of soils and how they vary throughout the world, soil composition, and the properties of soils. We will begin with the functions of soil. Soil plays a role in global nutrient cycling. Processes of decomposition and decay of plant and animal materials takes place in the soil. In this way, nutrients are cycled between components of the environment, such as when the leaves on these trees fall, decay within the soil, and return nutrients to the ground.

The Components of Soil

Soil is a mixture between organic and inorganic materials. Organic materials are made from matter that is alive or was once alive, and inorganic materials are made from non-living matter, such as rocks and minerals. We will first discuss the organic parts of soil. The remains of dead plants, animals, fungi, or any other living organism are just part of the soil's organic components. Other organic components in soil include microbes, which are small microscopic organisms—such as bacteria—which work to break down dead matter and leave behind valuable nutrients. Earthworms are another important organic element in soil. Earthworms work to recycle plant matter and deposit their waste, known as casting, which hold valuable nutrients for future plant growth. Earthworms also work to loosen, mix, and aerate the soil. These organic components in the soil give rise to humus, which is dark and rich in nutrients, has a high water-holding capacity, and is important in agricultural soils for growing crops.

Soil Texture Lab

Soil texture is the most important physical property of soils because soil texture determines how the soil will react to external forces, such as weather and wind. Sandy soils are well drained, which means water runs through them very quickly. Clay soils are just the opposite, and water tends to pool over them and drain slowly. Both of these extremes can be harmful to plants. A loam soil is best for plant production because it provides a good balance of particle sizes to hold some water in the soil for plants to use, but not so much water that the plants drown. Most soils are not made of just one type of particle. They are various mixtures and are classified accordingly, such as "sandy clay" or "silty clay loam." Scientists use special tools called hydrometers to measure soil texture and moisture, but you can also estimate soil texture in the field with the Soil Texture by Feel Method.

Soil Biology

Soils contain Earth's most diverse communities. We often cannot see organisms that live in the soil with just our eyes, but in one handful of soil, there may be millions of living things. Just one gram of soil can contain several thousand different types of bacteria. Although some of these bacteria are harmful, many of them are beneficial to plants and play important roles in sustaining life on Earth. For example, nitrogen-fixing bacteria are soil bacteria that help make nitrogen (a vital plant nutrient) available to plants. Other bacteria break down decaying organisms so that materials can cycle on Earth. Watch the slideshow to view some organisms that live in the soil. Have you ever seen any of them? Some you can only see with a microscope, but some you will probably recognize.

WORLD SOIL TYPES

Soils vary all over the world, depending on the climate, topography, parent material, and vegetation of the area. Because weather and climate vary worldwide, soils are affected differently all over the world. Another major factor in the development of soil is the type and amount of organisms that live within the soil. Earthworms and other decomposers increase the fertility of the soil, but these organisms are not found in all areas or at the same quantity.

Chemical Properties of Soils

Some chemical properties of soils include nutrient content and soil pH. Nutrients are the materials in soils that plants need to grow once they have made sugar from the CO2 in the air. A soil is called fertile if it provides a sufficient amount of nutrients for plant growth. The most important nutrients for plant growth are nitrogen, phosphorus, and potassium. Most plants cannot grow without these. But scientists are starting to realize that these three are not really enough for optimal plant growth; there are many trace chemical elements needed to build plant cells, as well as countless other chemicals we are only starting to categorize that are the byproducts of all the other life in the soil. Soil pH is the measure of the amount of how acidic or basic a soil is. pH values can range from 0 to 14, where 0-6.9 is acidic, 7 is neutral, and 7.1-14 is basic. Most soils have a pH of 5-8, and a pH of 5 or 6 is optimal for most plants. If the pH is too high or too low, plants will not grow well and may even die.

Inorganic Parts of Soil

We call the rock and mineral particles in the soil the inorganic components. These particles are made from rocks that have been broken down into smaller and smaller pieces over time by the weathering process. The original rock that a soil comes from is called the parent material. Over time, this parent material is broken down by nature in one of three ways: mechanical, chemical, or biological weathering. Mechanical weathering is the process in which rocks and minerals split and break apart into fine sediments. This can occur by a rapid change in temperature or extreme cold or warm conditions. Mechanical weathering can also take place when water freezes and then thaws in rocks. This causes stress in the rocks and causes them to break apart. Rocks and minerals can also be mechanically weathered when they tumble around in river and creek bottoms. Chemical weathering is the breaking apart of inorganic material. Water is the primary chemical responsible for chemical weathering. Water is able to dissolve rock and minerals over time. Rainwater is also naturally acidic and breaks down inorganic materials. Chemical weathering works together with mechanical weathering in most situations. Biological weathering is a special case of weathering when living things break apart rocks and minerals. Most cases of biological weathering occur because of roots of trees and plants that move into cracks of the rocks, splitting them apart and dissolving the minerals as they grow.

soil pH

a chemical property of soils that describes the measure of hydrogen ions in a soil sample; how acidic or basic a soil sample is

soil horizon

a distinct layer of soil that has characteristic properties

soil texture

a physical property of soil that describes how the soil feels, and the relative components of sand, silt, and clay sized particles

organic

a substance that contains carbon and hydrogen atoms, such as carbon dioxide, glucose, methane, and nucleic acid

inorganic

a substance that does not contain carbon and hydrogen atoms, such as salts, rocks, and minerals

soil profile

a vertical section of soil that shows the horizon and parent material

humus

dark colored organic material in soil; it is left over from the decay of living things

A horizon

dark upper horizon that contains substantial humus as well as some inorganic components like rock and mineral particles

O horizon

layer of decomposed organic material

regolith

the layer of loose rock on the surface of the earth; also called mantle rock

parent material

the rock material that was weathered to form the sediments in a given soil