Mrs. Pilon's "The Importance of Soil" Study Notes Page

The Importance of Soil

• Soil is part of the earth’s life support system
• All life depends on certain chemical elements
• These elements are the basic building blocks of life for living organisms
• These elements must be recycled constantly so that life can continue

• Food chains depend on soil
• Food chains show the order in which energy in the form of food is passed from organism to organism (plants-herbivores-carnivores; omnivores eat both plants and animals)
• Living organisms need energy
• Plants can convert energy from the sun into some of the food they need through photosynthesis
• Plants also need the nutrients from the soil
• Plants are the starting point in food chains

• Most other organisms obtain energy by eating plants or animals that are lower on the food chain
• This shows interdependence

Assignment:  make a list of all the foods in your refrigerator that come from plants; from animals. 

• When you eat animal foods, you are indirectly eating plants. 

• When a plant or animal dies, it is decomposed by organisms that live in the soil and its nutrients become available to the next generation of plants
• Food chains begin and end in the soil

Other ways we use plants

• Technology allows us to use plants to help make life more comfortable
• Trees provide wood that can be used to build shelter, furniture; make paper; used as fuel
• Some plants like cotton, hemp and flax can be used to make cloth and rope
• Dyes from plants can be used to colour cloth
• Almost all plants depend on soil for healthy growth

Assignment:  choose a plant (coconut palm, bamboo, spruce tree; corn) and find out what other products can be obtained from them

What is soil?

• If you break up a moist lump of soil, you will see spaces called pore spaces.  Some contain air—a larger proportion of carbon dioxide than we breathe.  Others contain water.
• The rest of the soil is a mixture of particles including mineral matter (largest portion--never alive, like sand) and organic matter (once living, like dead leaves).
• All soil is made up of mineral matter, organic matter, air and water

• Over thousands of years, soil has changed because of the actions of wind, water, gravity and changing temperatures, as well as the lives and deaths of millions of organisms.

Components of Soil diagram (circle graph) Mineral matter 40%; Air 25%; water 25%; organic material 10%

• Soil begins with rock, the parent material of soil. 
• The rock is broken down into mineral particles by the process of weathering, which takes a very long time. 
• Weathering can occur as a result of chemical or physical processes which change the rock.

A.  Physical Weathering

• Water freezes into cracks
• Earthquake
• Erosion
• Roots grown into cracks

B. Chemical weathering

• Pollution—acid rain
• Extreme heat/high pressure
• Exposure to oxygen (oxidation)
• Plants (lichens) grow on the rocks and cause chemical changes by breaking down certain minerals

C.  Biological Weathering

• Some rocks break down more quickly and easily than others.  If the mineral crystals in the rock are packed closely together, it takes longer to break down. 
• In a dry climate with extreme temperature changes, physical weathering occurs faster than in a wet, tropical climate.  Chemical weathering occurs faster in a hot, wet climate with lots of plants and other organisms.
• The movement of mineral particles by wind, water, ice and gravity is called erosion.

Organic Matter

            At first, soil contains mostly mineral material from the weathered parent rock.  Only a few plants and animals live in it.  As they die, they decompose and add more organic material to the soil.  Eventually, the soil will be able to support more types of life.

            The remains of dead plants and animals are broken down by microorganisms that live in the soil.  Decomposed organic material changes into humus, which forms 1%-20% of soil.  Humus provides the nutrients in which plants and animals need to grow.  Some organisms produce materials that “glue” or bind clumps of soil together and by doing that, it holds in the humus.  It also helps the soil to hold more water.

Summary:  What makes soil?

-Parent material (rock)

-climate

-organic material (humus)

-landforms

What is the difference between dirt and soil?

Describing Soil Texture and Structure

            Soil texture refers to the size of particles in a soil.  The three most common sizes are sand, silt and clay.  All soils contain a mixture of these three types.  The best soils for growing plants have a mixture of large and small particles.  The large particles provide space for air and ensure that water can drain freely.

            Soils are named according to the percentage of sand, silt or clay they contain. 

            A soil with about equal amounts of all three particles is called a loam. 

            Sandy soils feel gritty.  Sandy soils are easy to work, water drains through them easily and there is plenty of air, but there are not enough nutrients for plants to grow well.   

            Silty soils have a soapy feel.  Silt soil has at least 80% of silt and less than 12% clay.

            Soils with many clay particles can be squeezed and moulded into shapes.  They hold too much water for good plant growth, and the particles clump together and keep air and roots from penetrating.

            Sandy soils have large particles and large pore spaces.  Water drains through them very quickly, so plants growing in them must be able to live for a long time without water.  Usually sandy soils are low in nutrients.

            Clay-rich soils have very fine particles that are often shaped like little plates.  They have very small pore spaces, so it is hard for water to drain through them and they become waterlogged.  You need some clay in soil used for growing plants, because the clay particles contain important nutrients that are released into the soil water to be absorbed by plants.

            You can make soil more fertile by increasing the organic matter—adding compost or manure.

            Undisturbed soil particles clump together into larger units called peds.  Peds can have different shapes:  columns, flat plates, rectangular blocks, or rounded crumbs.  Block-shaped peds can make it difficult for plant roots and water can move through the soil.  Plate-shaped peds slow down the passage of water.  Tilling soil often changes the soil structure.  Heavy machinery can squash column-shaped peds into flat plates, making harder for water to drain.  Tilling can also break up the large blocks of soil, making it easier for plants to grow.

            An ideal soil structure for plant growth will have an upper layer of soil with a rounded crumb structure, with columnar structures below, to allow easy penetration by plant roots and good drainage of water.

            A poor soil for plant growth will have block-shaped peds that make it hard for roots to penetrate and water to drain.  There may be horizontal plate structures that restrict root growth and water flow.

Looking at Soil Profiles

            As soil forms, different layers appear.  These layers are called soil horizons.  Fully developed soils usually have three layers, or horizons, making up their profiles. On top is a surface layer of leaves and organic material.  The A Horizon is rich in organic material on top, but has soluble material lower down that has been washed down by ground water seeping (leaching) through the soil.  The B Horizon has weathered mineral matter and some humus that has washed from the A horizon.  The C horizon contains partly weathered parent material with almost no organic matter.  By studying the soil profile, scientists can tell how the soil was formed, the kind of matter that formed it, and how the soil has changed over time.  The thickness of the horizons gives clues about drainage, vegetation and climate.

Natural litter

Humus—decayed organic material

Topsoil—most fertile part; a few cm to a meter deep

Subsoil—lighter brown; fewer nutrients and less water

Partially broken-down rock

Parent rock

Only 8% of Earth’s surface is covered with topsoil good enough to grow crops

It takes hundreds of years to make a few cm of topsoil

Erosion is a major problem.  Before farming began, probably about 8 billion tonnes of topsoil per year was lost, and could be replaced by the rock cycle.  By the 1980s, more than 23 billon tones per year were lost—far too much to be replaced naturally. 

Exposed soil erodes more quickly than soil covered by vegetation.  Plants hold soil in place with their roots, and their leaves protect against wind, rain and harsh sunlight.  Farmers usually don’t leave fields bare nowadays if they are not growing a crop on them.

            Since 1989, Saskatchewan scientists have been mapping the soil found in this province.  This information can be used to predict areas that could be damaged by wind erosion, and measures can be taken to reduce erosion.

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