1. (Or the world under our feet)
High School, Updated 6/2017
Soil Basics
(Or the world under our feet)
Interstate Commission on the Potomac River Basin – Score Four: Students, Schools, Streams, and The Bay
This PowerPoint should be used in conjunction with ICPRB’s Score Four Soils Lessons.
This version includes information on soil organisms and the food web and is recommended for high grade levels. This presentation can be used to introduce the soil investigation, or to tie up what the students have discovered. The slides are meant for discussion by students.
Score Four: Students, Schools, Streams, and the Bay
Rebecca Wolf and Nguyen Le
Interstate Commission on the Potomac River Basin
Rebecca Wolf and Nguyen Le
Interstate Commission on the Potomac River Basin

2. Soil: The Foundation For a Plant’s Success
Soil provides plants with:
Nutrients
Minerals
Water
Oxygen

3. It’s Not Just Dirt Soil consists of: Mineral particles –
sand, silt, or clay.
Pore Spaces between mineral particles.
Organic matter – decomposing plants, animal matter and droppings.
Small organisms –
worms and insects and microorganisms, such as bacteria and fungi.
Examples of fungi familiar to students include mushrooms and toadstools. The part we see is considered the fruiting body of an extensive system of underground roots.

4. Soil: It’s a Mix The three minerals that comprise soil are: Sand Silt
Clay.

5. More On Minerals Clay particles are flat like paper.
These minerals are classified by size.
You can see…
Sand with your eye or magnifying glass.
Silt with microscopes.
Clay with electron microscopes.
Clay particles are flat like paper.
If sand particles were the size of a basketball, clay particles would be the size of a period at the end of a sentence.
A million grains of clay will fit in the space occupied by one average grain of sand.
Electron microscope image of clay (source: Well-sorted clay particles are sometimes compared to a deck of cards or stack of newspapers to help people understand their structure.

6. How soil feels tells us what it is!
The mix of minerals in a soil define how it feels. This is called its soil texture.
The particles feel differently, because of their different sizes and structures.
We can tell the general composition of soil from its texture.
How would you expect sand to feel in comparison to clay?
Note that grains of sand are not all the same size. Think about the different types of sand on beaches. Sandy Point State Park feels coarse. The sand at Ocean City feels finer or softer. Why?
Teachers, this is a good place to do the soil texture exercise, if you want to break up this presentation.

7. Pore Space – where lots of action takes place.
The spaces between individual soil particles are called pore spaces.
Pore spaces house water, oxygen, and microorganisms.
Plant roots grow into and make pore spaces.
Pore spaces also exist between lumps of soil, called aggregates, and in bedrock. This presentation focuses on particles to introduce the concept.

8. Pore Space
Sand Silt
Different types of minerals have different sized pore spaces.
Which type of mineral has the largest pore spaces?
How about the smallest?
Sand has the largest pore spaces, clay has minute pore spaces.
Picture piles of basketballs versus piles of cards or newspapers.
Clay

9. Porosity and Permeability – related, but different
Porosity – Soil scientists define porosity as the volume of pores for a given amount of soil.
Which mineral is most porous (has the greatest volume of pores for the same amount)?
But which is most permeable? (Which will rain pass through the fastest?
TEACHERS: Porosity and permeability are the key physical properties that determine the ability of a body sediment to store and transmit water, but the understanding of porosity and permeability is more complicated that it first seems, particularly since soil scientists and geologists define the term porosity differently than general on-line dictionaries.
To more fully delve into this concept with high school students, see Porosity and Permeability, by Anthony H. Fleming, Indiana Geological Survey: . Below are key concepts from this article.
“Porosity is defined as the volume of open spaces, or pores, between the solid mineral components that make up a rock or unconsolidated sediment, expressed as a percentage. … All unconsolidated sediments possess a significant volume of porosity, which ranges from 25 to 35 percent in well-sorted sand and gravel to as much as 60 percent in some silts and clays. Porosity is inversely proportional to grain size, with sediment composed of finer grains, such as silt and clay, having a substantially greater volume of open spaces than those composed of coarse grains, such as sand and gravel. This relationship may at first seem counterintuitive in light of the fact that sand and gravel transmits water far more readily than silt and clay.
But total porosity itself is not the determining factor in the ability to transmit water; instead, several other characteristics of the sedimentary particles and the pore spaces between them control the permeability of a rock or sediment.
Foremost among these characteristics are the sizes and shapes of the individual pores…the size of the individual pores in a body of well-sorted sediment is directly proportional to grain size, thus the individual open spaces in a sand …are orders of magnitude larger than …silt and clay. …As a result, the pore spaces between grains of sand or gravel are similar in shape and size to the surrounding particles, and tend to be quite large relative to the size of water molecules.
In contrast, individual clay grains are microscopic in size and consist of tiny, electrically charged plates…. As a result, the pore spaces between the clay minerals are also microscopic, generally only a few times wider than a molecule of water. The small pore dimensions, coupled with the electrical charge on the surfaces of the clay minerals, greatly retard the passage of water and instead tend to attract and cling tightly to water molecules, resulting in exceedingly slow permeabilities.”
For a fast-moving, but illustrative video on this concept, see Water Movement in Soil, USDA Natural Resources Conservation Service.

10. Healthy Soil Supports Many Diverse Lifeforms
Micro-organisms:
Bacteria
Nematodes
Fungus roots
Organisms visible to the eye:
Lower grades can skip slides They are provided to shed light on the ecological aspects of the soil community.
Additional Information:
Things we can’t see
Bacteria, Algae, Fungus, Protozoa, Nematodes, and Arthropods – organisms with jointed legs, including insects, crustaceans, such as sow bugs (pill bugs), spiders.
10-25,000 arthropods in a square foot of forest soil. A billion bacteria in a teaspoon of healthy soil.
Things we can see:
Worms, insects, animals
A variety (diversity) of organisms in soil is necessary for healthy soil, as they are part of the food web and ecosystem. Competition among these species keeps harmful organisms under control.
The more species and more organisms, the better the soil is for plants AND clean water.
Tiny to large insects & spiders
Earth worms
Tiny springtails

11. These Organisms Make a Food Web
Where small organisms are eaten by larger ones.
It starts with plants, which convert the sun’s energy to a food source.
Plant roots produce food (carbohydrates and proteins) for bacteria and fungi.
Larger organisms eat the smaller ones.
This slide can be simplified. It is a good illustration of the movement of energy and matter within the soil environment, as well as interrelationships among the organisms.
Soil organisms help cleanse our water. They decompose organic compounds, including manure, plant residue, and pesticides, preventing them from entering water and becoming pollutants. They sequester nitrogen and other nutrients that might otherwise enter groundwater, and they fix nitrogen from the atmosphere, making it available to plants. Many organisms enhance soil aggregation and porosity, thus increasing infiltration and reducing runoff. Soil organisms prey on crop pests and are food for above-ground animals.
Credit: S. Rose and E.T. Elliott. Please contact the Soil and Water Conservation Society at for assistance with copyrighted (credited) images. Give USDA credit
The soil food web is the community of organisms living all or part of their lives in the soil. It describes a complex living system in the soil and how it interacts with the environment, plants, and animals. Food webs describe the transfer of energy between species in an ecosystem. Much of this transferred energy comes from the sun. Plants use the sun’s energy to convert inorganic compounds into energy-rich, organic compounds…. Plant roots exude acids, sugars, and proteins into the area around their roots (rhizosphere), providing food for microscopic bacteria and fungi, which, in turn, provide nutrients and carbon to larger microscopic, wormlike nematodes. These deposit waste – providing nitrogen to plants. From Wikipedia and NCRS/USDA.
The competition among organisms in a healthy food web keep plants predators in check. (Teaming with Microbes, The Organic Gardener’s Guide to the Soil Food Web, Jeff Lowenfels & Wayne Lewis)

12. Where do Soil Organisms Live?
They live where there is space, food, and water.
In the top 4 inches of soil.
Small and large organisms live where there is organic matter to digest.
Micro-organisms live around plants roots.
Roots release proteins and sugars that bacteria eat.
Larger nematodes prey on the bacteria.
Larger organisms, such as worms, make spaces between clumps of soil.
The organisms of the food web are not uniformly distributed through the soil. Each species exists where it finds space, nutrients, and moisture. They occur wherever organic matter occurs - mostly in the top four inches of soil.
Organisms live in the microscale environments within and between soil particles. Differences over short distances in pH, moisture, pore size, and the types of food available create a broad range of habitats. 
The area around roots, the rhizosphere, teems with microorganisms due to the availability of food (dead cells of plant roots and the proteins and sugars exuded by roots).
Credit: Soil Food Web By Elaine R. Ingham,NCRS, USDA.
The soil food web is the community of organisms living all or part of their lives in the soil. It describes a complex living system in the soil and how it interacts with the environment, plants, and animals. Food webs describe the transfer of energy between species in an ecosystem. Much of this transferred energy comes from the sun. Plants use the sun’s energy to convert inorganic compounds into energy-rich, organic compounds, turning carbon dioxide and minerals into plant material by photosynthesis. Plant roots exude acids, sugars, and ectoenzymes into the area around their roots (rhizosphere), providing food for microscopic bacteria and fungi, which, in turn, provide nutrients and carbon to larger microscopic, wormlike nematodes. These deposit waste – providing nitrogen to plants. They also are eaten by

13. 3 Ways Healthy Soil Helps Cleanse our Water
Soil microorganisms decompose (break down and use) potential pollutants, such as manure and pesticides.
Worms, beetles, and fungus create pore in soil -- enabling more rainwater to filter into the ground and reducing runoff.
Bacteria secrete sticky substances that help soil to clump. Clumps of soil do not erode as much as loose soil, so less soil ends up in our streams.
Healthy soil is necessary for good plant growth. It also helps reduce water pollution. These are just 3 ways.

14. Review What different factors cause pore spaces in soil?
Which would retain water the best – a sandy soil or a clayey soil?
What types of organisms do you think you might find in your school soil?

15. Application Question To Think About
Rain gardens are meant to catch large amounts of storm water and then let it slowly seep into the soil over one to two days.
Which mix of soils would work best in a rain garden? And why?

16. Investigations and Activities
Collect soil samples from school ground:
Soil Components Texture Investigation
Soils Percolation Investigation.
Send to lab to have analyzed and/or use soil chemistry kits to learn pH and other chemical. characteristics.

17. Resources for Teachers
Flow diagram for Texture by Feel. Commonly used in the field. Provided by the USDA Natural Conservation Resources Service. (Click here for a high-resolution version of the graphic.)
Soil Science Society of America provides an excellent bank of soils lessons for multiple grades covering texture, biology, chemistry, forensics, and more.
Basic Hydrologic Science Course Runoff Processes Section Four: Soil Properties. In depth explanations with public domain graphics.
Soil Biology Primer, Natural Resources Conservation Service, USDA.

18. Definitions for teachers
Source (unless otherwise noted): Soil Health and Glossary, National Resources Conservation Service
Actinomycetes: A large group of bacteria that grow in long filaments that are too small to see without magnification. Actinomycetes generate the smell of “healthy soil,” and are important in decomposing cellulose, chitin, and other hard-to-decompose compounds, especially at higher pH levels. Many produce antibiotics.
Arthropods: Invertebrate animals with jointed legs. They include insects, crustaceans, sowbugs, springtails, arachnids (spiders), and others.
Bacteria: Microscopic, single-celled organisms. They include the photosynthetic cyanobacteria (formerly called blue-green algae), and actinomycetes (filamentous bacteria that give healthy soil its characteristic smell).
Fungi: Multi-celled, non-photosynthetic organisms that are neither plants nor animals. Fungal cells form long chains called hyphae and may form fruiting bodies such as mold or mushrooms to disperse spores. Some fungi such as yeast are single-celled.
Mineral: A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes" (Source: Nickel, E. H., 1995). "Minerals are naturally-occurring inorganic substances with a definite and predictable chemical composition and physical properties." (Source: O' Donoghue,1990).
Mycorrhizal associations: A symbiotic association of certain fungi with roots. The fungi receive energy and nutrients from the plant. The plant receives improved access to water and some nutrients. Except for brassicas (mustard, broccoli, canola) and chenopods (beets, lamb’s-quarters, chard, spinach), most plants form mycorrhizal associations.
Organic matter: any material that is part of or originated from living organisms. Includes soil organic matter, plant residue, mulch, compost, and other materials
Permeability: the qualitative estimate of the ease with which fluids, gases, or plant roots pass through soil.
Porosity: the volume of pores in a soil sample divided by the bulk volume of the sample. 
Silt: a granular material of a size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water (also known as a suspended load) in a body of water such as a river. (source: Wikipedia)