Lesson 18. Soil and Compost
Class Discussion: What is soil? How is soil created? Why is it important?
Soil is important because it produces our food and fiber, it supports the habitats which are
important to biodiversity, and it contributes to water quality and quantity.
Soil Composition: soil is made of 4 things:
1.
2.
3.
4.
Organic matter, or plants and animals that have been decomposed
Water
Air
Small rock fragments created through weathering.
Weathering is the process of breaking down rocks and can be physical or chemical. The
composition of the soil affects the plants and therefore the animals that can live there.
Layers. Different areas have different soil profiles, or layers. (diagram)
1. Topsoil, is made of organic litter such as fallen leaves and twigs. The topsoil helps
prevent erosion, holds moisture, and decays to form a very rich soil known as humus.
Humus is the main contributor to fertility and nutrients in soil.
2. Small rocks that do not contain organic litter. The small rock layer may have nutrients
from the top layer that have trickled down with water, called leaching.
3. Weatherized big rocks.
4. Solid rock known as “parent material”
Class discussion: What are the different types of soil?
Sand soil has the largest particles. It’s dry and gritty to the touch, and because the particles have
huge spaces between them, it can’t hold on to water. Plants don’t have much chance of using the
nutrients in sand because they are carried away by water runoff. When you roll the slightly wet
sandy soil in your palms, no ball forms and it crumbles through your fingers easily.
Silt soil has much smaller particles than sandy soil, so it’s smooth to the touch. When moistened,
it’s soapy slick. When you roll it between your fingers, dirt is left on your skin. Silty soil retains
water and drains poorly. It can easily compact, squishing all the air out.
Clay soil has the smallest particles among the three. It’s sticky to the touch when wet, but
smooth when dry. Due to the tiny size of its particles and its tendency to settle together, little air
passes through its spaces. Because it’s slow to drain, it retains plant nutrients. During the
summer months, it can turn hard and crack. If moistened, clay soil feels sticky and rolls up
easily to form a ball.
Loam contains a balance of all three soil materials—silt, sand and clay—plus humus. It has
higher pH and calcium levels because of its previous organic matter content. Loam is dark in
color and is mealy—soft, dry and crumbly—in your hands. It has a tight hold on water and plant
Lesson 18. Soil and Compost
food but it drains well, and air moves freely between soil particles down to the roots. The feel
test for loam yields a smooth, partly gritty, partly sticky ball that crumbles easily.
Class Discussion: What is compost? Who are the decomposers?
Compost is decayed organic matter, often made by people for their gardens out of food scraps.
Humus is naturally occurring compost. Compost is broken down by decomposers.
Decomposers are nature’s recyclers because they keep nutrients moving in the food web.
Nematodes are simple roundworms. Colorless, unsegmented, and lacking appendages,
nematodes may be free-living, predaceous, or parasitic.
Arthropods are invertebrates with exoskeletons, a segmented body, and jointed legs. Examples?
Bacteria are single celled microorganisms.
Protozoa are single celled and include amoebae, flagellates, and ciliates.
Fungi include mushrooms, yeasts, and molds, all which reproduce by spores.
Questions:
Why do different areas have different soil profiles?
Which soil do you think is best for growing plants?
What role do decomposers play in the food web?
What are some things that compost? What things cannot compost?
Name some Arthropods that you might find decomposing your compost!
Worksheets: Take notes on soil composition. Take notes on soil layers. Put soil profile image
on overhead and discuss why each one is different. Display the soil food web on the overhead or
draw it on the board. Hand out Soil Matching worksheet and have students match the item on
display with the item name and draw a line to the definition.
o
o
o
o
o
o
o
dish soap
4 jars of soils: Sand, silt, clay, loam
1 jar of invertebrates
1 jar of arthropods
1 jar of fungus
1 jar organic littler
1 jar humus
Lesson 18. Soil and Compost
Activity 1: Matching
Put jars on display with numbers. Have students match items on display to the name and
definition on the worksheet.
Activity 2: Test the soil! What kind do you have? Predict by feeling it with your hands.
Fill a jar about 1/3 full with the soil to be tested.
Fill the jar with water and a little detergent then cap it.
Shake the jar vigorously and set it aside for several hours or overnight.
The results:
SAND - If the water is clear and the soil has settled to the bottom; you have predominantly sand.
LOAM - If the water is still a little murky with bits of matter suspended in it; you have loam soil.
CLAY - If the water is still murky and there is a visible ring of sediment around the jar; then
your soil is mostly clay.
When the soil settles, sand stays at the bottom, clay at the top and silt in between. Their
percentages will be your clue on your soil type.
Vocab:
Organic Matter is any carbon-based substance originating from living things.
Physical weathering breaks down rocks into smaller pieces made of the same stuff.
Chemical weathering breaks down rock and can change what it is made of.
Leaching –dissolved nutrients and minerals are carried by water into the deeper soil layers.
Topsoil is the uppermost layer of soil, with a high concentration of organic litter.
Humus is decayed organic material that is rich in nutrients
Soil profile is a cross section showing the compositions of soil in different areas.