Guided Inquiry Lesson Plan #11: Soil Erosion
Length of Lesson:
One class period
Scientific Processes Addressed:
Measurement, Observation, Hypothesis, and Calculation.
Science Concepts Addressed:
the concept of water erosion on soil surfaces and the beneficial effects of
conservation practices.
National Standards Addressed:
A,B,C,D,F,G
Objectives:
Students will learn:
- That crop residue left on the soil surface slows erosion compared to erosion
on bare soil.
- How to use conservation terraces to slow the speed of water running off
- the soil’s surface.
- How farming on a contour can slow water runoff speed because each row works
as a miniature terrace
- How combinations of these conservation practices can significantly reduce
the amount of topsoil lost each year.
Materials:
1. One 5 quart pail of topsoil and one of sand for each group. Preferably
the topsoil is a loam, silt loam, clay loam, or silty clay loam texture. In
other words, a soil with a "nice" feel that one might want in ones garden
as compared to something that is "heavy" with clay and hard to work or something
that is very sandy.
2. Shallow containers. These are boxes, food service trays, pop flats, etc.
to put the soil or sand into for the exercise. Generally the containers are
about 1 foot wide by 1.5 feet long to several inches thick. The containers
may be larger if desired. One may construct wooden boxes that can be reused.
- A bag or box of grass clippings. This material is put on the soil surface
to demonstrate the effect of crop residue.
- A source of "rain", such as a sprinkler can or a hose with a fine mist setting.
Water needs to be applied to the finished "soil surface" in a manner that
is akin to a light-to-moderate rain and is that not so heavy or so forceful
as to immediately wash the soil away.
- A place such as outdoors to do the exercise. Since this exercise calls for
the mixture of students, soil, and water; it is messy.
- Grass seed or sod may be needed if a growing crop demonstration is to be
used.
Introductory Activity:
Have the students hypothesize about the
damaging effects they think a farmer would have if his fields were to endure
massive water erosion, (i.e. soil erosion?) What if he had just planted his
crops? How do you think that the farmer could prevent this from happening?
Activities:
Have the students break into their groups.
The exercise itself can begin in a number of ways. The instructor may want to
prepare several examples of soil surfaces and then demonstrate how each reacts
to rain or the instructor may want to briefly explain the various conservation
practices and allow the students to make their own displays. Brief descriptions
of several examples may be used. After each is set up, one end is propped up
to simulate a hillside.
- Bare soil-no conservation: Take a container and place a sufficient amount
of soil on it to cover the surface to a depth of 1 to several inches (depending
on the depth of the container), smooth the surface so it looks like a bare
field and lightly pack the soil. When water is applied to this example, it
should show the most runoff and the greatest amount of erosion. Note the more
the soil is packed the easier it is for the water to run off, conversely,
the looser the soil surface is the more water enters it and the more "rain"
necessary to begin erosion.
- Bare sand-no conservation: Take a container and set this example up like
#1. When water is applied it rapidly soaks or infiltrates into the sand and
very little runs off or causes erosion. This example is used mostly to show
how rapidly water infiltrates into the coarse (sand) soils as compared to
the loam, soils, etc. It might be explained that while the water infiltrates
rapidly, the sand can actually hold very little. It also shows why water erosion
is not considered as much a problem on very sandy soils as on loamy, silty
and clay soils. It also demonstrates what happens when an impervious layer
(the bottom of the tray) stops water movement and the soil becomes saturated
(stability is lost and the sand slips as in a landslide).
- Bare soil-terraces: Set up the example as in #1. Make sure there is enough
soil so one can make a series of terraces, which should be parallel to the
slope. When water is applied the series of terraces should slow or stop the
water flow. This slows erosion and allows water infiltration particularly
in the terrace "channel". As additional water is applied the terraces will
overflow and erosion will be intensified.
- Bare soil-contour farming: Set up the example as in #1. Using your fingers
or some utensil make a series of ridges parallel to the slope such as would
be formed from row cropping. When water is applied the rows act like small
terraces, but overflow much sooner. Mostly likely when the water overflows,
small erosion rills will be formed going down the slope and may do additional
cutting.
- Soil+crop residue-no other conservation practices: Set up example as in
#1. Sprinkle grass clippings onto the soil surface. One may set up an additional
number of examples to demonstrate what varying amounts of crop residue can
do to slow erosion. For this example, sprinkle enough grass clippings to completely
cover the soil surface. When water is applied the crop residue breaks the
force of the water droplets. Thus the droplets don't break apart the soil
particles which is a condition enabling the soil to erode. Also the crop residue
slows the water flow into the soil, allowing more to infiltrate.
- Soil+crop residue-terraces: Set up as in example #3. Sprinkle grass clippings
onto the soil surface as in example #5. When water is applied one should observe
less erosion than in either #3 or #5.
- Soil+crop residue-contour farming: Set up as in example #4. Sprinkle grass
clippings onto soil surface. Apply water as in the other examples. There should
be less erosion than in #4.
- Soil with a crop: Planning ahead is necessary for this one as a crop of
grass must be planted and growing to show how growing crops reduce erosion
potential. A plot of sod could be substituted.
- Rows running up and down the hill: Any of the above demonstrations can be
done, but with the rows or terraces running up and down the hill. This should
show increased erosion potential. (See the original)
Modifications:
- Varying the slope of the trays will also have a effect on the erosion potential.
- Intensity of the rainfall can also effect the potential of the soil for erosion.
- If the water is shut off before the soil is washed away, students can examine
the cross section of the demonstration. Packing by raindrops may be visible
on the bare soil versus less packing when protected by residue.
Expected Conclusions:
Student will find various results depending upon the mixture of soil, conservation
technique, and crop residue used for each sample.
Assessment:
Will be based on student interaction in cooperative groups, and on their ability
to meet the four objectives for the lesson. Students should make their own displays
and be able to give brief descriptions of the examples that they used to prove
that the soil conservation techniques they selected were in fact successful.
Extensions:
Instructor could arrange for the class to go on a field trip to a local farm
to actually see the effects that they have been studying on a real farm and
ask questions of the farmer.
Language Arts - Ask the students to hypothesize on what effects of beach erosion
has on shore communities each year. What do they do to try to minimize these
effects? For instance, plant dune grass.
Social Studies - Instructor could arrange for the class to go on a field trip
to a local farm to actual see the effects that they have been studying on a
real farm and ask questions of the farmer.
Computer Skills - Have the students find a picture of a Mangrove on the Internet.
Tip: look in Florida.