Grade Level:
Elementary, Middle School, High School
Ecological Concepts: Abiotic
factors
Arizona Science Standards: Science as Inquiry; Life
Science
Materials:
1) Soil test kits*
2) Soil sieves*
3) Magnifying lenses/loupes*
4) Plastic cups
5) Writing/drawing materials
6) Trowels*
7) Soil thermometers*
*May be borrowed from SCENE.
BACKGROUND
Soils are composed of organic
matter (stuff that used to be alive, like plants and animals) and
small inorganic
matter. There are three basic soil types: sand, silt, and clay.
Sand is comprised of tiny rock fragments and is the roughest in
texture. Clay becomes sticky or greasy when wet, and very hard when
dry. Silt is between sand and clay in texture. Loam is a combination
of sand, silt, and clay, and has a high proportion of organic matter.
It is the best soil for most plants. Almost all soils found in nature
contain some proportion of two or more of the three soil types along
with some organic matter. Therefore, these soils are loam, but vary
in the proportions of clay, silt, sand, and organic matter.
Plants need nutrients to grow and reproduce. The necessary nutrients
are provided by the decaying organic material in the soil. Levels
of nutrients such as carbon (C), phosphorus (P), potassium (K),
and nitrogen (N) in soil are easily tested. Plants also use many
other nutrients, but the ones listed here are the most prevalent.
Plant growth is also affected by other abiotic
factors such as air and soil temperature, air moisture (humidity)
and soil moisture levels.
GUIDED INQUIRY
Initial Observation/Exploration Period: Soil is
an essential abiotic factor in the environment, but it is often
ignored. Initial explorations could involve using hands to feel
the soil types found in the habitat, using soil sieves
to separate the different soil types, and looking at soil through
magnifying lenses.Wetting small amounts of soil and feeling it is
a good way to determine the texture and get an idea of the proportions
of the soil types present. Or, you can fill a jar with water and
place a handful or shovelful of soil in the water. Seal the jar
and shake the contents. Leave it to settle out for a few hours or
overnight. The heaviest soil particles (sand) will fall to the bottom,
the next heaviest in the middle (silt), and the lightest will be
at the top (clay). The thickness of each layer reflects the proportion
of each soil type in your sample. If no layers appear, your soil
is probably all clay.
Group Discussion and Question Period: Do some soil
types have more of some nutrients than others? Do soil types vary
in their ability to hold water? Do soil types vary in their ability
to heat up and cool down?
Important aspects of guided inquiry are encouraging students
to generate multiple
hypotheses, and letting students make decisions about
what data are important and create their own data sheets. Keeping
these ideas in mind, the sample in the box below illustrates how
ONE OF MANY possible investigations around this topic might develop.
Sample
Hypothesis: Let’s use the question, “Does
the texture of the soil affect soil nutrient content?”
Our hypothesis could be, “Sandy soil will contain fewer
nutrients than soil with more clay and/or silt content.”
Or, “As the texture of a soil becomes coarser, the fewer
nutrients it will contain.”
Sample Experiment Design: Dig up soil from
at least three different areas in your schoolyard. Include your
habitat area and other parts of the school grounds with less
variety of vegetation. If your school has a sand play area,
you may want a sample from that, as well. Include a sample of
compost if you have a compost pile. Use a soil
test kit to test for the levels of nitrogen (N),
phosphorus (P), and potassium (K) in each sample. These nutrients
are the dependent variables
and the texture of the soil at each different location is the
independent variable.
Each soil sample is an experimental
unit. Test at least three samples from each chosen
location to replicate
the experimental units. After testing, sieve a sample from each
location to determine the proportions of clay, sand and silt
in that location, and record this information, along with the
nutrient levels found in each sample.
Another common soil test is for pH, the acidity or alkalinity
of the soil. Sonoran Desert soils tend to be alkaline (pH above
a neutral 7) because the soil contains lots of calcium carbonate
(limestone), an alkaline mineral. If you wanted to extend this
experiment, you could look for a correlation between nutrient
level and pH.
Sample Prediction: Soil with the highest sand
content will have the lowest nutrient levels.
Record Results: Record the nutrient levels
in each soil sample, as well as the location from which that
sample was taken and the approximate percentage of clay, sand,
and silt in the soil in each location.
Sample Analysis of Data and Presentation: Make
a bar
graph with soil texture on the horizontal axis
and nutrient content on the vertical axis. Average
the nutrient levels from the two or more samples
taken from each location, and graph the average on the vertical
axis.
Discussion: Was your hypothesis supported?
If yes, go on to test other hypotheses. If not, why not? What
did happen? Why? This is a great opportunity to revise your
hypothesis and do another test. |
MORE:
All Levels
(a) Put the same amount of each
soil type (clay, sand, silt) in pots or cups of the same size (if
using pots, they must have solid bottoms) Use at least two replicates
of each soil type. Water all pots with the same amount of water;
enough to thoroughly soak the soil. After five to ten minutes, pour
off any water, measuring it in a liquid measuring cup or beaker
to see how much water was not absorbed. Which type of soil absorbed
the most water? Which absorbed the least?
(b) How long do different soil types retain water?
Put different soil types (clay, sand, silt) in pots of the same
size. Use the same amount of soil in each pot. Have at least two
replicates of each type of soil. Pour the same amount of water in
each pot, enough to thoroughly wet the soil, but not have standing
water on the surface. You can use a soil moisture tester (available
at plant nurseries) to test soil moisture. Or, you can use a hard,
solid tube like a narrow pipe, to take soil cores. Push the pipe
into the soil vertically, pull it out and tap out the soil in the
pipe. This is a subjective measure, but the students can each look
and come to a consensus as to how moist the sample is. Take samples
twice a day, in the morning and afternoon, until all of the samples
are completely dry. Record how long it took for the soil in each
pot to dry out. Graph the data on a
bar
graph, with soil type on the
horizontal axis and number of days or hours it retained moisture
on the vertical axis.
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