Soil Beneath Us

Objectives

Students learn that soil is a complex mixture of rock, organic material, and water, along with air spaces. Through soil testing and map reading, they learn that soil composition varies from site to site depending on the underlying rock type, overlying vegetation, time, topography, climate, and chemicals carried by water percolating through the soil. Lastly, students understand that soils in a watershed affect the chemistry and quantity of water as it percolates through them.

Lesson Overview

Students compare soil samples from the small watersheds and from other locations throughout their school district.

Materials
  • rowels
  • plastic bags for soil samples -- heavy-weight freezer bags are ideal
  • Soil maps (search for local county soil maps online or download soil maps for Dutchess County, NY below)
  • Soil Map Legend (download below)
  • Reading: What's the Big Deal About Dirt? (download below)

Optional:

  • soil thermometer
  • soil pH test kit
  • soil texture test kit
  • other materials as listed below
Procedure

To demonstrate how soils vary throughout the students' school district, collect soil samples from each of the small watersheds. Also invite students to collect soil samples from their home or from other locations. Perhaps they could request that distant family members or friends send them soil samples from other states or countries.

For local soil samples, find the sampling location on the SWEAP soil map and determine how the Dutchess County Soil Survey classified the soils. Are they Charlton loam soils? Dutchess-Cardigan complex soils? Nassau-Cardigan complex soils that come from undulating or hilly terrain? Students may not fully understand the soil classification scheme but they will appreciate the scheme's complexity.

In the field and in the classroom, students can conduct many different tests to compare their soil samples. A few suggestions are listed below.

Soil temperature. Do this test during your field trip to the study watersheds, and invite students to measure the temperature when they collect their other soil samples. Soil temperature varies with depth, season, soil type, time of day, light intensity and moisture content. The temperature range in soil affects the distribution of organisms within it. As a result, temperature readings can help you analyze why a population of organisms is found in a particular soil. Use the following procedure:

  • Read the air temperature at the site and record it.
  • Read the soil temperature at the soil surface and at depths of 5 cm, 10 cm, 15 cm, and 20 cm. Record the results.
  • Be sure to record other important variables: time of day, shady or open area, north or south slope of a hill, and general soil moisture conditions.

Soil pH. LaMotte Company manufactures a simple soil pH test kit. This test will be particularly interesting if on or more of the soil sampling sites overlies limestone, which makes soil alkaline and has a strong buffering capacity.

Soil buffering capacity. Students interested in soils' ability to buffer acid rain may wish to conduct a simple test.

  • Collect soil samples, preferably from several sites overlying different parent material (e.g., limestone and granite) or characterized by different land cover types (e.g., an agricultural field and a forest). Alternatively, students may wish to compare limed garden soil and humic soils.
  • Create "Acid Rainwater" by adding 100 mL vinegar to 900 mL water. Use standard white vinegar (5% acetic acid), available in most grocery stores. The "Acid Rainwater" solution should have a pH of approximately 4.
  • Measure and record the initial pH of the "Acid Rainwater."
  • Line a funnel with filter paper and place 15 g of soil in the funnel.
  • Pour 100 mL of "Acid Rainwater" into the funnel, collecting it in a beaker or jar as it drains through the soil sample.
  • Measure and record the final pH of the "Acid Rainwater" after it has percolated through the soil. Is the final pH higher than the initial pH? pH should rise after the water passes through soil with a strong buffering capacity.

Soil texture. Soil texture affects how water moves through it and is therefore an important factor determining water quantity and quality in streams and other surface waterbodies. A test kit that separates soil samples into sand, silt and clay fractions is available from LaMotte Company. Soil texture can also be examined by drying soil samples and then passing them through a size-ranked set of soil sieves. Alternatively, students can separate soils according to particle size using the simple and inexpensive procedure that follows:

  • Fill a large mayonnaise jar approximately two-thirds full of water.
  • Pour in soil until the jar is approximately half full of soil.
  • Agitate the jar vigorously for at least 30 seconds. Let it stand until the soil settles.
  • Describe and account for the appearance of the settled soil and the water above the soil. The settled soil should look a bit like a layer cake. Large heavy stones fall to the bottom first, followed by increasingly light soil components (sand, then silt and organics). Clay will stay in the water above the settled soil, making it murky.

Soil moisture content. The amount of moisture found in soil varies greatly with the type of soil, the amount of humus (organic matter) in it, and the climate. The following method for measuring the moisture content of soil involves comparing the weight of a soil sample before and after it has been dried in an oven. From this information, the percent of moisture can be calculated.

  • Weigh a beaker or other heat-proof container and record its weight ("A").
  • Add the soil to the beaker and reweigh it. Record the weight ("B").
  • Heat this in an oven set at 100EC for approximately 24 hours or until the weight is constant.
  • Reweigh the beaker filled with the dried soil. Record the weight ("C").
  • CALCULATIONS:
    weight of soil before drying = B - A
    weight of dried soil = C - A
    weight of water in soil sample = weight of soil before drying - weight of dried soil
    % moisture in soil sample = 100 x (weight of water in soil sample/weight of dried soil)

Determination of soil organic content by ignition. The organic content of soil greatly influences the plant, animal and microorganism populations in that soil. Decomposing organic material provides many necessary nutrients to soil inhabitants. Organic matter is made of carbon compounds which, when heated to the correct temperature, are converted into carbon dioxide and water. In the ignition process, a soil sample is heated to a high temperature. This results in a change in weight because carbon dioxide and water disappear from the sample as gases. This allows you to calculate the organic content of the sample.

  • Oven-dry the soil sample at 100EC for approximately 24 hours or until the weight is constant.
  • Weigh a crucible and its lid. Record the weight ("A").
  • Place the soil sample in the crucible and cover it with the lid. Reweigh them and record the weight ("B").
  • Using a clay triangle and ring stand, set the crucible with its soil over a Bunsen Burner.
  • Place the lid partially over the crucible so that the fumes can escape but not the soil particles. Heat the crucible until no more visible fumes are given off.
  • Cool the crucible. Weigh the crucible, lid and contents. Record the weight ("C").
  • CALCULATIONS:
    weight of soil before heating = B - A
    weight of soil after heating = C - A
    weight of organic matter in soil sample = weight of soil before heating - weight of oil after heating% organic matter = 100 x (weight of organic matter in soil sample / weight of soil before heating)

Soil macronutrients. Students can determine concentrations of nitrate, phosphorous, and potassium using a LaMotte Company test module. This module is fairly expensive.

Soil micronutrients. LaMotte Company manufactures a test kit to detect ammonia, nitrite, calcium, chloride, magnesium, manganese, iron, aluminum and sulfate. Again, these tests are fairly expensive.

Hidden Critters. A huge diversity and abundance of animals live in the soil and in the soil litter, or the dead organic matter on top of the soil. Many animals can be forced out of a sample by a combination of light and drying. Scientists use a Berlese funnel to collect soil organisms but a similar apparatus can be constructed inexpensively with a 2-liter soda bottle.

Watch this how-to video on creating a Berlese funnel:

 

  • Cut a 2-liter soda bottle in half to make two sections, a top and a bottom.
  • Invert the top and tape a mesh bag to the rim so that it hangs suspended about two inches above the mouth of the bottle.
  • Attach a small plastic bag to the outside of the mouth of the bottle.
  • Rest the inverted top inside the bottom. Fill the mesh bag with soil and soil litter, and place it under an incandescent light or in sunlight for one or two days.
  • When critters fall into the plastic bag, transfer them to a pan, small dish or petri dish.
  • Look at the critters with a hand lens or microscope, count them, and try to identify them.
Lesson Resources

Cary Institute of Ecosystem Studies | Millbrook, New York 12545 | Tel (845) 677-5343

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