NITROGEN RIDES A CYCLE
Grades 9 - 12

Soil is not dirt, but a complex and valuable living substance. In this lesson, students will investigate the biological, physical and chemical properties of soil. They will interact with a video to explore the interactions of soil organisms to provide plants with the essential element nitrogen.

Planet Under Pressure, #9: The Oxygen Partnership

Students will be able to:

• describe atmospheric nitrogen as a diatomic molecule
• name fungi and bacteria as types of organisms involved in providing nitrogen to plant roots
• describe the roles the different organisms play
• draw a diagram of soil, including humus, fungi, bacteria and minerals
• describe the cycle of nitrogen starting from the atmosphere
• graph results of experiments
• correlate amount of life in soil with humus level, ph, or nitrogen

• ph meter or paper
• metric or standard tape measure
• 1 liter plastic containers
• wooden stakes
• small shovel
• PVC 3 schedule 40 plastic pipe
• plastic funnels
• incandescent light
• large beakers
• plastic wrap
• soil testing kit (e.g., Lamotte s)
• ph paper
• microscope
• depression slides
• cover slips
• soil samples
• graph paper

Within one week prior to the viewing, the class should perform a simple transect of soil in an appropriate region near to the school. To perform the transect, measure out a straight line on a hillside or an area that has some sort of variation from spot to spot. Choose a regular interval between samples. This distance between samples is somewhat arbitrary, but will depend on the terrain and the extent to which the sampling is desired. Once a sample distance is chosen, an identical size sample of soil is taken at the regular intervals. A similar size and depth sample can be taken by simply digging with a shovel. A more consistent sample can be obtained using a 3- inch diameter by 6-inch long (Figure 1) schedule 40 PVC plastic pipe (cost ~$1.00 or free if your local hardware store has scrap pieces lying about), which is plunged into the ground to remove a neat cylinder of soil. Records of the location of each soil sample are kept and the samples brought to the class for analysis.

Macroscopic organisms are collected by placing a funnel filled with soil within 3 of an incandescent bulb overnight and collecting the organisms in a beaker sealed to the funnel with plastic wrap. The heat from the bulb drives the bugs down to cooler regions and out the bottom of the funnel into a sealed container. If this is done the day before the viewing of the video, the organisms will be ready for examination immediately after viewing.

To give students a specific responsibility while viewing, ask them to:

• Listen for any time nitrogen is mentioned, especially in relation to sources of nitrogen
• Look for the form in which nitrogen is depicted
• Listen for whenever life forms are acting together to allow the use of nitrogen
• Connect the manner in which micro-organisms act together to enable plant roots to take up nitrogen
• Observe the models for accuracy, i.e., are the numbers of atoms in a molecule accurate; are the shapes realistic in any way?

Start the video at the beginning where a child is playing in muddy soil. FAST FORWARD after the first 20 or so seconds to the spot where a greenhouse worker is bagging lettuce (~5 minutes into video). RESUME video at a point where there is a discussion on the erosion of rock into sand, silt and clay. Allow students to watch uninterrupted until the first animation begins when the narrator says, to unravel the secrets of soil.

PAUSE and remind the students of their focus for viewing. RESUME video.

PAUSE at about 45 seconds into the animation where the fungi are absorbing different minerals and ask the students to name some minerals that the animation might be trying to depict. Query the students as to why the video is using different shaped and sized molecules. RESUME video with the animation until about 1 minute and 25 seconds, where nitrogen is mentioned.

PAUSE: Ask the students why organisms require nitrogen. Why is nitrogen depicted as two blue balls? RESUME video.

PAUSE at 2 minutes and 15 seconds into the animation to ask why the nitrogens are jittering before they break apart? Discuss whether the model seems realistic and what it is trying to depict. RESUME video.

PAUSE at 3 minutes and 10 seconds to take note of the model of a humus particle. RESUME video.

PAUSE at 5 minutes to ask why sugar molecules are modeled as octagons. RESUME video.

STOP the video after animation ends and narrator says, this delicate balance between humus and living creatures. Total viewing time is about 8 minutes, not including discussion.

Short Discussion
Encourage the students to share key points of the video. Ask them to point out how the models in the animations coincided with their functions. Was the representation accurate and realistic, e.g.. at the 2 minute and 15 second mark into the animation, was the model of atmospheric nitrogen accurate? At the 5 minute mark, what shape were the sugar molecules? Is this reasonably accurate? Could they see any simple way to improve on the video?

Hands-on Activities
If you wish, you may have the students work in groups on the different activities and have them report to the rest of the class. Soil samples can be measured for pH with pH paper (the soil might need to be moistened with distilled water). This is not the most accurate method, but will give a reasonable comparison of the pH among the soil samples. Remember that whatever amount of water is added to the dry samples to measure pH, should also be added to all other samples. An alternative to using pH paper employs any of a variety of soil test kits. These are sold through typical scientific supply companies (see References). Total nitrogen is a reasonable choice of a single nutrient to measure since it is one of the most common nutrients which may be a limiting factor. Nutrients that are in such low supply are referred to as limiting factors. There are a number of ways to measure total nitrogen. The easiest, employ any of a variety of test kits. These are sold through typical scientific supply companies (see References). The soil should be viewed with and without a microscope for life forms and for general observations on soil aggregates. Make sure the students pay particular attention to any hyphae or mycelium in their samples by viewing them under a range of magnifications. Also try to ensure the students give at least equal time to the non-animal life in their samples. The macroscopic organisms collected by the funnel technic in the previewing activity should be counted and arranged into general taxonomical groups. Any arthropods can be stored indefinitely in test tubes for identification at a later time. Microscopic analysis can be done first on the macro invertebrates with a dissection scope and for microbes with a normal scope at 40X to 1000X mag. A hanging drop slide method with a small soil sample mixed in clean tap water is appropriate.

Additional Discussion
Discuss the diversity of organisms in the samples. Is there any difference in organisms found from the different soil samples. How did what they found in the samples coincide with what they expected and saw in the video?

Contact Extension Agency for information from local soil tests.

Contact a local excavation company and ask if you could arrange to have a class watch them dig a cellar hole.

Ask a local farmer to come in and discuss his experiences with soil.

Visit an organic farm and discuss with the owner the benefits and disadvantages of organic farming as it relates to soil.

Watch the entire video of Planet Under Pressure, #9: The Oxygen Partnership.

Research the history of the dust bowl in America.

Take soil samples from standard and strictly organic farms and compare their diversity of macro and microscopic organisms.

Take soil samples from neighborhood gardens and measure N2, PO4, Potash and pH.

Sterilize soil and see if it supports the same germination and growth as non-sterile soil.

Design a storyboard for a video which will accurately depict the types, shapes, sizes, orientation and stoichiometry of nitrogen molecules in the soil as the nitrogen is changed from one form to another.

Construct physical models of the different molecular states of nitrogen in the nitrogen cycle, i.e., N2, NH4+, NO2-, NO3-.

REFERENCES
Arms, K., Camp, P. 1995. Biology. Saunders, New York.
Jenny, H.
1933. Soil fertility loss under Missouri conditions, Missouri Agri. Exp. Sta. Bull.,
324.
1958. Role of the plant factor in podogenic functions, Ecology 39:5-16.
Smith, R. 1974. Ecology And Field Biology, 2nd Ed. Harper and Row, New York.


Soil Test Kit, Sudbury Laboratory Inc. Sudbury, MA 01776 (does about seven tests on N2 PO4
Potash and pH).

Soil Test Kit, Lamotte Chemical Products Co. P.O. Box 329, Chestertown, MD
(1-800-344-3100).

Master Teacher: Peter M. Faletra
Lin-Wood School, Lincoln, Hew Hampshire

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NOTE TO TEACHER

To use PVC sampler, it helps to have sharpened the lower rim that enters the soil and step on the upper rim and wiggle it forth and back. Then, of course, use the rope to pull the sampler out. An alternative to the rope is a metal rod that passes through both holes and is used not only to help extract the sampler, but also to wiggle it forth and back.

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