NECESSARY NITROGEN
Grades 9 - 12

Nitrogen is a critical component for life. Yet only certain organisms have the ability to interact with this atmospheric gas. In this lesson, students will be introduced to the essential role nitrogen plays in our environment. Students should have some prior experience with bonding and how it influences the physical behavior of a substance. The video segment correlates the inaccessibility of atmospheric nitrogen to plants with the necessary interdependence of plants with fungi and bacteria. Students will test the properties of the soil and the concentration of nitrates which can be used directly by plants. A statistical analysis of data from an experiment testing the effectiveness of fertilizers on plant growth should help to prepare students for assessing their own experimental data.

Planet Under Pressure, #6: The Living Soil

Students will be able to:

• describe the relative stability of the N2 molecule
• write formulas and correct oxidation states for at least three different forms of nitrogen
• draw a simple diagram illustrating the cyclic pathway of nitrogen through our environment
• use colorimetric reagents to determine the amount of nitrate present in a soil sample
• use graphing and statistics to compare data sets for significance in test results

For a class of 24 students:

• "Tennis Can Columns" Activity

For groups of four:

• 12 clear tennis ball containers w/ lid (label removed)
• 6 different soil samples (wooded and more sandy loam)
• 5 liters of distilled water
• 6 metric rulers
• 6 calculators
  
  
  
• "Arm Bending or Bonding Energies" Teacher Demonstration
• 3 identical springs minimally 30 centimeters in length
• Molecular Modeling Kit
• "Nitrogen in Nitrates" Activity

Pairs of students:

• students' soil samples [500 grams each pair]
• LaMotte Topsoil Tour - Nitrate Test Reagents & Color chart
• distilled water
• plastic sandwich baggies
• funnels (cut the top off a plastic bottle for an inexpensive funnel)
• filter paper (coffee filters could also be used)
• graduated cylinder
  
  
• "Statistical Significance" Activity

Groups of four:

• 24 copies of "The Significant Data Review"
• 6 calculators (minimum - individual calculators would be better)
• 24 overhead transparencies with graphing grid and scale
• 24 transparency markers - 4 different colors for each group

Optional - individual graph paper or graphing software

Biogeochemical cycles illustrate not only the interconnectedness of biotic and abiotic components of ecological systems, but also the supportive role chemistry plays in understanding our environment. Trees, plants, fungi, and bacteria are actively involved with our physical environment. With the aid of a microscope, a handful of dirt can introduce these "participants" by making them visible to students. Allow students time to view soil samples, or pond water samples, under magnification. Although the chemical "participants" are too small to be observed, their role is vital to the health and understanding of these dynamic, complex ecological systems. Chemical nutrients must be "sensed" using other devices or chemicals such as indicators. Indicators are molecules that react specifically with certain species to produce visible color changes.

The nitrogen cycle is one of the more perfect biogeochemical cycles. With the large reservoir of nitrogen gas in our atmosphere, this cycle has the capacity to maintain itself almost indefinitely on a global level. Locally, people become involved in this cycle and can influence the distribution of nitrogen. Ask students to think about ways in which people affect the amount of available nitrogen.

PART 1:
To give the students a specific responsibility while viewing, ask them to look for ways in which people interact with nitrogen in our environment.

START the video segment at the child playing in the dirt until the narrator says "...feels so good."

PAUSE. Ask students to consider how many different ways this child is interacting with the nitrogen cycle? RESUME video.

PAUSE the video where the child tastes some of the soil. Poll student responses to the question. Breathing should come up first. Do humans use the nitrogen in the air? Why or why not? Then push students to consider the interaction of the child with the soil. Is there nitrogen in soil? Can the nitrogen be absorbed into the skin? With what other forms of nitrogen are the students familiar? How do they know nitrogen is in the soil? How does soil taste? Is it dangerous to taste soil? (It would be interesting to note that farmers used to taste their soil to determine if they needed lime.) Many students involved in chemistry know that acids taste sour and bases taste bitter. Is the acidity of a soil important to plants? Use a concept map to begin connecting students' ideas about the soil and nitrogen. Answers will probably vary considerably, but this will open up ideas for experimentation and help the teacher to determine students' experiences with soil constituents. (The concept map provides a foundation upon which the students can construct their own understanding.)

Ask students to watch carefully the segment you are about to play. Take note of all possible sources of nitrogen and the important features of the soil. FAST FORWARD the video beyond the hydroponics segment to the rock or cliff. Play through the mineral nutrients section where sand, silt, and clay are discussed.

PAUSE: after the diagram shows the water absorption characteristics. Ask students if nitrogen could be involved in the: water? sand? silt? clay? How might it be involved? Briefly poll student ideas. Continue PLAYING the video through the hearty weed growing in clay to the narrator's clue of what ingredient is missing.

STOP the video when the narrator suggests the "forest floor covered with dead leaves."

PART 2:
How does the nitrogen in the air become part of the nitrogen compounds in plants? To give the students a specific focus for viewing, ask them to take note of the path nitrogen takes in the video segment you are about to play. Pay particular attention to the behavior of the animated nitrogen molecules.

START video and continue to play through the forest floor scene until the cycle is established and the blue nitrogen once again moves up through the tree. STOP the video.

PART 1:
Tennis Can Columns Activity. Demonstrate the nature of some of the materials found in soil by 1/2 filling clear tennis cans with soils, and then adding 3/4th of a can of distilled water to each. Cover and shake the cans vigorously to disperse the soil and then set them down and observe the layers formed as the soil settles. Students should observe the separation of humus from the sand, silt, and clay. Ask students how this procedure could help them to compare soil types. Encourage students to provide quantitative tests that could characterize a soil. Students may suggest measuring the relative depths of the soil components. Suggest that they take it further to calculate the percentage of each component. Ask students to predict the type of location each soil sample came from. How does the soil taken from a wooded area compare to soil taken from an urban area? Ask students to predict which soil samples have the most nitrogen in them. Students should now realize that most of the nitrogen for plants can be found in the humus. (Allow the tennis cans to sit in a shaded window where they will not be disturbed. Over time, students can observe the formation of algae, bacteria, and other life forms by viewing water samples under magnification. After a few days, the odor of the can may also help to convince students of the active ingredients found in soils)
PART 2:
Arm Bending or Bonding Energies Demonstration. The teacher should perform arm exercises by pulling on the opposite ends of one spring, two springs, and then three springs (dramatize the efforts needed). Use the modeling kit to show the triple bond in nitrogen gas. Use your text or a chemical handbook to determine nitrogen's bond energies. The triple covalent bond in nitrogen makes N2 a very stable molecule with a very high bond energy. Did the animators correctly portray the character of the nitrogen? If the molecule is that difficult to break apart, then how do plants get nitrogen from the air? What role does it play in the soil? Students will probably suggest the humus component and may remember that plants require nitrogen. What chemical compounds in plants involve nitrogen?

Review the general structure of an amino acid and focus in on the amine functional group[R--NH2]. What chemical compound is made up of just nitrogen and hydrogen? Ammonia [NH3] is one compound, hydrazine [N2H4] is another. Write their formulas and work with students to determine the oxidation states of nitrogen in these compounds. How do these nitrogen compounds get into the soil?

Think-Pair-Share! Ask students to quietly check their notes (think) from the previous segment. (Repeat the segment if necessary.) Then ask lab partners (pair) to construct a diagram to illustrate the cycling of nitrogen in the environment. After five minutes, allow the class to share their diagrams (share) so the teacher can construct one elaborate diagram with the map begun on the classroom board.

REPEAT the last part of the video if necessary for students to compare with their diagram.

How do the N-fixing bacteria convert this very stable molecule to forms that are accessible to plants? Introduce students to nitrite ions [NO2- ] and nitrate ions [NO3 - ]. Both of these ions, and ammonia are very soluble in water. Fertilizers contain ammonium and nitrate ions. How does nitrogen's oxidation state change as it reacts? What would you predict about the bond energy changes? Draw these ions into the nitrogen cycle map in their appropriate locations.

Nitrogen in Nitrates Activity - Testing for Nitrogen in Soil. Use LaMotte Nitrogen Soil Test procedure to determine how much nitrate is available to plants from the students' soil samples. Students should plan ahead to obtain samples from a variety of landscapes noting the conditions of plants and the texture of the soil. Written predictions regarding the amount of total nitrate present should be supported by the observations made. Color charts provided by LaMotte can be used to determine the relative amounts of available nitrate in the soil. A concluding paragraph comparing their predictions with the test results provides a check on their understanding of the role of nitrogen in the soil. (An optional procedure would utilize a spectrophotometer and known standards to construct a calibration curve. The absorption by the student samples can then be matched to the curve for a quantitative reading of the total concentration of nitrates in the soil.)

Statistical Significance Activity - Assessing Experimental Results. Student-designed experiments assessing the effectiveness of different environmental conditions on plant growth are a potential extension to this lesson. Present students with this data set simulating a student's experimental results. How can students determine whether a difference in growth is due to the conditions set by the experimenter or due to the variable growth of the plants chosen to test? How can the data be assessed? Direct students to compare the range of each of the data sets. Instruct students to find the mean, median, and mode for each data set. These comparisons will illustrate the inherent variability in living systems and the potential for other variables to influence an experiment. For a more graphical representation of variability, have students plot frequency distributions of all data on overhead graphs using different colors. Each student in the group should plot one of the data sets in a particular color. As the graphs are aligned over one another, students will observe the closeness of the data between some of the test fertilizers and the control group. (Students familiar with statistical methods can use standard deviations and draw box plots to show the distribution of the data.)

Ask students to write a concluding discussion regarding the use of these different fertilizers. Emphasis should be placed on the evidence used to support their conclusions. Ask students to suggest ways of controlling variables to help minimize the variations within each data set. What do they think Sarah could have done better?

Have students design experiments to test a question they have about their environment. Brainstorm with students the different materials and tools that are available to them and how they might interact those items to design a test. For example, testing nutrient levels in pond waters, soil samples from different geographic regions, or a comparison of effectiveness of fertilizers. Encourage students to formulate a hypothesis and insist that dependent variables must be measurable in some way. Student reports will generate a great variety of tests and relationships that can be communicated through graphing.

Nitrogen is necessary for chlorophyll production and a deficiency in nitrogen often appears as yellowing tips on the leaves of a plant. Gardeners can use nitrogen test results to determine if fertilizers are needed for their soil. Ask students to investigate how fertilizers are made and marketed. Many packaged fertilizers include 800-numbers for more information. What kind of rating system is used in the fertilizer industry? The LaMotte soil test kits can also be used to test for potassium, phosphorus, and pH in soils.

Ask students to write about the implications of over-using fertilizers or how the nitrogen cycle would influence aquatic systems. This activity would be a great assessment of how students understand systems and their dynamic nature.

Challenge students to use a computer modeling system to demonstrate relationships in the nitrogen cycle or use a commercial computer model of the nitrogen cycle to allow students the opportunity to influence one or more variables in the system, and experience in compressed time the results of the changes they invoke.

Have students test out the electrostatic character of soil particles and their attraction for particular nutrient species. Use the following materials to demonstrate the behavior of particles.

• 1-2 milliliters of clay & water (mixed to a glue-like consistency)
• 1-shallow plastic dish
• 100 milliliters Methylene Blue Indicator Solution (dilute)
• 100 milliliters Eosin Red Indicator Solution (dilute)
• 3 - 1" glass tubes with 1-hole stoppers at one end
• small amount of glass wool or cotton fiber to partially block holes in stoppers
• 3 - small containers for collecting indicator solutions

Behavior of Soil Particles. Place a large drop of the clay/water mixture on a shallow plastic dish and attach wires to the two terminals on the battery. Place the positive and negative wire leads into the drop, but opposite each other without touching. Observe the behavior of the clay particles. How do they move in an electric field? What does this infer about the charge of the particles? What charge do nitrate particles have? Would they be attracted or repelled by the soil particles?

Soil Leaching. Loosely cover the one-hole stoppers with cotton or glass wool and insert the stoppers into the bottom of the tubes. Fill the glass tubes about 2/3rds full with loam or sandy loam surface soil, and support the tubes in a vertical position. Position a container below each tube. Pour dilute methylene blue dye solution into the first tube. Methylene blue particles have a positive charge like ammonium ions. Pour dilute eosine red dye solution into the second tube. These particles have a negative charge like nitrates [NO3-]. Pour a mixed solution containing both dyes into the third column. Which solution passes through the soil column? What does this simulation imply about nitrates in the soil? Discuss the benefits/risks of using fertilizers containing nitrates.

Science and Math: Invite a soil scientist to speak to the class about local concerns and/or career opportunities for soil analysts. My experience suggests that there are many female role models in this field which will help to encourage female students to pursue scientific careers.

Science, Social Studies and Economics: Research current practices in plant cultivation such as crop rotation, hydroponics, or waste water sludge use.

Science: Research another biogeochemical cycle such as the sulfur, phosphorus, or carbon cycle.

Science: Set up a terrarium as a demonstration of nutrient recycling.

Science and Social Studies: How do other cultures prevent the degradation of their soils? Students should compare U.S. strategies to the ancient systems used in the rice paddies of the Far Eastern countries.

Science and Math: Nutrient concentrations are often given in parts per million [ppm] instead of molar concentrations. Set up an equation to convert "ppm" data to molarity concentrations for nitrate solutions.

Science: Investigate the Haber-Bosch Process of extracting atmospheric nitrogen for fertilizers.



RESOURCES
LaMOTTE Company, P.O. Box 329, Chestertown, MD 21620
Phone: 800-344-3100 Order Top Soil Tour Kit - Code 5425 $49.00
Additional Reagents R-5425 $35.00 for 40 tests (including nitrogen, phosphorus, potassium, and
pH)

HACH Company, P.O. Box 389, Loveland, Colorado 80539-0389 Phone: 800-227-4224

Purdue Agronomy Club, Department of Agronomy, Purdue University, West Lafayette, IN 47907

County Extension Service

Local 4-H Chapter

Local college or university soil scientists.

Waste water treatment plant scientists and technicians.

Local nursery or garden center

Master Teacher: Barbara A. Hopkins
Oyster River High School, Durham, NH