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Teacher and two students doing experiment.
Robert Pierce, school principal, directs a science class making experiments with soils, Gees Bend, Alabama. Marian Post Wolcott, 1939

Launching Units with Primary Source Phenomena: Bacteria-coated Popcorn

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This post is written by Jackie Katz, 2022 Albert Einstein Distinguished Educator Fellow at the Library of Congress. It is one in a series focused on using primary sources as phenomena in STEM classes.

Have you considered using primary sources as launching phenomena in your classroom? Primary sources often times present a puzzle that needs to be solved; the content related to your discipline can help put the pieces together. In my biology class, students often complain about the nitrogen cycle—”It’s boring, it’s confusing, all of the words sound the same!” But what would they say if presented with some of the experimental data responsible for discerning this vital biogeochemical cycle?

Present students with several data tables from a University of Michigan graduate dissertation entitled “Assimilation of organic nitrogen by Zea mays and the influence of Bacillus subtilis on such assimilation” (1916) at the start of a unit on the nitrogen cycle. Without providing detail on how these experiments were conducted, ask students to observe the tables and attempt to recreate the procedure that led to this data. If students struggle with where to begin, ask them to identify the independent and dependent variable in each of the data tables or explain that “check” is another way of saying “control.” The class can work together to devise a consensus procedure.

Two charts document experiment on how popcorn interacts with different substances
Article: Assimilation of organic nitrogen by Zea mays and the influence of Bacillus subtilis on such assimilation from Soil Science, 1917

With the procedure outlined, challenge the class to think backward and attempt to recreate the initial research question of these scientists. Students will likely arrive at a question about the effect various nitrogen sources have on popcorn. Ask students why this question was important to answer in 1916, when this dissertation was published. The question that the students devise can serve as the driving question for the unit.

In order to answer a driving question about the effect of nitrogen on plant growth, students will need to consider the structure of the nitrogen compounds tested, as well as the chemical processes of B. subtilis that relate to nitrogen conversion. Students could conduct some in-class experimentation with the materials tested, sodium nitrate, urea, and peptone, to determine differences in chemical and physical properties that might explain the results displayed in the table. An additional experiment could test the effect of B. subtilis on the nitrate concentration of a water or soil sample.

By combining class generated data with that collected in 1916, students can create a model to outline the relationship between nitrogen-containing compounds, plants and bacteria. The students’ models can also include information on the “why” that inspired this experiment. Students can consider their original ideas about why this experiment was conducted or read a newspaper article from 1910 to better understand the concerns of early 20th century farmers.

By using historical primary sources to launch a unit on the nitrogen cycle, students are able to recreate the process that was taken to build our current understanding of the cycle. The students are also able to explore why scientists set out to construct this information. Consider highlighting the process taken to understand other ecological concepts by incorporating other sources from the Library’s new Ecology primary source set.

Let us know in the comments how you used this primary source or the Ecology primary source set with your students.

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