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  • Writer's pictureAnneliese Abbott

Peanut Butter Decomposition: My Successful C:N Ratio Experiment

Peanut butter decomposition
Filter paper decomposed faster when we added peanut butter to living soil

Six years after my failed pot fertility experiment at OSU, it happened again. The instructor for my Soil Microbiology class at the University of Wisconsin-Madison told us that it would be our responsibility to choose the treatment for an experiment. She gave us some general guidelines, but the rest was up to us.


The experiment was on decomposition—a topic I had learned much more about since I graduated from OSU. I now knew that organic nutrient cycling was dependent on soil microorganisms. This time, fortunately, my instructor did, too—that’s what the class was about. She didn’t have us start with sterile potting mix. She told us to go outside and dig up some real, living soil. We were supposed to put that soil in little Petri dishes with a piece of filter paper and measure how fast the soil microorganisms decomposed the paper under different treatments. It was up to us to choose what variable we wanted to introduce to affect decomposition.


After doing so much research on organic nutrient cycling, the C:N ratio was the first thing I thought of. If everything I had read and seen was true, then adding some high-nitrogen amendment to the soil should make the paper (high in carbon) decompose faster. But what organic nitrogen fertilizer would be appropriate for this small-scale experiment? Our Spring 2021 class was online because of COVID, and I had to think of something that wasn’t too gross to keep in an apartment. That ruled out manure or anything else that would stink, like meat or eggs. What high-nitrogen but not stinky organic material was readily available in the city?


“How about peanut butter?” I asked my lab group at our first meeting. When they finally stopped laughing, they asked what effect peanut butter would have on decomposition. “It would lower the C:N ratio because it’s high in protein, which is nitrogen,” I explained. Though nobody uses peanut butter as fertilizer on a field scale, some organic growers do use oilseed meal—which is ground-up high-protein seeds with the oil removed. Peanut butter seemed like an easily available alternative that should—I hoped—have the same effect.


My group liked the idea of peanut butter better than anything else we came up with. After all, it just sounded fun! And fortunately none of us had a peanut allergy. We agreed to use unsalted, natural peanut butter so that peanuts were the only thing we were adding to the soil. We each mixed different amounts of peanut butter into three of our dishes and left the fourth one peanut-free as a control. Then we measured the decomposition every week to see what happened.


To my immense relief, this time it actually worked! The paper in the treatment with the highest level of peanut butter decomposed the fastest, and the one with no peanut butter decomposed the slowest. Lowering the C:N ratio really did speed up decomposition. We actually ended up having some of the most significant results of the whole class.


The key difference this time was that we were working with living soil, dug out of my organic garden plot. It was already full of fungi and bacteria, which were able to use the nitrogen in the peanut butter to build up their biomass faster and break down the filter paper. If I would have mixed peanut butter into sterile potting mix and put it on filter paper, nothing would have happened. I had finally learned the secret of organic farming—it only works when the soil is alive and healthy.

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