UW Farmers at the end of compost making (picture from Spring 2013)
UW Farmers, keep making your compost. The compost crew just tested our product, so we can show you that it works.
When I sent an email to the UW Farm listserv to enlist help for starting tomatoes in October 2013, I received a couple of puzzled responses from our concerned veteran farmers:
“Kay, ya know it’s winter right?”
“Why are you starting tomatoes at this time of the year? Just curious.”
It was not because we were craving greenhouse-grown tomatoes, nor was it because we were in denial of winter’s arrival. It was because we were using tomato plants as our experimental “subjects”.
This experiment was dovetailing of my research question and interests with those of our farm manager Sarah Geurkink. I was interested in finding out whether the compost that farmers have always been making in our Back Forty actually helps improve soil fertility. At the same time, Sarah was concerned about the impoverished state of the soil at the UW Farm’s new Mercer site. Unlike the rich, dark and sweet-smelling soil we have at the Botany Greenhouse site, where compost has been added for years, the soil at Mercer looked mostly like a mixture of dry, grey sand and silt interspersed with jaggy little rocks.
Is it just the look of the soil that differs? Or is it functionally different for plant growth as well? The pictures here are worth a thousand words (although the thousand words follow for those of you interested in nitty-gritty ecological methods). Mercer soil made stunted tomato plants, but even 25% of that soil replaced with UW-grown compost helped them thrive!
Alright, for those of you who are not interested in nitty-gritties, you can close this page and happily hum a tune as you head to your backyard and make compost. For those of you are not convinced and want further evidence, what follows is a detailed description of what we did to show that composting is important. (And to show that unlike what the cynics think, composting is not just a responsible way of discarding unused plant parts).
One of the main things to learn at a university is critical thinking, not just accepting what you’re told, but actually confronting and testing ideas. Since I wanted to know if compost matters, I had to do a test. A team of students got some advice from Professor Jennifer Ruesink in Biology about treatments and replication. Greenhouse Manager Doug Ewing provided key insight to help us choose tomatoes as our subject, because they are heavy feeders that thrive in fertile soil. Their substantial need for nutrients in the soil will accentuate any differences in the state of plant growths across soils of different compost content.
The experimental set-up that the compost crew agreed on was as such: we will have four soil conditions, the first being purely Mercer soil without any amendment, followed by a mixture of 75% Mercer soil and 25% compost, a mixture of 50% Mercer soil and 50% compost, and finally, just 100% compost without Mercer soil. We agreed to have fifteen replicates for each condition, which meant fifteen tomato plants in each type of soil condition. We also agreed to use compost from the Back Forty at the Botany Greenhouse site, produced using regular heap composting methods. We were to harvest the compost from the pile built way back around the month of April (thank you, spring quarter Dirty Dozen!) and also during the summer (thank you to our summer interns and BIOL240 classmates!).
The tomato plants were all started one afternoon in mid-October. We prepared about a hundred small rose-pots filled with Sunshine#4 growing mix and sprinkled several seeds into each of them. Even though we would eventually need fewer seedlings, we started more than we needed in order to compensate for those that would fail to germinate or germinate poorly. These rose-pots were placed into transparent plastic cups with lids that had small crisscross cuts at the top to allow some ventilation. After that, we moved all the cups with the rose-pots onto a thermal mat to ensure that the temperature will be high enough for germination to take place. By week one, each rose-pot had several tiny seedlings growing in them.
Two weeks later, we divided sixty medium-sized rose-pots into four groups of fifteen, with each group representing one of the four soil conditions mentioned beforehand. We also labeled all of these rose-pots to keep track of which one contained what kind of soil composition. We then carefully transplanted tomato seedlings from the small rose-pots into the prepared rose-pots, taking care to choose the average healthy-looking seedlings from all those that had germinated. We took an initial measurement of the heights of the plants not only to record each plant height at the start of the experiment, but also to allow us to account, in our later data analysis, for the differences in how deeply we planted each seedling. We then placed all of the rose-pots containing transplanted seedlings onto a rack in a research area in the Botany Greenhouse, where they were watered daily by our dedicated greenhouse staff and compost crew members using water without fertilizer. We made sure that the plants in various soil conditions were placed such that no one soil condition was clustered together, so that there will not be a confounding effect of the location of the rose-pots. For example, location could potentially affect how much water and light exposure each rose-pot gets.
We measured the heights and took pictures of the plants (whenever we remembered to bring a camera!) once a week for four weeks.
0% compost condition at the end of the 3rd week. Note that whenever we took pictures we grouped the plants of a soil condition together, but put them back in random order again after taking pictures.
25% compost condition at the end of the 3rd week (in tray nearest to camera-woman)
50% compost condition at the end of the 3rd week
100% compost at the end of the 3rd week
At the end of week four, we took a final height measurement and then prepared the seedlings for dry weight measurements. This was how we did it: we shook the soil off each plant, separated the shoot from the root, and then placed each part separately into an envelope. At the end of the process, we placed all the envelopes into a drying oven. This procedure is to dehydrate the plant parts so that we can take the dry weights of the shoots and roots for plants in each condition. Our hunch is that nutrient stress will be associated with larger root growth relative to shoot growth.
Lola preparing specimens for dry weight measurements
We also set samples of soil water from each soil condition aside for Electro-Conductivity (EC) measurements. To collect these samples, we poured some water into a few rose-pots of each condition and then collected the water that ran off. Since the nutrients in the soil are usually cations and anions, EC readings, by showing how well the soil water carries electrical current, indicates the amount of nutrients available in the soil for the plants.
What’s next: We have tons of data, and now we just have to analyze them! For now, we have enough visual “data” to let you decide whether you want to add compost to your backyard soil or not.
Your average tomato plants in various conditions at the end of the experiment
It’s inevitable to have some casualties that either died of natural cause or in the hands of Kay’s un-nimble fingers (wait…What? Hands of fingers???) while taking measurements. Sorry, plants. I guess they were too eager to be tossed into the compost pile.
0% compost condition at the end of the experiment
Acknowledgements: On behalf of the compost crew I wish to express lots and lots of heartfelt thanks to Professor Jennifer Ruesink and our awesome Greenhouse Manager Doug Ewing, both of whom figured hugely in the conceptualization of this project and supported us in countless ways as we attempt for the first time our very own controlled experiment. I also wish to thank the entire greenhouse staff for their unwavering support in taking care of the plants throughout the course of this experiment. Without their help, this blog post would have been a reflection on a failed experiment. I also wish to thank my wonderful compost crew for helping me at various stages of this experiment, from starting the tomatoes, transplanting the tomato seedlings into the various manipulation conditions, watering the plants, taking weekly height measurements, shooting pictures of the plants, to preparing dry specimens. They are Lola Chevreau, Chi-Yun Lee, Chen Yu-Chiu, Fency, and Angeline Blattenbauer.