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Repetition, Repetition

Repetition, Repetition
"Let's Do This." Let's collect leachates from 72 trees.

Repetition is an action of many diverse functions, highly dependent on person and context for its specific purpose. I remember my high school choir director teaching us repetition’s function for the musical, lyrical context: he would ask, “Why is something repeated?” Then, with a little more assertion and gusto, “WHY is something repeated?” Then, face reddened and voice projected with the strength of lower abdominal muscles refined after 30+ years of proper singing, “WHY IS SOMETHING REPEATED?” After a pause, he’d answer his own question: “To give EMPHASIS.” Of course.

 

Yes, in the context of musical dynamics, poetic description, public speaking or rhetoric of other sorts, repetition does very well in bring an idea to light, giving it added attention and consideration.

 

But science doesn’t use repetition for this purpose. As with music, poetry, or rhetoric, repetition does play an essential role in the success of the study, but in science, repetition’s necessity is not based on its ability to add emphasis to an experiment, but, quite practically, to help ensure the experiment’s reliability.

 

When I refer to repetition in my project, this is what I mean:

 

  • 4 soil amendments: biochar, biosolids, biochar + biosolids, and none

  • 3 soil types: lots of sand, lots of silt, and a mix between the two

  • 2 soil coverings: turf grass and woodchip mulch

  • 4 X 3 X 2 = 24 varieties

  • 24 varieties X 3 repetitions = 72 potted trees

  • Therefore, each task is repeated 72 times...

 

To measure the effect on nutrient leaching from a variety of soils, I get to spend some quality time with a plot of 72 trees. Seventy-two. Consequently, when I do any task regarding my trees, no matter the complexity of the task, I repeat it seventy-two times. To any office or factory worker, this number may seem insignificant, but I expect that for many others, like myself, it is not often that we repeat a task seventy-two times in daily living. I put two shoes each morning, fill up one car with gas, and wash three sets of dishes at the end of the day. Given my current lifestyle, I simply did not know what to expect out of 72.

 

I’m often at fault of overestimating my efficiency, but, like I mentioned in my last post, I am here to learn! Let me tell you what I’ve realized so far in completing tasks 72 times over:

 

Watering.

Expectation: 1 hour?

Actual: approx. 2.5 hours

Hose spraying a pot with a small tree planted in it.
115 seconds X 72 trees = 138 minutes

 

Measuring soil respiration (aka carbon dioxide levels coming out of the ground---a measurement of microbe activity).

Expectation: 4 hours?

Actual: approx. 6 hours

LI-COR soil respiration machine in place in one tree pot
5 minutes X 72 trees = 360 minutes

 

Collecting and measuring leachate (water drainage from each pot).

Expectation: 2 hours?

Actual: 3 hours with a helper; 4 hours by myself. (Yay volunteers!)

Birds-eye view of two graduated cylinders and a bucket of water vials
3 minutes X 72 trees = 216 minutes

 

Filtering all leachates.

Expectation: 2 hours?

Actual: approx. 3.5 hours

 

A long line of filtering contraptions arranged on a lab table
3 minutes X 72 trees = 216 minutes

 

Separating out 10 mL of each leachate for phosphate analysis (i.e. aliquoting).

Expectation: 1 hour?

Actual: approx. 2.5 hours

Birds-eye view of a 100 uL micropipette, held by a gloved hand
2 minutes X 72 trees = 144 minutes

 

In short, my expectations have usually been a bit too idealistic for the number of samples I’m working with, and things generally take much longer when repeating them 72 times over.

 

So why 72? Why the repetition? Why the extended time and “tedium”?

 

Imagine conducting a door-to-door neighborhood survey.  You want to know if your neighbors are happy with where they live! Do they feel safe? Respected? Known? Is it a good place to raise kids? Have pets? Host block parties? You don’t have time to stop by all 500 homes (it’s a big neighborhood!), but you don’t want biased or skewed answers. You’re going for a true representation of the overall perception here, so you decide to choose the homes randomly that you’ll pay your visits to. Great start! But how many homes should you stop by? Three? Six? Fifty? If you stop by only three or six, what happens if you randomly select cranky Mr. McGregor, the neighborhood Scrooge? (In all fairness, he has had a very difficult life, but he does tend towards extreme, abrasive negativity now-a-days.) On a satisfaction scale of 1 to 5 (5 being “extremely satisfied”), he chooses “1” all the way down the list without much consideration. Even if the other two or five responses are generally positive, the total response will still be tugged towards the negative end. Are these responses fairly representative of the entire neighborhood? Maybe, but we can’t be sure. We need a quite a bit more input in order to balance out Mr. McGregor’s unusual pessimism. Let’s say 50 homes will do! (Unless, of course, you want to make Mr. McGregor some cookies before you interview him, which may normalize the results and bring smiles all around.)

 

So we conclude that 50 homes is best---or, to bring the analogy back around, 72 pots of trees. The numerous trees balance out, in this case, the few trees that have nature either unusually for or against them. (Cookies won’t help in this situation, unfortunately.)


I want to know if my trees are happy with where they live. Are their soils allowing them enough nutrients? Enough water? Do they have an excess of nutrients so that the chemicals go to waste, drain out through the soil, and harm surrounding water quality? To discover reliable trends between a tree and its soil, I need to investigate many repeats, and take that extra time to do so. For the sake of having meaningful results about happy trees with resultant clean water, I am learning to not mind, and maybe even enjoy, repetition. :)

Kirsten smiling while holding a ruler next to new twig growth
Smiling still comes easy after measuring 720 twig growths. (10 twigs per tree!)