Image from The Martian movie gallery (copyright 2015 20th Century Fox)
But that's not what I've decided to put up on DftDL. Instead, I'm going to talk about potatoes. Well, potatoes on Mars. Because this week, The Martian is released in theaters and being the calculating opportunist I am, I think this is the best time to start sharing my "big plan" for my soil science class this semester: Growing Potatoes on Mars. (plenty of information and pictures after the jump)
For those of you not familiar with it (avoiding spoilers for now - but I'll get into some technical details in later posts that will require divulging some plot elements), The Martian is basically a "Robinson Crusoe on Mars" type of story. Mark Watney, a botanist/mechanical engineer, is stranded on mars - to survive long enough for a possible rescue, he needs food. Food that comes in the form of potatoes. Potatoes grown in Martian soil (but inside the habitat, obviously).
The movie is based (quite closely, from all appearances so far) on the book by Andy Weir. It's a good story, even if you're not completely thrilled by technical science fiction. There is some "grown-up" language in it, so I would suggest parents read the excerpt before giving it to middle-school-aged kids. It's easily one of my ten favorite science fiction novels of all time. And they're making a movie out of it - so there's going to be even more visibility and more people are going to be familiar with the story and the important soil science-related plot points.
If you haven't seen the trailers yet, I strongly recommend checking it out:
You can see my excitement about this story - what a perfect way to discuss soil science! One of the challenges in getting people to understand why soil science is important is because of people's pre-conceived notions about "dirt." It's old-fashioned and not important unless you're a farmer. But a firm understanding of soil science is just as important to the future of space exploration as developing new and faster propulsion systems.
Soil is a very complex system - any coverage of soil science includes diverse topics including weather, microbiology, and mineralogy. The pedosphere (the soil system) is at the intersection of the geosphere (earth and rock), the atmosphere (CO2, O2, N2 gases, sun and rain), the hydrosphere (water and associated chemistry), and the Biosphere (microbes and larger critters/plants - "macrobes?"). By planting a seed here on earth, we're plugging into a complex, interconnected system. The soil science part is often hidden behind a veil of dynamic processes. To understand the soil, you have to spend a lot of time unpacking dozens of concepts such as weathering/decomposition, gaseous interchanges, moisture potentials, redox and acid/base and organic chemistry, and ecology (even "ecology" itself is a complex interconnected set of concepts).
Planting a seed on Mars? You start with a tabula rasa. Martian soil? A mixture of sand, silt, and clay-sized particles with varying chemical compositions (mostly mafic silicate minerals plus oxy-hydroxides of iron, aluminum, and manganese). To that you need to add water, microbes, and "organic material" (something for them to "chew" on). Any experiment that might solve the challenge of growing something on mars must proceed from the ground up (pun thoroughly intended).
This brings up two important items that anyone trying to grow Martian potatoes needs to solve right away. The first is getting a reasonable Martian soil simulant. The second is keeping the Martian soil from interacting with Earth's environment. I've solved the first problem by baking my soil in a muffle furnace (at 500°C for ~2 hours). This vaporizes most of the organic material - any Mars soil is likely to have minimal organic compounds within it. I built a miniature "Hab" in the Dirt Lab - basically a miniature cold frame you might make to keep your early/late season vegetables from frost damage.
Almost ready to start growing potatoes!
The soil I've chosen is glacial till from the Copper Falls Formation - a red/brown diamict (diamict = sediment/soil with a mixture of very large and very small particles). This soil material is widespread across the northern US. Plus, it's got a mixture of igneous/metamorphic parent material, plus iron oxide and silicate clay weathering products - minerals that are very common on Mars. There are other "Martian Soil Simulants" that come from basaltic volcanoes (basalt is pretty common on Mars). Shipping about five cubic feet of any kind of soil material is prohibitively expensive, so I'll happily use a soil that looks good and that I can get to in less than an hour's drive.
A nice outcrop of glacial till exposed in a gravel pit (folding shovel is about 50cm long).
My "Hab" consists of a frame made from two 2x4's with holes drilled in it to accept 1/2" PEX tubing. The tubing comes shipped in ~3 foot (ca 1 meter) diameter coils that I just snip to create six loops - the ends then go in each of the holes in the 2x4's. The frame looks rather like a "Conestoga Wagon." I added a rectangular frame in the middle to make a better support for the interior lights and main access door. I placed the frame assembly on top of some black plastic, taping the plastic to the sides of the frame thereby sealing the underside. I draped a clear plastic tarp over the top of the frame and used PEX tubing clips to hold the tarp in place. Edges of the tarp were taped onto the sides of the frame, sealing the upper part of the Hab.
Wood frame and PEX tubing set up on the lab bench.
Covered with the plastic tarp and the central opening cut out.
Room for 12 wash bins inside.
Covered in plastic, the Hab is almost ready.
Access is provided by big 1/8" plexiglass panels on either end that fold down. The central "hatch" is covered with a rectangle of plastic that drapes over the opening and is held in place with magnets. Inside there's enough room for 12 3-gallon plastic bins - one bin for each experimental treatment of the soil. I could only sterilize/combust about 8 kg of soil at a time within the muffle furnace - it took me about three weeks to process enough soil to fill all 12 bins.
Soil in terra cotta pots before going into the oven.
I experimented with grilling the soil in order to process more material quickly. It worked, but it uses a lot of charcoal to get the soil hot enough.
Once it's heated, it really looks the part.
Now that I have the Martian soil, it's time to think about the experiments. The plot point for growing potatoes was to provide Mark Watney with enough food to survive until rescue. I'll be approaching this in "Mythbusters" fashion - "can potatoes grow well enough in Martian soil to sustain someone?"Then there are the questions that logically come out of that question - Watney's a botanist, so presumably he knows a thing or two about growing plants. What might he have done - or what kinds of earth-based experiments could be done to help Watney improve the yield.
In the book, Watney has his own plan for adding "fertilizer." I want to avoid exposing my students to potential pathogens, so our soil treatments are going to include worm castings plus some garden soil to "inoculate" the soil. Our compost is going to be assembled from organic materials that could reasonably be assumed to be available on a journey to Mars. If I want to replicate each treatment/control group, I can go with four sets of two treatments plus control (4x3), two sets of six, six sets of two, etc. But this is a component of experimental design that is important for young scientists to think about - so for now I've got a few potatoes planted to create a seed crop. My students will get to working on refining the experimental design as well as analyze the soil to determine pH, moisture capacity, and other physical soil characteristics that influence plant growth.
Next time I'll go over some of the finer points of "potato husbandry…"