I've been thinking lately about the art and science of making soil. The large variety of products available at the local gardening supply, and the fact that I have an uncle who made a career in soil geology implies that this post will probably just scratch the surface of such a rich and complex topic.

Soil Defined

Soil is the mixture of fine and coarse solid aggregates (i.e. regolith), gases, liquids, and organisms that together support plant life. It is a substrate that

1. Physically supports plants by their root systems
2. Provides surface area for the microorganisms that convert organic wastes into nutrients to plants (vis-a-vis the Nitrogen Cycle).
3. Retains moisture to buffer water delivery.

Soil is also a sub habitat for organisms, such as worms, that form a part of the human food chain.

Making Soil on Mars

In the MIT analysis I read a couple months ago, there was an ISRU "soil" module for extracting water from Martian regolith, not explicitly for creating soil per se, but it did remind me of a story I read a few years ago when Andy Weir was posting it chapter by chapter on his website. "The Martian" is now a best selling novel and about to be released as a major motion picture. Hopefully it won't spoil the story for anyone by discussing how the protagonist, stranded on Mars, grows potatoes in soil made from his own feces and Martian regolith. This is a kind of attention to detail that most writers don't consider. That said, I doubt it is quite so simple to grow a food supply on Mars (see previous post). For one thing, the light requirements are not trivial, as my basement AP system has shown.

Using Soil on Earth

Earth has the advantage of having had millions of years to work organic matter into the soil. I'm still figuring out how to take advantage of that.

Over the last couple of years, I've experimented a fair deal with growing tomatoes and to lesser degree with potatoes. I can say with certainty, I've grown more tomato plants than tomatoes. The same is probably true of potatoes. This year I have tomatoes growing in:

• AP-phenomenal growth, but no blossoms or tomatoes (my guess is not warm enough conditions in my basement. Pollination might be a problem if I got blossoms, not sure)
• Pots- slow growth largely due to intermittent drought (outdoor pots require daily watering, and I am sometimes negligent), no blossoms or fruit yet. Potted with garden soil.
• Garden- per typical, planted 3 in the garden, amended clay with garden soil and compost.
• Inverted bag planters- did three last year, all pretty scraggly, few tomatoes. This year only planted two of these, one has died. No fruit yet on the other. Potted with garden soil.
• Earth box style planter- so far the most prolific producer of fruit. This is basically a wicking bed system, but is filled with mix of garden soil, coconut coir, and pearlite.

My potatoes are mainly growing in a stackable grow bed filled with straw and dead leaves, with a little manure thrown in.
Last year's crop was unremarkable, possibly due to the cool summer. I got a handful of small potatoes at the end of the year, but had planted a mixture of white potatoes and sweet potatoes. Got no sweet potatoes. This year it's all blue potatoes.The plants at least seem to be doing well so far.

So, comparing soils, it seems that tomatoes really prefer light, good draining soil. I also have a pepper plant and broccoli in the Earth box that are doing better than my garden controls, so I don't think it's just tomatoes. I think the pearlite provides magnesium that maybe helps also. Certainly the wicking bed provides good buffering for water, not quite as maintenance free as my AP system, but close. I think that is the main purpose of the coir, which has a high surface area. Surface area is one of the advantages of expanded clay balls, and I suspect one of the reasons for terra preta and biochar effectiveness.

Back to Mars

I'm thinking coconut coir may not be readily available on Mars, or in the typical CELSS for that matter. Mars soil will likely need nitrogen and phosphorous, so pack lots of asparagus for your trip.

To the Moon

I play soccer with a guy who did some of the original research in making fake poop for NASA to mix with lunar regolith. While artificial poop is probably a more controlled experiment, it lacks the microbial biodiversity of the real stuff. I understand that lunar regolith is fine and sharp. Sounds like diatomaceous earth, which kills insects. Might need to mechanically process it before putting it in your worm bin. Bring biochar to the moon, it's deficient in carbon. Or just make a practice of cremating colonists when they die.

Biological Advantages

One of the things that has me thinking about soil has been watching our new quail poop in their sandbox (and everywhere else, their food, their water, each other). When I change their sand out, the worms will get the poopy stuff, since I understand the worms also like sand for digestion. One advantage of worm composting is that they aerate and stir the decomposing material. BSF larvae supposedly do this to an even greater degree, although I've not yet observed this. The nitrobacter also require a certain amount of oxygen, which is why my aquaponics uses a flood and drain system. Now compost is not the same as soil, it's more like a soil ammendment, the organic component of the soil. Without some structure, it doesn't drain well. Sand and coarser media provide that structure in soil, like the expanded clay balls in my AP grow beds. The biochar provides surface area and probably traps certain nutrients.

Anaerobic fermentation can also break down the wastes, but you end up with more methane, which while useful in other ways, doesn't help plants grow and won't produce breathable air (just ask my wife).