Michael Turner wrote:
"Someone last year grew some plant in a Moon regolith imitation. It
isn't the most fertile soil but you can grow stuff in it."
In the sense of providing support for roots and holding moisture,
perhaps. However, there seem to be qualitative differences between
lunar regolith simulant and real lunar regolith, none of them good:
http://www.lpi.usra.edu/decadal/leag/DavidJLoftus.pdf
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2193.pdf
To a reasonable first approximation, this stuff is nanometric silicon
oxide, subject to meteoritic and space-radiation processes that leave
the dust particles with lots more sharp edges -- at least when
compared to their terrestrial semi-analogues like official NASA lunar
regolith simulant. Nasty stuff you wouldn't want accumulating in your
lungs or wending its way through your GI tract.
I wouldn't think that you would grow food in the regolith. For food
crops you use fertile soil. You grow plants selected for good growth
in regolith, grow them, harvest them and send them for the methane
digester. You would probably add a little fertilizer to the regolith
to accelerate the process but not much, the idea is to transform the
regolith into soil for food crops.
This situation might argue for making your own dirt, by grinding up
rocks with less hazardous material and chemical properties. Grinding
up rocks to make soil? A waste of energy? Perhaps not. Maybe it
would just be a by-product of the sort of machining that would fall
right out of tunneling, and in general out of developing underground
habitat. Lava tubes are, of course, made by volcanic lava flows.
Volcanic processes produce rich soil on Earth. (The biggest carrot I
ever saw was at a market in the Aso-san caldera on Kyushu in Japan.)
Perhaps the Moon and Mars are much the same in this respect, they've
just never had plants taking any advantage of it.
That makes sense to me. Even if you are growing non edible plants
solely fore the methane digester, avoiding those nasty sharp edges
is a plus.
Alain Fournier