Martian soil is full of salts of any kind. They must be removed.
"We also found a variety of components of salts that we haven't had time to analyze and identify yet, but that include magnesium, sodium, potassium and chloride."
There are two ways to deal with this. One is to try to remove all the salt, the other is to use plants that are salt tolerant or even salt loving. Removing all of the salt would likely take up a huge amount of water, which will be an incredibly valuable resource on mars.
However, many organisms can tolerate huge amounts of salt, and some even require these amounts to survive. Some bacteria do this by storing the salt in special structures (just like with the metals). These could be grown by mixing soil with a little bit of water to make extremely salty water and then harvested, making the mixture less salty with each harvest.
http://en.wikipedia.org/wiki/HalophileHumans actually require some amount of salt as well, making NaCl (sodium chloride) a valuable resource. Magnesium is a trace nutrient required by many organisms as well, and potassium is required by just about all plants and animals. So growing salt loving bacteria and harvesting them could yield useful products in its own right, rather than just being a method for cleaning the salt out of the soil.
Another interesting plant that could be grown in high salt soil is Salicornia. Although it can't grow in as salty a soil as the bacteria, it could be the next stage for the soil after the first cleaning. The plant is edible by people and animals, some species yield oil in their seeds, and because it also stores salt in vacuoles it would extract salt from its growth medium and leave it less salty.
Some people in coastal areas are experimenting with using effluent from aquaculture (shrimp mostly) to fertilize Salicornia. So there's even a possibility of salt-water aquaponics being a part of the soil production system!
http://en.wikipedia.org/wiki/SalicorniaSpartina, a common marsh grass, has special glands that extrude salt from their leaves and can be easily scraped off. You could grow it, scrape the leaves once in a while, and compost the dead plants, leaving you with good organic matter and less salt in the soil.
http://www.dnr.sc.gov/marine/pub/seascience/dynamic.htmlSo there are many options in terms of plants and bacteria that you can grow in a highly salty soil, and these can be used to get the salt out, allowing you to grow less tolerant plants in subsequent stages. I picture a long greenhouse with different stages, on one end is closer to raw soil and on the other end is soil that we would actually recognize as such.
The mineral content of Martian soil suggests that it would form a sandy, highly acidic, saline soil if enriched with enough water for plant growth. It typically lacks carbonates (resulting in a high inherent acidity), and no ready sources of carbonates appear available in the northern plains of Mars to enrich Martian soils using in situ resources. Its lack of humus and clays also combine to give it a high water flow rate with poor water retention. These factors make it useless for agricultural purposes in its native state. Untreated Martian soil is likely toxic to most crop plants and unsuitable for plant growth as either a fertilized soil or a hydroponic medium.
Because of its strong oxidizing power, Martian soil acts like concentrated bleach and could burn rubber and plastics.
Another problem is with soil's pH. It can be turned normal, but that requires to add another compounds and buffers.
Acid is a lot more difficult to deal with, but there are Acidophile organisms as well... there is also the possibility of using ash from some other process, since ash is very basic and could neutralize the soil. You could use the spartina from above to make the ash, but the problem is burning things takes oxygen, which is precious and in short supply on mars.
There is also a problem of preparing transformation of small rocks, minerals and chemical compounds in situ into SOIL (which from definition is a life-supporting structure, beying alive itself. ).
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Let me just elaborate that the problem isn't with the technology of turning mars-soil into fertile-soil. The problem is how to do it economicly. Today - if we wished to transform 1 T of mars soil, we would have to bring to Mars 1,5 T of chemical compounds and instruments to do so...and that's pointless. We can still make compost tho'...but it'll take many ears before we could gather enough of it.
This is likely to be one of the most important activities for any colony on mars, since without soil things like growing food and making oxygen become hugely expensive industrial processes. Expect soil creation to be a huge, difficult undertaking, but not an impossible one. The good thing about using biology is that you don't have to carry tons and tons of chemicals with you, you can just bring spores or seeds and grow the chemicals you need.
Aquaponics, growing food, making soil, producing oxygen and even mining metals and salts are likely to be tightly connected processes. There are organisms on Earth that can do almost any chemical reaction, and by using some as they are and recombining the genes others we should be able to (relatively) easily make what we need with what we've got. If we try to make a machine for each of these processes individually, though, it will get out of hand pretty fast.