Martian Soil toxicity.

According to Wikipedia (https://en.wikipedia.org/wiki/Martian_soil)

Martian soil is toxic due to the presence of numerous perchlorate compounds.. On Earth any soil found to be so rich in perchlorates would either be mined for the compound or certainly be unable to support agriculture, or any life beyond a few super-specialized bacteria.

BUT: the first few million people going to Mars are not going to depend on Martian soil for their daily bread or tomatoes.

Hydroponics and aero-ponics (I think that is the correct term) will be the mainstay of food production. These methods are simply more efficient and economical when you are dealing with limited supplies of water, plant nutrients, space, sunlight and staff time. The only thing that Mars has in great abundance for agriculture is a carbon dioxide atmosphere.

Any plant parts that are not used for food (which if you are really determined about it can be a very small percentage - fermentation is your friend) will be recycled into more plant nutrients.

If and when colonists want to move to soil based agriculture there is a simple-ish solution to the perchlorate problem: H2O. Enough warm water will dissolve and or reactive with the perchlorates, allowing for simple mechanical methods like filtering or evaporation to remove and isolate the offending compounds. If water is not readily available in sufficient quantities good ol' bakin' and shakin' will do to. Setup a solar thermal concentrator that can heat your reaction chamber up to a few hundred odd degrees Kelvin (1000 would be a nice round number) with an extremely high atmospheric pressure (like a few Earth atmosphere equivalents). Expose soil, give it some time to get hot and shake it around to loosen up things. Then reduce the atmosphere pressure via release valve. Between the photons and the rush for lighter elements' atoms to fill the 'vacuum'. What is left behind will be dead and dry, but at least it won't be toxic anymore. And you now have a pressure vessel somewhere filled with the baked off volatiles. Any long term colony is going have uses for those.

For the curious: NASA cooperated with another Organisation on a study called MAGIC (which stands for Mars AGricultural something something). It proposes an automated hydroponic greenhouse as a supplement to crew meals.

Anyway.

Regards
Frank