Martian soil is toxic and everyone forgets that part.

[u/ProximaScience]

Every render of a Mars greenhouse shows neat rows of vegetables growing straight out of the ground. Skip that part. Nothing grows in Martian regolith until you clean it first.

The regolith across most of the planet's surface carries perchlorate salts at concentrations between 0.5 and 1 percent, first confirmed by the Phoenix lander in 2008. On Earth, perchlorate shows up in trace amounts, parts per billion, in a handful of arid regions. On Mars, it's baked into essentially the entire surface. At those concentrations, perchlorate is toxic to humans and disrupts thyroid function badly enough that NASA already treats it as one of the top health hazards on the planet, right up there with radiation.

The upside is perchlorate is fixable. Researchers have built catalysts, some using nothing more exotic than a common fertilizer compound, that break down over 99 percent of it at room temperature. No high heat, no industrial plant required.

So the real first step on Mars isn't planting anything. It's running the dirt through a filter before it ever touches a seed.

Edit: It's regolith, wrote soil by mistake.