Let’s presume the collective world has gotten out of its space-exploration budget-funk, and has decided it’s worth sending people to Mars. The first have gone and have returned, and now it’s time to set up a colony.
Of course that colony will have to be self-sufficient. Thankfully, there are a number of very useful things on Mars, so the colonists won’t need to bring everything, but the more they can find locally, the better. Ideally they should not need anything from elsewhere to survive after an initial supply of resources.
Most importantly, the colonists will have to grow their own food.
At its distance from the Sun, sunlight on Mars is about 40% up to half that on Earth, but still sufficient as power source.
Unless the colonists choose to grow plants in hydroponic systems, they will have to use Martian soil. Preliminary analysis shows that the basic composition of Martian soil is fairly similar to certain soil types on Earth. To be sure, no one has ever taken a sample from Mars and returned it to Earth, but we have two methods for determining composition of Martian soil and/or rocks. In the first place, there are meteorites that have reached Earth from Mars, of which 34 are known. Without going into a lot more detail, they are all igneous rock, in other words, hardened lava. These rocks tell us about the composition of the Martian crust, but very little about sediments, water and free elements in the soil. Secondly, in 2008, the Phoenix lander carried equipment that allowed it to perform an in-situ analysis of the soil near Mars’ north pole. The soil turned out to be alkaline, and contain elements necessary for plant growth, such as sodium, magnesium, potassium and chloride.
However, things are unlikely to be as simple as that. For one, it’s not correct to refer to the substrate as ‘soil’ (but failing a different, similarly evocative word, I will use it anyway), since the definition of soil includes the presence of organic material. The Martian regolith (which is a word I should use instead) contains none. Plants need soil for two basic purposes: for anchoring, and for their nutrition. Plants can anchor themselves in virtually anything (which is why you get plants growing in concrete and on roofs), as long as they have enough water and food. Water is available on Mars in the form of ice. The colonists will be deposited somewhere close to a source of this ice.
But what do you think would happen if you wet a soil that has been dry for billions of years? A soil that has been subject to direct radiation and dust storms. For one, there will be a lot of fine material. On Earth, growing crops in heavy, gluggy soils with lots of fine particles (clays) is hard. Plants need adequate aeration to grow properly. If the soil becomes too compacted (either because it is too fine or because people walk over it) plants don’t grow properly. This is why there is frequently no grass in a soccer goal. So you’ll have to get rid of excess dust before you wet the Martian soil else the stuff will turn into something akin to cement and be of no use for cropping. Also, there will be something like four billions years’ worth of accumulated salts that are freed if you wet this soil. When the Phoenix lander wetted the Martian soil, it released perchlorates, which are poisonous to plants. The soil pH was alkaline (8.3), which indicates accumulated salts of one type or another.
There are plants, notably those that are native to deserts, that tolerate a high salt concentration in the soil, but in order to grow highly-strung and finicky crop species, it’s necessary to get rid of excess salt. This probably requires a lot of (recycled) water, but at the very least, it will require lots of detailed testing, and time.
Next: the air. Mars has lots of carbon dioxide. Plants grow better with a higher concentration of carbon dioxide than present on Earth. It makes sense to run the glasshouses with a high carbon dioxide level. But at high carbon dioxide concentrations, the plants are likely to take up an increased percentage of poisonous elements from the soil, such as arsenic, cadmium and lead. Some areas of Mars are known to have fairly high concentrations of arsenic.
The big trouble is going to be nitrogen, because there is very, very little of that on Mars. Nitrogen is an element that does not easily form bonds with other elements. Whereas oxygen facilitates chemical reactions by reacting with other elements (for example by burning or rusting), atomic nitrogen just does… nothing. Since we have seen in an earlier post that Mars is too small to hold onto its atmospheric nitrogen, there may not be all that much of nitrogen to be found on Mars in any of its forms. So unless early explorers locate a deposit of nitrogen-rich substrates, the colonists will have to import nitrogen from elsewhere. And nitrogen is the most essential of the essential elements for plant growth.
All of which is not suggesting that using Martian soil for cropping is impossible, but that it likely isn’t easy or straightforward, and that early colonists are probably better off starting with hydroponic installations while all this other testing takes place.