Think about it: if we got rid of all computer developers, what would happen? The internet would fail. Technology would regress to the level of perhaps the 1960s or 1970s. But, we would still be able to live relatively normal lives, just with snail mail and rotary phones rather than email and smart everything. How about if we get rid of all electricians? Things regress rather further. Gas becomes really important. We start living by the sun again rather than flipping switches to stay up in the dark of night. But, suppose we get rid of all the farmers? Civilization collapses. Billions starve. Thousands of species go extinct as desperate humans try to find anything remotely edible for dinner. Nearly all technology regresses out of existence, since we no longer have time to maintain any of it, because we are too busy trying to find food.
So, farming is important. It is even more important on Mars, where there are (we presume) literally zero natural food sources. If we can't farm on Mars, we can never make the planet self sufficient. If we can't be self sufficient, then why go? It will always be prohibitively expensive to ship food from Earth, and what if we miss a shipment? The colony starves to death. Even if we did feed Mars Berlin Airlift style, we would only guarantee that the colony could never grow freely, and the main point of going to Mars is to achieve Earth independence for mankind. Basically, this is absolutely necessary. Farming on Mars must work.
But, can it work? On Earth, we have a billion years of life forms growing and decaying and growing again, all piled up in a rich, glorious, life-giving layer of sediment called topsoil, which provides plants with pretty much anything they need to grow. But, on Mars, this nutrient layer is completely nonexistent. The soil composition of Mars is hard to tell precisely; we honestly don't know very well what it is like. The satellites, of course, can use infrared and gamma imaging to detect precise chemical compositions, but these only function to a maximum depth of a couple millimeters for infrared, or a few centimeters, for gamma. So, we can talk about topsoil, but not about anything deeper than about the length of your hand. We can guess our way deeper than that by looking at craters, where a lot of digging has been done for us, but wherever there are no craters, we have no information.
The good news is that in most areas the soil seems pretty similar to Earth. There's more iron, and less water and nitrogen, but they are all present. The Ph results have been inconclusive, but most figures seem to be mildly alkaline, leading to a variety of carbonate minerals that will help plant growth. Really, all the stuff for growing exists already, except any sort of fertilizer. Now, I'm neither a geologist nor a farmer, and the articles I read on this matter were admittedly a little beyond the scope of my knowledge, but as far as I understood them, the prognosis for farming is actually good.
The colonists will also have some advantages. All irrigation will be mechanized, and all habitat precisely controlled, so the growing conditions in terms of temperature, rainfall, and whatnot should all be optimal. Furthermore, since we will be mixing our own atmosphere, and since CO2 is the main component of martian air, we can easily optimize the CO2 level for plant growth (roughly 2% is what plants like best, which is substantially more than Earth atmospheric levels, but still quite comfortable for human breathing). Most of all, as long as we are selective about the bacteria and fungi we bring to Mars, we can set up an ecosystem that has no plant diseases for the first few decades, and once multiple colony domes are established, the isolated nature of each settlement will protect neighboring food supplies from any blights that might occur locally. Unlike humans, Martian crops will never ask to go on vacation to Earth, so they probably won't need to update their immune systems as often as we do (though it would be wise to breed disease resistant strains just in case), and intentional exposure will not be important. No "Disease Day" for plants.
The bad news is that sun intensity ranges from half to about one third of the light on Earth, there are sometimes global dust storms which block the sun for days, and we'll be starting from zero in terms of any biological components of soil. That zero start point cannot be stressed enough; the other problems may mean slower growing and occasional difficulties, but they can be solved with sun lamps. Easy. Meanwhile, getting fertile soil requires coordinated colony-wide efforts from day one onward into eternity, or it will simply not work. It will be like trying to grow plants on unfertilized sand, which as any gardener knows, is exceedingly difficult. Even though the necessary chemical ingredients are present, they may not be easily accessible to a growing plant. Furthermore, the stakes are very, very high: if the plants don't grow, at least fifty people starve (hopefully more like a thousand), and the world loses about a trillion dollars worth of invested capital. So, it is important to get it right.
There are two ways to deal with this. The first is using every bit of human, animal, food, and biodegradable waste as fertilizer, right from the start. We'll need to get over our qualms and pour our sewage on our farms to help get things going. This will help create a closed loop for existing biological material to be recycled. We'll want to lay a sort of underground "floor" to trap irrigation water, and prevent valuable bacteria from leaching into the rocks below, where plants can't reach them. As we develop the soil more and more, we can expand and add local resources to our loop to make it wider.
The second thing we can do is hunt on Earth for forms of bacteria that eat rocks. These creatures, called lithotrophs, are some of the most primitive life forms on Earth, using neither any other living thing, nor the excretions of any other living thing, to survive. Fortunately, they are quite common even today. Unfortunately, most of them secrete chemicals that are not very useful for improving soil quality, such as sulpheric acid. However, if we can find the right sort that will take the soil chemistry of Mars and turn it into something useful, we can seed them onto Mars in domes that are set up by robots before we arrive. With no predators or competitors, they should thrive, eat lots of rocks, die, and enrich the soil for us. The advantage is that the colonists will land with a ready made dome already on the ground, complete with breathable air, temperature control, and a primitive sort of topsoil for farming. The disadvantage is that to do this, we must first invent the robot that can build the dome, and it means at least two flights out instead of one.
Normally, I prefer not to recommend inventing something in order to solve a problem, because inventions usually happen in their own time rather than when you want them to, and cost a lot more money than simply changing existing technology to suit your needs. But, given the supreme importance of this aspect of Martian colonization, in this case I think it would be foolish not to give ourselves every possible advantage, and plan for every possible contingency. If the farmers fail, then everybody dies. Seems they had best not fail.
Now, all of this says nothing about the difficulties that might be inherent in low gravity farming because really these problems are minimal. Gravity is necessary for plants to develop strong root structures, but the amount required seems to be pretty small; microgravity, in theory, is sufficient. The main thing is that the dirt needs to be compacted enough for the roots to grab on to, and hold the plants upright. However, this really isn't a major problem, as it turns out; we've been growing plants in zero gravity in space for a while now. In fact, the Russians grew wheat all the way back on old space station Mir.
There are the BASICS of farming on Mars. I didn't delve deeply into soil chemistry, so this topic may arise again later, but for now, this will have to suffice. Final summation: doable, but difficult. To hedge our bets, we can invent robots that build domes without us, and start biological processes before we arrive. Otherwise, our best bet is to spend a few billion dollars to send a few tons of this along on the colony ship:
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