Trying to scour the Internets for any reliable studies regarding IN-SITU resource production and make an easy to read, easy to understand Mars Colonization Handbook. Mars and science in general should not be restricted to established scientists, maybe the next big scientist is you, so I am trying to make you understand it and enjoy it. Science is so fun.
We have talked about the science of producing air, water, energy and about the techonology of getting there in the first place, and now we will address the wannabe Martian farmers.
Source: free to use or share
The science of growing plants
We are going to start by understanding what does a plant essentially need in order to grow and thrive. The process in which plants get their energy is called Photosynthesis, and it's the process of converting light energy into chemical energy. The carbohydrate molecules, like sugars, are synthesized from CO2 and H2O. Oxygen is also released as a byproduct of the reaction. I am not going to enter into details regarding photosynthesis, a fellow Steemian did a wonderful in-depth article that I could never compare to. I am only going to do a quick chemical outline of photosynthesis:
6CO2 + 6H2O -> (in the presence of light) -> C6H12O6 + 6O2
By now we should already know that Mars has a CO2 rich atmosphere and by the time we would need to do farming on Mars we should have already mastered the art of creating oxygen, like it was covered in my previous articles. Water and energy in the form of light would be already supplied by the IN-SITU systems.
The process of research starts with the basic question: Was it attempted or done before?
Of course, we are already pretty advanced with the research here. We have started in 1982, on the Soviet Salyut 7, continued through the Skylab, Mir and ISS, which had what was named "Veggie", the first real vegetable garden in outer space. Many plants were grown, tested and even brought back to Earth with the astronauts eating a part of the harvest on a regular basis for the last 10 years. Here is a short video to get you up to speed with what is considered known about farming in space.
Source: NASA’s Johnson Space Center
Creating the first garden on MARS
According to a study published in the US National Library of Medicine, the first few experiments will analyze using the higher plants for photosynthesis, and hydroponics was the chosen method of growing them. Because the reduced sunlight would affect the photosynthesis process and because of the cold temperature variations, the only method of growing crops before terraforming is to grow them in closed environments.
Example of hydroponics. Notice the soil absence.SourceHydroponics is the method of growing plants without soil, only by dissolving nutrients in water. By using perlite or gravel to cover the otherwise exposed roots of the plants (which Mars could supply), only the seeds and the nutriments need to be brought from Earth. NASA is the greatest researcher in this field, managing to use LED lighting in a different color spectrum with increased efficiency. The use of a bigger hydroponics farm could also be used as a bioregenerative life support system. Although there are other variations of the hydroponics such as aeroponics or fogponics, the system's process is largely the same, not needing any subsoil or soil material to grow. Water filtration systems could help us reuse the water or simply use waste water from the colony. More reading about filtration could be found in 
Plants and low gravity - low pressure environments
Plants are able to grow and survive in microgravity but there is more research needed to tell if there are any long term effects of ingestion in terms of the plants chemical structure. Some other studies found out that growing plants in microgravity or zero gravity could increase the risks or flooding or droughts by simply having too much or not enough water just floating around the roots without it being able to flow. In the case of Mars it appears that this is not an issue and the reduced gravity might even play a beneficial role in reducing the water and nutrients needed by the farm, by slowing down the rate of which they flow into the subsoil and away from the roots, thus increasing their exposure time.
Example of a big dome. On Mars it would be made out of sturdier materials and it would most likely be transparent.Source
Pressurizing a field at the Earth's pressure of one atmosphere needs a lot of engineering and structural support. Thus growing the plants in a pressurized system is much more expensive than just protecting them from the cold, so by using special types of plants that can survive under low pressures, there could be a reduction of a full degree of magnitude on the pressure. Plants can indeed function at a tenth of the Earth's pressure, according to the study. As a further point of research, the GMO industry could come up with even better optimizations for the Martian Atmosphere, including plants that can maintain photosynthesis at very low temperatures or plants that need a reduced intake of minerals from Earth .
What about minerals from Mars? Can we use nutrients produced IN-SITU?
To carry out the photosynthesis process, many nutrients are needed by the plants, the most important of them would be Nitrogen , Potassium and Phosphorus.
Nitrogen (N) is the main component of chlorophyll, responsible for the green color of plants and for facilitating photosynthesis.
Potassium (K) helps to open and close the leaves pores, regulating the water and carbon dioxide (CO2 quantities.
Phosphorus (P) is the main ingredient of photosynthesis.
Thus finding these minerals to be used as fertilizer will be very important in the food self-sufficiency of the colony. It is worth mentioning that human feces could be an important part of obtaining some of these minerals, but through mining, the other minerals could be located and produced locally.
By revisiting our old graphs on the atmospheric composition we can find out that almost 3% is Nitrogen. So we have the following calculation:
Total mass of atmosphere: ~2.5 x 1016 kg
Nitrogen (N2) - 2.7%
Total mass of atmospheric N2: 6.75 x 1014 kg
Surface Area: 1.4437 x 1014 m2
Illustration from NASA's 1980's A Case for Mars studyThis means that there is sufficient Nitrogen to sustain a thick enough soil to cultivate plants or even plant some trees with deeper roots. We just need to be extracting it from the atmosphere and fix it in the subsoil, at first in small amounts for the greenhouses built and then at a greater scale when terraforming options turn into a reality.
I am not able to discuss about this process at length here, called the Haber Process, since it would make my article super long and super boring but I have found 
There is evidence of other minerals in the composition of igneous rocks as some of the Mars experiments already have uncovered. To find more about igneous rocks, some fellow @SteemSTEM writers with far more understanding of the subject wrote about it (
Further research on each of the absolutely necessary materials to sustain plant life have given me optimistic results, all have been confirmed in sufficient quantities as to sustain plant development on a widespread scale.
Radiation
It could pose a big problem because of the thin atmosphere that can't protect the crops, and by using thicker and radiation resistant panels would completely obscure the even-so faint sunlight. We could always move the whole farming area underground or in volcanic crevasses or lava ducts but what if we choose to grow them on the surface?
Being born in the same year as the Chernobyl disaster (and close to it) made me very interested in the subject and as time passed I have observed the life around it refusing to wither. Pictures and studies made there show that plants tolerate the radiation pretty well, with only a 5% change in plant proteins, which don't necessarily get transmitted to the next generations. According to a study made by the Slovak Academy of Sciences, which I only read about on Astrobio.net, the plants proteomes seem to remember the early days of Earth formation, when radiation was ever-present and could easily adapt to radiation.
While radiation from Chernobyl is in no way compared to the radiation levels on Mars, the studies remain conclusive about thriving plant life in inhospitable conditions. Further research is needed to determine if the radioactivity changes in those 5% of the proteins would be transmitted to the humans or if GMO crops could identify and disable such changes.
Farming on Mars: It's a GO!
At least in enclosed spaces, using little more than a tenth of the pressure on Earth, which can easily be maintained using composite soft materials which don't take much space. Nutrients can be scavenged IN-SITU, usually as the byproduct of water mining that we discussed in an earlier article, and fertilizer from the colony can be treated and used in this matter, reducing the footprint of the colony even further. LED lighting can be used to increase the reduced solar flux as water and oxygen should already be supplied by the life support systems. GMO seeds could be used to make the plants yield even more by reducing some of the harder-to-get conditions on the surface and having colonists on the ground can make this progress even faster.
It took me a little too long to write this article, but I have tried to cover all aspects of such a vast undertaking. If you feel you can add something, please do so, it's our collective minds that will make Mars possible and I would love to hear from you. Also don't forget to talk about science to your children or friends. Maybe the next scientist is you or right there beside you :P
Additional sources and further reading: 1, 2, 3
https://www.space.com/9597-space-farmers-grow-crops-planets.html
https://www.astrobio.net/also-in-news/radiation-no-concern-for-space-crops/
Ask me, as I seem to be able to remember these things, even though I can't remember a five item shopping list :D
Additionally, if you are a new SteemSTEM member, or even if you are not but you want to contribute and better yourself in the process, I am open to give you all the guidance I can.
We are all scientists, it's how our brain works and we can all grow together as a community!
