Establishing agriculture on Mars may be achieved in a variety of ways. Various methods have been proposed by the scientific community, and may differ in strategy, scale, scope, and desired result, but all involve the basic elements of mitigating hostile native conditions and selectively introducing non-native flora and fauna. For the purposes of this article, "agriculture" excludes hydroponics, which is a topic equal to its own exhaustive examination, and focuses on more traditional soil-based agriculture (this is not to dismiss or minimize hydroponics, but to differentiate between the possible methods).
A daunting gauntlet of problems would need to be resolved to establish an agriculture on Mars capable of sustaining a permanent human colony. In the minimum, plants need air, soil, water, and nutrients, all of which exist on Mars. The problems, then are in making the elemental needs of plant life available to plants that humans choose to grow. In the presence of all the needed material and science needed to achieve the goal, the major impediment that remains is the collective barrier of cost, effort, and timescale required to achieve it.
Obstacles to Agriculture on Mars Edit
The first and most obvious obstacle to introducing human-sustaining agriculture to Mars of the lack of adequate nitrogen and carbon dioxide in the atmosphere. The air is simply too thin on Mars to sustain much beyond a wind only strong enough to redistribute dust around the planet. Any known flora would quickly die from lack of air.
A subset of the lack of atmosphere on Mars is the lack of ambient warmth. On the very surface - the actual material ground exposed to the sun and air, and a millimeter or so above it - actually gets warm in the summer months. Up to 70°F. But move a centimeter above the ground, and the temperature drops precipitously to well under 100°F below freezing. The reason for the dramatic difference is the thin atmosphere. Air needs to be fairly dense to transmit thermal energy. Because of this direct and close cause-and-effect relationship, atmospheric warmth is not addressed as a separate issue.
The term "soil" cannot be applied to the sterile and caustic regolith that makes up Martian dirt. The visually pleasing image of a red Mars turning green as introduced flora spontaneously spreads across the planet is an impossible fiction.
Early studies of Martian soil indicated that it contained life-sustaining elements such as potasium, magnesium, and sodium, and scientists as late as 2008 proclaimed Martian soil to be viable for plant growth. However, that declaration was premature, as later, more thorough analysis of Martian soil derived from wider geographic areas lead to the opposite conclusion: Martian soil is, to be blunt, poisonous.
Martian regolith is not just sterile, it is caustic. The native regolith is so saturated with superoxides like potassium hydroxide that attempting to grow plants in it would be akin to planting a seed in "Oxy-Clean"laundry bleach,and expecting the seed to grow. Impossible. There is a silver lining to this problem, though. As will be discussed later, these superoxides can be extracted and used in a variety of practical ways, leaving behind a less caustic dirt as a base to formulate life-sustaining Martial "soil."
The scale of agriculture needed to sustain a colony depends on the size of the colony and the dietary decisions made on a social level. If feed animals are a part of the nutrient base of the colonists, then the scale of the necessary agriculture grows dramatically, as those animals will need to be fed. In the context of planetary terraforming, versus enclosed biospheres, the scale is planetary in a larger sense, but in the specific area of soil, the scale need be only as large as the needs of the population. All dirt on the planet need not be made hospitable to plant life - only enough to grow the required food bulk, plus other flora as needed for atmospheric enhancement.
Unexpected circumstances and unexpected consequences of terraforming activity are an unknown but 100% expected future reality. Mutation of introduced organisms, native mineralogy, and a host of other aspects of evolving conditions will raise issues as the planet progresses toward a human habitable state. If mankind is lucky, those problems will present only technical issues to be surmounted, and not prove catastrophic.
Overcoming Obstacles to Agriculture on Mars Edit
There is so much oxygen locked away in Martian regolith, that future colonists will not want for breathable air. In fact, they will suffer from an embarrassment of oxygen riches to the extent that they will pump huge quantities of "excess" oxygen into the atmosphere. This oxygen will be a byproduct of the mechanics of removing superoxides from Martian regolith to produce "sweet" Martian dirt suitable for plant growth.
Similarly, carbon dioxide is a major component of the regolith. Releasing it from its bonds is a purely technical issue, and on a large enough scale, can produce more than enough gas "bulk" to generate significant atmospheric pressure.
Water vapor is another greenhouse gas which would become a major component of the new Martian atmosphere, once enough air mass was produced to hold solar heat. At the point where the air is above the freezing point for significant stretches of time - seasons - then the work of generating water vapor will become a focus of future terraformers. More than any other gas, water vapor will increase air pressure to human life-sustaining levels.