NASA experiments with plants
Humans cannot live without food. It is obvious that we need to bring our plants with us wherever we go. Also, if conditions allow, we can grow animals.
Plants on other planets[]
Main article: Plants on new worlds
There are many important parameters that dictate how plants will behave on other planets and in human colonies.
Luminosity[]
Plants can use even the slightest source of light for photosynthesis. However, they also spend some energy for their own chemical reactions, to maintain life. Solar light on Earth is about 50000 lux. However, when exposed to a light dimer then 300...500 lux, plants will produce no oxygen, since all the oxygen resulting from photosynthesis will be used for respiration. Below 300 lux, plants will produce carbon dioxide. Also, plants need both red and blue light.
Personal experiments: I made some experiments that are easy to conduct. I placed plants (grain seeds and grass) inside concrete tubes, partially covered. By controlling the opening of each tube, I could mimic the amount of light received on a planet. These experiments proven that plants are able to grow and even make seeds at much lower luminosities then those found on Earth. The lower the luminosity, the weaker and more fragile plants will grow, but still they will tend to grow high, even higher then in natural luminosity. It looks like at the orbit of Jupiter, agricultural production will be severely affected and beyond the orbit of Neptune superior plants cannot survive. Also, I noticed that in low luminosity, plants reach maturity far later.
Reduced light means reduced photosynthesis, in almost equal proportions with the Solar Constant (the amount of light received by the planet). This automatically means that a cultivated field will produce less food and will decrease the Population Limit of the planet.
Plants need both red and blue light, while green light is reflected. To test this, it is very easy. If you put a plant inside a concrete tube and you cover the entrance with a colored sheet of plastic, you will notice the difference. I also tested this and I found out that plants covered with a red or a blue filter tested at the luminosity conditions of Jupiter did not survive, while plants tested with no filter, but at the luminosity conditions of Neptune did survive. Some stars, like M - type stars or B - type stars have an excess of red or blue light, but a deficit of other types of light. In this case, the outer limit for plant life I where the amount of either red or blue light reaches the amount found at Neptune's orbit.
However, plants are trying to survive in their environment. With less light, they compensate by adding more chloroplasts or higher amounts of chlorophyll. Still, Earth plants use only 1% of the light that reaches them. There are other pigments then chlorophyll used by some algae and bacteria, able to use other light frequencies, including near infrared. Genetically modified plants will be able to survive on outer planets, but there is a limit. Beyond a certain point, there is not enough light for any living organism.
Soil[]
Any plant needs microelements in certain amounts. Each terraformed world will have its own chemical structure, with high amounts of certain elements and deficits of others. During the terraforming process, engineers will try their best to ameliorate the soil, but it will be impossible to create something identical with Earth.
The Earth has a soil that has been transformed by water, plants and bacteria for a very long time. Terraformed planets will have a virgin soil. Depending on conditions, the ground might be rocky or it might be soft, composed of dust, sand or grabble. Softer rocks can also exist, but they will by far not be like the soil of Earth. Roots might find hard to grow in these conditions.
On Earth, water has washed away minerals from the soil. What plants now extract is in trace amounts. On other planets, minerals were usually not washed away. So, plants will have higher amounts of nutrients. As they grow, they will produce in time a tiny layer of organic soil. There are places on Earth, close to the Arctic, where the layer of organic soil is very thin, but of great quality. A fast way to create a layer of soft soil is to mix it with carbon. During terraforming processes, atomic carbon powder can be produced from the atmospheric carbon dioxide, by using genetically modified plants or in an artificial way.
Weather[]
Plants are adapted to Earth's climate. The good thing is that Earth has many different climates and plants are adapted to majority of them. For example, we have grains able to resist in drought regions and fast-growing grain crops, adapted for Siberia. In Asia's hot and moisture climate, there are many types of rice. So, plants could adapt easy to various climates that will exist on newly terraformed planets.
Animals on other planets[]
Humans don't eat just plants. We also need meat, milk and eggs. Growing animals in a farm could be possible on another planet. One of the major advantages of growing animals is that they can eat plants unsuitable for human food. On planets with very low luminosity, we could grow species of grass that usually live in dense forests, to feed animals or insects.
On paraterraformed and other artificial environments[]
Producing food will be required anywhere where humans are.
Low gravity worlds[]
Plants can easily grow on paraterraformed worlds, in low gravity. However, growing animals in such environments will be a challenge. Suppose someone tries to make a pig farm to sell space stakes on an asteroid used as a touristic destination. On Earth, pigs will stay behind fences, while in low gravity, they will jump over them up to the roof. Large animals, like cows, have a much more rigid skeleton and could not even be able to walk in low gravity. Their movements will simply push them to the roof. Small animals like rabbits could adapt far more easy.
Where space is limited[]
In space stations and spaceships, where there is not enough room for classic agriculture, food will be produced from algae. Additional protein can come from insects. In the Soviet Union, experiments were done with Chlorella algae and it was proven that these algae can provide all the nutrients needed for astronauts.
Food trade[]
Some places will not have all the food needed. For example, touristic stations between Saturn's rings or military bases will not have enough room for growing food. They will have to import all their goods. also, they will send carbon dioxide, human dejections and used water back.
Food is heavy and occupies a large volume. Shipping food also implies shipping residual products back (that need special containers and insulation from the food containers). Also, food will need to be frozen, or all the goods will not reach their destination properly. This means that food trade implies high costs and smaller revenues for transporters. Food shipping will occur on short distances (like between two moons or a planet and an orbital station), but it will never be feasible between two planets.