Hydroponics is a technology for growing plants in nutrient solutions with or without the use of an artificial medium to provide mechanical support.
For nutrient solutions we understand water containing the appropriate salts and compounds. Plants rely on water and nutrients (that must be in their ionic form), for this reason, it is impossible to replace the water for the nutrient solutions. Either water is carried for an extended lunar mission or hydrogen is carried and converted to water with lunar oxygen or ice deposits on Luna must be mined for the water. The first steps of the lunar colonization could be carried out remotely with machines controlled by operators on Earth. It is necessary to calculate the amount of water to be used for the hydroponic system.
See also: Plant nutrients
The most important dissolved ions needed are Ca2+, Mg2+, K+, NO3- (nitrate), SO42− (sulfate), and H2PO4- (dihydrogen phosphate).
Commonly used chemicals for the nutrients include potassium nitrate, calcium nitrate, potassium phosphate, and magnesium sulfate. Various micronutrients are typically added to hydroponic solutions to supply essential elements; among them are Fe, Mn, Cu, Zn, B, Cl, and Ni. Chelating agents are sometimes used to keep Fe soluble.
The question regarding how to manufacture these nutrients on the Moon is still unanswered. However, water might be saved using aeroponics: growing plants in an air/mist environment with no soil and very little water.
Aeroponics is a type of hydroponics that economizes 98% of the water. It consists of spraying the roots of the plants with the nutrient solution. NASA has been using this technique in experiments with the space shuttle and International Space Station since 1990.
Aeroponic systems are much better with the oxygenation of the plants roots. Oxygen/air intake is very important to growing healthy plants. Both the mist or fog allows the root systems to grow many fine hair-like extensions on the roots because of the small particle sizes of the water droplets. This significantly increases the surface area of the root system, and allows the roots to take up both the nutrients and oxygen/air more efficiently than they would in a typical hydroponic system. This factor also allows the plants to grow faster than they would in a typical hydroponic system.