Hydroponics
Hydroponics refers to the technique of growing plants without soil.[14] In hydroponic systems, the roots of plants are submerged in liquid solutions containing macronutrients, such as nitrogen, phosphorus, sulphur, potassium, calcium, and magnesium, as well as trace elements, including iron, chlorine, manganese, boron, zinc, copper, and molybdenum.[14] Additionally, inert (chemically inactive) mediums such as gravel, sand, and sawdust are used as soil substitutes to provide support for the roots.[14]

The advantages of hydroponics include the ability to increase yield per area and reduce water usage. A study has shown that, compared to conventional farming, hydroponic farming could increase the yield per area of lettuce by around 11 times while requiring 13 times less water.[15] Due to these advantages, hydroponics is the predominant growing system used in vertical farming.[1]

Aquaponics with catfish
Aquaponics
The term aquaponics is coined by combining two words: aquaculture, which refers to fish farming, and hydroponics—the technique of growing plants without soil.[16] Aquaponics takes hydroponics one step further by integrating the production of terrestrial plants with the production of aquatic organisms in a closed-loop system that mimics nature itself.[1][16] Nutrient-rich wastewater from the fish tanks is filtered by a solid removal unit and then led to a bio-filter, where toxic ammonia is converted to nutritious nitrate.[16] While absorbing nutrients, the plants then purify the wastewater, which is recycled back to the fish tanks.[1] Moreover, the plants consume carbon dioxide produced by the fish, and water in the fish tanks obtains heat and helps the greenhouse maintain temperature at night to save energy.[16] As most commercial vertical farming systems focus on producing a few fast-growing vegetable crops, aquaponics, which also includes an aquacultural component, is currently not as widely used as conventional hydroponics.[1]

Aeroponics

Aeroponically-grown chives
The invention of aeroponics was motivated by the initiative of NASA (the National Aeronautical and Space Administration) to find an efficient way to grow plants in space in the 1990s.[1][17] Unlike conventional hydroponics and aquaponics, aeroponics does not require any liquid or solid medium to grow plants in.[18] Instead, a liquid solution with nutrients is misted in air chambers where the plants are suspended.[18] By far, aeroponics is the most sustainable soil-less growing technique,[18][1] as it uses up to 90% less water than the most efficient conventional hydroponic systems[1] and requires no replacement of growing medium.[18] Moreover, the absence of growing medium allows aeroponic systems to adopt a vertical design, which further saves energy as gravity automatically drains away excess liquid, whereas conventional horizontal hydroponic systems often require water pumps for controlling excess solution.[18] Currently, aeroponic systems have not been widely applied to vertical farming, but are starting to attract significant attention.[1]

Controlled-environment agriculture
Controlled-environment agriculture (CEA) is the modification of the natural environment to increase crop yield or extend the growing season.[19] CEA systems are typically hosted in enclosed structures such as greenhouses or buildings, where control can be imposed on environmental factors including air, temperature, light, water, humidity, carbon dioxide, and plant nutrition.[19] In vertical farming systems, CEA is often used in conjunction with soilless farming techniques such as hydroponics, aquaponics, and aeroponics.[19]