Hydroponics is the practice of growing plants in nutrient-enriched water rather than soil.
Hydroponics is incredibly productive and requires little space. For example, Factor e Farm's experiments found that they could grow $1 of lettuce per square foot per week. And the Institute of Simplified Hydroponics has found that they can grow 2kg of vegetables a day on 20m2 of space . Indoor growing in a greenhouse greatly reduces losses to pests.
Aquaponics is a kind of hydroponics that uses the water from fish tanks. This eliminates the need for an input of nutrients.
 Open-source hydroponic nutrient
You can mix hydroponic nutrient cheaply using mineral salts that most chemical suppliers have. The necessary ingredients are:
- Monopotassium phosphate (KH2PO4)
- Potassium nitrate (KNO3)
- Calcium nitrate (CaNO3)
- Magnesium sulfate (MgSO4)
- Boric Acid (H3BO3)
- Manganese chloride (MnCl2)
- Zinc sulfate (ZnSO4)
- Copper sulfate (CuSO4)
- Iron sulfate (FeSO4)
- Ammonium molybdate
- Hydroponics - Everything You Need To Know To Get Started, big messy 463-page book, covering everything: lighting, nutrients, suppliers, different plants and their requirements,
- How-To Hydroponics, 4th edition, a compact, practical book including plans for several systems
- http://www.hydroponicsonline.com/ An informative site. Also has forums.
- Captain Hydroponics Urban hydroponic designs and tips.
 Open-source systems
- HAPI - An Open Source Project focused on building an automation platform, grow unit designs and educational materials
- general-purpose single-plant bucket drip system, CFL grow light cluster, simple deep-water culture systemfrom Jared Bouck's Inventgeek
- How-To Hydroponics kit plans
- Hydroponics free DIY plans
- Home hydroponic system, open-source plans for a vertically-stacked hydroponic system designed by n55