How can lettuce grow with its roots in water?

Lettuce can grow with its roots hanging into a carefully managed watery solution. This is hydroponics: growing plants without ordinary soil. A support holds the plant upright, while water carries dissolved mineral nutrients to its roots.

Soil normally provides support, water, air spaces and nutrients. A hydroponic system must provide these needs in other ways. Roots take up water and dissolved substances such as nitrogen-containing nutrients. The leaves still need light and carbon dioxide to make sugars. The nutrient solution is not a replacement for sunlight, and plain water alone does not supply everything the plant needs.

Roots also need oxygen for the chemical processes that keep their cells working. In some systems an air pump adds bubbles to the water; other systems leave air around part of the root or pass a shallow stream of solution beneath it. Submerging every root in still, poorly oxygenated water can cause trouble even though plenty of water is present.

Growers manage the concentration and acidity of the solution as the plants use it. Recirculating water can reduce waste, but it can also carry a plant disease from one root area to another. Hydroponics reveals something useful: soil performs several jobs. Growing without soil works only when those jobs are supplied in another way.