Imagine growing food in outer space, feasting on fresh, leafy lettuce during cold winter months, or getting 1,000 pounds of tomatoes from one plant. These are some of the visions that have drawn horticulturists and other scientists to research and practice hydroponics -- or growing plants without soil.
A number of you expressed interest in exploring hydroponics in your indoor classroom gardens. So we spoke with horticulturists, educators and hydroponic equipment suppliers to find out about hydroponic setups that would be relatively easy and inexpensive to implement in classrooms. While some controversy exists about the role of hydroponics agriculturally, raising plants without soil allows students to consider basic plant needs and view root growth, and provides a good context for devising experiments to test their questions. (The controversy also provides fertile ground for research and debate!)
The word hydroponics actually comes from two Greek words -- hydro, meaning water and ponos, meaning work. With hydroponics, a solution of water and nutrients surrounds the plant roots, doing the work of soil. Hydroponic devotees believe that a more efficient system results from feeding plants directly, rather than feeding soil to feed plants. It allows the plant, they say, to put energy into leaves and fruits rather than into growing more roots to search for water and nutrients. Plants grown hydroponically still have the same needs as those grown in soil. A discussion of how these needs can be met follows.
Nutrients. Soil contains mineral nutrients necessary to plants' survival. Plants in hydroponic systems need a fertilizer solution that provides all of the primary plant nutrients (nitrogen, potassium, phosphorus) as well as important trace elements such as iron, manganese, and sulfur. Many garden or houseplant fertilizers lack the trace elements necessary to grow crops without soil. If you find one that includes minor elements, try using it. Otherwise, look in science supply catalogs or write to the business listed in the sidebar for nutrients for hydroponic growing.
pH. Most of the plants you'lll raise in the classroom garden grow best between the pH range of 6.0-6.8. Because tap water differs in pH across the country, you'lll have better results if you test and adjust the pH of your tap water planting. If you have narrow range pH paper, available at aquarium supply departments, or an electronic pH meter, test the pH of your tap water and adjust it to pH 6.0-6.8. You can use white vinegar or lemon juice to lower the pH, and baking soda to raise it. Start by adding a small amount, testing pH, and increasing until you've reached the appropriate range.
Although soil pH changes slowly over time, evaporation and transpiration will continually change the pH of a hydroponic solution and result in too concentrated a nutrient mixture. Whichever setup you're using, replace the solution every week or two, adding fresh nutrients and adjusting the pH each time.
Stability and Oxygen. Most hydroponic setups have some material to support the plants. Sand, vermiculite, pebbles and a spongy, fibrous material called rockwool are typically used. Since roots require some oxygen, aquarium-type pumps are often used for small scale setups. However, the fibrous material, rockwool, allows some air penetration, and may be used successfully in a setup without a pump. For short-term school projects, a setup such as that illustrated in figure C of the Exploration, with no aeration, may be adequate.