What...No Soil?

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!)

Background: Meeting Plants' Needs Hydroponically

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.

Laying the Groundwork

  • Consider laying the groundwork for a hydroponic project by having students brainstorm a list of the purposes soil serves for plants. This will probably include such factors as providing support or stability, and carrying water, nutrients and oxygen to plant roots.
  • Have students brainstorm another list of ways in which plants grown without soil could have the same needs met.
  • In addition to referring to the soil background and activities on pp. 101-115 of GrowLab: Activities for Growing Minds, found in the Kidsgardening store, consider using the "Laying the Groundwork" section from the "Earthgrippers" activity to stimulate thinking about the relationship between roots and soil.
  • Encourage student suggestions for hydroponic setups, and have students discuss how each suggested design would meet plant needs. Share background information from this article as appropriate.


  • Have students set up their own designs (see Exploring Classroom Hydroponics, on the Kidsgardening Web site) or used a commercial model.
  • Consider experimenting with different types of designs. Don't let lack of equipment, such as an aquarium pump or pH meter, stop you. You're likely to see results from even a basic design. You might try, for instance, growing plants with and without adjusting the pH or with different types of growing media such as and, vermiculite or rockwool. Students will likely want to compare hydroponically grown plants to those raised in soil mix.
  • We suggest beginning with such crops as lettuce, herbs, and houseplant cuttings, and experimenting with other plants once you have a feel for the setup.

Making Connections: Discussion Questions

  • How did your setup meet different plant needs? Were there some needs that were not well met? What did you observe to make you believe that?
  • If you compared plants grown with and without soils, did you observe any differences? What were they? What do you think caused them?
  • Where, in nature, have you seen plants growing without soil? Did they appear to have specific adaptations to grow in a soilless environment? What were they?
  • Can you think of any purposes of soil that are not fulfilled by hydroponics?
  • How do you think hydroponic growing could be used in world agriculture? What type of limitations do you think it has?
  • Would you rather raise a garden hydroponically, or with soil? Why?

Branching Out

  • Some people think that using hydroponics to grow food crops can reduce pest problems, result in less contaminated food, increase yields, be carefully controlled -- and as such, can answer many of our agricultural problems.
  • Others feel that hydroponics requires high energy and labor inputs and monitoring, and that overemphasizing them might diminish our concern for the soil and for ecologically benign agricultural practices. Your students may want to do further research on this controversy, and debate these questions in class.
  • Research aquatic plants and learn about the adaptations that help them lead soilless lives.
  • Try growing and comparing plants grown in salt water and fresh water.

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