Mixed Media

The material that a plant lives in or on is called its medium or substrate. For most plants, the medium is soil. Hydroponic growers find other ways to support growth to prevent drowning plants. Many setups use an inert, sterile medium. Some of the more popular choices included gravel, clean sand, perlite (a lightweight expanded mica), a lightweight pebble-like aggregate, and rockwool (an inorganic, spongy, fibrous substance that holds large amounts of water and air). These materials provide passages among the particles or fibers where air and water can circulate.

Clingy Plants Many plant species grow naturally without soil. Challenge your students to find out more about epiphytes, plants that grow on other plants, particularly in tropical forests. What are they? How do they survive? Where do they get their nutrients, air, and water? Why are they important to the total biosphere?

Each medium has strengths and weaknesses. Gravel and sand, for instance, provide support and good drainage, but can be heavy when wet and will dry out fast. Perlite is light and holds water well, but its fine dust can irritate lungs. (Sprinkle it lightly with water to avoid this.) Rockwool holds water and air nicely and makes it easy to move plants around, but breaks down fairly quickly. Your student scientists will benefit from exploring the properties and performance of a range of standard media and inventing some of their own.

Some hydroponics systems have no real media, but more or less elaborate ways of suspending plants in nutrient solutions. In commercial nutrient flow technique (NFT) and aeroponics, for instance, the roots lie or are suspended in a dark channel and nutrients are sprayed or trickled along the root zone. For a simple classroom system, see the Floating Styrofoam Raft.

A Medium Well-Done

 

 

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