A Breath of Fresh Air

Getting Oxygen to the Roots

Air . . . Where? To explore how much air can be contained in soil, have your students place a measured amount of coarse sand in a beaker or graduated cylinder. Ask them to determine how much water they can add before the water begins to puddle at the top, and to note the air bubbles that come to the surface as the air is displaced by the water. The volume of the water absorbed is an indication of the volume of air previously contained in the soil.

It is sometimes difficult for students to realize that even roots buried in soil must have oxygen for the plant to survive. Plants respire by taking in oxygen, which triggers plant cells to release and use the energy manufactured during photosynthesis, while also releasing carbon dioxide and water. Plant roots typically take in oxygen that's available in the small spaces between soil particles.

Classroom Tips: Supplying Oxygen

Hydroponic systems often use a pump to infuse oxygen into the water. For small setups, such as the Soda Bottle system described in this guide, aquarium pumps do the trick. In some systems (particularly commercial ones), the medium and roots are periodically splashed or flooded with a nutrient solution, allowing oxygen to bathe the roots in the interim.

In short-term passive systems, there are other means of getting oxygen to the roots. In some setups, water and nutrients reach the roots via a wick made of absorbent material, and part of the roots are continually exposed to air. A porous medium like rockwool has a tremendous capacity for retaining oxygen while also absorbing nutrient solution. In some otherwise passive systems, like the Simple Straw Aeration described in this guide, human bubbles do the aerating! Greens such as lettuce and herbs seem to be the best bets for a minimally aerated environment.

 

 

 

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