A Breath of Fresh Air: Getting Oxygen to the Roots in the Garden
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 growing 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 solutions. In 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.
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.
Application to the School Garden
Now that your students have a better understanding of the volume of air in the soil, have them think about what happens when portions of that air space in the soil is taken away. The roots have less air and won't be as healthy which consequently means, the plant won't be as healthy. Have your students think of ways the air spaces in soil may be taken away by humans. Some examples include compacting the soil by walking on it, overworking the soil, and overwatering. It is important that we, as gardeners, understand how to care for our plants from the ground up!