Plants need air. They use the carbon dioxide in the air during photosynthesis. And they
use the oxygen in the air during respiration. (We’ll tackle these subjects next
week.)You may know that the air we breathe contains
almost 80% nitrogen, and that nitrogen is an important plant nutrient. (It’s the
"N" in the N-P-K rating you’ve seen on fertilizer labels.) If there’s
so much nitrogen in the air, why do so many garden plants require supplemental nitrogen in
the form of fertilizer?
Because most plants can’t use nitrogen in the form in
which it exists in the air. Some types of plants, however, are able to fix atmospheric nitrogen through a
special relationship they form with soil bacteria. Fixing nitrogen simply means converting
it to a form that plants can use. Plants with this ability, such as the legumes, generally
don’t require nitrogen fertilizers.
It’s easy to imagine plants getting plenty of
air—after all, like us they are constantly exposed to the air. However, plant roots
also need air. When the soil stays wet for too long, plant roots are deprived of oxygen
and begin to suffocate and die. We said earlier that overwatering houseplants can lead to
root rot. The problem isn’t that there’s too much water—it’s that
there’s not enough air. The water saturates the soil and fills up all the air spaces,
so the roots suffocate.
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Root Respiration
If plant roots need air, then
what about plants grown hydroponically? Hydroponics is the practice of growing plants
without soil. There are many different ways to set up a hydroponic garden, but all rely on
water, rather than soil, to deliver plant nutrients. Many people think of hydroponics as
growing plants directly in a tub of water. If this is so, why don’t the roots
suffocate?
Because the plants don’t
grow directly in water. In most hydroponic gardens plant roots are bathed in a nutrient
solution at regular intervals; the plants are not constantly submerged. For example, some
hydroponic gardens are set up so that the roots grow onto a sponge-like material. Then
periodically a nutrient solution washes over the sponges. The damp sponges can absorb some
of the water solution and still contain plenty of air spaces. |
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