by Chris Woodford. Last updated: April 19, 2020.
Look, no soil! We're
so used to plants growing in fields and gardens that we find anything
else completely extraordinary. But it's true. Not only will plants
grow without soil, they often grow a lot better with their roots in
water or very moist air instead. Growing plants without soil is known
as hydroponics. It might sound weird, but many of the foods we
eat—including tomatoes on the vine—are already grown
hydroponically. Let's take a closer look at hydroponics and find out how it works!
Photo: These radishes are being grown hydroponically
at NASA's Kennedy Space Center. Space scientists are particularly interested
in hydroponics, because there's no soil in space. If we want to grow food
away from Earth, hydroponics may be the best way to do it.
Photo by courtesy of NASA Kennedy Space Center (NASA-KSC).
What is hydroponics?
Plants grow through a process called photosynthesis, in which they use sunlight and a
chemical inside their leaves called chlorophyll to convert carbon dioxide (a gas in the air) and water into glucose (a type of
sugar) and oxygen. Write that out chemically and you get this equation:
6CO2 + 6H2O → C6H12O6 + 6O2
There's no mention of "soil" anywhere in there—and that's all the proof you need that
plants can grow without it. What they do need is water and
nutrients, both easily obtained from soil. But if they can get these
things somewhere else—say, by standing with their roots in a
nutrient-rich solution—they can do without soil altogether. That's
the basic principle behind hydroponics. In theory, the word
"hydroponics" means growing plants in water (from two Greek words
meaning "water" and "toil"), but because you can grow plants
without actually standing them in water, most people define the word
to mean growing plants without using soil.
Why grow things hydroponically?
Photo: Onions, lettuces, and radishes all grow
well with hydroponics. The white surface of hydroponic containers like
these helps to reflect light evenly onto the plant leaves, improving growth.
Photo by courtesy of NASA Kennedy Space Center (NASA-KSC).
Although the benefits
of hydroponics have sometimes been questioned, there seem to be many
advantages in growing without soil. Some hydroponic growers have
found they get yields many times greater when they switch from
conventional methods. Because hydroponically grown plants dip their
roots directly into nutrient-rich solutions, they get what they need
much more easily than plants growing in soil, so they need much
smaller root systems and can divert more energy into leaf and stem growth.
With smaller roots, you can grow more plants in the same
area and get more yield from the same amount of ground (which is
particularly good news if you're growing in a limited area like a
greenhouse or on a balcony or window-ledge inside). Hydroponic plants
also grow faster. Many pests are carried in soil, so doing without it generally
gives you a more hygienic growing system with fewer problems of
disease. Since hydroponics is ideal for indoor growing, you can use
it to grow plants all year round. Automated systems controlled by
timers and computers make the whole thing a breeze.
It's not all good
news; inevitably there are a few drawbacks. One is the cost of all
the equipment you need—containers, pumps, lights, nutrients, and so
on. Another drawback is the ponic part of hydroponics: there's
a certain amount of toil involved. With conventional growing, you can
sometimes be quite cavalier about how you treat plants and, if
weather and other conditions are on your side, your plants will still
thrive. But hydroponics is more scientific and the plants are much
more under your control. You need to check them constantly to make
sure they're growing in exactly the conditions they need (though
automated systems, such as lighting timers, make things quite a bit
easier). Another difference (arguably less of a drawback) is that,
because hydroponic plants have much smaller root systems, they can't
always support themselves very well. Heavy fruiting plants may need
quite elaborate forms of support.
Photo: Waste not, want not: A researcher at the US Department of Agriculture
examines the roots of a hydroponic strawberry plant that's being nourished on wastewater from a trout farm!
Photo by Scott Bauer courtesy of US Department of Agriculture (USDA) Agricultural Research Service (ARS).
How does hydroponics work?
Artwork: In the nutrient-film technique, nutrient constantly drips past the roots of the plants, which grow out from an inclined tray. The nutrient is pumped back up to the tray and a second pump adds oxygen through an air stone (a piece of porous rock that lets air bubble through it—just like in a fish tank).
There are various different ways of growing things hydroponically.
In one popular method, you stand
your plants in a plastic trough and let a nutrient solution trickle
past their roots (with the help of gravity and a pump). That's called
the nutrient-film technique: the nutrient is like a
kind of liquid conveyor belt—it's constantly sliding past the roots
delivering to them the goodness they need. Alternatively, you can grow
plants with their roots supported by a nutrient-enriched medium such as rockwool,
sand, or vermiculite, which acts as a sterile substitute for soil.
Another method is called aeroponics and it's typified
by a popular product called the AeroGarden (see box below). Although
the name suggests you're growing plants in air, the roots are
actually suspended inside a container full of extremely humid air.
Effectively, the roots grow in a nutrient-rich aerosol a bit
like a cloud packed full of minerals.
In theory, you can grow any
plant hydroponically but—as is always the case with gardening—some
things inevitably do better than others. Fruit crops such as tomatoes
and strawberries, and lettuces and herbs, are among plants that do
How AeroGardens® work
Artwork: A typical AeroGarden grower and its key parts:
1 = Artificial light hood; 2 = Door for adding liquid and inspecting roots (with a liquid
gauge beneath); 3 = Seven separate openings for plants;
4 = Electronic display and control panel (including warning lights
telling you when to add water and nutrients).
Artwork from US Patent 20060272210: Smart garden devices and methods for growing plants, courtesy of US Patent and Trademark Office.
Want to have a go at hydroponics but don't know where to start? This revolutionary
AeroGarden® (made by AeroGrow) might be worth a look. It's an easy-to-use, highly automated
hydroponicum (hydroponic growing box) that's small enough to stand in
your kitchen (it's just 40 cm x 25 cm x 38 cm; 16in x 10 in by 15
in), in which you grow plants with their roots dangling down into a
sealed container filled with 100 percent humid, nutrient rich, highly
oxygenated air. Think of a hot and steamy greenhouse and you're on
the right lines: the makers describe it as a "near-perfect
rainforest growing environment."
Unlike traditional hydroponic
systems, which require quite a lot of careful observation and tending
to keep the nutrient levels just right, the AeroGarden's microchip controller practically
does everything for you. It automatically
regulates light, nutrients, and water according to what you're
growing; the built-in, energy-efficient daylight bulb (shining down from the
black upper casing) turns on
and off as necessary. Everything is amazingly easy. You simply buy
ready-made seed pods and nutrient tablets that release their goodness
slowly... and off you go. Hydroponics made simple!
There's a general description of how it all works
in AeroGrow's 2006 patent,
Smart garden devices and methods for growing plants by
W. Michael Bissonnette et al, and you can find out a lot more on the company's website if you're interested.
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