What are hydroponically grown plants?

Hydroponics (from the Greek “hydro,’’ for water, and “ponos,’’ for labor) is the growing of plants without soil.

We all know that plants need carbon dioxide and water, but there are all sorts of other things they need to live, which they usually get from soil. Those include nitrogen and magnesium, which is at the core of the chlorophyll molecule.

The key insight is that all the things a plant ordinarily gets from soil can instead be taken up from water containing dissolved nutrients. The dirt the plant is in is needed only to help hold the plant up.

This means plants can be grown with their roots in liquid solutions; they can be kept upright by sticking them in an inert substance like gravel or providing some other external support. You lose all the hassles of dirt, including possibly harmful insects and microorganisms that live in it, and can get incredibly good control over how your crops turn out, year after year.

Often, you can get away with no pest control (chemical or otherwise) at all.

There are a lot of variations on the basic scheme, including letting the water-nutrient mix flow, or letting the roots hang in a mist of droplets, a practice that has been called “aeroponics.’’

The biggest disadvantages of hydroponics result from the absence of the moisture-moderating effect of soil, which can take up or release water as needed. Excess wetness can allow the growth of salmonella bacteria (which can lead to food poisoning for people eating contaminated crops) or other microorganisms that can rot the plants.

Another worry is that unless great care is taken, trace minerals whose roles are still poorly understood might be missing from the solution used to grow plants, so it might not be a good idea to eat only hydroponic food for your whole life.

But in general, the quality of hydroponic crops seems very high.

Dr. Knowledge is written by John Swain, a physicist at Northeastern University. You can e-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.