Hydroponics, which lets gardeners grow plants without soil--instead relying on water and a growing medium like sand or air--has several advantages: It uses up to 90 percent less water than conventional methods of growing, plants grow faster since roots don't have to search for nutrients, and it eliminates or nearly eliminates the need for herbicides and pesticides. There are, however, some disadvantages, as well.
A main disadvantage of using hydroponic gardening systems is the need to pay a lot of attention to the needs of the plants. Unless you're using an automatic watering system and a timer, you have to check and adjust the water up to three times a day---maybe even more.
Additionally, getting to know the needs of a particular plant and how it performs in a hydroponic growing setting is a trial-and-error process, one where you will need to test for pH, flush the growing medium periodically, and perform other tasks, making sure to observe and note a plant's reaction to your ministrations.
Vulnerability to Power Outages
If you have set up an automatic system to water or mist your plants at timed intervals, your hydroponic growing system is vulnerable to power outages. You'd have to hand-water in the case of a black-out, which may be a bit of bother if you've got a greenhouse full of plants. There's trouble if you let the roots dry out.
Though a hydroponic system can be as simple as a well-draining pot filled with growing medium, finding ways to ease the time and effort needed to care for the plants could mean spending increasing amounts of money for increasingly sophisticated hydroponic systems. (In the long-run, considering water savings and increased yield, you'll likely recoup your investment.)
Organic Equals Bother
If you are fiercely devoted to organic gardening, achieving an organic hydroponic system is very difficult. Without soil, plants must get nutrients from the water, for which specially formulated fertilizers are created. There are organic nutrients that can supplement the nutrient solution used for hydroponics, which changes the taste of the crop produced to one organic gardening devotees might enjoy.