Gravity has been called a weak but pervasive force. It's also a force to be reckoned with in hydroponics. Even systems that aren't specifically considered gravity-fed often rely on gravity in one way or another.

This isn't a big surprise since gravity is hard at work in nature. Plants depend on it for more than just delivering water in the form of rain. Gravity and capillary action—the ability of a liquid to flow through a confined space against gravity—are both dynamic forces essential for plant health and development. In part, plants use gravity to help direct their roots downward into the soil (gravitropism) and their stems upward toward the sun. For the indoor gardener, gravity is often put to use moving water and nutrients through a growing system.

To understand how gravity-based systems work, it's useful to think of water and nutrient delivery in terms of ups and downs, or downs and ups. The down part of the equation involves getting a prepared nutrient solution from an elevated location, like a holding tank, to a planting bed or network of pots. We can see this in action in some tower systems, vertical gardens, ebb and flow systems and others. Water delivered to plants via a downward incline uses gravity power to seek its own level. In a well-planned system, that is in close proximity to thirsty plant roots.

Harnessing gravity to power all or a portion of a hydroponic system makes good sense. It's cost effective and saves labor, too. But what happens after all that nutrient-laden water completes the downward circuit? Gravity isn't much help when the goal is to get liquid to flow back uphill. Addressing this issue is what separates different types of gravity-based hydroponic technologies. The oldest, developed for watering, uses a pulley and a bucket. Once raised and emptied, the bucket is lowered to catch any runoff. It is raised again and again to repeat the process.

Today, some gravity-fed systems use a simple electric pump and a series of pipes or tubes to handle the task of recirculating water and plant food. The tank or reservoir is located above or below the plants, while water and nutrients are pumped through a tube for part of the cycle and allowed to flow back into the reservoir after passing through a series of tilted shelves, pots or pipes containing plants. Add a timer, fittings and a few minor accessories, and you have an energy-efficient set-up that's scalable and easy to maintain. Putting one together makes for a pretty straightforward DIY project.

As green as this sounds, there are gravity-fed systems that require no power source and still manage a high degree of automation. They eliminate the recirculating problem by supplying nutrients from an elevated cistern in small increments, allowing plant roots to exhaust the supply before more is released. There is no runoff or waste to deal with. Looking at it another way, the plants actually control the system as their uptake of the available nutrient solution triggers a specially designed valve to release more. Think of it as an intuitive ebb and flow system, but one that operates without electricity.

Many experts are hoping for big things from energy-efficient, gravity-fed hydroponics and other indoor growing innovations. In areas of the world where outdoor growing is impractical, or will be impractical in the decades to come, the development of large-scale, reliable and planet friendly agricultural solutions could hold the key to feeding the Earth's projected human population of 9.6 billion souls by the year 2050. Until then, whether you're growing a tomato plant in your basement or a bumper crop of lettuce and herbs in your greenhouse or grow tent, gravity is a powerful friend to have in your corner.