Deep water culture is a hydroponic method of plant production by means of suspending the plant roots in a solution of nutrient-rich, oxygenated water. Also known as raft/pond or float systems, this method uses floating rafts to suspend plant roots into a pond of water often 8-12 inches deep. Compared to other hydroponic techniques, such as NFT, deep water culture is relatively inexpensive to set up and can be easily reproduced by a home grower. Since there is a relatively large reservoir of nutrient rich water for each plant, there is buffering for pH, EC, and temperature, meaning those elements of the systems won’t fluctuate as fast as they might in an NFT system.
Hobby Methods
Most hobby growers use Deep Water Culture. Net pots, plastic pots with netting to allow roots to grow through their surface, are filled with a hydroponic medium such as Hydroton or Rockwool to hold the base of the plant. In some cases net pots are not needed. For Oxygenation of the hydroponic solution, an airstone is added. This air stone is then connected to an airline that runs to an air pump. As the plant grows, the root mass stretches through the rockwool or hydroton into the water below. Under ideal growing conditions, plants are able to grow a root mass that comprises the entire bin in a loosely packed mass. As the plant grows and consumes nutrients the pH and EC of the water fluctuate. For this reason, frequent monitoring must be kept of the nutrient solution to ensure that it remains in the uptake range of the crop. A pH that is too high or too low will make certain nutrients unavailable for uptake by plants. Generally, the best pH for hydroponic crops is around 5.5-6.0. In terms of EC, too low means that there is a low salt content, usually meaning a lack of fertilizer, and an EC that it too high indicates a salt content that could damage the roots of crops. Desired EC depends on the crop that is growing. A common EC for leafy greens is somewhere between 1.5-2.2.
Recirculation Deep Water Culture
Traditional methods using unconnected buckets require each bucket to be tested for pH and conductivity factor individually. This has led to the creation of Recirculation Deep Water Culture systems. Rather than having individual buckets, RDWC bins are linked together most commonly using a PVC pipe. A pump is also added at the front of the system that pulls water through a line from rear of the system into a control bucket. This return line generally has a spin filter on it that cleans particulate from the water before it reaches the pump. The individual bins, including the control are aerated. The primary disadvantage of rDWC is that disease can spread quickly in these systems which can facilitate the transfer of pathogens from one reservoir to another.
Commercial Deep Water Culture
In a commercial system, there is usually a large pond where crops float on a raft. Seedlings are germinated in cubes and then transplanted into the rafts. Plants may be re-spaced during the growth period. The nutrient solution is oxygenated through air pumps or recirculation, and water is chilled to a temperature between 18-24C in order to maintain proper dissolved oxygen concentration, which is crucial to plant growth. Chilling the water also helps to prevent pathogens such as pythium, and delay bolting. PH and EC are controlled with acid or base injectors and fertilizer injectors, respectively. Supplemental lighting can be added to make sure that the plants receive the correct amount of light. Lighting depends on the crop, and the stage of growth for the crop. For example, lettuce grows best with 15-17 mol·m-2·d-1 of light. Typically only short-statured crops such as leafy greens and herbs are grown commercially in deep water culture as rafts move through the pond and taller plants would necessitate being trellised. The most common commercial deep water culture crop is lettuce. Lettuce does best in a pH of 5.6-6.0, EC of 1.1-1.2, 17 mol·m−2·d−1daily light integral which may consist of a combination of natural and supplemental lighting, air temperature of 24 °C day/19 °C night, water temperature of 25 °C, and dissolved oxygen of 7 mg·L−3. The rafts are cleaned after each harvest by scrubbing to remove organic matter and applying bleach or other sanitizing agents.
