Deep water culture (DWC) 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 deep flow technique (DFT), floating raft technology (FRT), or raceway, this method uses a rectangular tank less than one foot deep filled with a nutrient-rich solution with plants floating in Styrofoam boards on top. This method of floating the boards on the nutrient solution creates a near friction-less conveyor belt of floating rafts. DWC, along with nutrient film technique (NFT), and aggregate culture, is considered to be one of the most common hydroponic systems used today. Typically, DWC is used to grow short-term, non-fruiting crops such as leafy greens and herbs. The large volume of water helps mitigate rapid changes in temperature, pH, electrical conductivity (EC), and nutrient solution composition.
Hobby methods
Deep water culture has also been used by hobby growers. 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.
Plant Growth Requirements in DWC
A grower utilizing a DWC system has to modify the environment as to provide the proper conditions for optimal growth. Although this is true for any protected-crop production system, such as greenhouses, indoor systems, or vertical farms; utilizing DWC comes with some specific responsibilities that the grower should heed. Most notably, the plants’ roots are suspended in a nutrient solution, opposed to a growing media. This requires special attention to specific parameters which are described below in more detail (oxygen, temperature, pH and nutrient concentration). As in other methods, soils and soilless media can assist in acting as a buffer for potentially harmful agents like disease or water quality concerns. Since the roots are directly cultured in the nutrient solution, the water quality is of the utmost concern.
Plants require oxygen to carry out the process of root respiration that is imperative for healthy growth, such as the uptake of fertilizer salts. The nutrient solution is oxygenated utilizing the injection of gases via various methods. Most commonly, the nutrient solution is oxygenated using “air stones.” Air stones are made from porous materials that, when air is pushed through, creates bubbles. As the bubbles float to the surface, diffusion occurs, and the surrounding water is oxygenated. Respective of the porosity of the material, these bubbles can vary in size. The smaller the bubbles emitted from the air stone, the greater the surface area of the population of bubbles. This leads to a greater rate of diffusion, and thus “microbubble” stones, for example, are far more efficient than casual aquarium stones.
Another way to inject oxygen into the nutrient solution is by using liquid oxygen (LOx). This method is more common in commercial settings due to the increased initial investment. However, it can be an economical choice for mid to large operations. Here, pure oxygen is compressed within a tank, and when released into the nutrient solution, forces a high rate of diffusion and oxygenation. It is possible to reach post-saturation levels with this method.
Water temperature is an important factor in oxygen retention in the nutrient solution. Water is generally chilled to a temperature between 18–24 °C in order to maintain proper dissolved oxygen concentration since oxygenation solubility in water increases as temperature decreases. Chilling the water also helps to prevent pathogens such as pythium, delays bolting, and can increase yield.