Deep water culture

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.[1] This method of floating the boards on the nutrient solution creates a near friction-less conveyor belt of floating rafts.[2] 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.[3] The large volume of water helps mitigate rapid changes in temperature, pH, electrical conductivity (EC), and nutrient solution composition.[4]

An example of deep water culture in lettuce production.

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.[5] 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 (CF) individually. This has led to the creation of Recirculation Deep Water Culture (RDWC) 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.[6]

Commercial deep water culture

In a commercial system, there is usually a large pond where crops float on a raft. Commercial systems are typically located in greenhouses, though they can be installed outdoors, in high-tunnels or indoors. Seedlings are germinated in cubes (such as rockwool, oasis, or other media) and then transplanted into the rafts. Plants may be re-spaced during the growth period, for example, in lettuce production the initial spacing can have 9 plants per square foot and the final spacing 3.5 plants per square foot.[7] The nutrient solution is oxygenated through air pumps or recirculation, and water is chilled to a temperature between 18–24 °C 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 (optimum 5.5–6.0) and EC (dependent on crop) are controlled with acid or base injectors and fertilizer injectors, respectively.[8] Oxygenation can also be achieved by using hydrogen peroxide (H2O2), which serves the dual purpose of also being a sterilizing agent.[9] Supplemental lighting can be added to make sure that the plants receive the correct amount of light.[10] 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.[11]

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 (such as tomatoes or cucumbers) 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 (of fertilizer), 17 mol·m−2·d−1 daily 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.[12]

The rafts are cleaned after each harvest by scrubbing to remove organic matter and applying bleach or other sanitizing agents.[13]

For commercial hydroponic production of leafy crops, the most common types of systems used are NFT and DWC systems.[14] An advantage of NFT over DWC is that there is less nutrient solution needed in relation to the area being used for plant production. This in turn reduces the energy needed to heat the solution when the temperature drop, especially in winter.[15] However, there is an increased chance of leakage due to NFT having a more extensive plumbing system than DWC. NFT also relies on pumps for a constant water supply, whereas DWC does not.[16] Additionally, DWC systems can have plants be transplanted at one end of the raceway and harvested at the other end, which reduces labor costs.[17]

See also

References
  1. Roberts, Olu (August 2019). "Food safety handbook for hydroponic lettuce production in a deep water culture". Cite journal requires |journal= (help)
  2. Jensen, Merle H.; Collins, W. L. (1985), "Hydroponic Vegetable Production", Horticultural Reviews, John Wiley & Sons, Ltd, pp. 483–558, doi:10.1002/9781118060735.ch10, ISBN 978-1-118-06073-5, retrieved 2020-12-11
  3. Gómez, Celina; Currey, Christopher J.; Dickson, Ryan W.; Kim, Hye-Ji; Hernández, Ricardo; Sabeh, Nadia C.; Raudales, Rosa E.; Brumfield, Robin G.; Laury-Shaw, Angela; Wilke, Adam K.; Lopez, Roberto G. (2019-09-01). "Controlled Environment Food Production for Urban Agriculture". HortScience. 54 (9): 1448–1458. doi:10.21273/HORTSCI14073-19. ISSN 0018-5345.
  4. "Growing Hydroponic Leafy Greens". Greenhouse Product News. Retrieved 2020-12-11.
  5. Bugbee, B. (2004). "Nutrient Management in recirculating hydroponic culture". Acta Hortic. 648 (648): 99–112. doi:10.17660/ActaHortic.2004.648.12.
  6. "DWC vs rDWC". GrowDoctorGuides.com.
  7. Brechner, M.; Both, A.J. Hydroponic Lettuce Handbook. Cornell Controlled Environment Agriculture; Cornell University. Available online: http://www.cornellcea.com/attachments/Cornell CEA Lettuce Handbook.pdf (accessed on 2 December 2014)
  8. Bugbee, B. (2004). "Nutrient Management in recirculating hydroponic culture". Acta Hortic. 648 (648): 99–112. doi:10.17660/ActaHortic.2004.648.12.
  9. Butcher, Joshua D.; Laubscher, Charles P.; Coetzee, Johannes C. (2017-07-01). "A Study of Oxygenation Techniques and the Chlorophyll Responses of Pelargonium tomentosum Grown in Deep Water Culture Hydroponics". HortScience. 52 (7): 952–957. doi:10.21273/HORTSCI11707-16. ISSN 0018-5345.
  10. Both, A.J.; Albright, L.D.; Langhans, R.W.; Reiser, R.A.; Vinzant, B.G. (1997). "Hydroponic lettuce production influenced by integrated supplemental light levels in a controlled environment agriculture facility: experimental results". Acta Hortic. 418 (418): 45–52. doi:10.17660/ActaHortic.1997.418.5.
  11. Mattson, Neil. "Greenhouse Lighting" (PDF). Cornell CEA.
  12. Breckner, M.; Both, A.J. "Hydroponic Lettuce Handbook" (PDF). Cornell CEA.
  13. Breckner, M.; Both, A.J. "Hydroponic Lettuce Handbook" (PDF). Cornell CEA.
  14. Hochmuth, R. & Cantliffe, D. 2014 Alternative greenhouse crops—Florida greenhouse production handbook. Vol 3. Univ. Florida, Inst. Food Agr. Sci. Ext. HS791
  15. Thompson, H.C., Langhans, R.W., Both, A.J. & Albright, L.D. 1998 Shoot and root temperature effects on lettuce growth in a floating hydroponic system J. Amer. Soc. Hort. Sci. 123 361 364
  16. Frantz, J.M. & Welbaum, G.E. 1998 Producing horticultural crops using hydroponic tobacco transplant systems HortTechnology 8 392 395
  17. Walters, Kellie J.; Currey, Christopher J. (2015-10-01). "Hydroponic Greenhouse Basil Production: Comparing Systems and Cultivars". HortTechnology. 25 (5): 645–650. doi:10.21273/HORTTECH.25.5.645. ISSN 1943-7714.
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