Page 17 - Griffin Gazette Issue 2 - 2022

Dissolved oxygen is a measurement of gaseous During flood events, plants have increased contact
oxygen molecules dissolved in liquid water, measured time with water and are exposed to extra waterlogged
in parts per million or mg per liter. Al Zylstra from conditions. Aerating irrigation water is important in
Dramm comments, “We have watched the awareness these scenarios to ensure plant roots have access
of the importance of dissolved oxygen go from “zero to oxygen during these events. Growers may utilize
to hero” in the horticulture industry inside a relatively mechanical agitation or aeration of reservoirs between
short period of 10 to 15 years. When we first started or just prior to irrigation events to ensure good
promoting the importance of dissolved oxygen oxygenation or may increase time between irrigations
about 16 years ago, there was little awareness of its to allow for effective dry downs.
importance on the part of most growers. Growers
were not paying attention to the DO levels in their NUTRIENT FILM TECHNIQUE
irrigation water at all; most didn’t know what DO levels Nutrient film technique (NFT) hydroponics utilize long,
were best, adequate and harmful; how to measure it shallow channels and continuously flowing water
or improve it. Today most growers are aware and are as their plant support and irrigation systems. Water
learning fast.” inside of channels typically covers no more than the
bottom centimeter or so of depth, leaving ample water
Oxygen enters water in several ways. Water passively
allows oxygen to diffuse into solution from the and root surface area for oxygen exchange. In these
atmosphere at the water surface. Bodies of water with systems, oxygenation of water happens through
larger open surfaces will be better able to passively passive diffusion across the water surface, as well
as mechanical mixing as the water is sprayed into
saturate with oxygen. Mechanical agitation through channels and flows over roots. Dissolved oxygen is
the use of circulation pumps can assist this process
by breaking smooth surfaces, increasing surface often not a concern for these systems, unless deeper
area, and generally physically mixing air and water. channels are used. As water depth inside of channels
Mechanical aeration takes this a step further: air is increases, these systems become more similar to
deep water culture than NFT systems. It is important
actively taken from the atmosphere and forced under to maintain very shallow depths to achieve proper
the surface through air pumps and bubblers, enabling
much greater injection of oxygen into water reservoirs. aeration and irrigation control when using this kind of
Different production methods will utilize each of these production system.
methods to different degrees.
DEEP WATER CULTURE
CONTAINER PRODUCTION Deep water culture (DWC) involves maintaining a
IN SOILLESS MEDIA deeper reservoir of water, with nutrition and pH
conditions adjusted for the crop floating on top of
Plants of all types can be grown in containers using the reservoir. In these conditions, plant roots are
soilless media and irrigated via wand, overhead completely and continuously submerged in water
sprinklers, drip irrigation, and more. In typical container during the entire growth cycle. Of all production types,
production, dissolved oxygen becomes less of an issue dissolved oxygen levels have the greatest impact
due to the regular dry-down between irrigation events. on DWC production. In pond lettuce production, for
These dry downs allow roots free access to oxygen, example, lettuce rafts float directly on the top of the
preventing waterlogged conditions from harming water, limiting surface area for oxygen diffusion, and
plants. Irrigation water is often mechanically aerated roots sit directly in the water, absorbing oxygen during
via wand or sprinkler heads, ensuring good mixing of respiration. The opportunities to replenish oxygen
air and water prior to reaching pots, and no further within the water are severely limited. Growers should
dissolved oxygen considerations are taken. include active aeration equipment such as air stones
and circulation pumps to adequately supply oxygen
EBB AND FLOOD SYSTEMS
across the crop.
Ebb and flood systems bear many similarities to
traditional container irrigation, with a couple key When all is said and done, growers know that dissolved
differences. As the name implies, subirrigation is oxygen certainly is important. Benefits to growers,
utilized to flood the rootzones of plants periodically, however, will vary based on production system growers
rather than top watering. After reaching a certain are utilizing. Growers should choose aeration strategies
irrigation level, water ebbs away, allowing plants to that best fit their environment and budget.
dry down to a certain extent between flood events.

IMAGE CAPTIONS: Top - Lettuce grown in deep water culture ponds often benefits the most from additional dissolved oxygen
equipment and monitoring. Bottom - Channels of hydroponic lettuce being produced in nutrient film technique (NFT) hydroponics options.