Understanding Dissolved Oxygen

J.James

Seed Slingin' Outlaw
Breeder
Understanding Dissolved Oxygen (growertalks.com)

Even the non-technical growers have an understanding and appreciation for the measurements used in greenhouse growing today. Monitoring and optimization of pH and Electrical Conductivity (EC) have become standard practice, improving plant health and quality throughout the industry. By measuring these two simple factors, most nutritional problems can be avoided. The measurement of Dissolved Oxygen (DO) is also proving to be just as critical to plant growth. As with the other two measurements, there are minimum levels required for a healthy plant. Also, by optimizing DO, as with pH and EC, we can see great improvements in plant growth and quality. However, far fewer growers are utilizing, or are even aware, of this measurement.

Many of the practices in botany already consider the effect that oxygen has on the root zone. In the field, we till the soil. Among other things, this adds air space to provide oxygen to the roots. In horticulture, growing media is selected with consideration for porosity for the same reason. The benefits of oxygen to plant roots, and to the rhizosphere in general, is well established in the mind of the grower. However, most are unaware of the level of oxygen contained in their irrigation water and don’t realize that there are methods to improve this level.

Dissolved oxygen is simply the amount of oxygen (O2) dissolved in water. It’s one of the best indicators of the quality, and the life-supporting ability, of water. People need the right amount of oxygen in the atmosphere to survive. And, just as fish need the right amount of dissolved oxygen in the water to survive and thrive, so do plants.
Measured in mg/l, as a percent of saturation (%) or in parts per million (ppm), dissolved oxygen levels are affected by the temperature and salinity of the water, and also by other chemical and/or biological demands (COD/BOD) of the water. Cold water can hold more dissolved oxygen than warm water (see Figure 1) and fresh water can hold more dissolved oxygen than salt water. The maximum amount of DO that the water can hold is called the saturation value. It’s possible, and very often desired—especially in a greenhouse—to exceed the natural saturation point of DO in water. This is called super-saturation.






At levels around 5 mg/l of dissolved oxygen, irrigation water is typically considered marginally acceptable for plant health. Most greenhouse crops, however, will perform better with higher levels. Levels of 8 mg/l or higher are generally considered to be good for greenhouse production and much higher levels, as high as 30 mg/l or more, are achievable and can be beneficial. If the DO levels are below 4 mg/l, the water is hypoxic and becomes very detrimental, possibly fatal, to plants and animals. If there’s a severe lack of DO, below around 0.5 mg/l, the water is anoxic. No plants or animals can survive in anoxic conditions. The irrigation water in many greenhouses has surprisingly low levels—often in the dangerous hypoxic range.

Unfortunately, without measurement and awareness of the dissolved oxygen in greenhouse irrigation water, problems caused by hypoxic water in plant growth often go undiagnosed or misdiagnosed. Happily, monitoring DO is fairly easy. Just as with pH and EC meters, there’s a wide range of devices available at varying quality, accuracy and ease-of-use to test the level of dissolved oxygen in water.

When measuring DO, it’s important to understand the oxygen demand in the irrigation system and in the roots. There are plenty of things that will use oxygen in this environment. This is the reason that you might test 8mg/l DO at a cistern or well head, but only 5mg/l at the plant. Organic material in the water, or biofilm in your pipes, will consume oxygen. For this reason, it’s important to test in multiple locations and look for ways to remove as much of the biological demand as possible.

However, the detrimental effects of low-dissolved oxygen levels shouldn’t be the only reason to measure. Increasing DO in irrigation water can not only prevent problems, but with super-saturated levels, it can increase quality and plant growth, reducing cropping time and overall health. High levels of dissolved oxygen promote healthy root growth. The root system requires oxygen for aerobic respiration, an essential process that releases the energy required for healthy root growth and a healthy plant.

Research shows that higher dissolved oxygen levels in the root zone of most crops results in a higher root mass. A plant with more root mass grows healthier and faster. A plant’s roots are where it gets the majority of its inputs for growth, including water and nutrients. Healthy roots with a good supply of oxygen have better respiration and are able to selectively absorb more ions in solution, such as the vital mineral salts nitrogen, phosphorus and potassium. When there’s less oxygen in the water than there is in the plant, this reduces the permeability of roots to water, therefore reducing (even reversing) the absorption of nutrients.

A healthier plant is a more efficient plant
DO isn’t just an additional nutrient one should pay more attention to because it makes a healthier plant; there are direct economic impacts, as well. When used properly, it can reduce the amount of nutrients and micronutrients required, as well as the amount of costly chemicals, such as fungicides. Additionally, evidence suggests that plant growth increases with super-saturation levels of DO, reducing cropping times and increasing fruiting or flowering yields.

Improving levels of dissolved oxygen can be done through various methods. Simple aeration or agitation can increase dissolved oxygen enough to prevent problems. Injecting air or, especially, pure oxygen can increase levels as well, but only as high as saturation levels. Paying attention to temperature can also help improve DO, as colder water can hold more oxygen. Additionally, water at atmospheric pressure will hold less oxygen than when under pressure. Think of a bottle of carbonated water: while under pressure, the water holds more carbon dioxide than it can when the cap is removed and the pressure is released. Once the bottle is opened, the CO2 begins to gas-off from the water, effervescing. The same is true with oxygen. Adding oxygen to a pressurized system can increase the level of DO.

Ozonation is another method for increasing oxygen levels in solution. Like oxygen injection, injecting ozone gas will increase dissolved oxygen. However, ozone or O3 is almost 13 times more soluble in water than O2. This allows for much greater levels of oxygen to be dissolved into the water. As O3 is very unstable and reverts back to O2 quickly, it leaves super-saturated levels of dissolved oxygen in the water. As the system remains under pressure, the DO levels can be maintained at more than 300% of the saturation level of DO.

While adding dissolved oxygen, ozone has an additional benefit in that it oxidizes organic material and biofilm in the pipes, reducing the oxygen demand and helping to maintain higher levels of DO.

Putting it to the test

Trials were conducted at Metrolina Greenhouses in Huntersville, North Carolina. Utilizing a portable water-treatment unit consisting of filtration and ozonation, the benefits of water super-saturated with dissolved oxygen were tested against their standard water sources. Three greenhouses were tested side-by-side using identical benching and booms. One used their pond water, one used their well water and the third used their pond water treated with the trial system. Consistently, the plugs in the third greenhouse using the ozone treatment had higher germination rates, faster cropping times, better root growth (see photo) and better leaf development. Measurement of their dissolved oxygen in the irrigation water used in the third house was 300% greater than in the other two houses (see Figure 2).

In a final summary of the trial and of their first year using their own, complete treatment system, Metrolina reported that they saw an average of two weeks shorter cropping time on most liners, with complete cropping time reductions up to four weeks. More robust root systems with less damage and more drought tolerance were noted, as well as the removal of all biofilm from filters and pipes, and an overall reduction in their shrink by 66% from their previous three-year average.

These results are extremely impressive and achievable by others with proper equipment. However, growers can improve the health of their water in the short term by simply paying attention to the dissolved oxygen in their water. At a minimum, monitoring DO in irrigation water can help prevent problems or suggest different courses of action as problems arise.

In the long run, the ability to optimize oxygen levels could improve plant health quality, reduce crop times and eliminate shrink, just as the focus on other input optimization has in the past.

By Kurt Becker
 
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J.James

Seed Slingin' Outlaw
Breeder
Dissolved Oxygen and Water

Although water molecules contain an oxygen atom, this oxygen is not what is needed by aquatic organisms living in natural waters. A small amount of oxygen, up to about ten molecules of oxygen per million of water, is actually dissolved in water. Oxygen enters a stream mainly from the atmosphere and, in areas where groundwater discharge into streams is a large portion of streamflow, from groundwater discharge. This dissolved oxygen is breathed by fish and zooplankton and is needed by them to survive.

Dissolved oxygen and water quality

A eutrophic lake, with excess algal growth
A eutrophic lake where dissolved-oxygen concentrations are low. Algal blooms can occur under such conditions.

Rapidly moving water, such as in a mountain stream or large river, tends to contain a lot of dissolved oxygen, whereas stagnant water contains less. Bacteria in water can consume oxygen as organic matter decays. Thus, excess organic material in lakes and rivers can cause eutrophic conditions, which is an oxygen-deficient situation that can cause a water body to "die." Aquatic life can have a hard time in stagnant water that has a lot of rotting, organic material in it, especially in summer (the concentration of dissolved oxygen is inversely related to water temperature), when dissolved-oxygen levels are at a seasonal low. Water near the surface of the lake– the epilimnion– is too warm for them, while water near the bottom–the hypolimnion– has too little oxygen. Conditions may become especially serious during a period of hot, calm weather, resulting in the loss of many fish. You may have heard about summertime fish kills in local lakes that likely result from this problem.


(Source: A Citizen's Guide to Understanding and Monitoring Lakes and Streams)

Dissolved oxygen, temperature, and aquatic life

Dissolved oxygen and water temperature for Passaic River below Pompton River at Two Bridges, N. J., 2017
Water temperture affects dissolved-oxygen concentrations in a river or water body.

As the chart shows, the concentration of dissolved oxygen in surface water is affected by temperature and has both a seasonal and a daily cycle. Cold water can hold more dissolved oxygen than warm water. In winter and early spring, when the water temperature is low, the dissolved oxygen concentration is high. In summer and fall, when the water temperature is high, the dissolved-oxygen concentration is often lower.


Dissolved oxygen in surface water is used by all forms of aquatic life; therefore, this constituent typically is measured to assess the "health" of lakes and streams. Oxygen enters a stream from the atmosphere and from groundwater discharge. The contribution of oxygen from groundwater discharge is significant, however, only in areas where groundwater is a large component of streamflow, such as in areas of glacial deposits. Photosynthesis is the primary process affecting the dissolved-oxygen/temperature relation; water clarity and strength and duration of sunlight, in turn, affect the rate of photosynthesis.

Hypoxia and "Dead zones"

You may have heard about a Gulf of Mexico "dead zone" in areas of the Gulf south of Louisiana, where the Mississippi and Atchafalaya Rivers discharge. A dead zone forms seasonally in the northern Gulf of Mexico when subsurface waters become depleted in dissolved oxygen and cannot support most life. The zone forms west of the Mississippi Delta over the continental shelf off Louisiana and sometimes extends off Texas. The oxygen depletion begins in late spring, increases in summer, and ends in the fall.


Map of Gulf of Mexico dead zone, 2009
Dissolved oxygen in bottom waters, measured from June 8 through July 17, 2009, during the annual summer Gulf of Mexico Southeast Area Monitoring and Assessment Program (SEAMAP) cruise in the northern Gulf of Mexico. Orange and red colors indicate lower dissolved oxygen concentrations.

The formation of oxygen-depleted subsurface waters has been associated with nutrient-rich (nitrogen and phosphorus) discharge from the Mississippi and Atchafalaya Rivers. Bio-available nutrients in the discharge can stimulate algal blooms, which die and are eaten by bacteria, depleting the oxygen in the subsurface water. The oxygen content of surface waters of normal salinity in the summer is typically more than 8 milligrams per liter (8 mg/L); when oxygen concentrations are less than 2 mg/L, the water is defined as hypoxic (CENR, 2000). The hypoxia kills many organisms that cannot escape, and thus the hypoxic zone is informally known as the “dead zone.”


The hypoxic zone in the northern Gulf of Mexico is in the center of a productive and valuable fishery. The increased frequency and expansion of hypoxic zones have become an important economic and environmental issue for commercial and recreational users of the fishery.
 
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