Dissolved Oxygen for Horticulture

Dissolved oxygen (DO) in water is a critical parameter in greenhouse and hydroponic operations. DO is required for plant respiration, which influences nutrient uptake. DO is also important for root health, as low oxygen levels favor harmful anaerobic organisms that cause root rot (e.g. Pythium and Fusarium). Sufficient DO levels promote an aerobic environment favoring beneficial microorganisms.

Current methods for increasing DO include hydrogen peroxide, air pump/stones to bubble thru atmospheric gases (21% oxygen), and mixers/stirrers to promote air/water surface contact. While these methods are inexpensive, they also have drawbacks. Air stones are the most highly adopted, but DO is limited by its air composition intake (if greenhouse air is the supply, it also pulls in detrimental CO2 levels). Other techniques include air diffusers and electrolysis (largely out of favor in recent years, due to high cost of power for low quantity of oxygen). It’s increasingly common to see pure oxygen in use in greenhouses.

DO levels are usually monitored by a meter, with recommended levels at about 8 mg/L, which represents environmental water oxygen levels at atmospheric saturation. Note that DO saturation decreases with increasing temperature and presence of salts in the nutrient solution. Effective DO levels usually range from 6-8 ppm. Chillers are sometimes used to increase DO levels.

Use of HVO Systems for DO

If greater than 100% saturation is beneficial in your setting, the use of HVO systems is a very good option to consider. Large grow operations would benefit from a single system with distributed tubing and head pressure requirements to support DO in an entire greenhouse. In addition, not having to transport tanks in and out of the facility will reduce the risk of contamination from the tanks themselves, and from delivery personnel. Most importantly, the cost of generated oxygen using HVO is lower than with compressed gas and liquid tanks.

High DO levels lead to improved plant growth

Soffer et al. demonstrated that DO is essential to root formation and higher DO quickens root formation (tested only up to 8mg/l). Suyantohadi et al. compared normal DO (7-8 mg/L) with an air stone and high (23 mg/ml, ~210% saturation) DO using pure oxygen enrichment. The high DO produced marked increases in lettuce growth compared to normal aeration in deep hydroponic culture.

Increasing DO for cannabis has not been well-studied in the literature, but DO is clearly important in these operations as well. Aeration is a necessary practice, especially in deep water culture (DWC) systems. There is a great emphasis on DO for grow operations and pure oxygen is now commonly used.

Increasing DO levels

Gas saturation in water is given by Henry’s Law.

c = pg / kH (2)
where
c = solubility of dissolved gas
kH = proportionality constant depending on the nature of the gas and the solvent
pg = partial pressure of the gas

For oxygen at 25C:

co = (1 atm) 0.21 / (756.7 atm/(mol/litre)) (31.9988 g/mol) = 0.0089 g/litre = 8.9 g/ml

“100%” DO saturation is for atmospheric pressure and oxygen composition of 21%. Dissolved Nitrogen in water is higher 13.8 g/ml than DO due to air composition (79%) and its Henry constant.

For example, Henry’s law suggests that:

At 21% oxygen = 8.9 mg/L (i.e. 100%)
At 90% oxygen = 38 mg/L (this is 427% saturation!)

Henry’s Law suggests that much greater DO levels can be achieved by using a higher % oxygen and/or increased pressure. Increasing pressure lacks practicality but higher oxygen % is easily implemented with an existing HVO system. An additional advantage is that air stones pulling in greenhouse air have a higher content of CO2 which is detrimental (outside air is recommended with air stones).

It is assumed that hydroponic DWC solution containers are fairly well-covered. Thus bubbling pure oxygen will lead to a headspace primarily of oxygen as diffusion with external air would be slow. It can be expected that higher DO levels can be achieved with pure oxygen.