The role of root oxygen in plant performance
By Mohyuddin Mirza
Cannabis growers are well aware of the importance of oxygen for cannabis production – both for quality and quantity – but we should understand the dynamics of oxygen and how proper levels of dissolved oxygen in water can impact the plant and its performance. The choice of any oxygen generation technology may well rest on this information and knowledge.
Oxygen is produced during the process of photosynthesis, the fixing of light energy on the leaves. This oxygen is produced during the light period and is exhausted through ventilation. This is a by-product of the chemical reaction of photosynthesis. It is beneficial for workers who can breathe this oxygen while working with the plants.
Oxygen in water is a different story. Dissolved oxygen (DO) has an important role to play in root health. However, dissolved oxygen depends on water temperature.
Simply stated, the cooler the water temperature, the better the DO. At 4ºC, the oxygen levels are around 9 to 11 mg/L or parts per million (ppm). That is why aquatic life survives.
At 20ºC, the oxygen level drops to around 5 to 6 ppm, and at 26ºC the level will be around 3 to 4 ppm. These are actual measurements I have taken in a facility. Some published figures may be slightly different.
The challenge here is that we cannot grow cannabis at 4ºC water temperature. We need around 18ºC to 20ºC. In some cases, the water temperature can go higher if black irrigation pipes are trapping light. The higher the temperature, the lower will be the DO.
Biochemical oxygen demand
Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed (i.e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. Many labs will work out this figure for you. The important thing to know is that the higher the BOD, the lower will be the DO. That may make it necessary to add oxygen to your irrigation water.
DO and root health
In substrate based growing media – like commercial soilless mixes, coir or rockwool – the media are designed for high porosity, medium porosity and low porosity, which indirectly indicate air to solid ratios. Highly porous mixes are used for rooting of clones, where more oxygen is available for rooting. These mixes need better water delivery because there is less water holding capacity.
The growing medium in the pots in Picture 1 is of medium porosity. It has good air porosity and water holding capacity. In Picture 2, the plant shows excellent development of roots in a high air porosity plug. Note an abundance of root hair.
When plants are irrigated to a saturation point, then the oxygen in the root zone depends on the levels of dissolved oxygen. The roots absorb nutrients through cation exchange and release hydrogen or hydroxyl ions which is reflected in the pH. When water is removed from the growing media by the roots then air starts going back based on the air porosity of the media.
My observation is that plants grown in aeroponic and hydroponic systems benefit most from dissolved oxygen. The photos below are from Choice Growers in Strathmore, Alberta, which uses an aeroponic system. The roots are directly misted with oxygenated water.
The plants shown in Picture 3 have roots with oxygen levels of around 8 to 10 ppm. Note the roots are pure white, contains lots of feeder roots and an abundance of root hair. This translates into very good top growth (Picture 4 ).
The relationship between oxygen levels and fungus Pythium is important to understand. Below 4 ppm, the fungus starts multiplying, and around 2 ppm roots will start turning brown and the top of the plant will show water deficiency symptoms.
There are different techniques and machines to add oxygen to water. Growers should investigate which protocols suit their setup the best. One last point: oxygen over 13 ppm is of no benefit to the cannabis plants.
Moyhuddin Mirza, PhD, is chief scientist with the Cannabis Nature Company in Edmonton, and a consultant with the cannabis industry. Email him at firstname.lastname@example.org