Grow Cannabis With Healthy Roots - Oxygen can be a limiting factor in the root zone by Ed Rosenthal
Cannabis plants are dependent on their environment for materials and energy. These key inputs, known as the limiting factors, drive photosynthesis and therefore plant growth. As outlined in the Marijuana Grower’s Handbook, the limiting factors are light, carbon dioxide, nutrients, water, and temperature. Through my work to update the book, which is now the Cannabis Grower’s Handbook, I’ve highlighted another critical limiting factor, oxygen.
Oxygen is required by all of the plant’s living tissues for aerobic respiration. This means everything from the bottom of the roots to the tops of the shoots. If oxygen becomes limited the plant’s respiration rate will dip and its ability to perform its normal activities will decrease. Although, in theory, oxygen could be a limiting factor in the stems, leaves, and flowers, that’s nearly impossible. The place where oxygen stress is a reality is within the plant’s root zone and occurs when oxygen uptake by the plant’s roots outpaces oxygen’s replacement there.
Clone with a great root structure Photo by Zoom Gardens
Here’s an excerpt from the new book that includes information to ensure your cannabis plants receive the correct amount of oxygen:
Oxygen Sources for Cannabis Plants
Plants have three sources of oxygen:
● Oxygen released during photosynthesis but held in the plant structure
● Oxygen in the atmosphere (Air is about 21% oxygen)
● Oxygen dissolved in water, which the roots absorb
Cannabis plants are terrestrial, which means that their roots explore the soil or whatever growing medium they are in. Underground roots do not see the light of day, and therefore do not have access to atmospheric oxygen.
Atmosphere
There is a lot of oxygen available in the atmosphere, which is about 21% oxygen. The roots’ source of oxygen is through diffusion to the root zone.
Oxygen diffuses 10,000 times slower through water than it does through air. Roots growing in compacted soil or degraded growing medium with little air porosity can quickly lose access to oxygen. The growing medium or soil that cannabis is growing in must have enough air porosity that oxygen can diffuse through the small air spaces in between the soil particles. If the plant’s roots are actively respiring, oxygen stress can occur within 20 to 30 minutes.
The primary way cannabis roots get oxygen is through diffusing through the air pores in the soil or growing medium or via water that is saturated with dissolved oxygen. When soil is watered from the top the draining water creates a vacuum in its wake. This pulls fresh air into the ground.
Dissolved in Water
Cannabis roots can also absorb oxygen that is dissolved in water or nutrient solution. As long as the water is saturated with oxygen, every time plants are irrigated or fertigated the roots get a fresh delivery. As temperatures rise, the rate of respiration in the roots increases.
Although oxygen in the atmosphere is at a concentration of about 21%, oxygen dissolves in water and maxes out at 8.3 ppm at 77°F (25°C). At 68°F (20°C), the dissolved oxygen concentration in water maxes out at 9.1 ppm.
As water temperatures decrease, the saturation level increases. The question arises: does it make sense to decrease the root zone temperature to increase oxygen availability to the roots? Unfortunately, a decrease in root zone temperature also drastically decreases the rate of root respiration, lowering the roots’ metabolism and productivity. Even though more oxygen may be available in a cooler root zone, slower root respiration rates slow the delivery of water and nutrients to the canopy. The ideal root zone and nutrient solution temperature should range between 68°F (20°C) and 72°F (22°C) in water culture and up to 75°F (24°C) in planting mix, where the water is interspersed with air.
Dissolved oxygen, sometimes abbreviated as DO, is the primary source of oxygen for cannabis roots in aeroponic production. Growing cannabis without a physical growing medium requires frequent fertigations with oxygen saturated nutrient solution.
Normal vs. Low Oxygen Levels
Normoxic conditions mean that the level of oxygen is not a limiting factor for respiration in the root tissue. The roots are actively growing and absorbing nutrients and water sufficiently. Root respiration is heavily dependent on temperature and oxygen concentration. If the oxygen concentration declines with a constant root temperature, the rate of respiration will also slow down. As respiration rates drop, the amount of energy available to do work also decreases. This is the energy the roots need to grow, absorb nutrients, and maintain cellular integrity.
The root environment goes from normoxic to hypoxic when the concentration of oxygen in the root zone drops to a level so low that not enough energy is available from respiration for the roots to properly function. Hypoxic root conditions lead to several problems for the plant. The rate of nutrient absorption declines and root growth decreases and dies. Plant roots signal to the leaves to close the stomata. Water absorption through the roots declines and then stops. This decreases the plant’s ability to photosynthesize since the stomata are closed and no longer allow CO2 to enter the leaves. The combined effects of a hypoxic root zone ranges from a smaller yield to plant failure.
As oxygen concentrations continue to decrease in a hypoxic root environment, the root zone eventually becomes anoxic, that is, it has virtually no oxygen. Root respiration stops completely and usually results in root die-off. Not only does this decrease the surface area for nutrient and water absorption, but the dying roots provide entry points for microbial pathogens such as Phytophthora cinnamomi (Jacobs et al., 1997).
An oxygen-enriched environment makes a big difference in plant vigor, ranging from 50 to 60% more growth for cuttings and 30% more for rooted plants. Root growth and development is best with oxygen saturated media, which is why it is so important to have high oxygen levels when taking cuttings for vegetative propagation. Root initiation from shoot tissue requires adequate oxygen levels.
Increasing Oxygen in the Root Zone
Passive approaches capitalize on how cannabis roots get their oxygen. Oxygen naturally diffuses through the air spaces of a porous growing medium/soil. Using a growing medium with particles large enough to leave space for air once the soil drains is essential for allowing the passive movement of oxygen-containing air through the root zone.
Proper management of the irrigation schedule is crucial in optimizing oxygen availability for the roots. Growing plants in fine grain or compacted soil or a growing medium that has low porosity leave little breathing room for the roots.
The reason why cannabis suffers from waterlogged soil is that it is more likely to be hypoxic since the pores in the soil are filled with water.
Irrigating with oxygen-saturated water is another way to passively introduce oxygen to the roots. Waterlogged soil is not harmful to the plants as long as the root zone has plenty of oxygen. Plants can be irrigated continuously as long as the nutrient solution is saturated with oxygen.
This is the theory behind growing in aeroponic production. The roots are constantly being showered with a nutrient solution that is always exposed to air. The high exposure to oxygen allows for constantly saturated water raining down on the roots.
Increasing oxygen concentrations in water is not difficult to do. There is so much oxygen available in the atmosphere that actively bubbling air through water in a storage tank with an aquarium pump attached to an air stone is sufficient.
A pump-over or cascade can also be used to oxygenate water in a storage tank. A pump creates a spray or jet that splashes back into the water and mixes with the air. This increases the DO to saturation. The larger the surface area of air-to-water interaction that occurs, the quicker the water becomes saturated with oxygen.
Increasing the dissolved oxygen (DO) concentration in fertigation solutions can be done through several methods.
The simplest is a venturi system. Air is drawn into a flowing liquid so when a hole is drilled into the pipe and tubing is attached the water is automatically aerated as it flows. The Mazzei Airjection Irrigation system makes it easy to introduce oxygen-laden air into the water-nutrient solution. Just hook it up to the irrigation line and it draws air into flowing water automatically. It requires no power and has no moving parts.
Bubbling air through the stock solutions and water storage tanks is one of the best ways to introduce oxygen into the system, as it is inexpensive and easy to maintain. Some air pumps can increase the temperature of the nutrient solution, which should be monitored.
We will add more information in the near future.
Macro shot from a root anchoring itself in the soil. Image by picture_fotografie
How to Tell When the Root Zone Needs More Oxygen
In the absence of oxygen, roots die off. If hypoxia is suspected, use a soil moisture meter. If the plant can be popped out of its container, the roots can be inspected. Healthy roots are typically white in color and fuzzy with root hairs near their growing tips. In some water culture systems, no root hairs are present. Slimy, brown roots are unhealthy roots, however, some staining from soil, planting media, or nutrients and additives can be normal.
Anaerobic bacteria flourish when a root zone is low in oxygen. These microbes produce ammonia-smelling compounds. Musty or acrid smells are usually indicative of persistent low-oxygen in the root zone.