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.
[image: Clone with a great root structure Photo by Zoom Gardens]

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.
[image: Roots in a hydrophonic system by AEssenseGrows]

Roots in a hydrophonic system by AEssenseGrows

*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.
[image: Clones starting to root from the stem by @Gonzophoto]

Clones starting to root from the stem by @Gonzophoto

*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.
[image: bad-roots.jpg]

*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 low oxygen water without allowing the soil to dry out
between watering is the worst scenario since oxygen never gets the chance
to diffuse through the pores of the growing medium. The water that is being
introduced into the root zone is not bringing oxygen with it. Allowing for
the growing medium to dry out a bit between irrigations promotes oxygen
availability in fine-textured soils.

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.

Other than air, there are other products that are available that increase
dissolved oxygen concentrations that also sanitize the water. Ozone (O3)
and hydrogen peroxide (H2O2) are strong oxidizing agents that neutralize
microorganisms in the water. Both compounds naturally break down to produce
oxygen:



Ozone breakdown 2O3 → 3O2

Hydrogen Peroxide breakdown 2H2O2 → 2H2O + O2



Ozone has a half-life of about 20 minutes at room temperature. It is an
extremely powerful oxidizer and has strong antimicrobial properties. Ozone
can be produced on-site with an ozone generator. Ozone generators can be
used to bubble ozone through water. Water should be ozonated at night in
storage tanks and allowed sufficient time to break down before being
applied to the plants. High-concentration ozone in the nutrient solution
can oxidize nutrients and potentially damage roots.
[image: Macro shot from a root anchoring itself in the soil. Image by
picture_fotografie]


Macro shot from a root anchoring itself in the soil. Image by
picture_fotografie

Hydrogen peroxide also has oxidative properties although nowhere near as
strong as ozone. Hydrogen peroxide should be diluted down to a maximum of
1% of the water volume. (It comes in various percentages from 3% to about.
30% solution). It also breaks down quickly, so it does not stay oxidative
for too long. Exposure to light breaks it down more quickly, so it should
be stored in an opaque container in a dark location. Much like ozone,
hydrogen peroxide has strong antimicrobial properties, which means that it,
too, can damage cannabis root tissue if exposed at too high of a
concentration. Luckily for both ozone and hydrogen peroxide, neither
compound leaves a residue and both break down into molecular oxygen that is
available to the roots.

Ozone and hydrogen peroxide neutralize pathogenic bacteria and fungi;
however, they also can eliminate beneficial microbes in the root zone. A
robust and diverse microbial community in the root zone is beneficial to
the health of the plant. When using ozone and hydrogen peroxide to
oxygenate the water, it is best to wait before applying so that the ozone
and hydrogen peroxide molecules have broken down.

*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.