Hydroponics: Root Indicators

By John Wallace, SIFT Farm Manager

Healthy roots after the system
has been flushed through with
clean water. Photo: NCAT
'Slimy' brown roots indicating
anaerobic (lack of oxygen) due
to either nutrient imbalance,
high pH, or the system needs
flushing through. Photo: NCAT

In a hydroponic system, the chemistry of the water must remain balanced. Temperature, pH, and dissolved oxygen must remain in equilibrium so that the roots can uptake nutrients. When the system is performing optimally, the roots should be white and rapidly growing. Dark-colored roots can signify that the system is falling out of that equilibrium. Anaerobic bacteria can dominate the system, locking out available nutrients and creating root rot.

While maintaining our hydroponic system, I came across a disturbing sight. The once-healthy white roots had turned dark and were caked with anaerobic bacteria. Typically, this is when we add a root guard additive, bacillus, which is a bacterial inoculant that will feed on the anaerobic bacteria and help clean the roots. The root guard aids in the recovery of the roots but does not fix the systemic problems that created the issue to begin with. Knowing something was very wrong, I began checking the pH.

The pH controls how easily other minerals can be dissolved due to the concentration of free hydrogen ions. The optimal pH level for a hydroponic system is between 5 and 6.5, just slightly acidic. If the pH becomes too alkaline, then minerals such as iron will drop out of solution. When iron is not biologically available to the plants, it causes a deficiency called chlorosis. This is because iron is needed for an enzyme to create a precursor molecule for chlorophyll. Chlorosis can be identified when the leaves begin turning yellow but the veins remain green.

The pH of the system had risen above 7, proving my assertion that something was defective. As well, the leaves were showing a touch of yellow. Iron deficiencies are often mistaken as a nitrogen deficiency because in soil, iron deficiencies are rare. However, imbalances of aerobic bacteria and anaerobic bacteria while in a solution can cause spikes in pH. This makes a plant more likely to have iron deficiencies in a hydroponic system than in soil. The next step was to identify why the anaerobic bacteria had taken over.

An 'air stone' -- in effect a small
electrical compressor used to
pump air into water to oxygenate
it. Photo: NCAT

The capacity for water to hold dissolved oxygen depends on the temperature. The colder it is, the more dissolved oxygen it can hold. Water temperatures above 75 degrees will begin losing capacity to hold dissolved oxygen. This will create a system that benefits anaerobic bacteria. It is recommended to keep the water around 70 degrees because it allows enough oxygen to be dissolved in the system while allowing the roots of the plant to absorb nutrients at an increased rate. In our greenhouse, it is difficult to keep the temperature stable. We use an aquarium tank heater to keep the water warm during cold days. The temperature of the water can often fall to 60 degrees or below. This will slow the growth of the plant, but it also allows for ample amounts of oxygen to be available. This situation would not be good for larger fruiting plants, but it hasn’t seemed to bother lettuces, greens, and herbs.

To increase the level of oxygen dissolved in the water, we pump air through an air stone in to the reservoir. I checked all the tubes to make sure it was operating correctly, but the tube from the air pump had become disconnected. It was a simple mistake that could have ended up with severe consequences. Without enough oxygen to the system, anaerobic bacteria exploded and overtook the beneficial bacteria. This caused strange pH changes that inevitably locked out the nutrients from the plant.

All in all, it was quick fix but now the roots were covered with root rot. To get the hydroponic system back into equilibrium, the reservoir needed to be flushed with fresh water. We found that the plants did not mind staying in fresh water for a few days. Once the roots began whitening up and new growth began, we added more compost tea and an organic fertilizer to jump-start the plants. It was a very valuable lesson that shows the domino effects that can occur in the water chemistry when oxygen is omitted from the system.

Posted on: October 24th, 2017