Adjusting pH, the endless chore
Plants in recirculating systems will change the pH of their solutions through nutrient uptake. This means that the pH of a recirculating hydroponic system will be inherently unstable and will require constant corrections. We usually carry out these corrections through the use of strong acids or bases, such as the commonly used pH up/down solutions we buy at stores. This makes the process of pH adjusting repetitive. Although many people have implemented automated systems for pH correction, these systems have potential for failure, especially due to sensor calibration or failure issues. Ideally, we would want a completely passive solution to maintain the pH of our hydroponic nutrient reservoir.
There are several ways we can tackle the problem of shifting pH. Chemical buffers are a potential alternative. I proposed the use of a citrate/carbonate buffer in a previous post, but this buffer does not work due to the microbial and plant metabolism of citrate ions, which pushes the pH up. Carbonate/phosphate buffers offer a better alternative, but there are inherent limitations in the strength of these buffers due to the limitations in phosphorous and carbonate concentrations that plants can tolerate. This means such buffers are usually restricted 1mM or lower concentrations — not able to compete with plant uptake.
The most popular choice in the research community are MES buffers, which can be used to keep the solutions at stable pH and can be used at concentrations even exceeding 10mM. The problem with these is that they can cause problems in some plant cultures and they can also become extremely expensive for large growing systems.
Ion exchange resins
How do we keep pH constant without using any chemical buffer? The solution comes in the form of weakly acidic ion exchange membranes. These substances contain polymer-like matrices which have functional ion attracting groups on their surface that react with acids and bases in solution and provide you with a constant pH level.
These composites are insoluble and the only thing they need to be efficient is to have solution passed around them frequently. It is a matter of putting them in a place where fresh passing solution will be in contact all the time – such as near or connected to a high-flow pump – and that’s it, no more pH problems, no more additions to control pH, problem absolutely solved. As the solution passes through the material, it will be able to react with the ion attracting sites in the polymer to stabilize the pH. These compounds have been studied in the literature and given good results (1, 2, 3).
In particular researchers in the 1980s tested weakly acidic ion exchange resins, like Amberlite IRC-50. One of the problems with their use was that these resins can have significantly high affinity for Ca and Mn, which means these cations need to be added in excess or replenished if the plants show problems with their uptake. Note that the Amberlite IRC-50 ion exchange resin is no longer available. For a public list of currently available resins, you can refer to this link.
I have tested ion exchange resins extensively in recirculating systems and they can provide you with high pH stability through time, especially when the plants are pushing the pH up. This is the most common steady state of a hydroponic system, as most plants will make solutions more basic through their nitrate uptake. Since the loaded state of the resin is acidic, they have their maximum buffering capacity towards pH increases in this state. As the resin gets used from this state, it gains buffering capacity against the other direction.
Another advantage of the resins is their ability to be regenerated a very large number of times. In weakly acidic ion exchange resins, this is done by soaking the resin for a short time in a strong acid. Normally muriatic acid is the acid of choice, as the chlorides of most cations trapped in the resin will be soluble. While other acids could be used, they might form precipitates with some ions trapped in the resin, especially Ca.
The resins in their charged state are naturally bad at buffering against pH decreases in the nutrient solution as they have only acid to contribute to the solutions. While protection to the downside will be created by the resin with time, you might need to modify the starting state of the resin if you want this protection from the very start.
Weakly acidic ion exchange resins are a great way to provide stability to a recirculating hydroponic system, they are also low cost and can be regenerated easily. They work best when plants push the pH of the nutrient solution up initially and do require modifications for cases where guards against pH drops are required from the beginning. Depending on the exact chemistry of the resin there might also be some issues with capture of some cations, like Ca/Mg/Mn, reason why it is important for us to monitor the plants closely when these resins are initially used.
Have you tried any weakly acidic ion exchange resins in your recirculating hydroponic solutions? Is your pH stable? Let us know in the comments below!