Why is the nutrient solution in my closed RDWC system becoming acidic?
I found Lynette Morgan’s article Open or Closed: What System is Best for You? to be a very good article. I found my closed RDWC system far more wasteful than I was expecting. In a 15-gallon capacity system I couldn't really get more than two or three weeks before it would go so acidic I couldn't control it. Not sure if I'm just using too much cal/mag or this is just a result of running out of nitrogen and the plants are creating carbonic acid as a response?
—James G. via Facebook
The main ones are often the composition of the nutrient solution itself and differential uptake of elements. When plants remove nutrient ions from the solution, the pH tends to either drift up or down (with upward drifts being more common, but not always).
When positive ions such as the cations Ca2+, K+, and Mg2+ are taken up by the plant, hydrogen ions (H+) are released from the root system and this lowers the pH of the solution. When anions such as nitrate (NO3-) are taken up, roots release hydroxyl ions (OH-) into the solution which increases the pH. So, if plants are entering a growth phase which favors the uptake of potassium and calcium in particular, then the solution pH may fall over time.
The same would occur if the system is running out of N — the plants can’t take up sufficient nitrogen, but will still take up the cations Ca, K, and Mg, making the solution increasingly acidic.
Another reason why the solution may go increasingly acidic is when the nutrients have a certain percentage of nitrogen present in the ammonium form (NH4+), usually supplied as ammonium nitrate. In some commercial nutrient products, ammonium is used to help counter the high alkalinity and pH which comes from some hard water sources and give a degree of buffering capacity (slowing the rate of pH change).
However, ammonium forms of nitrogen can have a strong acidification affect which is not required in all systems, particularly those that use a soft water or reverse osmosis water for example. Ammonium tends to have an acidification affect in nutrient solutions because unlike nitrate, it is a positive ion and when taken up by plants (which can occur very rapidly), it is replaced by hydrogen ions thus reducing the pH. If your system continually has problems with the solution becoming acidic with a pH that is difficult to adjust, then avoiding nutrient products that contain the ammonium form of nitrogen would be advisable.
Another reason for the solution becoming increasingly acidic is due to microbial activity in the system, which is more complex. The breakdown of organic material by microbial action under anaerobic conditions can have an acidification affect and often systems which have some form of early outbreaks of Pythium or other root rot pathogens first notice a drop in pH as the disease progresses.
Read also: Dealing with Pythium Root Rot
Also, the use of some supplements in water culture systems can have the same affect if these are organically based. Supplements that may be used as growth boosters or even hydroponic organic nutrients can contain acids such as phosphoric acid that also lower the pH over time, so it its always advisable to check the pH of these products before adding to a hydroponic system.
Suntec International Hydroponic Consultants
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Written by Lynette Morgan
Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.Full Bio