Potassium (K+) is one of the most limited nutrients in aquaponics and the bulk of K+ must be added in the form of a supplemental compound to bring the concentration to adequate levels, even for lettuce production which requires relatively little K. When growing fruiting crops, particularly tomatoes, cucumbers and peppers, much larger amounts of K are required to produce adequate fruit.
Potassium Sources, K2CO3 Advantages:
When growers talk about potassium in aquaponics most will immediately refer to Potassium Hydroxide (KOH) as the ideal compound for simultaneous pH adjustment and K addition. Although KOH is the most effective compound for raising pH to appropriate levels within aquaponics it is also highly caustic and therefore very dangerous to handle. If added inappropriately KOH can also ruin a systems chemistry as well as even kill fish and crops. While KOH has very potent basifying properties it provides the nutrient solution with relatively little alkalinity or buffering capacity which means the pH is more erratic and will change more significantly over time.
A less commonly utilized and sometime even discouraged compound is Potassium Carbonate (K2CO3). While it is not ideal for use in large scale commercial aquaponics it provides a vast range of advantages over KOH for small scale aquaponic production systems. Several of the advantages K2CO3 offers are:
For educational/classroom settings K2CO3 is far safer: KOH has many more risks associated with it compared with K2CO3. With its high toxicity and caustic ability KOH can do a lot of damage to many materials and to people. K2CO3 however is a mineral commonly used in baking and is available in food grade form so it is essentially non-toxic and is not nearly as caustic.
K2CO3 is all natural: Potash is naturally occurring rock made up of K2CO3 versus KOH which is a manufactured compound created by reacting KCl and H2O via electrolysis.
K2CO3 increases pH effectively: Although not as potent as KOH, potassium carbonate has the ability to increase the pH to perfectly optimal levels for aquaponics production since it’s solubility remains high at any pH. It’s also less likely to cause pH to increase by enough to harm the system. As an Exampel 130 grams K2CO3 added to 650 gallons’ nutrient solution at pH 6.6 increased pH to 7.4
K2CO3 increases alkalinity (buffering capacity): K2CO3 significantly increases the alkalinity, or buffering capacity, of the solution. This means pH is maintained at a consistent level for longer periods of time. This is due to the carbonate added, which must react with two H+ cations before forming CO2 and H2O neutralizing the acidic effects of H+. KOH has low alkalinity as hydroxide (OH-) only has the ability to absorb (1x) H+ before becoming H2O. Thus pH can change more significantly over a far shorter period of time when using KOH.
When to use KOH?
All that being said for K2CO3, KOH is popularly utilized in aquaponics for a reason and that is because KOH is more ideal for commercial aquaponics operations where profits are of vital importance. In other words, aquaponics systems that produces tens of thousands versus just hundreds of crops should use KOH
There are a couple advantages to KOH for large scale application but the main one by far is COST. KOH is very cheap and less KOH is required to adjust pH over time compared with K2CO3. Large scale systems can contain tens of thousands of gallons and therefore require very large amounts of the basic compound to adjust. Less carbonate in the water also means that there won’t be as much carbonate buildup throughout the system. A mixture of the compounds can be utilized to help boost alkalinity and maintain pH for longer periods than KOH alone.