Aquaponics - pH buffering

In aquaponics, one issue that can effect your success rate is the pH of your water. pH is a measure of water's (or soil's or whatever's) level of acidity and alkalinity. A pH of 7.0 is considered neutral and also is what pure water measures.

Actually when we measure pH what we are really doing is comparing the relative quantities of two molecules. The hydrogen ion H+ gives us acidity, and the hydroxyl ion OH- gives us alkalinity.

The process of nitrification is acidifying, but in aquaponics we aim for roughly neutral water around the pH 7.0 mark. Depending on the media you use in your garden beds, your system's pH might not be close enough to 7.0 for your liking. Because the bacteria tend to make their world a little more acidic, the water will dissolve anything more alkaline than the current pH of the water. What this means is, if you have something like calcium carbonate, say in the form of shell grit* in your system, your clever system becomes self adjusting. Shell grit seems to have a pH of around 7.6. The more acidic your water gets, the more shell grit gets dissolved. This brings the system back into equilibrium. The shell grit will last for many years (depending on how much acid there is).

The shell grit is trying to get the water to pH 7.6, and the bacteria are forcing the water toward 6.0. Hopefully the result is your ph settles somewhere near 7.0 and everything gets along just fine. Plants also seem to enjoy a pH of 7.0, and that pH level also allows for the maximum availability of various trace elements to be liberated from the system and offered around to anything that wants them.

Shifting an aquaponics system toward acidity is a little harder. Most people add lemon juice, or vinegar. The test pictured to the left is the one at the top of the page but with 2 drops of lemon juice added. The problem with adding acid is that it would have to be done on a regular basis which is why we need to be careful not to use anything too alkaline as our grow bed media. Some people have gotten into trouble with limestone and marble.

Over the last few days I've been doing experiments with softwood charcoal, and have found it to steer a system toward being more acidic on an ongoing basis. The tests have only been going for a few weeks. The problem of a too alkaline system needs to be avoided from the outset, but if you have very alkaline water (say from a bore) charcoal might be a good way to pre-treat your top-up water and the initial water you use in your aquaponics system.



*shell grit is available for a few dollars a bag wherever you can buy chicken feed. Its made by crushing oyster shells or similar. Chickens eat it for the calcium they need to make egg shells.You can also use egg shells or snail shells for pH control in your system.