What can I do to reduce acid addition?

I have a hobby hydroponic system using cocopeat in bags, fed by drippers. I have found that the pH of the solution running from the bags is much higher than what I am feeding. I have to add ‘pH down’ to bring my feed down to 5.0 pH in order to keep the run-off pH under 6.3. This is using a lot of pH down. I guess that the pH down I use is an acid, but I don’t know what kind. Is the amount of acid I am using doing any damage to my plants, and is there anything I can do to reduce the amount of expensive pH down I use? Answer by RICK DONNAN

Which pH?
Firstly, I am pleased that you are controlling your system through measuring the run-off solution (for pH and presumably EC). Often growers are conscientious in checking the pH of their feed solution, but ignore the run-off.

Most important is the pH of the solution around the plant roots, which is where the plant is actively taking up water and nutrients. This is known as the root zone solution, and the best indication of this is the run-off solution. The pH of the run-off will often be different to the feed solution pH, especially in dripper-fed media systems. Sometimes the difference can be quite substantial, or take a lot of acid to lower the root zone pH, as in your case.

Not often mentioned in books is that the downside to adding acid to your feed is that it alters the nutrient balance. The more acid you add, the worse the imbalance becomes.

How important is pH?
The importance of pH is well covered in virtually all hydroponic books. Quite often the need for tight control is overstated. For hobby growers a pH between 5.5 and 6.5 in the run-off is good and between 5.0 and 7.0 is usually acceptable, provided the iron in your fertiliser is in a chelated form.

Reason for pH drift
pH drift is not inherent in hydroponic systems; rather, it comes from the effect of nutrient uptake by the plant compared to what is in the feed.

The main influence here is the form of nitrogen nutrients in the solution. In hydroponic solutions, nitrogen usually comes as two different ions. These are negatively charged nitrate ions (NO3-) and positively charged ammonium ions (NH4+), with a much higher proportion of nitrate compared to ammonium.

The factors influencing pH in the root zone solution are the charge on these two ions and their relative rates of uptake by the plant. The relatively high nitrate content is taken up by the plant at a relatively moderate rate. Because it is a negatively charged ion, in order that the plant does not develop an electric charge, negative ions are exuded by the plant to keep it electrically neutral. The negative exudates, such as hydroxide (OH-) and bicarbonate (HCO3-) ions will raise the pH, which is what is happening in your case.

On the other hand, ammonium ions are taken up much more rapidly than nitrate ions (to the extent that chemical analysis of the run-off usually shows no ammonium—it has all been taken up by the plant). Because it is a positively charged ion, positive ions are exuded by the plant to try to keep it electrically neutral. This positive exudate is hydrogen ion (H+), the ‘acid’ ion, which lowers the pH. Put another way, ammonium in the hydroponic feed results in the plant exuding natural acid into the root zone solution to lower the pH.

Ammonium management
In commercial hydroponic operations, most growers add a small proportion of ammonium to their feed formula, to avoid upward pH drift. While the preferred option would be ammonium nitrate, this is difficult to obtain, and ammonium sulphate can be substituted. This is not added like acid, but rather it becomes part of the fertiliser formulation. Enough is added to control upward pH drift. Especially for a fruiting crop, the nutrient balance uptake varies through the life cycle of the crop and the amount of ammonium added may be changed typically about four times through the life of the crop.

As a hobby grower you have two ways to possibly use this technique. The first is to check the analysis of the fertiliser you are using and compare it with the others available to you. If you find one containing a higher proportion of ammonium, give that a try.

The second is to add a small amount of ammonium sulphate to your fertiliser. In either case, allow a couple of days for it to take effect. This is because of the time it takes for a dripper feed to work its way through to run-off.

How much ammonium sulphate would you add? Make a stock solution containing 3 grams ammonium sulphate in a litre of water. Add the same volume of this solution as the volume of Part B concentrate you use. This should give a modest reduction in pH rise. Check the response in the run-off (which will take at least a day or two). If not enough response, add the same amount again to your stock solution. Continue until you reach the stable pH you want, and mark how many grams per litre on the (dark) stock solution bottle. If the pH starts drifting too low, make up a slightly weaker stock solution. Ω

February 2015 / Issue 152