How do I manage acid addition and pH rise?

From a NSW hydroponic tomato grower.
How do I manage acid addition and pH rise?

I grow tomatoes in greenhouses south of Sydney. I use phosphoric acid to lower my pH. I have been adding increasing amounts of acid to bring my feed pH down about 6.0, but it is still rising to about 7.0 in the run-off. I have been adding some liquid ammonium nitrate to reduce the pH rise, but it has only reduced the pH rise by about 0.2 pH. I have had some symptoms of what has been suggested is iron deficiency. That is, the young leaves are pale and the veins show up darker on the pale leaf.

An analysis was done of my feed and the laboratory advised that it was generally OK, except that the phosphorus (P) level was high. I would like to add more acid, but that will make the P level even worse.

Can you suggest what I should do?

There are several suggestions I can make, although given limited information.

Iron deficiency
Firstly – iron deficiency. It certainly seems as though you have an iron deficiency. The typical symptoms are as you describe, that is, a yellowing of the youngest leaves between the veins, which show up as a darker green. When more severe, the entire leaf turns bright yellow. Iron deficiency is often the first symptom to show when the pH gets high. Whether iron deficiency occurs is highly dependant upon the type of iron that you have in your feed. The most common form is iron chelate EDTA. This starts to become unavailable once the pH rises above 6. The higher the pH and the longer the solution is used, the lower the available iron will be. I guess that this would be the type of iron chelate that you are using.

I recommend that you go to one of the more stable forms of iron chelate, namely DTPA or EDDHA. DTPA is stable to about 7 pH and EDDHA much higher. These contain a lower percentage of iron than the EDTA so you would need to increase the amount that goes into your feed mix in order to maintain the same ppm (parts per million) of Fe in your feed solution. However, it is possible that your current formulation has a higher than needed ppm of FE in order to allow for loss of availability. When using the more stable forms, then iron ppm aim in the feed need be only 1.0 ppm Fe.

Phosphoric acid is probably the most common used to control pH. However, it does present some difficulty as shown up in your case. Because the phosphorus (P) component of a typical tomato feed is relatively modest at around 40 ppm, adding a significant amount of phosphoric acid will give a substantial rise in the P content of the feed.

The main alternative is nitric acid, which has the advantage that the normal feed level is much higher at about 200 ppm N, hence adding nitric acid will have a much less relative impact on the N content of the feed. Unfortunately, concentrated nitric acid is a nasty liquid to handle, especially as it gives off irritating brown fumes, hence why phosphoric acid is more popular.

pH drift
In order to reduce the amount of phosphoric acid needed, you need to reduce the pH rise so you do not need to keep lowering your feed pH. This is done by increasing the ammonium content of your feed. Ammonium nitrate solid is banned for sale except if you have a special security licence. It is available in liquid form, but this is at half strength and is relatively expensive.

There is an alternative. This is to add mono ammonium phosphate (MAP) to your mix to give the added amount of ammonium that you need. You are no doubt adding phosphorus in the form of mono potassium phosphate (MKP). In order to keep your phosphate level steady, as you add MAP you reduce the amount of MKP accordingly. Without more detail, I can’t give a definite recommendation for the relative proportions, however about half each MAP and MKP would be in the ballpark, to eliminate your upward pH drift.

Ammonium addition mechanism
How does increasing the ammonium content of the feed reduce upward pH drift?

Two forms of nitrogen (N) are used in hydroponic solutions – nitrate ions (NO3-) negatively charged ions (known as anions) and ammonium ions (NH4+) positively charged ions (known as cations). In soil growing, any ammonium ions present are locked onto the soil particles and converted (the process of ‘nitrification’) to nitrate ions before being taken up by the plants. Thus, in soil, the plant is basically not exposed to free ammonium ions. Contrasting with soil growing, in hydroponic solutions ammonium ions remain available and are taken up very quickly, much faster than nitrate ions.

Especially during the vegetative stage, plants are taking up high proportions of NO3- ions. In order to remain in electrical balance they will be exuding positively charged ions, which raise the pH. If NH4+ ions are introduced into the solution, they are rapidly taken up by the plants. Therefore, the plants compensate by exuding positively charged ions to maintain the electrical balance. These are hydrogen ions, H+, the ‘acid’ ion. Consequently, the pH of the root zone solution will fall.

Therefore, increasing the proportion of ammonium in the feed will result in a relative lowering of the pH of the root zone solution. Reducing the proportion of ammonium in the feed will result in a relative raising of the pH of the root zone solution. This applies to all systems whether recirculating or not. For example, the pH of a recirculating solution may be over 7 even though the feed solution pH has been pulled down to below 6. This is a situation where the use of phosphoric acid to lower the pH will lead to a severe imbalance of high P levels developing in the recirculating solution. Increasing the proportion of ammonium ions in the feed should allow feeding at about pH 6 and to maintain that level with time.

When doing calculations, remember to include the ammonium that comes as part of the calcium nitrate. Typically, about 8% on the nitrogen in calcium nitrate is in the ammonium form, and about 92% in the nitrate form.