Maybe this is my PH problem
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.
Can someone smart explain what this means Add MAP???