I have tested the ph of my water several times. As it comes out of the RO system, the pH is usually between 7.0 and 7.15. According to orchidusa.com, orchid plants cannot take in nitrogen in the pH range of 7.0-8.5, and note that most orchid sources recommend a pH level of 5.5-6.5 for orchids. An MSU study has concluded that maximum nutrient uptake occurs in plants when the pH level is between 5.0 and 6.0. Because of these facts (and similar ones from other sources), I have decided to take steps to reduce the pH of the water going into the plants.
Today, I have experimented with General Hydroponics’ “pH down”, by first filling the mixing tank to the 9 gallon level and testing the water (pH – 7.11). I have then added my nutrients (1 1/2 tsp of MSU fertilizer), added 1/2 tsp of pH down, and let the solution mix for a 1/2 hour (using my small mixing pump in the tank). The resulting pH was 5.02, clearly less than I desired.
I emptied four gallons of water out of the mixing tank, and refilled to the 9 gallon level with RO water, and the measured pH was 5.83 — which is acceptable for the watering tomorrow.
Clearly, I need to repeat this experiment. My error was in not measuring pH after adding my nutrients, as I could not determine how much my nutrients affected the pH of the solution. It is also clear that the “pH down” solution can dramatically reduce the pH in the solution with just a few drops. My quart bottle should last a long time.
Update: February 6, 2015
After some testing in the mixing tank, I decided to concentrate on getting the pH correct in the RO tank. I tested the pH with my nutrients, and there was not a significant difference in the pH between the input RO water and the solution with the nutrients added (It went down about 0.1).
So, I measured the pH of the RO water this morning, and it recorded 7.2. 1/4 tsp of “pH down” reduced it to 6.75, and another 1/8 tsp reduced it to 6.35. I used this solution to water the plants this morning.
I will continue to monitor the pH in the RO tank each time I water.