I'm confused. A pH of 7.2 is very near the neutral point of 7. I can't imagine that 7.0 instead of 7.2 is going to make any difference.

I can appreciate that tracking down the source of troubled plants is really hard, but I can't imagine that pH is your culprit. And yes, I agree that adding supplements to your water is likely to introduce a whole new set of problems.

Have you tried bottled water as an experiment to see if it makes a difference?

General Hydroponics sells a liquid ph up and down. It's pricey, but it's powerful stuff. It should only take a drop or two per gallon.

What are your nighttime temps in the greenhouse? The spring seems very cold for everyone this year. Maybe they are getting too cold at night.

I agree there is some other problem. I would look more at the water temp, ambient temps and the soil temps. There are charts that tell you appropriate temps.

As for not being able to control the PH, have you checked the KH? KH is a measure of stability . If it is off then the PH can change rapidly. You can stabilize the KH with Bicarbonate of soda or make some slugs of Plaster of Paris that has no additional additives in it. Put one in the water and let it sit for a while. If the PH is whacked, the POP will gradually be consumed until the water is neutral. Once it is neutral, there will be no additional effect.

If you are using test strips, they can be inaccurate. The liquid tests are much more accurate.

How much light are the seedlings getting. When I had too little light, I used reflective mylar to increase the light. The difference was amazing. The closer to the mylar, the healthier the plant.

rws,
What kind of plants and growing media are being used?Blueberries?Peat moss?
Sulfur can take a while to be changed to Sulfuric acid,something like several months to a year.
Peat moss is naturally acidic and could help.
I personally use battery acid (Sulfuric)for my Blueberry plants and it doesn't take much,about a teaspoon per gallon to get the pH to about 4 or 5.My tap water is also about 7.2 pH.
But like Karin wrote,your water's pH may not be the problem.
Also,keep an eye on the pH in the next several months,if that Sulfur has not been flushed out,it could kick in and bring the pH too low. Brady

Battery acid is highly reactive and can be dangerous to use. it is a very strong acid. Please try other solutions before resorting to this. It just is not safe. In many places the sale is restricted by law. You would need to take precautions to mix it without injuring yourself. A full face mask like a welding mask is not too extreme. An acid proof lab apron is also a good idea as is a shower close at hand.

just fyi - it is perfectly legal for battery acid to contain up to 1% lead. Chemical companies can mix it in on purpose, because they get paid to "dispose" of the toxic waste in this fashion.

There's a Federal law that prevents anyone from suing over product liability if they use a product in a manner "inconsistent with its labeling." If you gave yourself lead poisoning or went blind from acid burns, you would have no recourse to sue.

Of course precautions need to be taken.I use eye protection.
People use it all the time to fill batteries.
I'd rather use rain water,but in the climate where I live,the Summers are usually dry.The tap water has bicarbonates that will gradually bring the pH to an unhealthy level for Blueberries.
I understand the risks. Brady

Have your water tested by a pool supply store or a business that sellls water treatment equipment. In most areas this is done at no charge. This should help you better understand what is causing your problem. Looking at pH alone is a very incomplete picture.

Thank you all for your responses.

Cole: I am currently using Gen Hydroponics PH Down, only I'm using the powder form...3 tsp per gallon to drop the PH from 7.2 to 6.5. (it just can't be normal.)

My night temps do drop down to about 50, but I have all my plants on 70 degree heating cables plus the flourescent lights are kept on. During the day the greenhouse warms up to 85 or higher. (I did not know that temps could affect PH)

Thanks for the suggestion to try bottled water...I'm going to do that today...but if that's the solution there is no possible way I'm buying 10 gallons of bottled water a day for my greenhouse. :)

Sleepless: I had not heard of your P of P test? I'm going to try that and see what happens.

Brady: battery acid, huh? (yikes!) For a normal person that may be a solution, but for me....I can't even open a can of pop without causing a disaster....imagine me with battery acid in my hands!

The big reason I became concerned about my PH (since it is near normal, I never really worried about it before) is because my potatoes had scab last fall, really bad...some sources recommend dropping the PH way down. Then I ordered some blueberries to plant this spring. Then I noticed alot of my plants in the greenhouse were showing signs of chlorosis(?), yellowing and I began to research the whole PH thing. Thats when I discovered that my well-water does not like to be adjusted. I've created my own growing media - 1/4 peat, 1/4 coir, 1/4 dairy compost, 1/4 vermiculite. For damper-offers I throw in a few handfuls of perilite.

To tell the truth, I had alot better luck with seed germination last year when I just watered with bliss. Maybe I'm fooling around too much, and trying to play mad scientist.

rws,
It does look like it's the bicarbonate load in your well water,which will definitely cause the Chlorosis.
Iron Chelate or Iron Sulfate might be tried. Brady

Thanks for adding more info, that is helpful.

So, I think you can discount the potato scab issue. I don't think high pH causes scab, rather low pH may help prevent it. Scab is not an indication of pH trouble.

Are the affected plants in your greenhouse yellow or yellow with green veins? If they are just yellow there could be other causes like low nitrogen or too much water. If they are yellow with green veins it could be chlorosis.

Our water has a pH of 8, btw, and that doesn't cause problems. Hopefully you can figure it out soon!