How can you alter the pH of water that you feed plants?

I am doing a science experiment and we are testing the effect of pH of water, on plant growth. I need a way to alter the water's pH without changing any other variables (salinity, D.O., etc) and without harming the plants any other way than the pH. It's complicated but I NEED HELP SOON!!!How can you alter the pH of water that you feed plants?
You can slightly lower the pH of water with an inexpensive acid.

Carbonic acid (bubbling CO2 through water) will lower the pH down to a minimum value of about 4.7. Vinegar is another dilute acid. This would add an organic compound variable. CO2 might not be that easy to come by. There is CO2 in paint ball cartridges....or you could purchase dry ice and put a small piece in water to carbonate it.

Raising the pH without using a salt isn't as easy. Ammonia will raise the pH...but is also a fertilizer (source of nitrogen for plants). A lye (NaOH) would add sodium.How can you alter the pH of water that you feed plants?
Adding different amounts of vinegar (weaker) or lemon juice (stronger) will make water more acid, a small pinch of baking soda will raise the pH. You don't need to add a lot of any of these, just a drop or so per cup of water fro the acid, maybe an 1/8 to 1/4 teaspoon of baking soda per gallon. Both are safe enough to use, since they're things found in foods you eat, and you probably have them in your house.



If you're going to be growing the plants for a few weeks, you might want to set aside an old soda bottle and mix up a stronger baking soda solution, say 1-2 teaspoons of baking soda and fill with water and mix. Then use an eyedropper to add some number of drops to a gallon of water for water ing them. If you keep using the same number of drops from the same source, your pH won't change as much as trying to add a ';pinch'; of the dry powder each time. Don't worry if all the baking soda doesn't dissolve, but let the dissolved powder and water come to room temperature before using this or measuring the pH, since the temperature will affect how much will be able to be dissolved.



But anything you add will change the chemistry of the water slightly. So to have the fewest variables, only use 1 or 2 of the materials to change the pH. If you only use 1, you'll have the fewest variables, but you won't be able to both raise and lower the pH, you'll only be able to go in one direction.