Ph is a measure of how acidic (low end of scale) or base (akaline, high end of scale) water or soil is. Different types of plants like different levels of ph, and some, including mj, are picky about what their roots sit in. There are scales of preferred ph for mj here:
http://www.marijuanapassion.com/forumWOW/Full-Marijuana-Growing-Guide.html#PH-AND-FERTILIZERS
Ok, how to get there.
First, mix your nutrients in b/f you test ph. To test the water, you can buy an electrode/batttery powered meter, look on Ebay, water testing, hydro sites, even Amazon. Or you can head to a nursery that has pond supplies and for about $15 a pond water test kit will work fine. You fill a little test tube with water and add drops, compare the color to a simple chart.
To change your mixed water's ph, you can buy products at grow shops, or with fish supplies at the pet store. If you're not doing big quantities, the liquid drops they sell to bring ph down are easy to control and adjust till you get the hang of it. They also have powders, but dissolve them in water first to make sure you get complete mix and an acurate reading.
Bringing ph down, which is what you have to do with most tap & bottled water, means making the water more acidic, so people recommend lemon juice as an organic alternative. I've never tried it.
I just threw this in cuz sometimes people (like me lol) may start their grow thinking it's not a big deal, and need someplace close to get something quick & cheap to get on track. Hope this helps.
