Just a word of warning about the pH meters you can get for a few bucks at the home centers. I've tried every one I could find and every one of them was pretty much useless. No matter what I stuck them into the readings always came back around 8. I'm talking sticking it in my home made mix, used coffee grounds, tap water, water from my filtering system. Nestle's chocolate milk mix in milk, mud after a wicked rainstorm and Pro mix BX and HP you name it I've stuck the darn things in it. The capsules with the powder are better, in my opinion, but if you're colorblind and I am, forget about it.
I've also found that when making your own mix if you end up buying all the extras, bone meal, blood meal, lime and everything else you think needs to go in it you're usually better of buying the already made stuff.