Man, you know, if you can buy 1, get one. I use the Dr.Meter, lx1330B model, $33 from the Zon, well worth the money.
also, a PPM meter, like $10, and PH meter if you want, always good to have also, $20.
You need to know what your Soil PPM is,
Like the above, nederwierie says, calmag, or at...