hey cwesto -

You r problem is usinf pro mix "HP"...it has a range of ph, i believe, 5.3 to 5.7 out of the container it was sold in (somewhere around, not #, but forgot) use the pro mix "BX"... it has a range of 5.5 to 6.3 out of the box i believe...

just add some extra perlite and you got yourself some "HP" using "BX"... you should be adding extra perlite anyweays so why not just get "Pro mix BX" and add some perlite, and what do you konw, you got some pro mix HP ( with added vermilculite, dont like this, but its better than havinf ph issues... never had problems with BX, but with HP had promblems.... hope this helps!

( i rerad this off a fourm, and tested myself what the soil ph waas on these hp and bx right out of the paltic containers they were housed in)