Few things.
Any wind actually fluttering the leaves?(to remove stale air trapped around each leaf).
Styrofoam may leach chemicals into your soil(though I've never used Styrofoam).
In a pinch, you can use an aqaurium PH tester. Try to find one with a large scale(5ph-10ph). Be sure to get the freshwater, saltwater tester are inappropriate. Test the water your using. Then just test the soil runoff. Imagine the PH of water being changed by the PH of the soil before running out the bottom.I.E. 7PH water, 6PH runoff = about 5PH soil going up due to the now added 7PH.
Walmart sells combo temp/humidity digital meters for about $10.
PH up and PH down can be found in the pool section, by the names sodium carbonate and sodium Bisulfate respectively. I've been told that those are plant safe.
After all that, there may be a lack of nutes problem. But please examine that one LAST.
MG soil, usually contains nutes already. MG soil tends to start dropping in PH soon after the nutes start being used. A flush may be in order(Rainwater for that one. Probably has the most correct PH to start.). Flush should also be look at last, after you've tested PH, if its extremely off, then Flush may be the way to go.