If it turns out to be a simple deficiency you won't need this, but here are the slurry test instructions just so you have them in your thread:
• Take samples from a few different places where there are roots (dig down a bit rather than just use the top - I often stick a spoon down the outside edge of the pot to grab some from there as well)
• Add an equivalent amount of distilled water as grams of soil (10 grams of soil, add 10ml water). If that doesn't make a slurry, use enough distilled water to make a stirrable-but-thick slurry.
• Stir it up, wait 15 minutes, stir again. Do that for at least an hour (longer is better), and then put your calibrated pH stick in the water. That's the pH of your medium.