That's all you need. Measure the pH of the water. Then measure the pH of the runoff from the bottom of the pot. You can then deduce the pH of the soil. For example: if the water has a pH of 7 and the runoff is 6, then the soil must be more acidic than the runoff, probably in the 5 range. In soil, you want a pH in the mid to upper 6 range for proper nutrient uptake.

Seedlings and new cuttings can easily be killed by too much nutes, regardless of the pH. I use a very light nute solution as a foliar spray for clones. And I never place them in soil until roots have developed. Also, in my limited experience, sunlight is much too harsh for clones. They must be gradually hardened first.

There is much to learn about growing cannabis. Most things can be found on this site in the sticky threads. I'm still in the early stages of learning myself so I may not always have the best advice. :-)

Post a pic if you can. That will help us diagnose the problem and steer you in the right direction.

DH