i'm about 3 weeks into my grow, here are the specs:

3 plants, 6x26W 6500k cfl, 3x26W 2700k cfl
miracle gro soil w/ worm castings and perlite

i added some 20-20-20 fertilizer a couple days ago at about 25%.

the tips of some of the older leaves on two of the plants have started to become yellow and brown, and curling UPWARD. what could be the cause of this? could it be nute burn or some sort of deficiency?

another question: can i use something like lemon juice to lower the pH of the soil? if so, how would I apply it?

it sounds like nute burn. since your using MG for your soil, you probably should wait to use any nutes. I ran into the same problem with pre fertilized soils. they should do fine with just water. for 3-4 weeks once they break the soil if you are using MG. Also what is your ph?

it may also be an Mg problem,but it sounds like you fluctuated the ph a little too far out of the comfort zone.I have always adjusted the ph of my nute solution,then watered,and I only feed every third watering to make sure the soil doesnt develop hot spots of nutrient.

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i still need to invest in a pH meter but have tested with the little strips and it seems to be in the 7-7.5 range. i'd like to lower it at least a full point if possible.