This is my first closet grow. I have 4 plants. On two of them the tips of the new leaves are starting to turn yellow. The nitrogen is normal. The soil PH is 6.5. They are white Widow fem. 6 weeks old. The first 3 weeks in solo cups. The last 3 weeks in 10 liter pots with drain holes. The soil is 20% perlite 80% Pro Mix. The temps art between 62 night and 80 day. I use all purpose Miracle Grow every other watering. The lights are 12/12.
Miracle grow all purpose won’t suit the plants needs during flowering you are actually getting first signs of nute burn likely a nitrogen surplus if you are going to grow MJ look for tomato fertilizers or cannabis specific as you enter flower it’s going to need more PK less N all purpose foods usually are even right across since the plant is using less N there is a build up starting in your plant and soil. Your plant will be craving Cal-mag and PK look into bloom foods.
Hi. Are you trying to start flowering? With the 12/12 lights. I was just checking since you said it was your first grow.
Tom
Sorry, I made a mistake in my post. The lights have been on 18/6. I am switching to 12/12 today to start flowering. I was using MG. Now I am switching to Dyna-Gro Bloom. Any suggestions on how I can fix the yellowing of the leaves. I just checked the PH on the run off water. It was at 6.0
Thank You,
Larry
Your ph is in a safe zone what I would like you to do is flush your soil before flipping 12/12 this is a simple process and will benefit root zone greatly run correctly ph’d 6.2-6.5 water 3-4 time the volume of it’s container through the soil this will clean a good portion of the unused nutrients/salts out of your soil. So you have healthier start point for your flowering after flushing feed this will help with the yellowing your plant should be tapered into a bloom diet but since you have been using all purpose food this isn’t as much an option you may be able to 1/4 dose what you’ve currently been using with the recommended bloom dose at 1/2 for first week then do away with the all purpose.
The yellowing is likely the start of build up in the soil reason I suggest the rinse as well it is easier to rinse plant at start of flower before it gets to big 
Hope this helps
Thank You Donaldj. I have finished the flushing. Do add the nutrients now or wait until the next watering?
Larry
You can add nutes now since you have mostly stripped your soil of any surplus
One more question. One day after flushing I put 2 liters of water with 1/4 MG nutrients into each 10 liter pot. About 1/2 of the water instantly drained into the saucers. I’m guessing it is because the soil was saturated from the flush. Now there is about 1 inch of water in each saucer. Should I remove that water or just let it evaporate?
Thanks for all the help,
Larry
remove the water You want your plant to mostly dry out between watering this is easiest to judge by it’s weight if it feels very light or by watching your plant it will slightly droop while saturated then perk up and droop again when it is dry but I find weight faster.
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And potassium helps it become more tolerant towards stress but yes, in flower you definatley need more PK than N