Odd problem...Help?

Your Ph is to high. Go get some ph down and make your water down to 6. The mg soil ph is to high. Seedlings wouldnt need juice untill a month old. That soil is for differnt plants. No good for marijuana.
You need to get some Promix. Its a soilless. aka peatmoss, limestones,perlite. Its the bomb. I start seeds in promix and then start feeding two weeks later. I do 1tsp grow 2tsp micro and 1 tsp bloom. Seedlings need some P during the seedling stage and Bloom contains Magnesium and sulfur. Those are acidic and will lower your ph. But you shouldnt feed them yet.
Adjust your water to 5.5 and flush your plants. That 5.5 will take your Ph down to 6.5 or 7 depending on how high your Soil PH is? Im sure Mg contains and you said it was 7.5? 7.5 should be 6-7. Good Luck.

Ridonc said:

I bought some distilled water by the jug. Used a ph tester. Results:

Tap water: 8.2
Rain Water: 6.9
Distilled: 6.7
Tap water (sitting for 48 hours): 7.8

MG soil ph: 7.5
Organic Soil: 6.6

The plants are thriving and growing like crazy with the new soil! Ph is everything

Click to expand...

Good now your more "aware" of your soils PH. Next time you mix up soil add a tablespoon of dolomite per gallon of soil. If you buy premade soil you are lucky because there is lime in that. I would add a tsp per gallon of premade.
Now if your mixing various organic soils you have to be aware that the mix contains no lime untill you add some(tbsp)per gal. Lime is a PH stablizer. It keeps the PH at a neutral 7, and it can never raise the ph above 7.
The best food for soil would be Jacks or Peters. It contains no calcium. And its slightly acidic. 10-30-20.

The thing that is consistant with all plants is that their roots are by far, many times more sensitive to the environment than any other part of the plant that is above ground. So while you could accidently cook the plants in 90f temps, or drown them in the rain, plants will survive because of how tough those above ground parts are. However, roots are a completely different thing. They are extremely sensitive to temps, and they do not like getting light on them, and they hate soggy soil for long periods of time.

Younger plants in particular can easily go into shock from issues that happen in the root chamber.

Number one is allowing the root area to dry out completely between watering if you are growing in soil.
Number two is no light should get into the root area, including UV and infrared light. Those cups you are using do not look light proof.
Number three is that while plants can handle a change in temperature from 60f to 90f without dying, roots cannot. The container the plants are in does not look like it can keep the root area cool.
Number four, seedlings do not need much in the way of nutrient for the first few weeks. Make sure you do not seed them in anything other than proper seed raising mix. Premix soils with slow release nutrients will damage the root system on young seedlings.

Hope that helps.