New Grower, black spots and dry leaves

[Plantenthusia]

I am a new grower and just trying to keep two plants alive. They are bag seeds, I use a 12W LED grow light and a 5500K 60W CFL. I supplement that light during the day with sunlight from the window. I used seed sprouting Miracle Gro, which I found out later is not the best. I also use rainwater PHed up to 6.5-7.0 with small amounts of baking soda. The rh is a little low, it fluctuates between 35-50%. The temp is always between 20-27 C. The pictures are below, any advice would be awesome!

 

[The Germinator]

Looks like splash to me. I see lots of what looks like dirt on the stems of the leaves. Otherwise it looks good.

 

[Plantenthusia]

Splash? The bottom most leaf is dry and crinkly. Those dark spots you see in the second picture appeared on the bottom leaf a couple days ago. I'm wondering if it'll spread to the other leaves eventually?

 

[Weaselcracker]

The new growth looks healthy. Often problems with the very bottom leaves can be ignored. Let's hope that's the case.
You don't need to ph plain rain water. I realize that it has a ph that looks low. Mine is 5.6. But it's basically a meaningless/empty value. I would skip the baking soda as it may cause more harm than good.

Stronger solutions like nutrient mixes do generally need to be ph balanced through. An exception is if you're using a decent soil mix, especially one containing dolomite lime.

If you feel the need to adjust ph (soil growers usually find it unnecessary) I would buy the proper ph up and down solutions- or if you have trouble finding them, there are home substitutes. But plain rainwater does not need to be adjusted. I've used it exclusively for more than ten years.

Put it this way- one drop of ph-down or ph-up will drastically shift the ph of a five gallon bucket of rainwater.
Now reverse the situation and think of it the opposite way- a bucket of rainwater is going to have basically no effect on the ph of your soil. It's too weak.