natural grow - brown spots?

Hi, I'm new to the site, this is my first real grow, and I have a question.

I have my plant (yes, only 1/10 seeds sprouted) in a 8"x8" pot for right now. Good soil, 2-3 weeks into vegetation, about 7" tall, and looking pretty healthy; maybe 4-5 leaf nodes (levels of branching), the first two are about 2" apart, but the last 3 are very closely spaced...So it basically looks like everything is going as planned!

Except.

I had a feeling that the slight yellowing on the bottommost leaves may have been caused by a lack of nitrogen...I know that the rounded (absolute bottommost) leaves yellow and fall off regularly, but the next level of leaves up had yellowed tips, so I added some vegetable peelings (celery leaves+ends, some lettuce) to add some N.

At the same time, I put a clear plastic bag (with some holes in it) around the pot and raised above the plant, essentially putting the plant into a greenhouse.

Were the brown spots caused by the addition of too much Iron + Magnesium from the celery, or from too much heat in it's new environment?

( I've been trying to keep this grow au naturale, adding only compost materials; no synthetic/store-bought fertilizers...)

well what is the temperature in your room

what it sounds like is a calcium lock out u can get this when the ph goes to high or to low .run with ph6 water for 2 to 3 day then u will have to get plant food that has got a 20 20 20 npk u can use tomato feed cactus feed orchids feed any thing thats got that kind of npk.u will need flower feed some thing on the range of 7.30 30 thats when u give less nitrogen more phosphorous and more potassium u say that u dont want to use any nutes this will result in small yield .the feeds r really cheap u have to sort out your lock out anyway so u r just as well as going for anything that will get u back to normal hope everything goes ok ............................any probs give is a message back.......garry

It's an outdoor grow, so it's been in the 80s lately, and humid (So. Flo. for you).

It's not the fear of having a smaller yield that stops me from getting fertilizers, I just want this grow to be 'natural'. I've been adding other fruit skins/peelings (cherries, peaches, banana) to the soil for added nutrients and, hopefully, flavor.

This is a very recent problem. Everything was going fine (except for the slight yellowing) until I added the celery+lettuce cuttings (for N) and enclosed the entire pot+plant in a clear yellow bag (to keep it moist and get more CO2 from the decomposing compost in the soil?).

I didn't check on the plant for one day, and when I came back there were some brown spots (but the yellow has receded).

I haven't checked the pH of the soil or the runoff, but I do have some litmus paper that shows the water as having a neutral pH. The soil is good and the water hasn't changed, so I know that neither of them are the culprits.