Are you trying to harden them off? Are they close to a window and have you had freezing temps outside? If you are trying to harden them off, you may have put them out for too long. If they get too much sunlight and they are not use to it, it can burn the leaves. However the leaves turn white. If you still have then inside, and the leaves are touching the windows and you had freezing temperatures at night, you may have frozen the leaves. Unfortunately, I've learned both of these things from experience.

Which leaves are turning yellow, the young leaves or the older leaves? How long ago were the plants transplanted? The leaves that are yellow, is the yellowing between the veins or in the veins and not in between or all over? Since you did not add fertilizer to the second plot (symptomatic plot) then you probably have a N deficiency, just fertilize and you should be fine. I would also get some gympsum or calcitic lime and add that as well. The gypsum is very soluble in water and can supply Ca to the plant fast whereas the calcitic lime is relatively insoluble and will be slowly available to the plant but will offset any of the acidity that the compost has added. If you do a pH test on your own you are only going to get half of the story, so be weary of the results. The pH of the soil is affected by many things, the plant, the soil type and the organic matter. The plant and the organic matter can be measured the simple way but the soil part you need to do something different (namely an exchange) in order to get the value. The soil clay acts in the same way as your water softener, in that it has the capacity to hold a variety of positively charged ions (Ca, Mg, K, H, Na, NH4)and will exchange those with the soil solution thus making them available to the plant. themedicinalpepper.com Here is a link that might be useful: The Medicinal Pepper

depends how many. it is normal for lower leaves to yellow and fall off as they age. Is there still growth on top of the plant? Does it otherwise look healthy? john

I hope you shaded it after transplanting. For the winter inside it lost it's hardness. 18 inches tall and wide doesn't sound too big. My peppers by the end of the season are usually at least 2' high, may be more, and at least 20 inches wide (more, if I let them be.) you can trim it a little, I guess. But not the branch that is producing. Support that branch, tie it somewhere, it can easy snap under wind. Other than that, nothing at least 2 weeks after transplanting, then you can start fertilizing...