Plant leaves can be burned by fertilizer. Are the plants in the sun outside, just recently moved there? The sun can scald plants that have been indoors. Either thing could have damaged your plants.

In all honesty, I have never-ever added fertilizer any of my herbs - basil included. In good soil, it isn't needed. Some herbs, in fact, rather hate overly rich soil.

FataMorgana

They have been going outside way before I did this but they where in clusters vs single plants. I only put fertilizer because I transplanted them into individual cells and they said that to use some to help it along.

I was having a browning/wilting issue with my Basil as well so I hit up my local county extension and this was the response they gave me. Hopefully it helps. I've also attached a photo of what my Basil looks like with the browning.

"The basil would benefit from more light and air circulation. Gradually move it to full sun (it will cook if you do it too quickly, think your first day at the beach). It may be staying overly wet. Also do not wet foliage when watering. Make sure the cup is not blocking air movement."

Mine looks the same way though my sweet basil is actually doing great so is my cinnamon basil for the most part it is mainly my lemon and Thai basil which is either not growing, browning, or bending over. The have been in full sunlight for a while now if you look at some of my older posts you can see them as babies. The only thing is now they are in individual cells. I may have over-watered (I really dk how to water them so much) them but I do water from the bottom. The only things that have really changed major is is split up the clusters and re transplanted them into individual cells. They got a little more sunlight now the weather is warmer and I did spray some fish fertilizer when I transplanted I used 1 tablespoon for a gallon of water like it said and I gave the plants a light misting.

More pics

And now look at my sweet basil

The ones in the last pic/tray looks just fine, to me.

Basils love warm , humid weather. Mine are just sitting there, outside, in 4" pots. The nights are just too cold for them(mid 40 ish).

If they are in the sun for many hours a day and standing in those foil containers they might be burning up with the reflected heat. I also agree that you are probably overwatering. Plus spraying them straight after transplanting with fertiliser was not a good idea. Just-transplanted plants need a drink and a rest, not intensive force feeding;-)