Lime.. and why we use it...Next time just amend with a little lime and don't worry about pH.
I think its just a little burn from the feed.
What would really help is a pic in blue or natural light.
reminds me of compost heaps..
we compost anything, sticks ,grass any organic mater really.
but for any organic matter to break down we need bacteria.
all bacteria feeds off nitrogen.
We have to replace the nitrogen that the bacteria took out of the compost, we add it to the compost heap either via CAN or urea.
nitrogen lowers PH
thus the need for lime
limes alkaline and counteracts the added Nitrogen.
My preference for lime is Dolomite