Souring, as I understand it (not be experience) is due to humidity and rain when ripening, so fertilizing should not be an issue.

I think that fertilizing with figlets affects the fruit:

http://www.crfg.org/pubs/ff/fig.html

Fertilization: Regular fertilizing of figs is usually necessary only for potted trees or when they are grown on sands. Excess nitrogen encourages rank growth at the expense of fruit production, and the fruit that is produced often ripens improperly, if at all. As a general rule, fertilize fig trees if the branches grew less than a foot the previous year. Apply a total of 1/2 - 1 pound of actual nitrogen, divided into three or four applications beginning in late winter or early spring and ending in July.

I fertilize my Fig trees organically with horse manure. The trees are well established with large figs.
Felix