Are you sure the "flipped switch" is Nitrogen and not heat? How much did you use? How long/often? Where are you located?

It would take a ton of N to stop a tomato plant to stop flowering. It's a often-repeated myth here that N is bad for tomatoes. It's more likely that it's weather or water related.

I've been wondering about this, since I've read in numerous places that too much nitrogen will give you lots of tomato foliage, but not many tomatoes. Then I read where you should give your tomatoes some nitrogen when the fruits are developing. And I've also read it's good to keep your tomatoes somewhat hungry and thirsty. So I'm sure there's gotta be some kind of balance. Anyone know how much is too much? I'm trying to figure out how I should feed my tomatoes this year.

The best results are obtained here by planting toms in the ground, rather than in pots; by using a lot of composted horse manure... as I am reticent to use commercial fertilizer... and by watering as needed.

Those roots in loamy compost-rich soil do most of the work.

Yes, it is also important to abate disease.

The manure I seek fumes with ammonia; it is damp; therefore N-rich. I bury it promptly and water it in so the N gets into my plant's soil.

I don't know how horse manure rates for Ph and K but I suppose it's not very well endowed with Ph... which supposedly promotes flower formation.

Don't forget to consider genetics and, most importantly, seasonal circumstances... pertinent to flower formation.