I usually apply compost manure combined with osmocote in spring. Then I use osmocote again in late summer. Any suggestions ?

That sounds like what I do but I also use a liquid foliar feed during the season. I prefer fish emulsion but I've used Miracle Grow as well. It just gives them a little boost between the slow release granular feedings.

I add Chelated Iron when I use a liquid fertilizer.
When I was more organized, and fertilized on a semi-regular basis, I used foliar as part of a Tomato-Rose regime, usually putting Rose fertilizer on the roses but probably more often than I remember simply mixing up five to twenty gallons of one or the other and using the same concoction on both but at that I always used the Iron but also used Ironite. :

I do not know if they changed the formula but ten years ago when I put Ironite on Sharon's plant pots, they would get DARK green similar to what the liquid Nitrogen farmers put on Corn.
Does not seem to do that any more.

Fifteen years ago, partly because we had far, far, far more full time garden shops, I used darn near every brand of fertilizer available and had so many different containers I just started mixing them together in the pails I mixed in.
Kinda sorta ruined trying to find the best one :whistling

I have 150 roses growing in large containers. I have been fertilizing them every 3 weeks with a reduced concentrate of fish emulsion. My roses are fine with this fertilizer, but applying it in two gallon watering cans is getting tedious.I was wondering if the regimen mentioned by Seil would be a more practicable alternative. I was thinking of the following:
Osmocote every four months year around (in Hawaii our growing season is year around);
fish emulsion in the in-between months.
Osmocote comes in two forms--2874150 and 274850. Is one better for roses?