Does the fertilizer for a rhododendron in a container need to be different than for other plants? I noticed that most fertilizers that are labeled for azaleas and rhododendrons have a lower amount of nitrogen.

I understand that for a plant that is in the ground, many of you use organic fertilizers or light compost. I guess neither of those work well for containers.

The standard fertilizer many people use for containers is FoliagePro 9-3-6, which they typically dilute and use either weekly or with each watering. Is that the best fertilizer for a container rhododendron as well? I would really prefer to use a time-release fertilizer like Osmocote just for convenience, but I am open to using the best method.