I'm all for natural fertilizer and beefing up the soil with twice yearly mulching.
Soil fertility has a lot to do with the health of the plant, be it host or nectar source. Any plant that is struggling for any reason will be of poor nutritional value and/or produce little or no nectar. Plants grown with the proper nutrients and soil type will be vigorous and fruitful.
This goes for plants that are grown out of their climates as well. For instance, Porterweed is A-1 as a nectar plant in its native Florida but is rather wimpy here in the Midwest. Zinnias are magnets here in the Midwest but are useless in South Florida due to root knot nematodes. Differing soil types can also play havoc with introduced plants, Florida sand vs clay, for instance. It's always best to stay with native species whenever possible.

John