Seedfork,
Nitrogen would be likely to be the most important to add in most soils.
When I decided to test high nitrogen on daylilies I also remembered that high nitrogen is supposed to produce leaves rather than flowers on many ornamentals.
Then I checked the actual research.
For most plant species the plant must be mature or adult before it can bloom. That is usually measured by its size. When the plant reaches a certain size it is able to flower. Below that size very few plants flower and above that size as the plant gets larger it is more likely to flower. Researchers often count the number of leaves to estimate the plant size (its quick and easy). If a plant grower starts seeds and grows them in low nitrogen (relatively speaking) they count a certain number of leaves before the plant flowers. If they grow the same plant exactly the same way but give it more nitrogen the number of leaves required before it flowers is often larger. It takes more time to grow that number of leaves (and that is not a good thing for a commercial plant grower). But the plant is larger when it flowers. And there is a strong rule in plants that the larger the plant the more flowers and often the larger each flower. So more nitrogen can cause more leaves (and the leaves are larger) and it may delay the plant flowering. Delays in flowering, etc., are not good for commercial growers.
Daylilies will not necessarily suffer from delays in flowering if they have set seasons to flower and anything that increases the number of buds on a scape or the number of scapes in a clump is a good thing. So I tried high nitrogen. And I got rebloom where I had none before. So I went out and bought pure nitrogen - urea, last year. I will see the results of a small test using urea hopefully this year but if not then next year (the fans on the test cultivars were very small last year). I don't expect to get a high proportion of registered rebloomers to do so in my location - I probably quite simply do not have a long enough growing season or a warm enough growing season. For some cultivars the rebloom is not very effective for example, Mosel had never rebloomed here, I can get it to rebloom but it only does so at the very end of the season. Ophir is registered as a rebloomer - I doubt that I can get it to rebloom here. The ditch lily can rebloom in Florida - I sort of had rebloom on it here one year - a fan sprouted but all the leaves were eaten to the ground by deer. Then two new fans sprouted from the crown, then a lone scape grew (no leaves) between the two small fans and flowered and then the larger fan of the two new fans also flowered. I think that was forced rebloom that one sometimes sees on plants that have been newly divided and had their leaves cut back. The replanted division produces a scape, that the grower removes to help the division establish and then later on in the same year that fan produces another scape - I consider that forced rebloom.
