Which fertilizer is best to grow a giant pumpkin? I hear that question a lot. The answer that any good grower will tell you is that it is the one that the plant needs. You'll only know that through a soil test, tissue test or an experienced eye.
About every four days or so lately I've been giving the plants a little RAW Grow 7-4-5 fertilizer. That isn't as aggressive as I know some growers will do, but I think it is sufficient at this stage. The Grow fertilizers I like because they are water soluble and have a little more than just NPK in them. The main nitrogen source in it is plant protein hydrolysate, which I've read good things about. It also has humic acid, cane molasses, boron, copper, seaweed, iron DTPA, magnesium, manganese, silica, zinc and azomite. And the qualities of each are sufficient to see results from them.
The little extra nitrogen will help support the rapid growth of the plants at this stage. Ideally I would like something like a 7-2-5 at this stage, but a 7-4-5 is close enough.
Soon, I'll be adding a little blood meal to that fertilizer mix to add more nitrate nitrogen. The studies I've read suggest that during rapid growth nitrate demands of the plant go up, so this will help support that. I've also read that too much nitrate nitrogen can help vine growth, but diminish root growth, so to me, it makes sense that June is the time of the season for nitrate nitrogen (like blood meal and calcium nitrate) and then stop that once you start getting into the fruit growing stage.
So, which fertilizer is best to grow a giant pumpkin? It is a balance of what your soil has and what the plant is asking for at that time of the season. But at this time of the year, something with some nitrogen should be given regularly in spoon feeding quantities.