One thing that is critical, but kind of difficult is to know how much to fertilize a pumpkin plant for maximum growth. This is kind of something I don't feel I've fully figured out, but something I think I'm starting to get a better idea on. In my experience there are four ways to know how much to fertilize a plant.
First, is a soil test. A good professional soil test can let you know exactly what you have in your soil and can give you some idea of what you need to add. One challenge that can come from a soil test is knowing how much of the nutrients in your soil are actually available to a plant. For example, your test may show you have a lot of calcium and a lot of potassium. The question is how much of that is in a form that a plant can uptake and is having too much potassium affecting the plant's ability to uptake the potassium. A soil test is crucial, but doesn't give all of the answers and unless you are testing frequently, it doesn't tell you how the numbers are changing as the season goes on. It is more of a one-time snapshot.
Second, leaf tissue test. The nice thing about a leaf tissue test is it tells you what is in the plant (more accurately, what is in the leaf), so it gives you a better idea of what the plant is up taking and what might be deficient. So in some ways more accurate than a soil test, but the two go hand in hand in a lot of ways. A lot of growers will get a tissue test around the time they pollinate. It is kind of expensive, but a great way to know exactly where your plant is at.
Third, EC testing. Using a little EC testing unit it can tell you about what the fertility of your soil would be based on its conductivity. Most of the macro nutrients are in a form of a salt, which conducts electricity. So you shoot for a range and it will give you an idea of where you are at with macro nutrients in the soil at a given time. The nice thing about EC testing is you can do it frequently and then make adjustments accordingly. The downside to it is that it doesn't tell you if you have too much of one thing or not enough of another. You just get one number for everything. My soil is sandy, so nitrogen leaches easily. If my number is low and my soil test said he was a little high with phosphorous and potassium then I an assume that my nitrogen is low.
Fourth, is the eyeball and experience test. Plant color, growth and what the vine tips are doing can give you good clues to what your nutrients are doing for you as well. It can be a little tricky at times because yellow leaves can be caused by a lot of things, but where and when the leaves yellow are excellent clues to potential issues. The downside to the eyeball test is that you usually get information a little too late. Only after the problem occurs. But if you can't afford to tissue test, weekly, it is the next best thing.
The best solution is a combination of all the above. The right testing at the right times will give you the information you need to grow a big pumpkin.