Using nitrogen-based fertilizers at the start of watermelon plants is an excellent idea. However, once the melon plants begin to flower, you should switch to a phosphorus and potassium-based fertilizer. Watermelons require a lot of potassium and phosphorus to grow well. Adding calcium to the fertilizer is also a great idea, it creates strong cell walls and membranes during development, which can really improve the quality of the melon.