For watermelon plants, using nitrogen based fertilizers at the onset is a good idea. Once the melon plants start flowering, however, you should switch to feeding the watermelon a phosphorus and potassium based fertilizer. Watermelons need ample potassium and phosphorus for optimal melon production.
Foliar fertilizer is a fertilizer product which is designed to be applied directly to the leaves of a plant. A number of companies manufacture a range of foliar fertilizers, from organic products safe for use on food crops to more aggressive chemical fertilizers for ornamentals.