Right after plating watermelons, a nitrogen-based fertilizer is the optimal choice. After the plants begin flowering, a phosphorus and potassium-based fertilizer is best.
When planting watermelon seeds, the ground should be above 70 degrees Fahrenheit and the soil pH should range between 6 and 6.8. Manure and mixed soil composts are a good choice for covering the seeds as they contain high levels of nitrogen. As the plants begin to grow and produce, the level of nitrogen should be reduced for a good harvest. Liquid seaweed is one option for using as a fertilizer after the flowers or fruit have begun growing.