Watermelons like loamy, fertile, well-drained soil. To determine whether your soil gets good enough drainage, take a look at it after a heavy rain. If you see puddles in the dirt, the soil isn't draining well enough.
To further enrich the soil, till compost into the top of layers.[2]
Watermelons grow best in soil with a pH of 6.0 to 6.8. Test the pH of your soil and determine whether the levels are appropriate for watermelon plants. If not, you can change the balance by adding compounds available for purchase at a plant nursery.