Yes, watermelon grows better in soil with pH levels ranging between 6.0 and 6.5. Test the soil using a soil pH test kit. If you find that you need to adjust the levels, here’s how: Alkalinity: To increase the alkalinity of your garden soil, add some ground agricultural lime to it. Be sure to take into account the type of soil in your garden. For example, soil with a lot of clay will require more ground lime than soil with little clay. Test, mix, and test again. Acidity: To increase the acidity of your garden soil, add some aluminum sulfate to it. This chemical should adjust the pH levels almost immediately. Again, you should test, mix, and then, re-test so you get the correct pH level.