Determine the pH of the planting site following the instructions in a soil testing kit. Watermelons tolerate a pH between 5.0 and 6.8, but produce best at pH levels between 6.0 and 6.8.
Add agricultural lime to the soil in the amount indicated as necessary by the soil test to bring the pH into the desired range. Apply lime at least four weeks before planting. Turn the lime into the top foot of soil with a spade or tiller.
Place a 2-inch layer of organic matter on top the soil in the bed. Use garden compost, leaf mold or composted manure.
Spread 15 pounds of 5-10-10 fertilizer over every 500 square feet of watermelon bed. Use a slow-release fertilizer so it can release nutrients slowly to the plants over a period of two to three months.
Till the organic matter and fertilizer into the top 8 inches of soil with the spade or tiller. Incorporate these amendments with the soil completely.