Hard to tell, without more details, but it sounds like a nitrogen deficiency. Regular fertilizing could help. Also, it might be too acidic. I've seen watermelons suffer and dwindle away under too much acidity, but they can take down to somewhat below pH 5. There are many other possibilities as well. I wouldn't recommend trying to fix it until you're sure what's wrong.
I'd recommend sending a sample of your soil to a lab for analysis. Many labs (like you're county's extension office, in the US) give you a sheet to fill out, and you can put down what you intend to grow in the area (e.g. lawn, woodland, orchard, corn, etc), and they can give you recommendations on what minerals/amendments/fertilizers can help you have the most success, and in what quantities to put them on.