My husband and I, first time gardeners, planted some watermelon for our grandchildren. We water the raised garden they are in every other night.
We have been noticing yellow leaves, but only on the watermelon. Why would this be, and what can we do to cure it?
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.
Hard to tell without a picture, more info on your location and other cultural practices but depending on your local climate and soil you may be watering too much. Watermelons only need about 1" of water a week as do most plants. Your soil mix in your raised bed should be well draining but still have water retaining components. Too much water and too frequent watering can cause yellowing as it can make it harder for the plant to take up nutrients and the water washes the nitrogen out of the soil.
Before you water, stick your finger about an inch into the soil to see if it's moist. If it is, don't water.
It's also best to water in the early morning rather than at night so the leaves can dry out if any water gets on them to reduce the risk of disease. When watering try to avoid wetting the leaves.
