What Nutrients Do My Watermelons Need So They Won't Rot on the Vine?

Chemical Fertilizers

• Nitrogen, phosphate and potash are the most common ingredients of fertilizer and are critical to the growth of watermelons. Most bags of fertilizer will have three numbers separated by dashes on the bag. These numbers represent the percentages of nitrogen, phosphate and potash. Prior to planting, Clemson University recommends applying 5-10-10 fertilizer at a rate of 30 pounds per 1,000 square feet. Five-10-10 means 5 percent nitrogen, 10 percent phosphate and 10 percent potash. After planting but before the vines run, Clemson University recommends applying ammonium nitrate at 1 pound per 100 feet of row or calcium nitrate at 2 pounds per 100 feet of row.

Organic Fertilizers

• If you're interested in organic gardening, the way you fertilize will be a little different. According to the Virginia Cooperative Extension, watermelons grow best in soil amended with organic material such as a mixture of cow manure and straw. It is safest to compost manure with carbonaceous materials like dead leaves, straw or any other brown plant matter prior to mixing them into the soil. Uncomposted animal manure can contain harmful bacteria that are killed by the heat of composting. Other organic methods for fertilizing include rotating watermelons with legume crops like beans. Bean crops have a unique way of using nitrogen that enriches the soil and benefits watermelons. You can also find a number of organic fertilizers at your local garden store. These include kelp meal, blood meal, gypsum or fish fertilizers.

Soil Testing

• The best way to know what nutrients are present in your soil and what ones are missing is to have a soil test performed. Most university extensions will perform soil tests for you. The cost and procedure may vary, so contact one near you for instructions prior to taking samples. Soil tests look for nutrients like nitrogen, potassium, phosphorus, calcium and manganese, among others. The test will also check for pH so you'll know whether or not you have acidic soil. You can just test the soil where you'll be growing watermelons, but it might be a good idea to take samples from various parts of your garden to ensure all the other fruits and vegetables get the correct fertilizer as well. Once you have the results of your test you'll know exactly what and how much to give your watermelon. Soil testing can be done at any time of the year, but a couple of months before planting is ideal.

Other Important Growing Tips

• It is important to note that improper nutrition is only one possible reason your watermelons are rotting on the vine. Insect damage creates openings for bacterial and viral diseases. Some insects, like the cucumber beetle, actually transmit disease. Fungi can live in the soil or be spread via the wind. All of these are potential causes of watermelons rotting on the vine. Take a look at other symptoms; check the root crown just below the soil line for rotting that could indicate root rot. Look for mold or dead spots on the leaves. In some cases fertilizing can exacerbate a fungal infection so proper diagnosis of your problem is key. Your local university extension may be able to help with this, too. Check to see if you can send them pictures or samples of your watermelon and vines.