Select a product which has the same percentage of nitrogen and potassium and with low or no phosphorous such as a 15-2-15 (15% nitrogen, 2% phosphorous and 15% potassium) or a 15-0-15. Products with analyses like 16-4-8 have been recommended for years. However, the current thinking is to use equal amounts of nitrogen and potassium, which seems to promote healthier growth.

Also, choose a fertilizer with 30-50% slow release or water insoluble nitrogen. To determine the percentage of slow release N, look on the label under nitrogen and you will see one or more of the following categories: nitrate nitrogen, ammoniacal nitrogen, and urea nitrogen, all of which are water soluble.

Usually last on the list is water insoluble nitrogen or slow release nitrogen. In many cases, the water insoluble nitrogen is urea, which is an organic form of soluble nitrogen covered with sulfur. The sulfur slows the release of the urea nitrogen. Sometimes the urea is covered with a polymer coating, such as in Osmocote products. If the total nitrogen content is 15%, the water insoluble part should be at least 5%.

Additional Resources
http://edis.ifas.ufl.edu/LH010
http://edis.ifas.ufl.edu/ss170