Recommendations for fertilizing lawns and other garden plants are commonly made in terms of "pounds of actual nitrogen."
The first of the three most prominent numbers on any bag of fertilizer is the percent of nitrogen in that product. A 100 pound bag of 10-10-10 contains 10 pounds of nitrogen (as well as 10 percent phosphoric acid and 10 percent soluble potash).
The nitrogen content of common manures and fertilizers ranges between 1 and 46 percent. Therefore, to apply 10 pounds of actual nitrogen, you need to apply 1,000 pounds of the 1 percent nitrogen material, or 22 pounds of the one with 46 percent nitrogen.
Many fertilizer products provide very specific application rates for specific uses on the bag or container. You should always follow those directions first. If the fertilizer you're using contains mostly water soluble nitrogen, apply no more than 1 pound of actual nitrogen per 1,000 square feet at one time. You can safely apply more than that if you're using an organic fertilizer (derived from plant or animal waste), a fertilizer that contains mostly water insoluble nitrogen, or a controlled-release fertilizer.
Different kinds of garden plants need different amounts of actual nitrogen over a season. Here are some basic guides to how much actual nitrogen different plants need per season: