Fertilizer blends are rated according to the amounts of nutrients they contain. The three numbers listed on fertilizer packages represent the three most important plant nutrients: nitrogen, phosphorus and potassium. A blend of either 5-10-10 or 10-20-10, indicating the percentage of the three nutrients, is recommended for green snap beans.
Fertilizer for snap beans can be added to the soil as it is being turned over and prepared for planting, or it can be added to the garden rows before planting. About 1/2 cup of fertilizer is sufficient for every 10-foot-long row of beans, and it should be worked into the top 3 to 4 inches of soil. Reapply fertilizer when the bean plants start flowering; scatter the granules on the soil between the bean rows and water well.
Beans generally don't need much in the way of additional fertilizing, and well-rotted manure or aged compost are often all that's needed to assure a good crop. Chemical fertilizers supply their nutrients more quickly, providing the plants with nitrogen for healthy leaves and stems, phosphorus for healthy roots and potassium to help fight diseases and produce fruit.