The three numbers that you see on bags of composted manure, boxes of blood and bone meal and water-soluble fertilizers refer to three major plant nutrients associated with healthy growth; the number value (e.g., 20-20-20) is the percentage of each nutrient that is immediately available to plants for uptake.
N: Nitrogen is necessary for strong, vigorous growth, leaf production and is used in the process of photosynthesis.
P: Phosphorus promotes flowering, stimulates root growth and is used in the process of ripening fruits and seeds.
K: Potassium works with nitrogen to aid in the photosynthetic process, and is necessary for normal fruit formation and in the uptake of other nutrients.
The numbers for N-P-K are usually higher for chemical fertilizers than they are for soil conditioners (an analysis of 2-1-1 is typical) since the nutrients in organic compounds—such as compost or rotted manure—are released into the soil profile more slowly and over a longer period of time. Nevertheless, producers can legally only claim the immediate nutrient availability on product labels.