Fertilizers are labeled with numbers that represent the percentage of the three primary macronutrients (nitrogen (N), phosphorus (P), potassium (K)) that are immediately available in the fertilizer.  These elements are listed as the NPK ratio, for example: our All Purpose Mix 4-6-2 is comprised of each major nutrient whereas our Blood Meal 12-0-0 is a single ingredient source of nitrogen.  Each nutrient plays specific and complementry roles: nitrogen energizes vegetative growth; phosphorus produces expansive roots, flowers, fruits and viable seeds; while potassium (or potash) promotes resistance to disease and temperature stress.  Most fertilizers will also contain varying amounts of the secondary macronutrients (calcium, sulfer, and magnessium) along with trace elements or micronutrients that also play an important role in plant nutrition.