Every bag of fertilizer has three numbers connected with hyphens on the label, such as 6-2-0.  Those numbers are the NPK ratio-the percentages of nitrogen, phosphorus, and potassium in the fertilizer.  Organic fertilizers have lower numbers than synthetic types because the numbers show the amount of nutrients that will be available int first year.  Organic fertilizers release nutrients slowly, giving your plants more nutrients in the long run. 

• Nitrogen : promotes leafy growth
• Phosphorus : promotes strong root systems and good flowering
• Potassium : promotes flow of nutrients through entire plant, quality of fruits and seeds, helps plants withstand stress such as disease / drought / extreme temperatures