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Fertilizers have a standard code to show their relative makeup and to indicate their best use. Complete organic fertilizer has an N-P-K rating of 4-4-4. These numbers indicate N (nitrogen), P (phosphorus) and K (potassium). The letters are consistent with the periodic table of elements, and always occur in this order: N-P-K. Nitrogen, phosphorus, and potassium are considered the “primary” nutrients, followed by the secondary group Ca (calcium), Mg (magnesium), and S (sulfur). Many plants absorb more calcium from the soil than they might phosphorus. These six elements are considered “macronutrients,” and must be available in some quanities for plants to grow. “Micronutrients” are also important to plant health, and they include B (boron), Cu (copper), Fe (iron), Mn (manganese), Mo (molybdenum), Zn (zinc), Cl (chlorine), and Co (cobalt).