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 β€œprimary” 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 β€œmacronutrients,” and must be available in some quanities for plants to grow. β€œMicronutrients” 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).