What Do Those Numbers on the Fertilizer Label Mean?
Most growers are aware that the three numbers on the fertilizer label refer to nitrogen (N), phosphorus (P) and potassium (K), respectively, but many do not know exactly what the numbers signify. In general, they refer to the weight of each of these elements expressed as a percent of the total weight of the net contents of the container. The first number refers to the percentage, by weight, of elemental NITROGEN regardless of the form in which it is present. That form may be urea nitrogen – CO(NH2)2 , ammoniacal nitrogen – NH4, or nitrate nitrogen – NO3. The weight of the carbon, oxygen and hydrogen atoms in these various nitrogen containing molecules is not counted.
However, things are different when we get to phosphorus and potassium. Phosphorus is expressed as the percentage, by weight, of phosphate – P2O5. From the atomic weights of phosphorus (31) and oxygen (16) we can calculate the percentage, by weight, of elemental phosphorus in the phosphate molecule at 43.7%. The atomic weight of the phosphate molecule is calculated as follows: (2 x 31) + (5 x 16) = 62 + 80 = 142. The atomic weight of the phosphorus portion of the phosphate molecule is 62. The weight of the phosphorus portion of phosphate molecule is 62 divided by 142 or 43.7% of the P number on the label.
Potassium is expressed as the percentage, by weight, of potash or K20. Similarly, from the atomic weights of potassium (39) and oxygen (16), we can calculate the percentage, by weight, of elemental potassium in the potash molecule at 83.0%. The atomic weight of the potash molecule is calculated as follows: (2 x 39) + 16 = 78 + 16 = 94. The atomic weight of the potassium portion of that molecule is 78. The weight of the potassium portion of the potash is 78 divided by 94 or 83.0%.
What this means is that a fertilizer which is labeled as 10-10-10 is, in reality, 10% elemental nitrogen, 4.37% elemental phosphorus (10 x 43.7%) and 8.3% elemental potassium (10 x 83.0%) by weight or 10-4-8 in elemental terms. Understanding these differences is important when you are attempting to utilize the numbers on the fertilizer label to calculate application rates to meet the nutritional requirements of your plants.
The other macronutrients, calcium (Ca), magnesium (Mg), sulfur (S) and silicon (Si) are expressed as a percentage, by weight, of the elements themselves whether the element is part of a larger molecule or not. For example, the 2% calcium or 2% magnesium on a fertilizer label means 2% elemental calcium or magnesium of the weight of the contents of the package.
Trace elements iron (Fe), copper (Cu), manganese (Mn) and zinc (Zn) are usually present as part of a larger chelate molecule which keeps them in solution and hence available to the plant. Boron (B), molybdenum (Mo), chlorine (Cl), sodium (Na), cobalt (Co) and nickel (Ni) are typically present in ionic form as the elements themselves. The number on the label refers to the percentage by weight of the trace element itself and not the accompanying EDTA chelate in the case of the chelated minerals.
Note that chelates are used to keep some of these elements in solution. Solubility is critical for plant nutrition. A plant can only take up minerals that are dissolved in soil solutions, when growing in soil, or that are dissolved in nutrient solutions in the case of hydroponics. If the mineral does not go into solution, it does not go into the plant.