If you need to know the percentage of an element that is contained in a material and it doesn’t appear on the label, you will need to determine the atomic weight of each element that is in the material’s chemical formula. The water molecule (H2O), for example, contains 2 atoms of hydrogen (H) and 1 atom of oxygen (O). The atomic weight of H is 1 and the weight of O is 16. So the total molecular weight is 18. Calculating the percentage of each element would look like this:

  • 2 ÷ 18 = 0.11 or 11% H
  • 16 ÷ 18 = 0.89 or 89% O

This is a simple example of how to determine the percentage of an element. It gets more complicated as chemical formulas get more complex. Gypsum (calcium sulfate CaSO4), for example, doesn’t appear to be too complex but most gypsum products are hydrated; i.e., water molecules are added. The most common chemical formula for gypsum is CaSO4•2H2O. Using the table below, we can determine that the atomic weight of a hydrated gypsum molecule is 172. Calculating the percentage of Ca (atomic weight 40) and S (atomic weight 32) would look like this:

  • 40 ÷ 172 = 0.23 or 23% Ca
  • 32 ÷ 172 = 0.186 or 18.6% S

Most Fertilizer labels give the percentage of elements but some may overlook elements that aren’t claimed in the guaranteed analysis. Labels for sulfate compounds, for example, may not give the percentage of sulfur if it isn’t claimed. Make sure you have the entire chemical formula before attempting to calculate the percentage of any of its elements. If it doesn’t appear on the label, it is often available on the product’s MSDS or SDS document.

Element Atomic Weight
Aluminum (Al) 27
Boron (B) 10.8
Calcium (Ca) 40
Chlorine (Cl) 35.5
Cobalt (Co) 59
Copper (Cu) 63.5
Hydrogen (H) 1
Iron (Fe) 56
Magnesium (Mg) 24.3
Manganese (Mn) 55
Molybdenum (Mo) 96
Nitrogen (N) 14
Nitrate (NO3) 62
Oxygen (O) 16
Phosphorus (P) 31
Phosphate (PO4) 95
Potassium (K) 39
Silicon (Si) 28
Sodium (Na) 23
Sulfur (S) 32
Sulfate (SO4) 96
Water (H2O) 18
Zinc (Zn) 65.4