What does N-P-K mean?
Without trying to stir up disturbing flashbacks to your 10th grade chemistry class, but just for a moment try to picture the chart on the wall called the "Periodic Table of Elements."
Each of the elements on it has a symbol like those used in formulas such as H2O and NaCl.
N represents nitrogen, P is for phosphorus, and K is potassium (since P was already taken).
To botanists and gardeners, N, P, and K are known as "macronutrients," the most important elements to plants and therefore key ingredients in fertilizers. The N-P-K ratio listed on fertilizer packages defines the relative amounts of each nutrient.