Regardless of its type, any fertilizer you buy will come with information about the nutrients it contains. Prominently featured will be the N-P-K ratio, the percentage the product contains by volume of nitrogen (chemical symbol N), phosphorus (P), and potassium (K). For example:
• A 16-16-16 fertilizer contains 16% nitrogen, 16% phosphorus, and 16% potassium.
• A 25-4-2 formulation contains 25% nitrogen, 4% phosphorus, and 2% potassium.
History of NPK
Scientist Justus Von Liebig was responsible for the theory that Nitrogen, Phosphorous, and Potassium levels are the basis for determining healthy plant growth. However, this theory, which dates to the 1800s, doesn't take into account the dozens of other nutrients and elements that are essential to plant growth such as sulfur, hydrogen, oxygen, carbon, magnesium, etc. Nor does the theory talk about the importance of beneficial soil organisms that help your plants to flourish and to fight off pests and diseases.