If you are into any plants, you often hear about NPK fertilizers commonly labeled as 20-20-20 or 19-19-19 or 13-40-13 or 30-10-10 etc. Have you ever wondered what it means and how this will impact your plants health? We get this question very often and that is why we decided to write about it.
The three numbers on the label represents the value of three macronutrients used by the plants. NPK represents Nitrogen (N), Phosphorus (P), and Potassium (K) and the numbers like 20-20-20 or 19-19-19 means the concentration of these elements in the fertilizer. How plants respond to the fertilizer will largely depends on the ratio between these elements. To find the actual the ratio, divide all three numbers on the label by the smallest number.
- 20-20-20 fertilizer has a 1-1-1 ratio
- 19-19-19 fertilizer has a 1-1-1 ratio
- 15-30-15 fertilizer has a 1-2-1 ratio
- 30-10-10 fertilizer has a 3-1-1 ratio
The ratio is very important while choosing the fertilizer as they have different impacts on your plants. Knowing the NPK values of a fertilizer can help you select one that is appropriate for the type/stage of plant you are growing. We recommend the following ratios for different stages/purposes.
- Balanced – 1-1-1 (Advised for Matured Plants)
- Boosted Growth – 2-1-1 (Only advised for Seedlings)
- Rooting – 1-2-2 (After repotting)
- Flowering – 1-2-1 (Once a month for matured plants only)
It is not true to say bigger numbers are better than smaller numbers on the label. This is because the plant absorbs a certain amount of nutrient and rest goes unused. The higher the number (For example 30-10-10) there is a risk of leaching and burning the roots. We would recommend smaller numbers compared to larger numbers at least for orchids
If you like to learn more about plant nutrients, read What are the nutrients required for plants?