Decoding Fertilizer Labels: Understanding NPK Ratios and Plant Nutrition

NPK 10-10-10, 3-1-2… What does it all mean? These numbers that we typically see on plant fertilizer packaging can be confusing, but understanding the idea behind them is not. Here’s a quick primer;

• Those ratios refer to the nitrogen (“N”), phosphoros (“P”), and potassium (“K”) content, by weight, in a particular fertilizer product. So, a fertilizer with a 5-10-5 NPK means that the fertilizer contains 5% nitrogen, 10% phosphorus, and 5% potassium, by weight.
• Note that an NPK of 5-5-5 is the same as one with 10-10-10 since both fertilizers contain equal parts nitrogen, phosphorus, and potassium. The only difference between the two is that you’ll need to use twice as much of the 5-5-5 to give you plant the same level of nutrients as the 10-10-10 (since the 10-10-10 means that each ingredient is 10% of the fertilizer bag by weight while the 5-5-5 means that each ingredient is only 5% of the bag by weight).

Why does NPK only measure those three ingredients when plants need a total of 17 macro and micro nutrients? Because nitrogen, phosphorus, and potassium are the three primary macronutrients plants need to survive, and plants require them in larger quantities than other nutrients. If you’re curious as to what the other 14 elements are, here they are;

• The other macronutrients are calcium, sulfur, and magnesium, and the micronutrients (trace minerals) are iron, boron, chlorine, manganese, zinc, copper, molybdenum, and nickel, along with the non-mineral essential elements hydrogen, carbon, and oxygen.

Generally-speaking, any fertilizer that has a balanced mix (e.g. 5-5-5 or 10-10-10) or one where the sum of the phosphorus and potassium percentages equals the nitrogen percentage (e.g. 3-1-2 or 8-5-3) will work for your container-based plants. The MOST IMPORTANT thing is how much of the fertilizer you apply since too much fertilizer can burn plant roots and damage the plant.