Understanding NPK Ratios
Fertilizers have a series of three numbers on them, called “NPK ratios”, which stand for the concentrations of the macronutrients contained within them. “NPK ratios” correlate to the levels of the macronutrients: Nitrogen (N), Phosphorous (P), and Potassium (K).
Reading The NPK Values
A fertilizer with a NPK ratio of 20-10-20, for example, contains twice as much Nitrogen as Phosphorous, but an equal amount of Potassium. These ratios are important in deciding what fertilizer to use on your plants, because each macronutrient plays a different role in the plants overall health. Nitrogen is necessary to maintain plant foliage, phosphorous is instrumental in fruit and bloom production, and potassium is required for overall plant health.
Make Better Educated Guesses
Once you know what macronutrient you need to provide to your plant, you must then know how much fertilizer to add. For example, a plant that you wish to encourage to bloom, or one that you want to produce more fruit, you would pick a fertilizer high in Phosphorous, and lower in Nitrogen and Potassium. A fertilizer with a 5-20-5 “NPK ratio” would work for this example.
Calculate You How Much Fertilizer You Need
An Example With Phosphorus
You can calculate how much fertilizer you would need to use to add one pound of the macronutrient, phosphorus, to the soil. Since we are interested only in the phosphorous macronutrient, we divide 100 by the value of the phosphorous in the “NPK ratio”. Since 100 divided by 20 is equal to 5, we know that five pounds of a fertilizer with a “NPK ratio” of 5-20-5, needs to be used in order to add one pound of Phosphorous to the soil.