Whenever you see a fertilizer product, it will have three numbers prominently listed on the package, usually on the front. These numbers are very important and tell a great deal about what this fertilizer will do.
Nitrogen, Phosphorus, and Potassium (N,P,K) are what the numbers refer to. They are always listed in this order. It is the percentage within that package of each component. For example, a common type of all-purpose fertilizer is referred to as 10-10-10. This is a balanced blend of equal portions of Nitrogen, Phosphorus, and Potassium. If you purchased a 50-pound bag, five pounds (or 10%) would be Nitrogen, five pounds would be phosphorus, and five pounds would be potassium. The remaining 70% is simply filler, or inert ingredients, which are there mostly to help disperse the chemicals.
Up, Down, and all Around
A common way of describing the purpose behind each chemical is to think “up, down, and all around.” Applying this simple phrase will help you remember that Nitrogen (N), the first chemical listed, helps with plant growth above ground. Nitrogen does a great job of promoting the green leafy growth of foliage, and provides the necessary ingredients to produce lush green lawns. Lawn fertilizers will frequently have a high first number for this purpose.
Phosphorus (P), the middle number, is very effective at establishing growth below ground, in the form of healthy root systems. It is also the component most responsible for flower blooms and fruit production. You’ll notice that fertilizers designed for flower production, or starter-type fertilizers for your lawn, have a high middle number.
Potassium (K), the last number listed, is considered important for overall plant health. This is primarily due to its ability to help build strong cells within the plant tissue. In turn, the plants withstand various stresses, such as heat, cold, pests, and diseases. For example, winterizer fertilizers will have a high component of potassium.
When shopping for fertilizers, be mindful of their intended use. Fertilizers that have equal numbers can generally be used as an all-purpose fertilizer. If you had only one product to work with, 10-10-10 would be my recommendation.
For promoting good fruit or flower production, look for a middle number that is higher than the first. Otherwise, your plants will be stimulated to put out lots of nice green foliage, likely at the expense of fruit or flower production. Instead, you want the energy and nutrition of the plant to go towards the desired result, flowers or fruit, so a higher middle number is a more appropriate choice.
To toughen up your plants or lawn for environmental stresses, then you’ll want a fertilizer that promotes the last number, and middle number. A high first number in this case may not be appropriate, because you are not likely to be promoting new lush foliage when at the same time putting plants or turf to bed for the winter. Instead, your goal should be to promote cell structure and strong roots which continue to grow through winter.
Finally, whenever you apply fertilizers, don’t assume that more is better. You can burn plants by over fertilizing, and damage the surrounding soil as well. Instead, err on the side of less is better. If your soil is rich in organic matter, it should have all the nutrition plants need.