What do fertilizer numbers mean?
The best phrase to remember what each number in your fertilizer does, is
"Up, Down, All Around".
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.
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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.
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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.
Fertilizers that have equal numbers can generally be used as an all-purpose fertilizer (10-10-10, etc).
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.
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.
*** When applying fertilizer, don't assume more is better. You can burn plants or grass by over-fertilizing. ***