Fertilizing
When and How to Fertilize Your Lawn
What the N-P-K numbers on fertilizer actually mean, when to fertilize in Utah (hint: never miss Halloween), and why you should skip nitrogen in summer.
May 8, 2026 · 5 min read
Fertilizing isn’t complicated once you understand what’s in the bag and when to put it down. Here’s the simple version.
What is fertilizer?
Fertilizer is made of small pellets containing percentages of three key nutrients:
- Nitrogen (N) — the 1st number
- Phosphate (P) — the 2nd number
- Potassium / Potash (K) — the 3rd number
So a common bag labeled 16-16-8 is 16% nitrogen, 16% phosphate, and 8% potassium. When a fourth number appears (like 9-9-9-11 Fe), the package will name the element — in the Western region it’s usually Iron (Fe) or Sulfur (S).
How to use fertilizer
Our soil fertility in the West is usually poor, so your lawn will need feeding at least once a year. A few rules of thumb:
- Never miss Halloween. Fertilize at or around Halloween every year — it’s the single most important feeding.
- A second application around Presidents Day is always beneficial.
- Know your square footage and apply the product according to the package directions. (Not sure of your lawn’s size? Use our Turf Area Calculator.)
- Apply evenly across the whole area. Two applications at a ½ rate are better than one full application.
The seasonal rule that matters most
Your lawn reacts to each season according to soil temperature — and the cooler the weather, the safer it is to fertilize.
- Fall & Winter: apply a nitrogen-phosphate-potash combination (for example, 16-6-8 or 15-15-15).
- Summer: do not apply a nitrogen fertilizer. Instead, use only Ferrous Sulfate (Ironite).
Why skip nitrogen in summer? Because heat plus excess nitrogen feeds lawn fungus and stresses the grass. Learn more about lawn fungus →
Quick schedule: Feed at Halloween → feed again at Presidents Day → iron-only through summer. Follow that rhythm and your lawn will thank you.
Frequently asked questions
- What do the numbers on fertilizer mean?
- They're the percentages of Nitrogen (N), Phosphate (P), and Potassium/Potash (K), in that order. For example, 16-16-8 is 16% nitrogen, 16% phosphate, and 8% potassium. A fourth number, when listed, is usually Iron (Fe) or Sulfur (S).
- When should I fertilize my lawn in Utah?
- Fertilize at least once a year around Halloween, with a beneficial second application around Presidents Day. Cooler weather is the safest time to fertilize.
- Should I fertilize my lawn in summer?
- Do not apply nitrogen fertilizer during the hot summer months — it can fuel fungus and stress the lawn. In summer, use only Ferrous Sulfate (iron) and you can aerate the lawn instead.