NPK Fertilizer Ratios Every Pro Should Know

The best way to fertilize will depend on what it is that you're planning to fertilize and what is in the fertilizer.

In this FAQ, Christina Burton, Horizon's Maintenance Channel Manager, discusses the role of NPK (Nitrogen, Phosphorus, and Potassium) and common NPK ratios that make choosing the right type of fertilizer much easier.

Covering NPK Fertilizer Ratios - Video Transcript

So to answer the question, “What is the best way to fertilize?”, we really have to address two things:

   1. What's in the fertilizer?

   2. What is it that we're planning to fertilize?

Starting with “What’s in the fertilizer?”, it’s important to note that any bag of fertilizer that we look at has 3 numbers on it. And those three numbers represent Nitrogren, Phosphorus, and Potassium in that order. It’s always in that order. Those 3.

Nitrogen is responsible mainly for the greening and the overall shoot growth of the plant, whether that’s turf or ornamental plants.

Phosphorus is responsible for the establishment of the plants, whether that be new seed or new sod. In landscape ornamentals, it’s also responsible for blooming.

Potassium is important for the overall cell strength of the plant.

So turfgrass fertilizers, as a general rule, are in a rough ratio of about a 3:1:2. So meaning there’s about 3 times as much Nitrogen as Phosphorus, and about 2 times as much Potassium as Phosphorus.

Now, it’s always important to start with a soil test to really know what your turfgrass needs, but again this would be a very common ratio and it’s really important to note that it’s about the ratio and not necessarily the actual numbers on the bag.

For example, you might see a fertilizer that’s a 24-3-12. You might see one that’s a 24-5-11. But roughly speaking, those are about in that 3:1:2 ratio. A very common fertilizer for turf.

If we were starting a new lawn though or starting sod, again that Phosphorus, that middle number becomes really important. Those seedlings can’t reach the available Phosphorus in the soil.

So that’s where you might see something like a 6-20-10. And again, maybe it’s more 6-20-20. It’s not so much about the exact numbers, but just recognizing that when you see this center number being very high in comparison to the other two, that is mostly likely meant for starting a new lawn or new sod.

Landscape ornamentals on the other hand typically are more balanced. So their ratio would be often times a 1:1:1. Equal parts Nitrogen, Phosphorus, and Potassium. So again, very common to see products like a triple 15 (15-15-15), a triple 9 (9-9-9). And so while those numbers might be slightly different, the ratio is the same. That fertilizer contains equal parts of Nitrogen, Phosphorus, and Potassium.

And so that’s where, and we’ll talk in more detail later, we gotta know not just what these numbers are, but what’s the makeup of each of those. What type of Nitrogen for example? Or do these contain other things like Iron or Manganese or other nutrients in addition to these three that the plants might need.