A 101 on the Rule of 3: Fertilizer NPK by the Numbers

Fertilizer NPK by the Numbers

You’ve probably seen these three numbers before, but do you really know what they mean?

There are three numbers on bags of fertilizer that represent the amount of nitrogen (N), phosphorus (P), and potassium (K) in the product. These are the three primary nutrients turf needs to thrive.

The first number is the amount of nitrogen, the second stands for the amount of phosphorus and the third one shows the level of potassium. For example, a bag with 18-0-3 indicates the fertilizer has 18% nitrogen (by weight), no phosphorous and 3% potassium.

(N)itrogen provides quick growth and greening of grass so in most cases the first number shown in the N-P-K ratio is the highest.

(P)hosphorous promotes root development and aids the establishment of new grass seed or new sod. When the middle number is double digits such as 18-12-6, that fertilizer is considered to be a starter for new lawns. Many municipalities only allow the use of fertilizers with phosphorous for establishing new lawns, so be sure to check your local restrictions. Fertilizers for existing lawns will have a zero as the middle number on the bag.

(K) Potassium helps grass resist diseases throughout the year and is important for overall plant health.

Having a soil test done is a useful way to determine what fertilizer analysis is a good fit for your particular need.  The test results may indicate the soil is deficient in nitrogen, phosphorous, potassium or other elements — such as iron or manganese.

Delicious vegetables and flowering ornamentals need more phosphorous and potassium so a balanced blend such as 14-14-14  is a good choice.

With regard to the nitrogen component, there are slow release nitrogens available that meter out over a longer period of time. Rather than a quick burst of green-up, there is more extended turf growth. According to Scott Gilbertson, Reinders Territory Manager, some things to consider when looking at slow release nitrogen options include:

  • The type of soil you have.
  • The time of year you are applying the fertilizer. Certain products release better at certain times than others.
  • Whether you are looking for a long feed (10-12 weeks) or something that releases quicker (4-6 weeks). However, fast-release fertilizers can be tricky.

Characteristics of Nitrogen Fertilizers for Home Lawns (via University of Illinois Extension)

NPK Table

Fertilizers categorized as weed and feed products have a broadleaf weed killer for weeds such as dandelions and are usually applied in the fall. Others contain a pre-emergent herbicide to control crabgrass and are applied in spring before the weeds begin appearing.

Understanding the information on fertilizer bag labels will help you choose the right products for you or your customer’s lawn and garden needs. Creating a lush lawn and a garden with beautiful blooms and bountiful harvests can be made easier by following the Rule of 3.

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