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Fertilizer 102 – Understanding the Label

Last week we talked about the impact fertilizer could have when it left your yard and made its way into the storm drain. This week we’ll talk about how to read your fertilizer label to make sure that you are applying it correctly.

As a reminder, there is a good chance that your turf grass doesn’t need to be fertilized. Mature, established grass should have root structures capable of thriving without fertilizer. In fact, a little struggle encourages more resilient root structures that help turf grass endure environmental stress. If you’ve been practicing techniques like aeration and grasscycling, you are even less likely to need to fertilize—saving time and money!


But let’s say that you have to fertilize and of course you want to fertilize correctly, so you have to understand the numbers on the label. There are three key numbers you’ll see that correlate to Nitrogen (N), Phosphorus (P), and Potassium (K). Each number represents the percent of the fertilizer made up of that nutrient. Sooo, a 21 for N means 21% of the bag is Nitrogen (or 10.5 lbs in the example of a 50-lb bag), a 3 for P means 3% is Phosphorous, and a 20 for K means another 20% is Potassium. The remainder of the bag is inert material (think neutral dirt) included to make it easier to spread the fertilizer on your turf grass.

So what do each of those nutrients do? Here’s a quick gnomeonic: Up, Down, and All Around. Nitrogen, the first number and the “Up” portion of the phrase, helps turf grass growth above ground. Nitrogen promotes the tall, green blades that many homeowners are looking for—which is also why many fertilizers have a high Nitrogen number. Caution, though, too much nitrogen can burn the grass.

Phosphorus, the middle number, promotes healthy root systems and is the “Down” portion of the gnomeonic. We are big fans of healthy root systems that allow the turf grass to draw more water from deeper in the soil, allowing you to water less frequently.

Potassium, the final number, promotes a tougher plant capable of withstanding environmental stresses and is the “All Around” component of fertilizer.

Knowing what the numbers mean is the first step in proper application, but next week we’ll talk about how you can determine what your turf grass actually needs. When in doubt, remember that less is more when it comes to fertilizer because too much can burn your plants, waste money, and pollute your County.

If you need extra help, you can always easily reach an expert through the Master Gardeners Program:
UCCE Master Gardeners Hotline

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