Ask a Master Gardener

Fertilizer Basics

2/9/21

All the numbers on bags of fertilizer are confusing. Can you explain what they mean? MR

Understanding the numbers on the fertilizer bags is essential before you can properly use the fertilizer. So, let’s dig in.

Soil nutrient management can be a little intimidating. I gave a Zoom presentation a few months back to a group of employees at a company about soil science and my friend who scheduled the lesson said: I have to admit, I didn’t know there was this much math in gardening. I thought that was a fair response, because to manage the nutrient levels in your soil, you are going to need to do some math. Many of us will need a calculator. But, once you understand it, it’s not that difficult.

The illustration I like to use to help people understand fertilizer is that of a lawn mower. Lawn mowers need gas and oil. The gas gets consumed, but the oil level remains stable…pretty much. If we relate this to the garden, nitrogen is the gas and phosphorus and potassium are the oil. Nitrogen gets consumed by plants and needs to be replenished or your plants run out of gas so to speak. But, as long as phosphorus and potassium levels stay within the preferred limits, you don’t need to supplement them at the same rate as you do nitrogen…we’re talking maybe adding phosphorus and potassium every few years rather than multiple times a year.

Most fertilizers you encounter at the store show you their nutrient content via three numbers such as 10-20-10. These numbers are the percentage of nutrient levels in each pound of fertilizer. The first number is nitrogen, the second, phosphorus, and the last is potassium. So, a fertilizer marked 10-20-10 would contain 10% nitrogen, 20% phosphorus, and 10% potassium per pound of fertilizer. The rest of the content would be inert or binding agents. For another example, a fertilizer marked 49-0-0 would be 49% nitrogen, 0% phosphorus, and 0% potassium.

There is another link to this puzzle though. Before you go in search of fertilizer for your lawn or garden, you really need to know the current nutrient content of your soil because if you don’t know the nutrient levels, how do you know how much or how little to add? To find out your nutrient levels you will need to get a soil test and we have instructions on how to do that on our website in the Lawn and Garden - Soil section.

So, let’s assume you did a soil test and now have the recommendations back from OSU. In many cases, the soil we test in Tulsa County is low on nitrogen and high on phosphorus and potassium. If this was the case in your garden, you would not need to purchase a fertilizer with any phosphorus or potassium in it. Furthermore, if you did have high levels of phosphorus and potassium and you were to purchase a 10-20-10 for example, you would be worsening your soil nutrient levels as well as wasting money purchasing phosphorus and potassium you did not need.

When you get your soil test back, it will contain recommendations for nutrient amendments. If your soil test said that you need to add 1 pound of nitrogen, four times a year, which fertilizer should you purchase?

In this instance you should purchase a nitrogen only fertilizer such as 49-0-0. Remember, the first number tells you the percentage of nitrogen fertilizer per pound of fertilizer. So, if your recommendation was for 1 pound of nitrogen per 1000 square feet, you would need about 2 pounds of this 49-0-0 fertilizer to get you 1 pound of nitrogen.

The 10-20-10 fertilizer would only be 10% nitrogen so to get a pound of nitrogen, you will need 10 pounds of this particular fertilizer.

Now, let’s assume your interest in fertilizer is for your lawn and the recommendation from your soil test was to add 1 pound of nitrogen per 1000 square feet. The question that usually comes up here is “how do I figure that out?”

Oftentimes, if you purchase the same brand of fertilizer as your fertilizer spreader, the bag will have info on where to set your distribution levels on the spreader. However, if you don’t have this info, you will need to do a little experiment.

The best way to do this is to step off a 1000 square foot area in your lawn. This would be approximately a 33’ by 33’ square. If your lawn is 100‘wide, just use a 10’ by 100’ portion or some variation of that.

Now, put enough fertilizer in your spreader to give you 1 pound of nitrogen. If it is 10-20-10 that would be 10 pounds. If your fertilizer is 49-0-0 that would be 2 pounds. Then, take a guess at the setting of your spreader, maybe using a product you are familiar with as a reference. Now you need to spread that fertilizer on the test area to see how you do and adjust the feed levels up or down to help you meet your distribution target of 1 pound of nitrogen per 1000 square feet.

In my yard, I tend to use the same fertilizer each year. If you do the same, you just need to remember your settings for the next season.

And that’s all there is too it. If this is still confusing, we have several videos on our website (tulsamastergardeners.org) in the Lawn and Garden Section - Soil that you can review. But the more you know about your soil and fertilizer, the better able you are to provide the best growing environment for your plants.

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.

Photo: Bill Sevier