Jared Barnes, Ph.D., obtained his degree in horticultural science.

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Don’t Fret About Fertilizers
by Jared Barnes    


‘Human Food’ vs. ‘Plant Food’
One of the first and most important things gardeners should understand about fertilizers is that plant food is not the same as human food. For humans, food is a source of nutrients for growth and energy, all derived from the sugars, fats and proteins we intake. But for plants, nutrients only serve as the building blocks for growth, and instead of nutrition coming in complex molecules they come in the form of basic elements such as calcium and iron. 

The energy that plants need to live comes from the sun. They use carbon dioxide and water to capture and store this energy in sugars, and they use the nutrients they absorb through their roots to build more complex compounds. 

Overlooking this difference is where many gardeners go wrong. Some people assume that if a plant isn’t growing, it needs “food.” Food makes perfect sense when we think of nourishment in human terms. When we are famished, eating will often help us to feel rejuvenated. However, you could have a “well fed” plant in a poorly lit location and it would not survive, grow or reproduce. Adequate light is needed to keep energy levels up. 

It’s easy for gardeners to have this misconception. Heaven knows we have a hard enough time managing our own diets, let alone the diet of another entire biologically dissimilar group of organisms. But knowing the difference helps when fertilizing your plants. 

Fertilizer 101
Let’s talk fertilizer basics. Fertilizers contain several essential plant nutrients. The three that we focus on the most are nitrogen (N), phosphorus (P) and potassium (K). Of the nutrients taken up by plants from the soil, these can comprise upward of 80 percent of the mineral nutrients in plants, hence the need to ensure an adequate supply. 

Because of the importance of these nutrients, fertilizer manufacturers have helped consumers by implementing a three digit system that tells how much of these nutrients are in the bag. We often refer to these fertilizers by their three-digit ratio, such as 17-3-19 or 20-20-20. Just like a dietary label, these three numbers give us an idea of how much of these three nutrients we are applying. For example, a 100 pound bag of 17(N)-3(P2O5)-19(K2O) fertilizer has 17 pounds of nitrogen. To calculate phosphorus we multiply the 3 pounds of phosphorus oxide by a conversion factor of 0.44 to get 1.32 pounds phosphorus (P), and for potassium multiply the 19 pounds of potassium oxide by a conversion factor of 0.88 to get 16.7 pounds potassium (K).

While much of the focus is on these three, there are other nutrients, such as calcium, magnesium, sulfur, iron, boron and others, that are also important. But, unless you are in a unique situation, soils are not typically deficient in these nutrients, and most fertilizers provide trace amounts of them. Therefore, unless you have a severe deficiency, these shouldn’t be a problem. 

I would love to give you a magic number and tell you exactly how much fertilizer you should use in certain areas of your garden, but that’s harder than it seems. With humans, it’s quite easy to tell when we are hungry. However, with plants it’s much harder. We can’t use our senses to tell when the soil surrounding the root zone are running low on nutrients. Fortunately, a lot of research has been done to determine nutrient levels in the soil, and the most accurate means of getting this info is to do a soil test. Don’t be intimidated by a soil test. I did my first soil test before I was of voting age, and if a whippersnapper like me could do it, then so can you. Just contact your local extension office for a soil test kit. 

Setting a Schedule
It helps to get on a regime of fertilizing, whether it’s once a year or once every couple of months. For example, container gardens have heavy requirements for fertilizer because they are constantly leaching mobile nutrients. Therefore, I apply an all-purpose organic fertilizer to my containers once a month at a rate of 1 teaspoon for every 3 inches of container diameter. I also fertilize with fish emulsion once every two weeks. 

Another reason timing is important is because fertilizing encourages growth. I fertilize my annuals frequently, up until frost. However, I stop fertilizing woody plants about six to eight weeks out from the first freeze. Ceasing application early helps prevent any excess tender growth from occurring late in the season that might be damaged by winter’s cold. 

Once you’ve got your soil test requirements and choose a type of fertilizer to use, you’re ready to get going. You’ll see that applying fertilizers is as easy as ABC… and NPK!



This article appeared in a previous State-by-State Gardening issue.


Posted: 07/03/19   RSS | Print


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paschallb - 07/06/2019

Very nice concise article- thank you.  Sometime, I would enjoy a fertilizer article that wrote about differences, if any, between an organic v. “regular” fertilizer especially as it relates to the three basic elements.  K, P, N are basic organic elements indeed so may a difference be in the method they are produced or is there something I am missing.  Thank you.

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