The Basics Series: A Fertilizer Primer

The Basics Series, as the name suggests, is a series of articles that lay out some of the basics of gardening and plant care – Soils, Mulches, Planting and Staking, Roots & Water, and Plant Fertilization. Our hope is that this series will be accessible to the beginning gardener while also offering some additional information that will prove valuable to those who have been digging in the dirt for a little longer (first lesson: dirt is what’s tracked onto your carpet, soil is what we plant into). If, along the way, you would like additional information on a particular subject, or if you have suggestions for additional blog posts please feel free to leave them in the comments below.

I began experimenting with fertilizers on my parent’s back stoop when I was about 6 or 7 years old. The small concrete square gave me about 16 square feet upon which to germinate various seeds and test various fertilizer concoctions. If you’re thinking I was a peculiar child, I think you’d be right but the concept of seed germination and the promise of plant fertilization were almost maddeningly fascinating to me as a kid (and still today). Only the Lone Ranger held such sway over my young mind. I kept collections of seeds (which were sometimes collected from the plants in our landscape and sometimes swiped from the mailbox when fortune allowed me to get to the Gurney’s order before my mom) in an empty tackle-type box under my bed. The fertilizer I used for these experiments (which was also likely procured by illicit means) was kept in various solutions in a few milk jugs on the stoop. Beyond this, I had a small stack of Dixie cups and a backyard from which to dig all the soil I could need.

So how did the experiments turn out? Not so well. Of the endless dozens of seeds that I tried to germinate, only a few actually popped up. And of the few that I did get to grow, I don’t remember that any of them grew enough to be transplanted into the garden. Why? I was a kid, for crying out loud! Kids don’t read the instructions for board games let alone the fine print on a box of fertilizer. I don’t remember the formulations or solutions that I was using but my guess is that I was dosing the seeds so ferociously with the fertilizer that as the seed cracked open to drop its first root, it simultaneously burst into little flames and disappeared into nothingness due to the amount of nitrogen that they were receiving. First lesson? Read the box. Any package of fertilizer or fertilizer-containing product will have a label on it which describes two very important details – the analysis and the directions

The Analysis

The analysis of a fertilizer details the percentages of the three main macronutrients that are contained in the package. The macro’s – Nitrogen, Phosphorus, and Potassium – will be displayed as a series of three numbers on every fertilizer package, no matter the type or brand. The first number is the percentage by weight of Nitrogen in the package. The second number is the phosphorus. The third is the potassium. In the picture of the “Tomato and Vegetable Food” below, the numbers tell us that, by weight, the contents are 7 percent nitrogen, 22 percent phosphorus, and 8 percent potassium. To simplify the math piece of this equation, we’ll assume the bag weighs 100 pounds. The fertilizer analysis – 7-22-8 – would tell us that there are 7 pounds of nitrogen, 22 pounds of phosphorus, and 8 pounds of potassium in the bag. What about the other 63 pounds? Micronutrients and carriers. Micronutrients are nutrients which are more often already available in garden soil and are slightly less important to the growth of the plant. The back of the bag shows that it contains traces of boron, copper, manganese, and zinc. The carriers that make up the rest of the weight are often clay particles or some similar inert material. The fertilizer gets bound to these particles to make application easier and more reliable.

The Macronutrients N-P-K

To understand the fertilizer analysis a little bit better, let’s look at the function of the three macronutrients.

Nitrogen brings the green and the growth. Phosphorus brings the blooms and the fruit. Potassium strengthens cell walls.

There. Done.

While these descriptions are wildly simplified (Google: “The effects of phosphorus on plants," for example) they are, at a fundamental level, very useful descriptions. Look at a bag of lawn fertilizer. It will often contain 20-30% nitrogen. Why so high? Because green, leafy grass is what we desire our lawns to consist of. Nitrogen brings the green and the growth.

The pictured bag of tomato food has a higher percentage of phosphorus. Why? Phosphorus brings the blooms which lead to the fruit.

Why is this important? Because if we use the wrong formulation for the right plant, we end up with a plant that doesn’t do what we want it to do. Sometimes we want for our shrubs to flower more profusely. But if we feed them with a high nitrogen fertilizer, all we’ll get are a bunch of beautiful green leaves and a little extra growth. Not blooms. Why? This is what nitrogen does – big green. To get more blooms, we need to use a fertilizer with a higher percentage of phosphorus (the middle number). Why? This is what phosphorus does. Blooms.

Shop wisely.

The Label Directions

After choosing a fertilizer based on your plant’s needs, the next step is to read the rest of the label. Crazy, I know. But consider this: when your doctor identifies a problem in your body and prescribes a medication, you feel compelled to read the instructions and dose accordingly, yes? If you don’t, any pharma commercial will tell you (as soothing music plays and children frolic and fly kites) about the risks of the medication: hair loss, colon cancer, sensitivity to light, swollen digits, risk of infection, renal failure, gout, bad breath, fever, rash, and brittle fingernails. I’m not a doctor but I suspect these symptoms occur more often in people who don’t dose according to the label. Don’t do this to your plants! Read the label and dose accordingly.

Remember: More is not better. More gets you a bunch of Dixie cups on the back stoop with nothing but burned up little nothings growing in them.


Timing of application. Keep in mind that there are good, neutral, and don’t do it times to fertilize plants whether they’re trees, shrubs, perennials, or lawns. Two questions can help guide you with getting the timing right.

  1. When does the plant most need the food? As a plant’s sole purpose in life is to reproduce, either by budding, flowering, and seeding, or by asexual means – tubers, root divisions, etc., it is during this stage that the plant will use the most energy. Therefore, fertilizing a little before or a little after the plant’s bloom cycle will benefit the plant most. A week or two in either direction will be fine.

  2. When is the fertilizer most accessible? Scratch off January and February. Plants are dormant then. And dormancy being for plants what general anesthesia is for you and me, the plants won’t be eating much. Scratch off Most of June-August. Plants also slow down their growth processes when the temperatures are high and the soil is dry. The plant can be damaged if it’s fertilized during these hot months. As it can’t process the nutrients as well as in cooler months, it can and often will burn.

So when is the best time to fertilize? For most plants, fall is it.

Our next post will be a checklist of garden and landscaping tasks that you may want to consider getting done in the next couple of months. In this one, we’ll look at why fall is best for fertilizing in most cases

Previous Posts in the Basics Series:

The Basics Series: Understanding Soil

The Basics Series: Balance in the Soil - Amendments and Their Kin

The Basics Series: Planting and Staking

The Basics Series: Roots and Water

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