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How To Calculate Fertilizer?

By: Emma Grace

How to calculate Fertilizer?

So you’ve checked out some of our other articles about fertilizer and picked up some high-quality products for your garden.

Now what?

Application, of course! But properly applying your fertilizer can be complicated, especially if the directions are unclear or if you have a large plot of land to fertilizer.

Knowing how to properly calculate your fertilizer is vital for ensuring that your plants get the exact recommended amount of nutrients. Too little fertilizer and you won’t see any results; too much, and you can damage or even kill your plants.

There’s no need to worry, though—I’m here to teach you all the tips and tricks for proper fertilizer application, whether you have a granular or liquid product. We’ll also talk about why properly calculating your fertilizer is important, and I’ll share my super-simple formula for how to do it. Let’s get started!

In this guide you’ll learn:

  • Why calculating your fertilizer is important for garden health
  • How to understand the composition of your fertilizer
  • How to calculate fertilizer, both liquid and granular
  • And so much more!

Why Should You Calculate Fertilizer?

You might be wondering why calculating your fertilizer is so important. Isn’t sprinkling a handful over your plants good enough?

The answer is no, many times over! Making sure your plant has the correct dosage is super important. Too much fertilizer, bad things happen. Too little fertilizer, nothing happens. Let’s talk specifics.

What Happens When You Put Too Much Fertilizer On Your Plant?

Putting too much fertilizer on your plant is kind of like drinking too much caffeine. I know when I have too much coffee, I get a headache, lose my appetite, and feel just plain awful (does that stop me? …no).

Putting too much fertilizer on your plants is kind of the same thing. The excessive amount of nutrients will actually hurt them, rather than help them. Check out this YouTube video on recognizing fertilizer burns, showing you how to spot an overfed plant:

Those brown spots don’t just occur on peppers, but on the majority of overdosed plants. And like he mentioned, you can take the plant out of the pot (or ground) and flood it with water to try and wash some of that fertilizer away. It doesn’t always work, but it’s worth a shot.

Another huge problem that comes from over-fertilizing your plants is environmental waste. Excess fertilizer can cause a host of environmental issues, including algal blooms in local waterways. These algal blooms cause serious environmental disruption, as they prevent sunlight from reaching the bottom of the water, where aquatic plants grow.

Fertilizer runoff can also be toxic to the local wildlife. If you’re using organic fertilizer, this is less of an issue, but it can still be harmful to native flora and fauna.

As you can see, over-fertilization causes a host of problems not only for your garden, but for the world around you. Make sure to properly dispose of fertilizer if you have extra, and always use the proper amount.

Now that we’ve covered the negative effects of over-fertilizing, let’s talk about another huge problem: under-fertilizing.

Read More >> How To Dispose Of Fertilizer?

What Happens When You Under-Fertilize Your Plant?

Under-fertilizing your garden doesn’t have the same environmental impact as over-fertilizing it, but it can still cause problems for your plants.

Really, the only problem you’ll have is that nothing will happen! If your plants aren’t getting the nutrients from fertilizer, they simply won’t be able to grow to their fullest potential. They might improve a little, but you won’t see the results you want/the product guarantees.

This is definitely an issue I had when I was a novice gardener. I was so scared to burn my plants that I underfed them, and then I was surprised when nothing happened (silly me).

Back then (really about a year ago) I wished I had a simple formula that would help me calculate how much fertilizer to use. It would’ve saved me a lot of time and heartbreak over my moderately-yielding crops.

Luckily for you, I’ve since figured out how to calculate my fertilizer with a super-simple formula! Before we get there, though, let me teach you about the composition of your fertilizer, so you better understand proper application.

Read More >> How to Make Liquid Fertilizer from Dry Fertilizer?

What Is Fertilizer Made Of?

This is a super broad topic, so I’ll get into the specifics for you. Fertilizer is comprised of multiple elements and ingredients that promote plant growth in various ways (I know that as vague, too, but I’ll explain). Today we’ll talk about nutrient composition and possible additives.

Nutrient Composition of Fertilizer

Understanding the composition of your fertilizer is important in selecting the right product, and in calculating how much fertilizer your plant needs.

Each and every fertilizer product you buy should have three numbers somewhere on the packaging that look like this: 10-10-10. If you don’t see these numbers (often called the ‘NPK ratio’) don’t buy it! You should always know the nutrient composition. In fact, you can’t calculate your fertilizer without it.


The first number represents the amount of nitrogen in the product. A higher concentration of nitrogen will promote leafy green growth, making it great for houseplants, trees, and shrubs.


The second number represents phosphorus, which promotes blooming and fruiting. I like a higher phosphorus content for my vegetables and flowers.


The third number is the amount of potassium, which helps plants fight off diseases and improves the overall quality of a crop. It’s always good to have, but most fertilizers concentrate on nitrogen or phosphorus.

There are a few other ingredients you can keep an eye out for, as well. Sulfur increases the acidity of your soil, calcium improves root and stem strength, and magnesium can aid in photosynthesis. These should also be represented in numbers on the nutrition label of your fertilizer.

I recommend learning more about nutrient science if you’re a committed gardener. A strong knowledge of this can help you product bigger crop yields and have a healthier garden in general.

Possible Additives in Fertilizer

Most additives in fertilizer are man-made, making the products inorganic. While organic and inorganic fertilizers are both great for your garden, some gardeners prefer to stick with one type or another.

Inorganic fertilizers may contain pesticides or insecticides, which can be really beneficial when used properly. Make sure to check the label for the exact type of chemical in the product before purchasing or applying.

Organic fertilizers are more likely to have animal products. Common organic fertilizers can include bonemeal, bloodmeal, fishbone meal, or even manure. If you’re a vegan gardener, or if this kind of thing freaks you out, definitely make sure to check the label. These products can also be found in inorganic products, but in my experience, they’re more likely to pop up in organic fertilizers.

These are just some potential additives in fertilizers. Again, be sure to read the nutrition label and the fine print at the bottom before making a decision as to whether or not to use a product.

Read More >> Which fertilizers to use for Raspberries?

How To Calculate Fertilizer?

Full disclaimer: I hate math. I am a writer and gardener by trade, and math has never been my strong suit. For the health of my plants, though, I’ll do anything (including a little basic math).

I did my absolute best to make this formula as simple as possible, and I’ll provide an example as we go so that you can see what I’m talking about. Let’s go!


Let’s keep this simple. Say you have a vegetable fertilizer with the composition 10-15-11 that you want to apply over 100 sq. feet of garden. To make things easy, we’ll say you want to put one pound of nitrogen on your garden. The question is: how many pounds of this 10-15-11 fertilizer will you need to put one pound of nitrogen on your vegetables?

Step One

For ease, convert each nutrient percentage to its decimal form. For this fictitious fertilizer, it would look like this:




To get that conversion, all you do is divide the nutrient number (in this case, either 10, 15, or 11) by 100. Super simple, right?

Read More >> Which Fertilizer is Right for Ferns?

Step Two

Next, divide the amount of product you want to apply (in this case, one pound of nitrogen per 100 sq. feet) by that decimal conversion. It would look like this:

1 (pound of nitrogen per 100 sq. feet) /.10=10

What this means if that if you wanted 1 pound of nitrogen to make its way into the soil, you would need to use 10 pounds of fertilizer to make that happen.

Again, these are made-up numbers, but you get the point. You can plug any variables you want into this formula and it’ll be right every time, since it’s been tested and proven beneficial by yours truly.

Of course, a lot of fertilizer packages do have instructions. This formula is designed for people who would want to try something different, gardeners who are fertilizing a lot of land, or people who purchase a product that doesn’t come with instructions (most of them do, though).

My Final Thoughts On How To Calculate Fertilizer

Admittedly, it took me a couple tries to get the math right with this, because I was using more complicated formulas or just tripping myself up (again, I’m terrible at math).

But this formula has been proven perfect, and it provides some pretty amazing results. If you’re a gardener who likes to do things your own way, I definitely recommend this.

While most fertilizers and plant foods have directions, some gardeners notice that those directions aren’t what’s best for their plants. There’s no such thing as being a perfect gardener, or as the perfect product for your garden. It’s all about finding what works for you and your plants.

That being said, I highly recommend using this formula and calculating your own fertilizer, especially if you’re working with a bigger chunk of land. If I can figure the math out, you can, too!

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