Wondering which fertilizer to use in your garden? This guide explains the differences between organic and synthetic fertilizers and how to choose the best option for your garden’s long-term health.
To get started, keeping compost is a good, long-term way to enrich your soil and feed your plants. From there, you can decide which additional fertilizers best suit your situation after testing your soil for any deficiencies.
How to Choose a Fertilizer for Home Gardens
Gardening requires patience—something many of us lack when starting out (that was me!). We want quick results, but shortcuts often come at a cost. Fast-growing plants can be invasive, and excess fertilizers harm soil and waterways, leading to issues like algae bloom.
Instead of guessing, start with a soil test to determine what your garden actually needs.
For years, home gardeners have been told to rely on synthetic fertilizers. While they work fast—their usefulness is narrow and they have a big environmental impact when overused.
Organic fertilizers, on the other hand, release nutrients slowly, improving soil health over time.
So, what’s best for your plants? It depends on your soil, what you’re growing, and your ecological concerns.
Let’s break it down.
Contents
The Big Picture
When it comes to fertilizers, there are two main types: organic and synthetic. Each has its own benefits and drawbacks, depending on your gardening and environmental goals.
Here’s a quick overview to help you decide what’s best for your garden.
1. Organic Fertilizers
These fertilizers are made from natural sources like compost, manure, seaweed, bone meal, fish emulsion, or plant-based materials. They release nutrients slowly as they break down, improving soil health over time.
Best for: Garden beds, raised beds, long-term soil health.
Pros:
- Feeds soil microbes, improving structure and fertility.
- Provides a slow, steady release of nutrients.
- Less risk of over-fertilizing or harming plants.
- Sustainable and eco-friendly.
Cons:
- Nutrients take time to become available to plants.
- More expensive than synthetic options (unless you can produce your own).
- Harder to measure exact nutrient levels (generally not an issue).
2. Synthetic Fertilizers
These are manufactured fertilizers, often derived from petroleum-based chemicals or mined minerals. They provide nutrients in a form plants can absorb immediately.
Best for: potted annuals needing a quick nutrient boost.
Pros:
- Works fast for annuals needing an immediate nutrient boost.
- Easy to measure and apply.
- Typically less expensive than organic fertilizers.
Cons:
- Can harm soil microbes and reduce long-term fertility.
- Easy to over-apply, which can burn plants and cause long-term soil and water imbalances, disrupting aquatic ecosystems and harming wildlife.
- Synthetic fertilizers contribute to soil degradation, making it harder for plants to thrive naturally over time.
- Made from fossil fuels and mined minerals, adding to environmental and carbon footprint concerns.
Which One Should You Use?
- For long-term soil health: Stick with organic options like compost and manure.
- For a quick fix or potted plants: Synthetic fertilizers can help, but use them carefully to minimize environmental harm. Also, always check that any application is food-safe.
- For the environment: Organic fertilizers are the better choice, unlikely to contribute to water pollution or soil degradation.
Whichever you choose, less is more—only fertilize when your soil or plants truly need it.
Want to improve your soil naturally? Start a compost bin or experiment with cover crops to add nutrients without synthetic chemicals derived from fossil fuels or mining.
Frequently Asked Questions
1What Are ‘Natural’ or ‘Organic’ Fertilizers?
Natural or organic fertilizers are derived from organic matter found in plants, animals (including manure), and/or powdered minerals.
These fertilizers are slow-acting, calling the microbes into action which in turn helps improve your soil.
Examples include homemade compost created from decomposed food scraps and composted animal manures.
The use of the word ‘organic’ here means the materials have been minimally processed and retain their natural forms.
When added to our gardens, they are converted by microbes (tiny life forms like bacteria and fungi) in the soil, making them available for uptake by plant roots.
These fertilizers may be produced in a factory, farm, or your compost pile (see how to compost here and how to compost in the winter here).
They are slow-acting, delivering a low-dose of nutrients beneficial to soil health and plant growth, and unlikely to overdose (although it’s always possible with any over-application).
Because of their composition, organic fertilizers not only offer the macro-nutrients plants need: nitrogen (N), phosphorus (P), and potassium (K), but also provide secondary and micro-nutrients, vitamins, and other goodies that help improve soil structure and health.
Commercial organic fertilizers are generally more expensive than synthetic fertilizers, but, if you factor in the benefits to soil health, environmental considerations, and sustainability (more on this below), they (in my opinion) provide greater value.
Fertilizers may be soil-like (compost), liquid, granular, or spikes inserted in the soil.
Here’s some examples:
- Compost from food scraps and yard waste
- Composted animal manure
- Cover crops like alfalfa or clover rye
- Fish emulsion
- Liquid kelp
- Liquid seaweed
- Fish hydrolysate
- Earthworm castings
- Bone meal
- Blood meal
- Feather meal
- Rock phosphate
- Cottonseed meal
Listen
Transcript
Organic fertilizer — which I think many of us may not even think of as fertilizer, but it adds nutrients to the soil, so by definition it is fertilizer. One thing you learn pretty quickly in gardening is that the word “organic” has several different meanings. Sometimes it’s clear from the context which definition is being used, but not always, especially since there’s usually a lot of overlap between what the different definitions include.
When you hear people talking about organic fertilizer, they usually mean either fertilizer that is acceptable for use in certified organic growing or fertilizer that has come from living sources. So, in that second sense, compost is organic fertilizer because it’s a bunch of formerly living things — mostly from plants — decomposing together. Manure is organic — it wasn’t living, but it came from animals. Not everything that falls under one definition will be both. Rock dust is a fertilizer used in certified organic growing, but it’s not from living things, it’s bits of crushed rock.
I’m going to use the “living things” definition of organic. And that’s in contrast to what are called synthetic fertilizers, or sometimes commercial fertilizers. And the exact definition here gets fuzzy too, which we’ll avoid and just say that most of the big bags of fertilizer sold at garden centers with their N-P-K labels fall into the synthetic category. They’re what most of us think of when we think of fertilizer.
Most of the nutrients our plants take up are ions, which are atoms or molecules with an electrical charge — and we just did a whole episode on ions. And that’s true no matter what the source of those nutrients are — organic or synthetic. When the plant takes them up, they’re ions, with only a few exceptions.
When positive and negatively- charged ions come together to form a compound without a net electrical charge — where the positives and negatives cancel each other out — that’s what chemists call a salt. Which is different from what we call salt and put on our food … or avoid putting on our food. It’s another example of a word that has been hijacked by scientists — like fruit and nut — that had a common meaning long before scientists got a hold of it and made it something different.
But using the chemical definition, an easy way to deliver ions is through salts. Salts are compounds of ions and when they come in contact with water, they usually get ripped apart back into their ionic forms. Or, as chemists prefer to say, they dissolve. We think of water as this harmless, neutral thing, but salts see things differently. Even though water molecules overall have no net electrical charge, because of their structure, there are positive and negatively charged parts to water molecules. Which means that as salts come in contact with water — which they do in soil — they get pulled apart into their ions by those electrical attractions from water molecules. And, once they’re ions, they can be taken up by plants.
That’s simplifying things a bit, but that’s the main principle behind synthetic fertilizers. They’re mostly salts that quickly become ions in soil and then can be taken up as nutrients by plants.
Organic fertilizers, on the other hand, include much bigger, more complex molecules that need to be broken down by microorganisms — sometimes over a period of years — before they get down to those same ions that plants can use. Some parts of organic fertilizers become available nutrients quickly — there are ions in organic matter which can deliver nutrients in the short-term — but the larger molecules from things like proteins and carbohydrates are going to take a while to get to that usable, ionic form.
So organic fertilizers require patience. They’re a way to build the health of your soil and soil structure through the years and for years to come. Many gardeners who use organic fertilizers find they never need to buy synthetic fertilizer. Synthetic fertilizers still have their uses. If you need nutrients delivered to the soil now, they’re a quick fix that you won’t get from organic fertilizers. Which can also be a challenge with synthetic fertilizers — it can be too much too soon, and the nutrients that are mobile in soil can wash away before plants use them — and end up in places we don’t want them — and the ones that aren’t so mobile can accumulate in the soil to unhealthy levels.
And not all organic fertilizers work so slowly for so long — fish emulsion has a timeline very much like synthetic fertilizers. But as a general rule, that’s the biggest difference. Organic fertilizers are a long-term part of building soil health and soil structure and synthetic fertilizers are better for a quick, temporary fix to deficiencies in the soil now. They can help soil structure a bit, but not as much as organic.
2What Are Synthetic Fertilizers?
Synthetic (commercial, chemical, manufactured) fertilizers are often petroleum-based, deriving from fossil fuels or mined from naturally-occurring mineral deposits which are manufactured or refined in factories.
To their credit, complete synthetic fertilizers deliver the three macro-nutrients: nitrogen (N), phosphorus (P), and potassium (K) in a form that roots can immediately absorb—if they need it—and they do it quickly. And that’s why they’re so popular in farming.
The drawbacks are how they are made, where they are sourced from, and how they affect the soil and our waterways—all things that can be undesirable to eco-friendly gardeners.
Synthetic fertilizers are usually less expensive than organic fertilizers, but the overall cost to the earth may cancel the value for home gardens.
And, for a home garden, faster does not mean better!
Because they are powerful and fast-acting, it is critical to apply synthetic fertilizers with caution and care, following the instructions carefully, erring on the side of under—rather than over applying. Otherwise, any surplus in the soil can burn or kill the plants, adversely affect the soil, and/or enter our waterways caution a variety of serious problems (more on this below).
Examples of product ingredients in synthetic fertilizers include ammonium phosphate, ammonium sulfate, ammonium nitrate, potassium sulfate, and superphosphate.
Product labels may show N-P-K numbers like 20-20-20. The N-P-K numbers for synthetic fertilizers are higher than the amounts in organic ones.
3What Are The Basic Primary Macro Nutrients?
To understand fertilizers, it’s good to know your N-P-K.
These are three of the main macro-nutrients plants need.
- Nitrogen (N): promotes green growth above ground. | “Up” |
- Phosphorus (P2O5 ): promotes root and flower growth but does not trigger flowering as is sometimes reported. | “ Down” |
- Potassium (K2O): promotes overall plant health including the plant’s immune system, and temperature tolerance. | “All Around” |
A simple (very generalized) way to remember how they affect the plant is Up (nitrogen), Down (phosphorus), and All Around (potassium). It’s not a firm rule though—there are plenty of exceptions.
In Canada and the United States, the N, P, and K are always listed in the same order, no matter which brand you choose, as numbers on the product label like this:
4-3-2
or some other combination of three numbers.
But just because these are important macronutrients doesn’t mean your soil is deficient in them. This is why a soil test can save you time and money.
And even if you have a soil test done and know what amendments are needed, the numbers on the package represent the ingredients not just the specific elements so you do need to read the instructions carefully to deliver the desired amounts.
Example
Espoma Rose and Flower Food (4-3-2), is an organic, slow-release fertilizer. It is formulated to support the growth and flowering of the plant through the growing season. This product also contains beneficial microbes. The product is marketed to rose growers but also used for other flowering plants.
The numbers on the package are 4-3-2.
That’s 4% nitrogen (N), 3% phosphorus and oxygen (P2O5 – phosphorus oxide), and 3% potassium and oxygen (K2O – potassium oxide).
- The first number (4 in the example) is nitrogen.
- But the 3 is for P2O5 (so it’s not entirely phosphorus: the phosphorus is just over 40% of the total phosphorus oxide by weight).
- And the 2 represents K2O. The potassium is just over 80% of the total potassium oxide by weight.
The rest of the product is filler, which is good because this keeps the macro-nutrient levels at the desired ratios for healthy roses and flower blooms.
So, if you are trying to apply specific amounts per instructions from a soil test, keep in mind that the elemental amounts are lower than the package implies and some calculations will be needed to get the amounts right. This is likely more math than a typical gardener might do. But we can follow the instructions on the package to at least keep the application within the recommended limits.
Listen
NEW! Click play to listen:Transcript
We’ve been talking about plant nutrition for the last couple of episodes. First we talked about how there are about twenty essential elements that plants need during their lives, and I say “about twenty” because there are a few that have only been shown to be essential for some plants, so you can come up with slightly different totals depending on what you include. And there very well may be others that we don’t know about yet. We’re down to very small amounts now, and it’s actually pretty hard work to show that an element is essential — and it sounds like it may not be the most exciting work in the world — so we may add to that number in the future as more get discovered.
Thankfully, as gardeners, we rarely have to worry about most of them. They’re present in the amounts needed in our soil and deficiencies are possible, but rare for most of the essential elements. Certainly for the micronutrients that are only needed in small amounts. If your plants are growing fine, then they are getting the essential nutrients in the amounts they need. That’s the soil test that really matters.
And last time, we talked about what I think is the most important lesson gardeners come to learn about plant nutrients. Plants get most of their nutrients from the soil. Some through their leaves, but most through their roots from the soil. When we add nutrients through fertilizers, we aren’t feeding our plants, we’re adding to the soil. We aren’t addressing the needs of the plants, we’re addressing any deficiencies in the soil for meeting those needs. And most of the time — and there certainly can be exceptions — but most of the time our soil already has the nutrients plants need. Especially if we’ve been taking good care of our soil. I read all sorts of advice about what formula of fertilizer to add to your soil — “well, for new transplants, you want a 5-30-30 fertilizer … but for blooms use 5-35-5 or vegetables 5-10-5” … or whatever … and I have no idea where these numbers come from since the person recommending them has no idea what the current levels are in your soil. So how can they know what needs to be added? Maybe they make sense for growing in soilless mixes or with hydroponics, but for growing in soil you cannot know what to add — if anything — without knowing what’s already there.
I think the most value I got from the first soil test we paid for — from a lab — was to see how high our levels of phosphorus were. Which, it turns out, is common in our part of the world. Because that meant that all the advice I had heard about adding balanced fertilizer to the soil — or adding any fertilizer that contains any amount of phosphorus — anything where the second number wasn’t a zero — was clearly wrong. Not only was there no reason for us to add more, there was reason to avoid adding more. Yes, phosphorus is essential and does all sorts of great things, but it’s already there in our soil and excess amounts actually do harm. And that’s true for most nutrients. Our soil test really pulled back the curtain and exposed how much of the advice you hear doesn’t make sense. I think the value of soil tests is often exaggerated. Worth doing — but more limited than some of the claims made for them. But just seeing the phosphorus level was worth the money we paid. Your test results may not be so black and white, but for US, that alone showed how much of the fertilizer advice you’ll hear needs to be taken with a grain of salt.
Okay, so that’s N-P-K (up, down, and all around). Now let’s look at the two main types of fertilizers.
4Do ‘Balanced’ Fertilizers Make Sense for Your Garden?
5-5-5 or 10-10-10
Instead of relying on “balanced” fertilizers, test your soil first. Plants need specific nutrients, not a one-size-fits-all mix.
You may have heard the advice to add a “balanced” fertilizer to your plants and it sounds sensible until you dig into the science.
This episode (embedded below) of our podcast, Two Minutes in the Garden, goes into more detail.
Plants need what they need—not seemingly equal proportions of nutrients.
For long-term growing, you’ll want a soil test to have a basic idea of what you have—and what’s lacking—to guide your planting and soil amendment decisions. It’s a big problem to overfeed soil because of what happens to the runoff nutrients, so more is not more.
And, as mentioned, synesthetic fertilizers—if you choose to use them at all—are suited for short-term amending in things like pots of flowering annuals. Organic fertilizers like compost and manure are for ongoing use in the ground and raised beds.
If you’re selecting a synthetic fertilizer, don’t assume “balanced” is what your soil and plants need.
Listen
NEW! Click play to listen:Transcript
Balanced fertilizers—such as 5-5-5 or 10-10-10—contain equal amounts of nitrogen (N), phosphorus (P), and potassium (K), the three primary nutrients plants need. At first glance, this might seem like a sensible choice. After all, if plants require all three, providing them in equal amounts should ensure they have access to everything they need.
However, this approach overlooks a few key factors. First, plants do not absorb all nutrients at the same rate. More importantly, soil is not a blank slate. It already contains nutrients, and the levels of nitrogen, phosphorus, and potassium can vary significantly depending on location, soil type, and previous amendments. Using a balanced fertilizer assumes that all three nutrients are deficient in equal amounts, which is rarely the case.
To put it in perspective, consider human nutrition. We don’t consume an equal amount of every vitamin and mineral just because they’re essential. A person might need more calcium but no additional iodine. Adding equal amounts of both wouldn’t make sense—yet many gardeners take this one-size-fits-all approach with fertilizers.
In many soils, phosphorus levels are already sufficient or even excessive. Adding more can contribute to nutrient runoff, polluting waterways and causing ecological issues. The only way to know what your soil actually needs is through soil testing. For example, if test results show high phosphorus but lower nitrogen levels, using a balanced fertilizer would supply unnecessary phosphorus while failing to provide enough nitrogen.
Another often-misunderstood aspect of fertilizers is the labeling. The N-P-K numbers on packaging don’t represent the pure elements but rather their compound forms: nitrogen (N), phosphate (P₂O₅), and potash (K₂O). Phosphate contains only about 44% phosphorus, and potash is roughly 83% potassium. This means even a “balanced” fertilizer doesn’t supply equal amounts of each nutrient in elemental form. If true nutrient balance were the goal, even more phosphorus would need to be added—exactly what many soils don’t need.
Ultimately, using a balanced fertilizer without understanding your soil’s nutrient levels can lead to wasted money, poor plant growth, and environmental harm. Instead of assuming your soil needs equal parts of N, P, and K, start with a soil test and apply only what’s necessary for healthy, sustainable growth (and this does not have to come from synthetics).
5Is Fertilizer Even Necessary?
Whether or not you even need fertilizer depends on the conditions in your growing space and what you’re growing.
Ground Soil
For long-term soil management (ground, raised beds), it’s usually recommended to amend the soil ongoing, year after year with organic fertilizers like compost and/or composted manure.
- Natural or organic fertilizers are derived from organic matter found in plants, animals (including manure), and/or powdered minerals. They are slow-acting, calling the microbes into action which in turn helps improve your soil.
Depending on the composition of the soil and what you’re growing, this may mean adding an inch or two of compost or manure once or twice a year.
Synthetic fertilizers act quickly but are temporary. They can be easily over-applied, potentially harming plants and the environment (both during production and application).
I opt not to grow any in-ground plants that require ongoing use of synthetics.
Flower Pots
Plants like single season flowering annuals generally come with a synthetic fertilizer in the potting mix to release nutrients throughout the season.
- Synthetic (commercial, chemical, manufactured) fertilizers are often petroleum-based, deriving from fossil fuels or mined from naturally-occurring mineral deposits which are manufactured or refined in factories. These are fast-acting and single-purpose. Excess may harm plants and waterways.
Organic fertilizers are not recommended for potted annuals because they are not fast-acting.
My work-around (to avoid synthetic fertilizers) for growing annuals in containers is to add compost to the potting mix ongoing, year after year.
6How Do I Know Whether a Fertilizer is Organic or Synthetic?
Great question! Start with the label. What does it say? Look for the word ‘organic‘ on the product. Both organic and synthetic fertilizers say ‘fertilizer‘ or ‘plant food‘ on the label. Sometimes these products are also called ‘soil amendments’ or ‘enhancers’.
But don’t stop there. Read the whole label. Check the ingredients. Look up the company. Find out how they make the stuff. Where does it come from? What is it? What will it do? What are the warnings? Is it right for your garden?
Ensuring it is organic and therefore slow-release and (we hope) beneficial to the soil is step one. Step two is feeling good about the source, processing methods, packaging, and so on. It’s a complicated world out there and the choices we make all add up.
7Which Type of Fertilizer is Best for the Environment?
If you have read this far, you know this is a loaded question and huge topic. The answer is: both organic and synthetic fertilizers have ethical and environmental flaws, but overall, homemade organic wins.
I’ve been gardening for decades now, using compost to enrich my garden beds.
My second choice is composted animal manure, but it often comes in big plastic bags, which presents a whole other wasteful problem. The one time I bought it in bulk—happily avoiding any plastic bags—it happened to contain a lot of living seeds from weeds—indicating it never got hot enough during the composting process to kill the seeds—and that, in turn, caused a lot of headaches in the garden. If you have your own critters like hens or rabbits, those are both excellent sources, and rabbit poop does not require a waiting period: it can go straight into the garden.
If you are sensitive about animal rights, you may not want to use an animal-based product like manure, fish emulsion, or blood or bone meal. But, some support this use because it makes use of every part of the animal.
Fertilizers derived from fossil fuels or mines are not sustainable long-term, and the processes for obtaining the materials are destructive. Synthetics can also cause a buildup of chemicals like arsenic, cadmium, and uranium in the soil, harm the microbes, reduce soil fertility, and change soil pH levels.
And, yes, there’s lots of hypocrisy in all of this because most of us live on the grid and ride vehicles fueled by fossil fuels and so on.
There are companies that produce organic fertilizers using lower-impact, sustainable practices with an aim to trample on the environment as little as possible. Mind you, there’s still those dang plastic jugs and bags to dispose of.
Gardening, like everything else we do, is not very green.
Do your homework and find the solution you can live with.
8How Are Fertilizers Harmful to Waterways?
We’ve heard a lot about this in the past few decades. Basically, synthetic fertilizers deliver nitrogen, phosphorus, and potassium to the soil. Plants take up what they can, and the excess is carried away by rain water. This ends up in our rivers, lakes, streams, and oceans, causing a toxic overload.
It’s a serious problem.
Imagine thousands of acres of farms, all (inadvertently) introducing surplus fertilizer chemicals to waterways. You’ve probably seen algal bloom—where the water becomes clogged with overgrowth of algae and aquatic plants, choking out everything else. Ultimately, they die off and sink down, depleting the water of oxygen. This in turn kills off fish and other aquatic life.
Toss in pesticides, insecticides, herbicides, and plastic pollution, and we’ve created dead zones: entire sections of waterways that are virtually lifeless.
But that’s farms doing that, some say. Certainly, a little home use doesn’t matter? Well, we all play a role: there are millions and millions of us gardeners all doing similar things. Grass lawn fertilizers are another huge problem. So yes, it all adds up.
Again, this is why less is more with synthetic fertilizers, and all fertilizers should be selected to fill specific needs, not broadcasted without an identified need or deficiency.
9So, Which Fertilizer Should I Use?
As mentioned, my number one choice is to just use homemade compost ongoing, applying whatever I have to my garden beds once a year. This will slowly release nutrients into your soil when the it is warm and moist. It’s not always easy to create that much compost but if you have a good local source, you’re set!
Whichever type you choose, decide based on what your plants actually need and what’s available in the soil. Have you ever done soil tests? Perhaps now is the time to get a baseline on your growing space.
If you’re going to use a synthetic fertilizer for short-term crops, follow the instructions exactly.
For long-term soil health, start with basic organic soil amendments like homemade compost and composted manures, which can be added to your soil at any time. If you need to purchase it, research local commercial organic fertilizer options.
Examples of Organic Fertilizers
You can buy them online, or, if you have a local garden nursery you love (and it supports organic growing practices), have a look there and see what they recommend.
- Roses | Clematis | Flowering Perennials 4-3-2
- Tomato-tone Fertilizer 3-4-6
- Herb and Veggie Food 3-4-4
- Bulbs, Flowers, Roses 4-12-0
Want to see an example of how an organic fertilizer can help your plants?
Amy of Get Busy Gardening has a great example here where she applied an organic fertilizer to some of her tomatoes and peppers to compare the difference.
Again, because I put my efforts into producing as much food-waste compost as I can, I have not tried or tested many products.
It can also get costly to fertilize with commercial products.
See what is available where you are, at a local nursery or by mail-order. Look up the companies, see if you like what they’re doing, and learn about the different products (percentages and ratios of N, P, and K) to understand what is right for your situation.
When applied as directed, these can make a huge improvement in your garden.
Summary
Plants do not know the difference between the macro-nutrients in each type of fertilizer: available nitrogen (N), phosphorus (P), and potassium (K) are all the same to them. It’s the side-effects that make the difference.
Organic fertilizers are slow-acting, calling the microbes into action which in turn helps improve your soil.
Synthetic fertilizers are fast-acting and short-term for a single-purpose. They are usually made from fossil fuels or mined mineral deposits. Excess may harm plants and waterways.
While synthetics are used for short-term applications including flowering annuals, organic fertilizers are recommended for ongoing amendment of garden beds and raised beds.
~Melissa the Empress of Dirt ♛
Read More
- Chemical and Organic Fertilizers | Oregon State University OSU Extension Service
- Synthetic vs. Organic Fertilizers | Enviroingenuity.com
- How Fertilizers Harm Earth | Scientific American
- Debate Over Organic v. Chemical Fertilizers | Todayshomeowner.com
- Fertilizer 101 | Todayshomeowner.com