Skip to main content

Organic vs. Chemical Fertilizers, and Those 3 Little Numbers

Maria is a master gardener and master of public health. She & her husband, known online as The Gardener & The Cook, live in coastal Alabama.

We bought this fertilizer for the centipede grass the first spring after moved into our new (to us) home a couple of years ago. We've never had centipede before so I had to research its needs.

We bought this fertilizer for the centipede grass the first spring after moved into our new (to us) home a couple of years ago. We've never had centipede before so I had to research its needs.

Those Three Little Numbers on Fertilizer Bags

The three numbers on fertilizer bags and bottles are the chemical analysis (often called the fertilizer analysis). Those numbers and their chemical symbols are, in order: nitrogen (N), phosphorus (P), and potassium (K). Gardeners often refer to potassium as "potash". They will always appear in this NPK order. (The word “potash” reminds me of a funny story which I will tack onto the end of this article.)

What We’ll Cover in this Article

  • The meaning of the 3 numbers on bags of fertilizer;
  • What each chemical does; and
  • The individual organic fertilizers and what they do best.

From now on, think of the three numbers as “NPK”.

— Maria

Choosing the Right Fertilizer for Your Needs

Before spending money on fertilizer, think about which plants you will be using it on, and the specific needs of those plants. By choosing the one best suited to your garden’s needs, whether synthetic or organic, you can target the needs of your plants much more accurately.

I prefer this method over those large bags of more generic fertilizer that may be good for one plant, but not so good for another. The primary exception to this is turf grass for which you most likely will need fertilizer in large quantities.

Another thing to consider before spending money on fertilizer, is to do a soil test to learn what nutrients your soil needs. You can get the kits and complete instructions free from your local county extension office or local master gardeners. The pH tests usually cost anywhere from $3 to $7. Soil tests should be done about every two or three years.

Now, About Those Numbers:

Each number represents the percentage of the product that is composed of that particular chemical. The numbers won't add up to 100%, because the remainder of the product is filler, including inert components that will aid in dispersing the chemicals. You already know I prefer organic fertilizers for my veggies, and organic pesticides for all my plants. So I will also give you a list of good organic sources of these chemicals needed by all plants.

Nitrogen Provides Lush Leaves and Grass

Nitrogen promotes lush green lawns, and beautiful leaves with strong stems. All plants need some nitrogen, but those needs will vary. Look to high-nitrogen fertilizers for turf grasses and foliage plants such as the hosta and hydrangea leaves shown below.

This is a hosta called "Guacamole". I had problems growing hosta in central Florida, but now that we're a bit farther north, I will be trying them again.

This is a hosta called "Guacamole". I had problems growing hosta in central Florida, but now that we're a bit farther north, I will be trying them again.

The beautiful dark green leaves of hydrangea

The beautiful dark green leaves of hydrangea

A Quick Word About Organic Fertilizers

Organic fertilizers are great, but — and this is a big but — It takes huge amounts of organic fertilizers to provide the amount of nutrients your soil needs, and to equal the amount found in synthetic fertilizers.

The Best Sources of Organic Nitrogen

  1. Composted Cow Manure: cow manure should be at least 6 months old.
  2. Composted Chicken Manure: chicken should be at least 4 months old.
  3. Blood Meal: In order not to burn plants, dissolve blood meal in water before applying to plants, or work it into the soil before planting.
  4. Fish Emulsion: this can be very stinky, but is an excellent organic fertilizer. It can be diluted with water to weaken the odor.
  5. Coffee Grounds: leftover coffee that you didn’t drink can be poured onto the soil around plants, too — just be sure it has cooled. Additionally, used coffee grounds are only about 2% nitrogen, yet they are a great soil amendment, especially for acid-loving plants such as azaleas. Granted most of the acid is in the coffee you made, and not much is left in the grounds, but I still put it on the ground around my azaleas.

Phosphorus Provides Strong Roots and Flowers

When thinking about flowering, remember our fruits and some veggies are produced from the flowers of their plants. So phosphorus is highly important in the production of not only beautiful flowers, but squash, tomatoes, okra, beans and peas, and many others.

If you want beautiful flowers and strong healthy fruits and veggies, you must make sure your soil contains the needed amount of phosphorus.

About turf grass: after buying an older home with a centipede lawn, we learned that phosphorous should NEVER be added to centipede without a soil test that specifically calls for it.

Some of my dad's brilliantly colored day lilies, now in my garden. Day lilies make great pass-along plants.

Some of my dad's brilliantly colored day lilies, now in my garden. Day lilies make great pass-along plants.

The Best Sources of Organic Phosphorous

  1. Bone Meal: a great slow-release form of phosphorous and calcium, and my personal favorite.
  2. Banana Peels: banana peels break down quickly, so they are an excellent source of phosphorus. Just insert them into the soil near your plants. Phosphorus is not mobile, so it is important to place the peels near the base of the plants, without disturbing the roots, of course.
  3. Crab and Shrimp Shells: be aware the odor may offend your neighbors, and may attract critters to your compost bin. You can dry them, then grind them in a coffee grinder used only for this purpose and for grinding eggshells.
  4. Bat Guano: this can be purchased at many garden centers.
  5. Fish Bone Meal: this can be purchased at most garden centers.
A proper ratio between phosphorus and nitrogen produces both gorgeous leaves and beautiful flowers.

A proper ratio between phosphorus and nitrogen produces both gorgeous leaves and beautiful flowers.

Potassium Does Its Work on the Inside of Plants

Potassium works on the inside of the plant, building strong cells and healthy tissues. This enables plants to withstand common plant stressors: diseases, pests, even heat and cold.

All plants need potassium, but some need more than others. When shopping for plant foods, you may notice that those intended for winterizing lawns will have a higher number for potassium.

Imagine the healthy cellular structure needed to produce these gorgeous swirls and striations in this jumbo elephant ear leaf.

Imagine the healthy cellular structure needed to produce these gorgeous swirls and striations in this jumbo elephant ear leaf.

The Best Sources of Organic Potassium

  1. Granite Dust (or Meal): this is finely ground granite that is a great slow-release source of potassium. It also serves to maintain good soil drainage and to create good soil structure.
  2. Egg Shells: egg shells consist primarily of calcium carbonate, but they also contain potassium. They are especially helpful if your tomatoes suffer from blossom-end rot, as it is due to a lack of calcium.
  3. Ashes of Hardwoods From Your Fireplace: while hardwood ashes are an excellent source of potash, using too much will raise the pH levels of your soil considerably, so be sure to keep a check on your soil's pH levels. These days, you can buy a soil test kit at most any garden center.
  4. Kelp (Seaweed) Meal: this can be in a dried and powered or liquid form. Both forms are fairly quick-release. Kelp meal can be purchased from many garden centers. If you live by the ocean, and can collect your own, lucky you.


Be sure to use the liquid form of any fertilizer only on potted plants – not those in the ground. Rainwater and your irrigation system will wash liquid fertilizers deeper into the ground, away from the roots – this is true for any plants in the ground. So, save the liquid fertilizers for your container-grown plants.

Up close and personal with a beautiful healthy leaf of one of my banana plants.

Up close and personal with a beautiful healthy leaf of one of my banana plants.

In Summary: Choosing the Right Fertilizer for Your Needs

Before spending money on fertilizers, do two things:

  1. Think about which plants you will be using the fertilizer on, and the specific needs of those plants. By choosing the best individual compounds, you can target the needs of your plants much more accurately. I prefer this method over those large bags of synthetic fertilizers that may be good for one plant, but not so good for another.
  2. Do a soil test every 3 years to determine the pH of your soil, and any nutrients that may be missing.

The Funny Story I Promised

Several years ago one of my uncles sent off a soil sample to one of the labs at Auburn University that does the tests. When he received the results, he decided that he needed to be sure "those kids" knew what they were doing, so he sent another sample. (It’s actually faculty, staff, and grad students.)

He intentionally added a large amount of ashes from his fireplace to the second sample. In about two weeks he received a brief note along with the new soil analysis. Among other things, the note said, in bold print, "No more potash!" He then announced, "Those kids know what they're doing down there."

If you want to have a lab do the soil test for you, the little bags or boxes, along with detailed instructions are available from your local master gardeners. While some master gardener locations have the equipment to conduct the tests themselves, most will send your samples off to their state’s land grant university. Here in Alabama, it’s Auburn University. In Florida it’s the University of Florida.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 MariaMontgomery

Your Comments and Questions Are Always Welcome

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on October 15, 2021:

Maria, you're always welcome.

MariaMontgomery (author) from Coastal Alabama, USA on October 15, 2021:

Thank you Miebakagh

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on October 14, 2021:

Maria, your followers with commonsense will.

MariaMontgomery (author) from Coastal Alabama, USA on October 14, 2021:

Hi Miebakagh, Thanks for reading my article. Composting is so important, and keeps us from wasting food scraps and yard waste. Good luck with your new compost pile or bin. Thanks for commenting. I'm concerned that, with the mix-up, not many people will see the article. Oh, well. You win some, you lose some.

MariaMontgomery (author) from Coastal Alabama, USA on October 14, 2021:

I'm glad you found it, Miebakagh.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on October 14, 2021:

Maria, I've read the story a moment ago. Thanks.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on October 14, 2021:

Maria, the read is very helpful. I'm makinp a compost at present, and will apply your suggestions.

MariaMontgomery (author) from Coastal Alabama, USA on October 14, 2021:

Hi, Miebakagh! I replied to you last night, but I my reply hasn't been published. I just got an email from HP that they have now cleared the article for publication. Here's what happened: I was really upset because I spent a day and a half polishing a very old blog post, and changing out most of the photos. I deleted the old blog post several weeks ago by clicking on "trash". What I didn't notice was that it put the post in a group of others I thought I had deleted, with all of them waiting for me to click on "delete permanently". I didn't know that until HP unpublished this article with in 2 or 3 minutes of my publishing it. I immediately went back to the old site, found what was wrong, and replied with my explanation to the HP Edit Team. In less than 24 hours they responded. Thanks Edit Team.

MariaMontgomery (author) from Coastal Alabama, USA on October 13, 2021:

Hi Miebakagh, unfortunately HP unpublished this article within 2 or 3 minutes of me hitting the publish button. It is from a very old blog post that I deleted several weeks ago. What I didn't know was that, even though I clicked on "send to trash", it was not permanently deleted. It is now, and I have explained to HP, and am hoping they will republish it for me. Thanks for trying to read it. I hope it's up and running again soon. I worked on it for 2 days trying to get it updates and just right.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on October 13, 2021:

Where's the article? Instead, I got a page 404 response.