Quick Guide: Fertilizer

For indoor growers, people growing edible landscapes, gardeners and landscapers, fertilizer is a critical success factor. There’s an amazing amount of fertilizers to choose from today and understanding some basic concepts about fertilizer will make your edible garden produce more food and your landscape more beautiful. Below is a great start to help you dive into a subject that makes a big impact on food production and water use.

4-6-3 What do these numbers mean?

These numbers correspond to the percentage of nitrogen, phosphorus, and potassium in the product you are buying. This is often referred to as NPK. An easy way to remember the order of what each letter (NPK) represents is they are in alphabetical order, nitrogen, phosphorus, and potassium. NPK is a reference to their atomic symbol on the Periodic Table of Elements.

So if the numbers on the container of fertilizer are 4-6-3 this represents 4% of the contents of the container by weight are nitrogen, 6% phosphorus and 3% potassium. Each of these elements has a different impact on your garden and understanding the amounts and differences is key to gardening success. For those math majors out there, yes these numbers don’t add up to 100%, because there are other nutrients and fillers in the container.


Nitrogen helps plants grow healthy green foliage and stems. The amount of nitrogen needed varies by plant. Too much nitrogen may give lush green leaves but inhibit blooms or fruit. There are many variables in this formula and it takes some experimenting to get it “right” and a good way to speed up the process is to test your soil.


Phosphorus increases your plants ability to flower and the bloom size as well as promoting strong healthy roots. Newly transplanted plants and young plantings often benefit from phosphorus fertilizer. The highest phosphorus fertilizer you can use is 61%. Once again too much can be a bad thing. Too much phosphorus can stimulate algae growth and make it hard for your plants to take up other important nutrients like iron and zinc. Try to resist the temptation of more is better because with fertilizer it definitely is not the case.


Potassium is also very important for your plants. It helps fight off diseases and helps with water management by providing drought protection. A great time to use potassium is after your plants are damaged by disease or insects.

One of the problems with fertilizers is many people overdo it thinking if some fertilizer is good, more is better. Another misconception is fertilizer will heal a sick plant. Often the plant is sick because of overwatering, virus or bad soil pH. It is always good to take a soil sample before fertilizing. If you want to take a soil sample before applying fertilizer take a look at this how to take a soil sample article.

Here is a great article on 6 Signs You Are Over Fertilizing Your Plants

Finally, if your soil is rich in organic matter, beneficial bacteria and fungi you will have to fertilize less or maybe not at all. This is really just the start of what should be a deeper dive into the subject, a little more research will ensure you are giving your plants exactly what they need.

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2 Responses

  1. It really helped when you described fertilizer ingredients and what they’d do for our plants. Next week, we’re going shopping for gardening tools to start a veggie garden in our backyard. My wife and I are grateful for your advice about using potassium found in fertilizers to fight off diseases. https://www.schubertsod-depot.com/fertilizer

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