What Do the Numbers on Fertilizer Mean?
Short Answer:
These numbers represent the % amount of Nitrogen - Phosphate - Potash in a given fertilizer. (e.g. a 10 - 10 - 10 fertilizer is 10% Nitrogen - 10% Phosphate - 10% Potash). This is also referred to as the N-P-K number. These are the three primary nutrients needed for plant’s growth and ongoing health. Key Point - Higher is not better, and can kill or seriously harm a plant if too high. Learn why below.
Longer Answer:
Every Fertilizer has 3 numbers on the bag. Many recommendations specify one number combination or another. What do those mean, is there a best one?
Every fertilizer sold in the US must be labeled with 3 bold numbers displayed on the package known as the fertilizer grade (Ex: 10-12-6). These represent the % amount of Nitrogen - Phosphate - Potash in a given fertilizer. (e.g. a 10 - 10 - 10 fertilizer is 10% Nitrogen - 10% Phosphate - 10% Potash). This is also referred to as the N-P-K number (see why below).
Nitrogen, Phosphate, and Potash are the sources of the three primary nutrients plants need:
Nitrogen (N) - Used for leaf development and chlorophyll production
Phosphorus (P) - Used for roots, flowering, and managing environmental stress
Potassium (K) - Used to help strengthen early growth as well as the plant’s immune system, and defenses against insects / other pests
Additionally, there are three major “micro” nutrients that are increasingly being feature more prominently on fertilizer as agricultural science understand their importance. These are:
Magnesium
Sulfur
Calcium
The level of need for these three is more species specific, so you are likely to encounter them prominently featured on specific use fertilizers, like Tomato-Tone (Nightshades need lots of Calcium and Magnesium) or Holly-Tone (Sulfur acidifies the soil, breaking down other nutrients into forms that an acid loving plant can absorb).
Finally, just like any other living thing, plants need a robust mix of other micronutrients from Iron, to Zinc.
One thing to keep in mind though, too much fertilizer will kill plants (the terms “nitrogen burn” or “fertilizer burn” is used. A plant will develop visible burns on its leaves, and its roots will die off from too much fertilizer)
Too little, and a plant will grow slowly, eventually dying when its exhausted the nutrients in its soil.
One last note - just like with humans, there has been a major movement towards microbiome health for plants. Increasing amounts of research has shown that just like with humans, plants require many different species of fungus, bacteria, and other species to break down nutrients, fight off disease, and live healthy lives. (This is what people mean when they say “living soil”). Many higher-end and organic fertilizers now include “Probiotics”. These can make a major difference for poorly performing plants, or if a site has had nothing but grass planted on it for any period of time.
So what's the best number to use?
Unfortunately, there’s not an easy answer here. The right fertilizer depends on the plant, and its stage of growth.
General rules of thumb:
Higher N-P-K numbers are not better - they typically represent fast release fertilizers which get quick results at the cost of long term plant health
Slow release fertilizers are safer, much less likely to overdose a plant, and tend to be be better for a plants long term health
Good specific use fertilizers (ie. for tomatoes / for citrus / for succulents) focus on the micronutrient amounts, not just the N-P-K number. In many cases a good fertilizer will have a much lower N-P-K number than others, but be richer in other micronutrients
Use organic material based fertilizers whenever possible. Fertilizer are made from two sources, organic material (think manure, bone meal, fish, kelp, etc) or chemical process (combining chemicals to create the need fertilizer molecules). Studies are continuing to show that organic material based fertilizers are much better for the soil and microbiome, as well as significantly less likely to cause runoff issues. It will say on the bag if it is made from organic material sources
Labels are your friend! They can be intimidating, but most fertilizers are labeled with detailed use instructions (typically the small print on the back)
When in doubt, get a nutrient test done! You can buy nutrient tests at a big box store or online. Your community extension office will also perform them.
Most plants require 3 different fertilizer mixes (or a well round species specific fertilizer) every year
Once during early spring to support new growth
Monthly during their growing season
Once in the late fall to help them through winter / dormancy