Understanding plant fertilizers and nutrients

All about fertilizers and plant nutrition from major and minor nutrients
How to translate the numbers on a fertilizer bag

Some gardening knowledge is gained by doing, other knowledge is gained by study. Fertilizers fall into the second category. But once you understand the basics of fertilizers and fertilization, it will all become rather simple. This page shares my knowledge about plant nutrients. Here’s to vigorous plants through proper fertilization! Bob

Nitrogen - Phosphorus - Potash
Nitrogen – Phosphorus – Potash

What is “fertilizer analysis?”

All fertilizers have three numbers on the label which indicate the fertilizer analysis, or “percentage by weight” of nitrogen, phosphorus and potassium, in that order.

Therefore, a 50 pound bag of fertilizer labeled 20-10-5 would contain 20% nitrogen (10 pounds), 10% available phosphates (5 pounds) and 5% soluble potash (2.5 pounds). See the calculations below:

EXAMPLE: 50 pound bag of 20-10-5 fertilizer

20% nitrogen .20 x 50 lbs = 10 lbs

10% available phosphates .10 x 50 lbs = 5 lbs

5% water soluble potash .05 x 50 lbs = 2.5 lbs

This fertilizer would be considered a “complete” fertilizer, since all three nutrients are present.

An “incomplete” fertilizer might have a label like 0-0-60 or 46-0-0, since it would only have one of the three major nutrients present. Another example of an incomplete fertilizer would be 0-20-20, since one of the three nutrients (nitrogen) is missing.


Fertilizers also have “ratios” which indicate the relative amounts of nutrients to each other. For example, a 10-10-10 fertilizer is a 1-1-1 ratio, and a 20-10-5 fertilizer is a 4-2-1 ratio.

Ratios can be helpful when looking for the “right mix” for a certain type of plant or situation.  For example, vegetable gardens often call for a 1-2-1 ratio, which would translate into a 5-10-5 or 10-20-10 fertilizer.  Most trees like a 2-1-1 ratio, which would be a fertilizer product such as 10-5-5 or 20-10-10.  Lawns prefer a 3-1-2 ratio fertilizer, so a fertilizer product with 30-10-20 on the label would be a good ratio match.

High analysis fertilizers (those with larger numbers on the label) would be applied at a lower rate to yield the same results.  In other words, 5 lbs of a 20-20-20 fertilizer would yield the same amount of actual nutrients as 10 lbs of a 10-10-10 fertilizer.


What do the 3 numbers on a fertilizer bag mean?
Answer: N – P – K

These are the 3 “major nutrients”
  Nitrogen – Phosphorus – Potassium


The Nitrogen percentage is the first number on the label.

Example: 10-10-10

Nitrogen is a primary nutrient that makes plants “grow.”  When you put fertilizer on your lawn, most of the “green up and grow” comes from nitrogen.


There are ‘quick release’ and ‘slow release’ forms of nitrogen.  Slow release forms are more expensive but remain effective for a longer period of time. Organic fertilizers are slow release and have less potential to “burn” plants.

> Nitrogen produces vegetative growth in plants, but too much nitrogen can cause problems. One problem is succulent growth, which makes a plant more susceptible to certain diseases due to lush foliage.

> 78% of our atmosphere is nitrogen with rain and snow accounting for 2 to 12 pounds of actual nitrogen per acre (43,560 square feet), per year. “Lightning charged rain” is high in NH4 and NO3. Snow has been called “poor man’s manure”. . . now you know why!

> Plants in the Legume family “fix” atmospheric nitrogen into the soil. Peas, beans, clover, and alfalfa are legumes, as well as Black Locust (Robinia pseudoacacia) trees.


The Phosphorus percentage is the middle number on the label. Example: 10-10-10

Phosphorus is a primary nutrient that encourages rooting, blooming and fruit production in plants. Vegetable gardeners have typically been told to apply 5-10-5 since the higher middle number (P) helps vegetable production.

> FLOWER POWER! Phosphorus is important for root-growth and blooming in plants, and is the main ingredient in “starter fertilizers” as well as liquid fertilizer “bloom boosters”.

> Phosphorus is lacking in most Southwestern Pennsylvania soils we have tested since 1979. Applications of super-phosphate (0-20-0), triple super-phosphate (0-46-0), or bone meal (an organic source) can be used to help correct deficiencies.

> Since phosphorus moves very slowly through the soil, it should be incorporated into the soil prior to, or during, planting. In existing lawns, we recommend core-aeration prior to an application of phosphorus.

NOTE: Many states have now limited or banned the use of phosphorus fertilizers due to pollution concerns of waterways.


The Potassium percentage is last on the label and easy to remember since Potash is on the “ash end.” Example: 10-10-10

Potassium helps plants resist disease problems and aids in winter hardiness. (“K” is the symbol for “kalium” or potash, and is commonly used to represent potassium)

> Most “winterizer” fertilizers used on lawns in late fall are high in Potassium, since it promotes winter hardiness in turfgrass.

> Potassium fertilizers have a high “salt index” and should be used with caution, since they can “burn” plant foliage.

> Most “complete” fertilizers contain potassium since it is fairly mobile, readily leaching out of the soil profile.

Secondary nutrients also play an important role in plant growth. The 3 secondary nutrients are Calcium (Ca), Magnesium (Mg) and Sulfur (S).

The essential elements are basic to plant growth, and need to be mentioned here, even though they aren’t commercially available fertilizers.  The 3 essential elements are Carbon (C), Hydrogen (H) and Oxygen (O).  Plants obtain these elements from carbon dioxide (CO2) and water (H2O).

MACRONUTRIENTS: C – H – O   N – P – K   Ca – Mg – S
When you group the essential elements with the major nutrients and secondary nutrients, you end up with the 9 macronutrients: Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, and Sulfur.

MINOR ELEMENTS: Fe – B – Mn – Cu – Cl – Mo – Zn
Nutrients needed by plants in lesser amounts are known as the minor elements.  These include Iron (Fe), Boron (B), Manganese (Mn), Copper (Cu), Chlorine (Cl), Molybdenum (Mo), and Zinc (Zn).

How can you possibly remember everything?

Simple . . . start with the name and moniker of a famous restaurant manager used below:

C Hopkins, Cafe Mgr.
But mother’s cooking is more zestful

C – Carbon

H – Hydrogen
O – Oxygen
P – Phosphorus
K – Potassium
I   – (nothing)
N – Nitrogen
S – Sulfur

Ca – Calcium
Fe – Iron
Mgr – Magnesium

but – Boron
mother’s – Manganese
cooking – Copper/Chlorine
is more – Molybdenum
zestful – Zinc


Spring fertilization – Trees and shrubs

Soil pH – Do you have ‘sweet’ or ‘sour’ soil?

Liming soil – Adjusting the pH of ‘sour’ soils