site map

BOBscaping videos

Sandys Garden

Master Gardener Advice

Christmas pages

City and Township information

green news

Bob's Blog

WPIAL news



Some gardening knowledge is gained by doing. Other knowledge is gained by studying. Fertilizers fall into that second category. But once you understand the basics of fertilizer it will become rather simple. Therefore, this webpage is to share my knowledge (both remembered and already * forgotten -- see my note below) about plant nutrients.



* I had to reference some old university notebooks from my agronomy and horticulture classes to compile this plant nutrient information for you. 

Here's to vigorous plants through proper fertilization!


Fertilizer Analysis

Major Nutrients - N, P, K 
 - Nitrogen
 - Phosphorus
 - Potassium

Secondary Nutrients - Ca, Mg, S

Essential Elements - C, H, O

Macronutrients - C, H, O + N, P, K + Ca, Mg, S

Minor Elements - Fe, B, Mn, Cu, Cl, Mo, Zn

Remembering it all - C. Hopkins, Cafe Manager

What does fertilizer "analysis" mean?

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:

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

Furthermore, this product would be considered a "complete" fertilizer, since all three nutrients are present.
An "incomplete" fertilizer would 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 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 the fertilizer bag mean?

Answer: N - P - K

These are the "major nutrients"
  Nitrogen - Phosphorus - Potassium

NITROGEN (N) - the Nitrogen percentage is the first number on the label

Example: 10-10-10

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

  • 78% of our atmosphere is nitrogen, and rain and snow account 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 trees.

PHOSPHORUS (P) - 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.

  • 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 (organic source) can be used to 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 phosphorus application.

  • 2015 Update: Many states have now banned the use of phosphorus in fertilizers due to concerns for waterways.

POTASSIUM (K) - the Potassium percentage is last on the label

Example: 10-10-10

Potassium helps plants resist disease 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 turfgrasses.

  • 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, and readily leaches out of the soil profile.


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

Essential Elements
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).

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: C, H, O     N, P, K    Ca, Mg, S

Minor Elements
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

Remembering it all

How can you possibly remember everything?

Simple . . . start with the name of a famous restaurant manager in the table 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




soil amendments

soil pH



home | site map | terms of use | contact
Copyright 1998-2015  DONNAN.COM  All rights reserved.