What's in the Label?

What Do Fertilizer Labels Mean?

Commercial fertilizers, by standard, contain at least one of the three primary nutrients (N), phosphorous (P), and potassium (K).  Phosphorous and potassium in their elemental form cannot be absorbed by the plants.  In fertilizers they come in a form that is useable by plants.  Phosphorous comes in the form of phosphate (P2O5) while potassium comes as potash (K2O).  Fertilizer qualifies to be a commercial grade only when it contains more than five percent of N, P2O5 or K2O.

Every bag of commercial fertilizer is labeled with three numbers separated by dashes as shown in the above picture.  These numbers reflect the percentage of the three primary nutrients in the fertilizer, the first number being nitrogen (N) - the second number is phosphate (P2O5) - the third number is % potash (K2O).  The label always lists the nutrients in this order.

16-20-0 is a fertilizer containing 16% nitrogen, 20% phosphate, and 0% potash.  Mixed fertilizers (fertilizers that contain two or more of the primary nutrients) either complete or incomplete, are made by combining different single-element sources to form a desired blend.  Because of this combination, the resulting fertilizer mix may have particles of varying colors.   In as much as plants require different proportions of the primary nutrients, manufacturers produce different grades. 

How to determine the nutrient content of fertilizers

1.  Fertilizer grade (example 6-10-10)

2.  Total weight per bag (example 50 lbs)

3.  To determine the nitrogen (N) content:  50 lbs x 6%

                     50 x .06 = 3 lbs N

4.  To determine the phosphate (P2O5) content:  50 lbs x 10% P2O5/lb fertilizer

                      50 x .10 = 5 lbs phosphate

5.  To determine the potash content:  50 lbs x 10% K2O/lb fertilizer
                     50 x .10 = 5 lbs potash 

6.  The total amount of nutrients in the 50-lb bag of fertilizer is 13 lbs.  The rest is inert filler material.

7.  To convert phosphate into phosphorous and potash into potassium the following conversion rate are necessary:  1 lb of P2O5 = 0.44 lbs P; 1 lb of K2O = 0.83 lbs K.  So, we already know that there are 5 lbs of phosphate and 5 lbs of potash in a 50 lb bag of 6-10-10. 
To convert phosphate in to P:  multiply phosphate weight by 0.44 lbs.
        5 x 0.44 = 2.2 lbs P
To convert 5 lbs potash into K:  multiply potash weight (lb) by 0.83 lb/lb K2O
        5 x 0.83 = 4.15 lbs K

It is important to master this simple math skill because it is a tool in determining the economics of your choice of fertilizer.

Functions of the Three Major Elements

To appreciate and understand the meaning of the fertilizer grades, one must understand the functions of the elements that are featured in the fertilizer labels. 

Nitrogen (N) is the major element that affects vegetative growth of plants.  Sufficient nitrogen results in good foliage which key to producing a good crop.  Nitrogen is a major component of plant proteins, chlorophyll, vitamins, and enzymes.  It is gives the green coloring of the plant.

When nitrogen is deficient, plants exhibit stunted growth, chlorosis or yellowing on older leaves, premature death and poor yields.

Nitrogen is a plant-mobile element, therefore the symptoms of deficiency are observed on the older leaves. 

Phosphorous (P) promotes early root formation and growth.  It is a component of some enzymes and proteins, ATP, RNA and DNA necessary for plant development.  Phosphorous also promotes healthy development of the reproductive system of the plant.  When we say reproductive system we are referring to the flowering, fruit setting and seed development.  

Deficiency on phosphorous is manifested by stunted growth of plants, unusual dark green and purple pigmentation on leaves, poor fruit and seed development, and delayed maturity. 

Unlike nitrogen, phosphorous is an immobile element hence, the deficiency symptoms are usually noticed on the younger plant parts.

Potassium (K) is necessary in the following plant processes:  formations of sugars and carbohydrates; protein synthesis; and cell division.  It is enhances flavor and color on flowers, fruits, and vegetable crops.  It also helps increase fruit size and quality which results in better marketability of produce.  Potassium helps build resistance of plants to diseases. 

Plants that are deficient to potassium exhibit the following symptoms:  slow growth resulting in low yields; scorched or burned appearance on leaves; necrosis on mature leaves; weak stalks resulting in lodging; and small fruits and shriveled (undeveloped) seeds.

Therefore  a fertilizer that has the label 6-10-10 (6% N - 10% P  -10% K) is intended to promote a healthy plant growth giving special emphasis on root and fruit development just like what a gardener would on his tomatoes.  On the other hand, a fertilizer with a label of 21-0-0 (21% N and 0% P and K) is intended to meet the needs of a plant to develop healthy foliage.  It is usually applied on lawns and non flowering and fruiting shrubs.  The fertilizer 16-20-0 is a formula that's used to provide nitrogen and phosphorous needs of plants.  The emphasis here is foliage growth and root and flower formation.  This also can be used in areas where K is needed but the soil has sufficient amount of K2O.

Pointers to Consider Before Choosing your Fertilizer:

1.  Have your soil analyzed. 
Soil analysis result will give you a customized fertilizer recommendation for your garden.  Collect soil samples.  Assign five locations that represent the different sections of your garden.  Collect soil by cutting straight into the soil up to about one foot deep.  Avoid scraping only from the top soil.  Air-dry soil samples.  Mix a you pulverize the samples.  Put them in a bag and send them to a soil analysis lab in your area.

2.  Observe your plants
Observe how your plants grow.  Look for unusual appearance and behavior. (I will remind myself to post a list of the most common deficiency symptoms as reference.) 

3.  Acquaint yourself with general knowledge
The soil in general contains all the essential elements for plant growth.  Some elements are more abundant than some.  Nitrogen, because it is absorbed by plants in large quantities, and can be lost easily through leaching and immobilization, needs to be replaced more regularly than P and K.  Phosphorous is often bound in the soil due to soil unfavorable soil pH (soil acidity).  Water plays an important role in making the elements available for plant uptake. 

4.  Avoid fertilizer overdose
Fertilizers are like medicines.  They are helpful in the right amounts but can be toxic beyond a certain level.  Addition of any element to the soil will result in a change in the soil chemistry.  Sometimes, too much of one thing will bind other element making them unavailable for the plants thereby solving one problem by creating another.  Always follow the recommended rate and procedure of application.

5.  Pricey does not always mean better
Now that you know the meaning of the labels, you should be able to determine which bag gives you the nutrient at the least cost per unit weight.