How to Read Fertilizer Labels
The letters and numbers on fertilizer products are always stated in the same order, N:P:K which represent Nitrogen, Phosphorous, and Potassium respectively. The numbers represent the actual percentage of each of those chemical constituents in the fertilizer blend.
Commercial fertilizer manufacturers are required by law to identify the chemical content of fertilizers using a standardized NPK and percentage rating. Doing so allows an accurate calculation for application rates and the correct use of fertilizers.
Nitrogen is always the first number on the bag, so for 15-30-15, there is 15% Nitrogen (N) by weight in the fertilizer. In a standard 100 lb bag of 15-30-15 fertilizer, there is 15 lbs of nitrogen.
Nitrogen is required for top growth of grass. Adding nitrogen makes the grass produce chlorophyll and as a result, the grass turns a deep, rich green.
Nitrogen also makes grass plants and grass grow much faster. If your grass grows poorly, is pale green or even yellowed a bit, it needs nitrogen.
Phosphorous is always the second number on the bag, so for 15-30-15 there is 30% phosphorous by weight.
The same 100lb bag of fertilizer contains 30 lbs. of phosphorous, usually in the form of potash. Phosphorous promotes development of plant roots, blossoms, and creates a stronger, healthier plant.
If your grass roots are thin or not healthy, it may need phosphorous. If some your grass is purple, that may also be a sign of a lack of phosphorous in the soil.
Potassium is always the THIRD number on the label. For 15-30-15, there is 15% potassium (K20) by weight, so the 100 lb. bag of fertilizer contains 15 lbs of potassium.
Potassium is required by plants for toughness. Potassium helps plants tolerate cold, drought, and heat. An early fall application of high-potassium fertilizer will help grasses survive severe cold and frost. It also helps plants protect themselves when subjected to insect damage and diseases.
Clearly the numbers 15-30-15 do not add up to 100% mathematically, so the rest of the bag must contain other materials. Unlisted traces of minerals and micro nutrients are present, but the bulk of the fertilizer consists of inert filler, which enables the uniform application of the highly concentrated nitrogen, phosphorous and potassium. Fillers are inert or inactive, highly variable and may be a proprietary mix of sand, simple powdered clay, or even sawdust.
Fillers are also necessary to help prevent the application of concentrated fertilizer in any area. Too much high-nitrogen fertilizer dropped on a small area can burn the roots of plants, harming, even killing grass or vegetables. The uniform application of fertilizer is made easier and safer with the addition of fillers.
All three constituents may not be present in all fertilizer products, but the NPK numbers on the fertilizer labels will clearly identify that fact.
Specialized fertilizers may not require all three of the major chemical constituents. A fertilizer identified as 20-0-0 contains only nitrogen and fillers, and a product identified as 0- 35 -10 contains phosphorous, potassium and appropriate fillers, but the numbers clearly tell us it does not have nitrogen content.
To determine which fertilizer is needed,* only a soil test really can provide any accurate indication of fertilizers actually required for healthy growth.
Now you know what those NPK numbers really mean. By the way, you might be surprised to learn you may not even need fertilizer this year. That might be true if you decided to go organic in the garden, leave grass clippings on the lawn, apply rich compost, other enhancements, and aerate that lawn twice a year instead.
Note: Avoid applying fertilizer if it is not needed. Excessive, unused fertilizer in the soil will dissolve and contaminate surface and groundwater. It eventually flow into waterways, causing algal bloom and subsequent oxygen depletion of lakes and rivers. It is also a waste of valuable global resources.