Understanding Fertilizers

A fall application of fertilizer can help your lawn survive the winter and thrive in the following spring and summer. To decide which kind of fertilizer to use, it is important to understand the formulation. The formula is represented by three numbers – for instance 20-15-10. The first number represents nitrogen (N), the second number is phosphorus (P), and the third number is Potassium (K). These three elements are the most important nutrients for plant growth and development. The numbers themselves are the percentages of these elements in the fertilizer.

Nitrogen (N) – Vital for photosynthesis and leaf health. Provided by elemental nitrogen.

Phosphorus (P) – Encourages root growth and seed production. Provided by phosphorus pentoxide (P₂O₅).

Potassium (K) – Contributes to the overall quality of the plant, including size and color of leaves and flowers. Provided by potassium oxide (K₂O).

The ratio of these three nutrients is different in different fertilizers, so a ‘starter’ fertilizer will have a higher ratio of phosphorus to nitrogen and potassium (for instance 5-10-5) to promote root growth but a fertilizer for annuals will have more nitrogen and potassium (20-10-20).

For a fall lawn application, an established lawn will need a good feeding of a nitrogen-heavy fertilizer. This helps the grass recover from the stress of summer heat and store more energy for winter. Newly established grass will want a higher concentration of phosphorus to create root growth, too much nitrogen will create a lot of leafy growth, but the plant won’t have enough support to stay healthy.

Prepared by Lindsey Smith, Collections Curator