All plants need 6 key nutrients. The first thee come from the air and water: Oxygen, Carbon, and Hydrogen. The next three come from soil: Nitrogen (N), Phosporous (P), Potassium (K).
What do fertilizer numbers mean?
Fertilizers are labeled with 3 numbers representing the three soil nutrients, always in the same order: Nitrogen, Phosphorous, Potassium. The higher the number, the more that nutrient is in the fertilizer.
- Nitrogen fosters leaf and vegetation development. Choose a fertilizer with a high first number for lush, green lawn and shrub growth. Apply these in the spring.
- Phosphorous aids root development, flower, and fruit production. Choose a fertilizer with a high second number for flower beds and fruit trees. Apply these in the spring before and while plants emerge.
- Potassium, helps roots regulate water absorption and the movement of nutrients, plus disease resistance. Choose a fertilizer with a high second and third number for fall applications to winterize.
Some fertilizers have equal parts for each nutrient, represented by the same number three times. For example, you might see 10 – 10 – 10. These fertilizers are great for overall health and general application.
But, what is the number?
The number you see on fertilizer packaging is actually a percent of the fertilizer’s total composition. For example, a 24 – 0 – 11 fertilizer contains:
- 24% Nitrogen
- 0% Phosphorous
- 11% Potassium
By adding these percentages, you’ll learn that 35% of this fertilizer is nutrient; the remaining 65% is productive filler like clay or limestone that help distribute the nutrients at a safe rate, avoiding oversaturation of nutrients and what we call “burning” your lawn. Just be sure you’re applying it according to the labeled instructions.
What about Iron (Fe)?
Some fertilizers will have added callouts that they contain Iron. Plants only need a very tiny amount of Iron to be healthy, but when they aren’t getting that very tiny amount they become deficient and can’t move Oxygen as well. When they can’t move Oxygen, they can’t produce chlorophyll as well and become yellowish.
Too much lime added to soil can cause this deficiency. Using a fertilizer with Iron will quickly solve your problem. Soil test kits are helpful in diagnosing this.
A few warnings to call out:
- Read the label and follow instructions. Even with fillers, using more than instructed can burn plants.
- Newly planted perennials rarely need additional fertilizers. Don’t risk burning them the first year you plant them.
- Granular fertilizer needs to be watered into the soil after applying.
- Don’t apply fertilizer to the vegetation; keep applications contained to the soil.