Fertilizer Basics: What N-P-K Numbers Mean
The three numbers on a fertilizer label tell you what is inside. Once you can read them, choosing and using fertilizer is simple.
What the three numbers mean
Every fertilizer label shows three numbers, such as 10-10-10 or 5-10-5. They give the percentage by weight of the three main plant nutrients, always in the same order.
| Letter | Nutrient | Main role |
|---|---|---|
| N | Nitrogen | Leaf and stem growth, green color |
| P | Phosphorus | Roots, flowers, fruit and seeds |
| K | Potassium | Overall vigor, water regulation, disease and cold resistance |
A 10-10-10 fertilizer is called balanced because it has equal parts of each. A 3-1-2 ratio favors leaves and suits foliage houseplants. A tomato or bloom fertilizer such as 5-10-10 holds back nitrogen so the plant puts its energy into flowers and fruit.
Plants also need smaller amounts of calcium, magnesium, sulfur and trace elements such as iron. Good potting mixes, compost and complete fertilizers supply these.
Organic and synthetic fertilizers
Synthetic fertilizers are concentrated and act fast, and their nutrients are available immediately. They are easy to overapply, and they do nothing for soil structure.
Organic fertilizers such as compost, aged manure, fish emulsion, seaweed and bone meal release nutrients slowly as soil organisms break them down. They are harder to overdo, and they improve the soil over time.
Plants take up the same nutrients from either source. Many gardeners use compost as the foundation and add a targeted fertilizer only where it is needed.
Forms
- Liquid or water-soluble: mixed into the watering can. Fast and easy to control, and best for houseplants and containers.
- Granular: scattered on the soil and watered in. Suits garden beds.
- Slow-release: coated pellets that feed for three to six months. Convenient for pots and shrubs.
When to feed
- Feed while plants are actively growing, which for most is spring through late summer.
- Stop feeding houseplants in late fall and winter, when low light slows growth.
- Stop feeding outdoor perennials, shrubs and roses about six weeks before your first fall frost, so that new growth can harden.
- Do not feed a plant that is dry, newly repotted, sick or dormant. Fertilizer is not medicine.
How much
Follow the label, and when in doubt use less. For houseplants, half the recommended strength at every second or third watering works well. Water the soil before applying liquid feed, so the roots are not exposed to concentrated fertilizer.
Signs of too much fertilizer
- Brown, scorched leaf tips and edges
- A white crust on the soil or pot rim
- Lush leaves with few flowers or fruit
- Wilting although the soil is moist
Flush a potted plant by running plenty of water through the soil several times, and pause feeding for a month or two.
Test before feeding garden soil
A soil test from your local extension service or a home kit shows the soil pH and which nutrients are actually low. Many garden soils already hold enough phosphorus and potassium and need only compost and a little nitrogen.