Fertilizer is a gardener’s best friend—but those numbers on the bag? They can be confusing. Whether you’re growing tomatoes, tulips, or a bed full of salad greens, understanding what NPK means will help you choose the right fertilizer and grow a stronger, healthier garden.
What Is NPK in Fertilizer?
NPK stands for Nitrogen (N), Phosphorus (P), and Potassium (K) which are the three macronutrients (primary nutrients) plants need.
- Nitrogen (N) promotes leafy green growth.
- Phosphorus (P) supports strong roots, flowers, and fruit.
- Potassium (K) improves root development, plant health, and stress resistance.
Every fertilizer label lists these nutrients as a three-number ratio—like 3-6-4 or 4-4-4. These numbers represent the percentage of each nutrient in the mix. They are always stated with Nitrogen first, Phosphorus second, and Potassium third.
How to Read a Fertilizer Label
Using Burpee Tomato + Vegetable Fertilizer 3-6-4 as an example, you can see that the NPK is listed as:
- 3% Nitrogen
- 6% Phosphorus
- 4% Potassium
This higher phosphorus content makes it ideal for vegetables and fruiting plants. It helps develop strong roots and promotes flower and fruit production.
What is a Complete Fertilizer?
A complete fertilizer is a plant food that contains all three macronutrients. Most of Burpee’s fertilizers are complete. One great example is Burpee Rose + Bloom Fertilizer 4-6-4. It is specially formulated for roses and flowering plants. The higher middle number (phosphorus) helps promote vibrant, long-lasting blooms, while nitrogen and potassium support healthy foliage and strong roots.
What is a Balanced Fertilizer?
You may also hear people talk about a “balanced” fertilizer, such as Burpee All Purpose Fertilizer 4-4-4. This type of fertilizer contains equal amounts of the three macronutrients. We recommend this fertilizer for salad greens like Lettuce and Spinach or a general fertilizer at the beginning of the season. But here’s the key: balanced doesn't always mean best. While a 4-4-4 fertilizer delivers even nutrition, some plants need more of one nutrient than others.
What is an Incomplete Fertilizer?
An incomplete fertilizer has zeros in one or more parts of the NPK ratio. These targeted formulas can support specific plant needs.
For example, Burpee Bone Meal is 6-8-0. Bone meal is a slow release source of Nitrogen and Phosphorus that you should apply in Autumn when planting your flower bulbs such as Tulips and Daffodils. Nitrogen and Phosphorus work together to keep the bulb itself robust while producing beautiful blooms. Burpee Bone Meal also contains 7% calcium, a key micronutrient that strengthens cell walls, supports root growth, and helps plants absorb nutrients.
Another incomplete fertilizer is Burpee Blood Meal 12-0-0. Blood Meal is the highest nitrogen source in organic fertilizers.
- Perfect for side dressing Corn which requires a lot of nitrogen
- Cabbage (try fan favorite ‘Saku Saku’) and brussels sprouts are also nitrogen hungry early in their season
- Leafy greens like Kale, Swiss Chard, and Collard Greens
Final Takeaway
NPK may sound like alphabet soup, but once you understand what those numbers mean, it’s easy to feed your garden with confidence. Explore our organic fertilizer collection and find exactly what your garden needs to thrive.