What Do Fertilizer Numbers Mean? How To Choose The Right NPK Ratio

Emma D.

1 hour ago

3 min read

The three fertilizer numbers show the percentage by weight of nitrogen, phosphate, and potash in the product, always listed in that order. For example, a 10-5-5 fertilizer contains 10% nitrogen, 5% phosphate, and 5% potash. These numbers help gardeners compare products and choose a fertilizer that matches the needs of their soil and plants. The “right” fertilizer NPK ratio depends on the plant, soil fertility, and existing nutrient levels. For Colorado gardeners, it’s important to note that phosphorus and potassium are often already adequate in established soils, making soil testing especially useful before adding additional nutrients. Let’s explore how to understand the NPK ratios and how to choose the best fertilizer for your garden or landscape.

What Do The Three Numbers On Fertilizer Mean?

All fertilizer labels use the N-P-K sequence:

First number = nitrogen
Second number = phosphate
Third number = potash

Technically, fertilizer labels report nitrogen as N and phosphorus and potassium as their oxide equivalents:

Nitrogen as N
Phosphorus as phosphate (P2O5) - P
Potassium as potash (K2O) - K

Here are some examples of how to read fertilizer labels:

10-10-10 = equal percentages of all three nutrients
20-10-5 = more nitrogen than phosphorus or potassium
0-10-10 = no nitrogen

These numbers represent a percentage by weight, not ratios alone. For example, a 20-15-5 fertilizer contains 20% nitrogen, 15% phosphate, and 5% potash. Fertilizer ratio is different from fertilizer grade:

15-10-5 has a 3-2-1 ratio.
8-12-4 has a 2-3-1 ratio.

What Do Nitrogen, Phosphorus, And Potassium Do For Plants?

N, P, and K are macronutrients plants use in relatively large amounts.

Nitrogen (N)
- Supports leafy, vegetative growth.
- Important for chlorophyll.
- Most commonly supplemented in gardens.
Phosphorus (P)
- Supports root development, flowering, seed formation, and energy transfer.
- In Colorado, many established soils already contain adequate phosphorus.
Potassium (K)
- Helps regulate water movement, enzyme activity, stress tolerance, and general plant vigor.
- Colorado soils often have naturally adequate potassium levels or higher.

When it comes to fertilizer, the adage, “More is not necessarily better,” applies. Too much of any of these nutrients can increase soil salts or interfere with the availability of other nutrients.

What Is A Balanced NPK Fertilizer?

A balanced fertilizer is one with equal or similar N-P-K numbers. For example:

10-10-10
5-5-5
20-20-20

A balanced fertilizer is not automatically the best choice for every plant or soil. Soil may already contain plenty of phosphorus and potassium, especially in Colorado. Repeated use of balanced fertilizer can add nutrients that are not actually needed. Test your soil before applying fertilizer, rather than automatically choosing a balanced product. Colorado State University (CSU) recommends avoiding heavy applications of phosphate and potash when unnecessary, as they can contribute to soil salt buildup.

How Do You Choose The Right Fertilizer Ratio For Different Plants?

Start with each plant’s individual needs and a soil test, not the biggest NPK numbers. Separate soil tests for your lawns, ornamental beds, and vegetable gardens are recommended. Soil test results can help identify how much existing phosphorus and potassium are in your soil, the soil’s pH, levels of organic matter, and any nutrient deficiencies.

Generally speaking, leafy plants and lawns often need more nitrogen, while flowering or fruiting plants may benefit from fertilizer tailored to the crop, but do not assume high phosphorus is necessary without evidence. Vegetable garden fertilizer should reflect soil test results and organic matter levels. Container plants need regular nutrient replacement because nutrients can leach from the potting mix.

For Colorado gardens, nitrogen is commonly the nutrient most likely to need supplementation. Phosphorus and potassium are often already adequate in established soils. Be sure to read and follow the fertilizer label exactly once you’ve chosen one; overfertilizing, even with the “right” fertilizer, can damage roots and increase salt levels.

Shop Plant And Garden Fertilizers At Bath Garden Center

One of the benefits of shopping at Bath Garden Center & Nursery is access to expert staff. Bring all of your fertilizer questions to us, and we’ll be happy to help you. For best results, bring your soil test results, the names of the plants you want to fertilize, photos of the plant or garden, and the type of fertilizer you’re currently using. You can also fill out a contact form if any questions arise!

Whether you need help with a soil test or with understanding your results, our friendly staff can answer all your questions about how to fertilize houseplants, vegetables, flowers, lawns, trees, shrubs, and container gardens. We can help you find the right fertilizer and other soil amendments, such as compost, to grow your garden successfully.