Fertilizer Labels: What N-P-K Numbers Mean
Beautiful lawns and gardens start with healthy, well-nourished plants. From grasses to flowering shrubs, every plant's need is different, but some basics apply across the board. Choosing the right fertilizers for your lawn and garden doesn't have to be difficult. Armed with fertilizer basics, making the right choice is as simple as 1-2-3:
- What The Numbers Mean
- Why These Three Nutrients?
- How N-P-K Affects Plants
- When To Use Different Ratios
What the Numbers Mean
All fertilizer products must provide some uniform information to help consumers compare products easily. Every label carries three conspicuous numbers, usually right above or below the product name. These three numbers form what is called the fertilizer's N-P-K ratio — the proportion of three plant nutrients in order: nitrogen (N), phosphorus (P) and potassium (K).
The product's N-P-K numbers reflect each nutrient's percentage by weight. For example, the label on a 12-pound bag of Pennington Ultragreen Lawn Fertilizer 30-0-4 reveals that it contains 30 percent nitrogen, 0 percent phosphorus and 4 percent potassium, in addition to its other beneficial ingredients. This comes in handy when soil test results recommend pounds of actual nitrogen rather than specific fertilizers. By multiplying 12 pounds by .30, you'll see that this 12-pound bag contains slightly more than 3.6 pounds of actual nitrogen.
Why These Three?
Plants need larger quantities of nitrogen, phosphorus and potassium than any other plant nutrients. Often called primary macronutrients, these three are among the 17 nutrients essential to all plant life. Other elements, such as iron, are just as critical to plant survival, but plants need them in much smaller quantities. Additionally, different plants have different needs. Azaleas, for example, need more iron than many other plants, so specialty fertilizers, such as Pennington UltraGreen Azalea, Camellia & Rhododendron Plant Food 10-8-6 is formulated with iron, magnesium & 3 chelated essential plant nutrients for plant vigor & beautiful blooms. This amount of iron is shown in the ingredient panel of the product label, but not in the N-P-K ratio.
Plants derive all their essential nutrients naturally from air and soil, but soil can get depleted, especially in thriving lawns and gardens. Because plants use larger amounts of N-P-K, these nutrients need frequent replenishing. Other factors enter in, too. Nitrogen moves freely in soil, so water from heavy rains or irrigation steals nitrogen away in a process called leaching. Fertilizer products step in and replenish used and lost nutrients so that lawns and gardens stay nourished.
How N-P-K Affects Plants
Each of the three nutrients in the N-P-K has its own special job to do. Nitrogen (N) spurs vigorous, leafy growth and rich green color in lawn grasses and other plants. Without enough nitrogen, growth slows and lawns and plants turn pale. With too much nitrogen, flowering and fruit-bearing plants put their efforts into green growth and forfeit their blooms and fruit.
Phosphorus (P) focuses energy on strong root development and flowers, fruits and seeds, while also helping plants use other nutrients efficiently. A shortage of phosphorus leaves roots weak and flowers and fruit lacking. An increasing number of states and counties restrict the application of phosphorus to lawns due to the environmental concerns of phosphorus runoff into waterways. In these states or counties, homeowners can usually apply a lawn starter fertilizer that is high in phosphorus, such as Pennington UltraGreen Starter Fertilizer 22-23-4 if a soil test indicates a nutrient deficiency. Check with your local county extension agent on any nutrient application restrictions.
Potassium (K) enhances overall growth. It helps regulate root and top growth and keeps plants healthy and balanced. This affects all aspects of lawn and garden well-being, from cold and drought tolerance to disease and pest resistance.
When to Use Different Ratios
Beautiful turf needs the vigorous growth and deep color plentiful nitrogen supplies, so lawn fertilizers have an N-P-K ratio with the first number much higher than the other two. Soil usually provides enough phosphorus and most of the potassium healthy grass needs, so the numbers for those nutrients are low.
Tasty tomatoes and other vegetables need nitrogen, too, but flowering and prolific harvests require more phosphorus and potassium. That's why the second and third numbers are highest in products such as Pennington UltraGreen All Purpose Plant Food 10-10-10.
Flowering bulbs depend on phosphorus for healthy roots, so bulb fertilizers, including Lilly Miller Bone Meal 6-12-0, have a high middle number. Versatile fertilizers, such as Lilly Miller All Purpose Planting and Growing Food 10-10-10, deliver a balanced N-P-K designed for all-round garden health.
By understanding the basics behind fertilizer labels, you can choose the right products for your lawn and garden goals with confidence. Pennington's portfolio of lawn and garden fertilizers can help your lawn and garden reach its full potential for beauty and enjoyment.
1. Jolene Hansen, "How to Identify and Correct Plant Nutrient Deficiencies," Earth Juice, May 2021