Deciphering Fertilizer Ratios: What Do They Mean?

By: Ken Wilson

When you see the letters NPK on a fertilizer bag, it contains the three essential nutrients, nitrogen, phosphorus, and potassium, that all plants need to grow and remain healthy. Also on the bag are three numbers, in ratio to each other. That’s where it gets confusing. There seems to be an endless variety of number combinations, often in the same ratio but with different values. What do these ratios mean? 

Deciphering the ratio of the three macro-nutrients is simple enough as it is what it says. For example, a 3:2:1 fertilizer contains, by weight, 3% nitrogen, 2% phosphorus, and 1% potassium. A 6:4:2 fertilizer has the same ratio of nutrients but twice the concentration.

The combination of nitrogen, phosphorus, and potassium in different ratios and concentrations is clear once one examines what each fertilizer is designed to do. While some are specifically formulated to green up turf or grass at the beginning of spring, others may provide nutrition for the development and growth of flowers and fruit. I’ll highlight what each nutrient provides and then detail the role of various fertilizers.

Deciphering Fertilizer Ratios: What Each Means

Before delving into the many different ratios, it’s helpful to know what the role is of each of the “Big Three” elements in fertilizers:

Nitrogen (N) is the most important nutrient and is essential in protein formation. It ensures that the plants, their foliage, and their fruits are healthy as they develop and are harvested.

Phosphorus (P) is essential for the plant’s ability to photosynthesize, convert, store, and use the sun’s energy to help it grow and develop.

Potassium (K) helps to strengthen plants’ root systems and, by doing this, increases their ability to absorb nutrients, thus improving the quality of their crop and helping to resist disease.

Knowing which of these elements is most needed to create a perfectly balanced soil for your plants is the key to which fertilizer to use. That requires an analysis of a soil sample, which can best be done by a professional horticulturist.

What is 30-0-10 fertilizer used for?

This specialized fertilizer contains 30% nitrogen, no phosphorus, and 10% potassium. The remaining 60% is filler, micro-nutrients, and other ingredients. It is used as a professional quality fertilizer for turf, including golf course fairways and tees, so it’s also ideal for lawns.

To avoid burning the grass, use no more than 1 lb. per 1000 square feet, and apply it in the early morning or late afternoon to miss the heat of the sun.

30-0-4 fertilizer – what does it mean?

The ratio of nutrients in this fertilizer is designed to encourage vegetative growth with 30% nitrogen content, 0% phosphorus, and 4% potassium to encourage root growth.

Gardeners and farmers use 30-0-4 in early spring for young plants, as a foliar feed during summer, or as a late-season supplement to develop blooms and maintain the production of fruit. If you are fertilizing lawns or gardens, you should water immediately after applying the fertilizer so that it penetrates deep into the soil.

What is 13-13-13 fertilizer, and what is it used for?

This balanced all-purpose fertilizer is valuable when the soil hasn’t been tested or has no imbalance of the “Big Three” nutrients but will benefit from a good all-round boost. It contains 13% each of nitrogen, phosphorus, and potassium, as well as copper, iron, manganese, and zinc oxides.

It is often used as the initial fertilizer at the beginning of the growing season, particularly to encourage the production of ornamental flowers and fruit. It will green up the lawn, but there are better choices for this purpose.

What does 20-20-20 fertilizer mean?

The three prime nutrients are present in this general-purpose fertilizer in equal proportions, each comprising 20% of the total weight. It is similar to 13-13-13 but is 50% more concentrated. This means that less needs to be used to provide the same amount of nutrients to the soil. Conversely, it can be easily diluted down to a 10-10-10 for regular maintenance of the soil.

A triple 20 fertilizer is multi-purpose but is ideal for increasing the fertility of poor soil before planting. It promotes the growth of foliage to improve photosynthesis and also supports the development of flowers and fruit.

What is a 12-12-12 fertilizer used for?

12-12-12 is a popular all-round fertilizer for lawns, shrubs, flowers, and vegetables. Being multi-purpose, it is a cost-effective way to feed your garden. It is available in different formats, including liquid concentrates, powders, granules, and fertilizer spikes.

Experts recommend applying 12-12-12 in late fall or early spring to give the soil a boost before spring planting, and it can be applied every three to four months during the summer to maintain the nutrition levels. It shouldn’t be used on indoor plants as it may burn their roots, and it is also not the best fertilizer to maintain the greenness of lawns, as one with a higher nitrogen content will be more effective.

What is a 21-0-0 fertilizer used for?

With a high nitrogen content but no phosphorus or potassium, this is an excellent fertilizer for greening up a lawn and promoting the growth of vegetables such as lettuce, cabbage, kale, peas, carrots, and other root vegetables.

21-0-0 is generally in the form of ammonium sulfate, and as well as the nitrogen component, it also provides sulfur, which improves the quality of the soil.

What is a 18-24-12 fertilizer used for?

This specialized fertilizer is designed to promote seed and seedling development. It is recommended as a starter for new lawns and turf and to use when reseeding an older lawn. It should be applied just before applying the seeds.

18-24-12 is used to enrich phosphorus-deficient soils, and it is also effective in establishing strong root systems, which, together with the nitrogen content, makes it such an effective new lawn fertilizer.

What is a 16-16-16 fertilizer good for?

This is another of the balanced fertilizers, more concentrated than 13-13-13, but still generally used for all flowering plants as well as grass, trees, vegetables, and evergreen shrubs. As well as being all-purpose, it enriches the soil and is often used as a starter fertilizer for new lawns.

What is a 10-0-0 fertilizer?

As you can tell from the NPK ratio, this is a pure nitrogen fertilizer, and even the nitrogen content is not as high as in many others. It often combines soluble and slow-release nitrogen, making an excellent spring top dressing to encourage lush green lawn growth.

It often contains weedkillers, and the active nitrogen also inhibits the growth of weeds like dandelions and crabgrass.

What is 16-4-8 fertilizer good for?

16-4-8 is one of the best all-round products and is particularly good for lawns and evergreen shrubs. While it is not aimed at promoting root growth or the development of flowers or fruit, with relatively low quantities of phosphorus and potassium, it does provide nutrition to plant foliage.

This fertilizer can be used to boost vegetative growth all year round and is used extensively to keep turf and lawns lush and green.

When to use 28-0-3 fertilizer?

With its high nitrogen content, this one is both a very efficient weedkiller and also a provider of macronutrients for a wide range of plants, including vegetables, flowers (especially roses), and bulbs. It also benefits lawns by promoting root growth. The best time to use 28-0-3 is when weeds are a problem – spring, late summer, and early fall.

What is 10-0-10 fertilizer good for?

This product is phosphate-free, so it is a good choice for soils already rich in phosphates but lacking the other two elements, nitrogen, and potassium.

In liquid form, 10-0-10 is often applied using drip irrigation or aerial spraying. It is very effective at an agricultural level in boosting the vegetative growth of crops like hay, pasture, nuts, and fruit and row crops such as wheat, corn, maize, and cotton.

Final Thoughts

Fertilizer ratios show two things: the amounts of the three most essential macro-nutrients, nitrogen, phosphorus, and potassium, in relation to each other, and also the concentration of each. For example, a 10-10-10 fertilizer has a 1:1:1 ratio, with each nutrient being 10% of the total weight. In a 20-20-20 product, although the ratio remains the same, it contains double the amount of each nutrient. It’s that simple!

About the author 

Ken Wilson

Long time career in the home services industry from remodeling to patio construction. Currently residing to in SWFL and active contributor to multiple home & garden publications.