Fertilizer Numbers: What They Mean and Why They Matter

Confused about fertilizer numbers? We don’t blame you! Walk into any gardening supplies store and you’ll see shelves full of a variety of fertilizers all with different ingredients, but more importantly with three numbers that can seem intimidating.

Understanding these three numbers doesn’t need to be so difficult though, and we’re here to take the guesswork out of gardening so you can have beautiful, lush green lawns and healthy plants.

This article will not only explain what these numbers are but also give you great rules of thumb so you can easily navigate through the aisles to find the right fertilizer for your backyard gardening adventures!

What Do Fertilizer Numbers Mean?

Every fertilizer you come across will have three numbers in bold. These are the N-P-K ratio of the plant and will always be in that order. This label is also known as the fertilizer grade, which is a national standard.

These three numbers correlate to the three most important plant macronutrients, namely nitrogen (N), phosphorus (P), and potassium (K). While there are 16 total nutrients that are essential for plant health, these three are the most important.

In this manner, the first number is always the amount of nitrogen (N), the second number is the amount of phosphate (P2O5), and the third number is the amount of potash (K2O). Together, they represent the primary nutrients your plant needs: nitrogen (N) – phosphorus (P) – potassium (K).

The numbers found on our All-Purpose Fertilizer, for example, are 5-5-5. This is the percentage by weight of the N, P, and K found in the fertilizer.

Breaking Down Fertilizer Numbers Further

Let’s simplify some numbers to understand the nutrient content in a bag of, say, 100lbs of an all-purpose fertilizer with an NPK ratio of 5-5-5.

This means it has 5 percent nitrogen, 5 percent phosphorus, and 5 percent potassium.

Calculating the exact pounds of nutrients is super simple then.

N: 5% of 100lbs (0.05 x 100) which is 5lbs

P: 5% of 100lbs (0.05 x 100) which is 5lbs

K: 5% of 100lbs (0.05 x 100) which is 5lbs

The rest of the fertilizer is filler, which is usually sand or granular limestone.

In this way, you can calculate the exact amount of nutrients in just about any fertilizer if you know the weight and the NPK ratio.

Why are Nitrogen (N), Phosphorus (P), and Potassium (K) So Important?

But wait, what’s so important about nitrogen, phosphorus, and potassium? We mentioned above that these are the three most important macronutrients, but let’s understand what each of these is good for:

Nitrogen (N)

Nitrogen (N) is probably the most widely recognized nutrient, known primarily for its ability to “green up” lawns. Nitrogen mainly affects vegetative growth and general health. And this is why you’ll notice that many lawn fertilizers are high in nitrogen to promote leafy growth.

Chlorophyll, the green substance in plants responsible for photosynthesis, is largely composed of nitrogen. It is also used heavily in new shoots, buds, and leaves.

Air contains about 78% nitrogen, but atmospheric nitrogen is not readily available to plants. They must absorb it through the soil.

All these nutrients come in organic and synthetic (chemical) forms. Ammonium nitrate, urea, and ammonium sulfate are readily available forms of nitrogen, and these are what are common in chemical fertilizers and leach heavily and quickly out of the soil.

Nitrogen can be applied organically in many ways as well, including blood meal, feather meal, and various liquid fertilizers such as Alaska Fish Fertilizer. Keep in mind that many organic dry fertilizers are slow-release, helping the long-term nitrogen content and building up organic matter in the soil.

Nitrogen deficiency is recognized by the yellowing of older leaves, slowing or stopping of growth. Leaves may drop sooner than expected. Excess nitrogen is recognized by extremely fast growth, resulting in long, spindly, weak shoots with dark green leaves.

Phosphorus (P)

Phosphorus (P) is important for healthy roots and is used more heavily during blooming and seed sets. Phosphorus is easily rendered unavailable to plants when the pH is slightly unbalanced. It is released in the soil through decomposing organic matter.

Phosphorus deficiency is recognized by dull green leaves and purplish stems. The plant is generally unhealthy, sometimes yellowing.

Lack of blooming with lush green foliage may also indicate a lack of phosphorus. Organic phosphorus can be found in rock phosphate, bone meal, and various liquid organic fertilizers such as Neptune’s Harvest Fish & Seaweed.

Potassium (K)

Potassium (K), sometimes known as potash, is important for general health of plants. It is key in the formation of chlorophyll and other plant compounds. Potassium is also known to help with disease resistance.

Potassium deficiency is hard to symptomize, but plants are generally sickly, with small fruit, yellowing from the older leaves upwards, and sickly blooms.

Sources of organic potassium include sul-po-mag (sulfate of potash magnesia), palm bunch ash, and liquid fertilizers such as Earth Juice Meta-K.

What Other Nutrients are Important for Plant Health?

We mentioned above that there are 16 nutrients that are essential to overall plant health and growth. We’ve already covered the three primary macronutrients, but what about the rest of them?

Of the remaining 13, plants get three from air and water and these are hydrogen, oxygen, and carbon.

We’re down to 10 now. Of those, three are considered secondary nutrients. This means that while they are important to plants, there are needed in lesser amounts than N, P, and K. These are:

• Calcium (Ca): Calcium helps plants in the formation of cell walls and cell membranes, and even plays an important role in soil structure. Signs of calcium deficiency in plants include curling leaves, discoloration, slow growth, and even different kinds of rot.
• Magnesium (Mg): Magnesium assists plants in nutrient uptake, and is even essential for phosphate metabolism. It helps give leaves their green color. When plants are deficient in magnesium, yellow starts breaking through veins and around the leaf edges. Older leaves usually suffer first and eventually die if proper care is not provided.
• Sulfur (S): Sulfur is necessary for chlorophyll formation which helps them maintain that dark green color and encourages proper growth. Plants that are deficient in sulfur are often pale green, yellowish-green, and sometimes even completely yellow. These signs show up on younger leaves first.

The rest of the nutrients that plants need are known as micronutrients, or trace minerals, since they are needed in much smaller amounts. These include iron, manganese, zinc, copper, boron, molybdenum, and chlorine.

Fertilizers may contain some or all of the trace elements, but your garden may not require more of these. Overfertilization comes with its own host of issues, so conducting a soil test is the best method for figuring out which nutrients are lacking in your soil.

Test Your Soil to Pick the Right Fertilizer

The best way to know if your garden soil has enough nutrients or not is to have it tested. The soil test results include an analysis of your current soil as well as recommendations on which fertilizer to use to correct the deficiencies your soil may have.

While home test kits are available, lab tests provide the most comprehensive and reliable results. Every state in the United States has a Cooperative Extension Service linked to its university system. A soil test usually costs between $15 and $20, and it takes about a month to get the results.

Soil continually changes, so it’s recommended to get your soil tested every 2 to three years and to keep records to test results, fertilizer applications, and any amendments you may use.

There you have it, that’s everything you need to know about fertilizer numbers and how you can use them to have the plants, crops, and gardens of your dreams.

Eric Vinje

Eric Vinje founded Planet Natural with his father Wayne in 1991, originally running it as a grasshopper bait mail-order business out of a garage.

Eric is now retired, but is still a renowned gardener known for his expertise in composting, organic gardening and pest control, utilizing pesticide-free options, such as beneficial insects.

Eric believes when you do something good for the environment, the effects will benefit generations to come.