If you’ve ever stood frozen in the fertilizer aisle in an overwhelmed brain fog, you’re in familiar territory. Although you can always grab the closest bag with the best marketing and head for the cash register (I’ve been there, friend), I prefer the scenario where you walk confidently down that aisle of fertilizer knowing exactly what N, P, and K represent and why different ratios provide specific benefits to your plants.
By the end of this post, you’ll be able to do just that.
Decoding NPK Fertilizer Ratios
The three letters stand for the three macronutrients that your plants need to thrive: nitrogen (N), phosphorus (P), and potassium (K). Each of these elements impact your plant’s overall health by targeting certain aspects of the plant’s development (which we’ll get to in a moment). Because N, P, and K specialize in different aspects of healthy growth, commercial fertilizers offer various ratios of these macronutrients so you can give your plant precisely the nutrition boost it needs.
You’ll typically see ratios like 13-13-13 or 19-19-19 written across a bag of fertilizer. This simply indicates that N, P, and K are equally represented within the fertilizer mix. These fertilizers offer a well balanced boost to your vegetable garden and kitchen herbs. However, if you’re looking for a more specialized fertilizer for vegetables or flowers, decoding the NPK fertilizer ratio is your ticket to garden success.
How to Choose NPK fertilizer
Knowing exactly how NPK benefits your plants gives you the agency to make informed and confident decisions in applying fertilizer to your garden. While there’s a lot of crossover in their benefits to the garden, each element specializes in a particular aspect of healthy plant development. Let’s break it down.
Nitrogen is generally considered the most important of the major nutrients because it provides the plant with the energy necessary to grow. While it is typically found in native soil, nitrogen levels are often depleted as plants “feed” during the growing season, thus the need for supplementing via fertilizer. Remarkably, there are certain plants that alternatively “fix” nitrogen back into the soil. Instead of absorbing nitrogen, several varieties of beans actually replace it in the soil as they grow.
Why it benefits plants
Nitrogen specializes in boosting rich green leaves in your plants. While it also serves as a catalyst for other plant functions, its essential role in photosynthesis and energy production is its primary purpose in fertilizers.
Since the sole purpose of lawn fertilizer is to deliver the lush, green yard of your dreams, nitrogen is heavily prioritized in their mixes. Blood meal (12-0-0) and lawn fertilizers (up to 46-0-0) offer dramatically emphasized nitrogen compared to phosphorus and potassium.
While nitrogen boosts your green leaves, phosphorus (P) specializes in strong root development. Phosphorus strengthens the plant’s ability to absorb available soil nutrients by creating a healthy and complex root system. Phosphorus also plays a crucial role in ensuring plants mature on time, which is particularly important for fruiting plants like tomatoes.
Why it benefits plants
In addition to ensuring strong roots and appropriate rate of growth, phosphorus boosts your plant’s ability to fight disease and stimulates sustainable and healthy overall development. (More on that here…)
Bone meal is an excellent organic fertilizer that emphasizes phosphorus in its NPK ratio of 6-8-0.
Potassium (K) is often associated with fruiting and flowering plants. Although potassium may not have a direct role in producing flowers, potassium-deficient soil often results in reduced quality of the predetermined amount of flowers produced. (See why here.)
Potassium assists in the movement of water and sugars inside of the plants, and this is thought to improve the quality of flowers and (in the case of fruiting crops like tomatoes and strawberries) create juicier and sweeter fruits.
Why it benefits plants
In addition to its association with healthy flowers and fruits, potassium also thickens the cell walls of plants. This has the double benefit of strengthening the plant’s physical structure and also defending it against disease.
A great illustration of a potassium-rich fertilizer is the 4-18-38 Masterblend fertilizer marketed for tomatoes and vegetables. Because it is high in potassium (K) and also high in phosphorus (P), we know that this particular fertilizer is designed to target root and fruiting elements, while also including nitrogen (N) to create a balanced fertilizer to promote overall development.
The Best NPK Ratio
Obviously, we want all the benefits nitrogen, phosphorus, and potassium have to offer our plants! The trick here is knowing what your plants want to meet their nutritional needs. Although a well-balanced NPK fertilizer is usually all you need for healthy plant development, here are some of the best organic fertilizers to address specific deficiencies.
Nitrogen-rich Organic fertilizers
To use, simply broadcast (garden-speak for “toss a handful here and there”) the alfalfa meal on the garden soil you wish to fertilize and mix it in. This can be done before, after, or during the growing season. You can easily grab a bag online, and it is often sold in bulk (and reasonably priced) at farm and animal feed stores.
Although compost is technically more of a soil amendment than a fertilizer, it does offer nutritional benefits as well!
My go-to organic fertilizer is fish emulsion. Fair warning: this stuff has a strong odor. That said, its ability to boost garden production is just as strong! When it comes to leafy greens and newly established transplants, a healthy dose of fish emulsion delivers a massive payout in terms of overall plant development. And get this: it’s cheap, and a single bottle can easily get you through a full growing season. Here’s the one you’ll want to get.
Blood meal works anywhere you need a specialized boost of nitrogen, but it really shines if you’ve just created a new raised bed or container garden. The optimal time to prepare your garden for its next growing season is in the fall, allowing it to overwinter before planting in the spring. But if this isn’t possible, grab this impressive soil amendment for a quickly-accessible source of nitrogen to jumpstart your growing season!
Phosphorus-rich organic fertilizers
Tomatoes and other flowering vegetable plants are “heavy feeders”, requiring high levels of nutrition from their soil. Bone meal targets the plant’s root system and overall health, creating a resilient plant that is able to focus on fruit production rather than fighting disease. I started using this bone meal to fertilize my tomato plants a few years ago and wonder why I didn’t use it sooner!
Dr. Earth Organic 5 Tomato, Vegetable & Herb Fertilizer
I’ve also used Dr. Earth’s Organic Tomato and Herb fertilizer with great success! Since it also emphasizes phosphorus in its mix, it focuses on healthy root expansion and plays a crucial role in ensuring plants mature on schedule. While I’m a huge fan of fish emulsion, using too much nitrogen-rich fertilizer can result in beautifully lush, green plants at the expense of developing flowers, meaning you’ll have luscious green tomato plants without any fruit on them!
Potassium-rich organic fertilizers
If you’re lucky enough to live near the coast, this free resource is an incredibly beneficial organic fertilizer for your vegetable garden! Simply cover your garden bed as you “tuck it in” for the winter season, or create a fertilizer tea by steeping it in water to create a liquid fertilizer for your plants.
Although it’s already been mentioned as a nitrogen-rich fertilizer, alfalfa meal also ranks pretty highly in potassium.
While this is admittedly a surface-level breakdown of how NPK ratios of nitrogen, phosphorus, and potassium impact your garden, it’s a great starting point for determining which ratio of NPK fertilizer best suits your specific needs in the garden. A balanced NPK ratio is often the only fertilizer you’ll need, but knowing how each element supports the growth of your garden gives you the agency to make your own decisions as to what your garden needs. Knowledge truly is power, and you’ve just been given the keys to the garden.