Understanding the NPK Ratio on Fertilizer Packaging
It can be overwhelming to walk into a garden shop and find the shelves stacked with package upon package of different fertilizers, bagged soils, compost, and other soil additives. You might find dozens, maybe even hundreds of varieties for what may seem like bafflingly specific situations. Is it really that complicated to feed your plants? Do you really need a different fertilizer for your grass than you do for those hostas by the driveway? Is this all a marketing ploy to make you buy twenty products or is there really something to it?
The answer is, frustratingly: yes and no. Plants are living things, just like people and animals, and just like you, they need a lot of different nutrients to survive and thrive. That being said, there are probably more additive products on the market than is strictly necessary. Some products, certainly, are more convenient to use than others, some are cheaper, and some are easier to store, but the overwhelming majority of fertilizers rely on three basic things to work: nitrogen, potassium, and phosphorous, abbreviated N, P, and K, respectively.
Soil Macronutrients: Nitrogen, Potassium, and Phosphorous
Nitrogen (N), potassium (P), and phosphorous (that one’s K, don’t worry about why) are the things plants consume the most, and because of that, they’re the things your soil is most likely short on. For that reason, they’re called “macronutrients” in this context. Most of the fertilizers you see in the garden aisle will have an NPK value marked on them somewhere, something like 15-15-15 or 10-8-10. Those numbers tell you how much nitrogen, phosphorous, and potassium is in that specific fertilizer, and they’ll always be in that order: N, P, K. If you don’t see an NPK ratio marked on the bag, the internet makes it easy to look up the ratio for any product you might see in a store.
If you’re a beginner gardener or you want your lawn to look nicer and you get as far as knowing what NPK ratios your plants need, you’re already ahead of the game. Check what type of grass you have on your lawn or what species of plant you have in your garden and use a search engine to find what their preferred NPK ratio is. Then, do a soil test to figure out what the current nutrient ratio is in the surrounding dirt. If the ratio doesn’t match what your plant prefers, you just need to buy the right fertilizer to compensate. For example, tomatoes like an NPK ratio of 8-32-16. They need lots of phosphorous and potassium to turn into sugars and acids. If you find that your soil is perhaps short on potassium, you’ll just need to find a high-potassium fertilizer to balance things out.
Does the source of the nutrients matter?
You’ll see a lot of fertilizers advertised based on what they’re made of. You might see kelp fertilizer, bat guano, blood meal, fish meal, granite dust, and many more varieties. Fundamentally, all these specialty substances are just different sources of the same three macronutrients: nitrogen, potassium, and phosphorous, but they will tend to have a very high concentration of one and not the others. These types of products are what you might buy to fine-tune the nutrient ratio in your soil. Bat guano, for example, is very high in nitrogen, but not a particularly good source of phosphorous or potassium. Fertilizers advertised as “multi-purpose” tend to have a nearly identical NPK ratio (such as 15-15-15) and will raise the content of all three nutrients.
What about Micronutrients and Soil pH?
It’s true that your soil’s pH and its macronutrient content can also have an impact on your plants, but managing those factors is a little different, so we will be discussing them in another blog post. Even if these are issues of concern for you, understanding the broad categories of soil fertilizer that are available and how the NPK ratio impacts your lawn or garden first will set you on the right track.
If you need to test the levels of nitrogen, potassium, and phosphorous currently in your soil so you know what fertilizer to buy, Element Certified’s Basic Soil Test Kit will give you that information, plus your soil’s pH, electrical conductivity, and sulfur content. Remember, soil can be very different in different areas of your property, so you may need to purchase multiple kits.