When you buy a package of fertilizers, you’ll often see a group of three numbers on the label. If you find those numbers confusing, you’re not alone. Many people, including seasoned gardeners who’ve been using fertilizers for years, don’t know much about what’s in the fertilizer package. But that’s about to change.
Image courtesy of Pat Dalton, Flickr, CC License
Contrary to what some people think, the numbers on the label actually mean something and they can help you choose the right fertilizer for your plants. This article uncovers the mystery of the fertilizer numbers, sheds light on the famous NPK and tells you how to get the most out of your fertilizers.
What is NPK?
If you browse online gardening forums, you might come across a lot of jargon and acronyms such as NPK.
What does NPK stand for? It refers to the three main ingredients in any fertilizer.
- N stands for nitrogen, an important component that plants such as spinach and lettuce need to have a healthy and rich foliage.
- P for phosphorus which is needed for strong roots (carrots, beets, onions), blossoming flowers (petunias), and fruit production (tomatoes).
- K for potassium which improves the plant’s resistance to diseases and speeds up its metabolism.
The next time you stand in the gardening aisle and face an endless row of shelves stacked with fertilizers, take a look at the bolded numbers on the package. They’re there for a reason. So what do they mean?
The numbers usually refer to the amount of nutrients in each package. You can call it the ratio of nitrogen, phosphorus, and potassium in the package. So a label that has 10-10-10 would mean that this fertilizer has a balanced quantity of the primary nutrients. It contains 10 percent of nitrogen, phosphorus, and potassium respectively.
While specialty fertilizers, especially those with high nitrogen content such as 12-6-6 are good for certain perennials and evergreens, a balanced one is a safe bet since it is versatile and works just about with all types of plants you grow in your garden.
Another risk with specialty fertilizers is that they might burn the plants. It’s easy to confuse one fertilizer that’s high in phosphorus (usually the second number) with one that’s high in potassium (the third number). This is why many gardeners just prefer a 20-20-20 balanced package of fertilizer. Then it’s a matter of using the right dose as we’ll see later.
How to Calculate Nutrient Content in Fertilizers
Measuring and weighing your fertilizer is an involved process. It’s not as straightforward or easy as, say, weighing a sack of potatoes or finding out how much sugar is in a bottle of juice. So let’s delve into that and simplify this process for you.
The NPK numbers you have on the label are usually percentages not actual weights. If you want to know exactly how much nitrogen is in your package, you’ll have to do a little bit of calculating.
Let’s assume that the label has 10-10-10. This is a balanced package of fertilizer. To find out the exact weight of nitrogen for example, we multiply the first number times the weight of the package and divide by 100. Here’s the formula:
Package weight x ration / 100 = NPK weight in pounds
So a 50-pound package of fertilizer with a 10-10-10 label will have:
50 x 10 / 100 = 5 pounds of nitrogen.
If you’re wondering about the remaining weight. It’s usually filler material. Most often it would sand or pebbles of limestone. It’s the kind of material that doesn’t impact your soil one way or the other. It doesn’t change its chemical structure nor affect its water retention.
The same formula applies to specialty fertilizers with more custom ingredients. Take the NPK ratio of 7-0-20 for example. If the package of fertilizer is 100 pounds, then you have the following active ingredients:
- Nitrogen: Multiply the weight by 7 and divide by 100, so the package has 7 pounds of nitrogen.
- Phosphorus: you don’t have any phosphorus in this fertilizer.
- Potassium: Multiply by 20 and divide by 100 giving you 20 pounds of potassium.
Your 100 pounds bag of fertilizer labelled 7-0-20 has only 27 pounds of nutrients. The remaining 73 pounds are filler materials. This particular bag of fertilizer is good for a garden plagued with pests and diseases, but it will not help a blooming plant and won’t make your fruit plants give you a good produce.
Organic Fertilizer NPK vs Synthetic Fertilizer NPK
So far we have been talking about synthetic fertilizers. These are easy to measure and calculate the exact amount of each major nutrient in it. You might also have noticed the high numbers on the label. Synthetic fertilizers are processed and manufactured with each ingredient added to create either a balanced fertilizer or a specialty one.
But when it comes to organic fertilizers, it’s a different ball game. For one thing, organic fertilizers don’t have the ingredients immediately available. The raw materials are there, but it’s up to the microbes in the soil to process these materials and turn them into fertilizers. This is why the NPK numbers on bags of organic fertilizers are usually smaller and not as exact as is the case with their synthetic counterparts.
So when you look at the label on the organic fertilizer package and see numbers like 1-2-5 or 3-1-6, don’t let that discourage you. Even though these percentages are low, your garden might get more primary nutrients out of them than from synthetic fertilizers with higher ratios.
There are two reasons for this. The first is that plants absorb organic nutrients at a much higher rate. In some cases, your perennial plant might get as much as 90 percent of the nitrogen in an organic fertilizer compared to between 40 to 60 percent of the same ingredient in synthetic ones.
The second reason is the release period of the nutrients. Chemical fertilizers are usually available to the plants as soons as you sprinkle them and water the soil. They can stay for up to 6 weeks. But organic fertilizers need time and the active participation of microbes to create a slow-release nutrient that can last for up to 15 weeks in the soil.
So while a high NPK on a synthetic fertilizer bag might seem like a good bargain, the fact is, not all of those nutrients will end up making their way inside the plant’s roots. Not to mention that you’ll need to spread the fertilizer more than once to give the plants the amounts of nutrients than they need.
What’s in your Organic Fertilizer Package?
We already mentioned that with synthetic fertilizers, the amount of active ingredients often makes up only a small portion of the whole package. But what about a bag of organic fertilizer?
You don’t get as much filler or inert materials in a package of organic fertilizer as you would with the synthetic one. In most cases, you get plenty of useful materials that help your soil in more ways than one. Here are the most common ingredients in your organic fertilizer package:
- Manure Ingredients: Needless to say that manure is the backbone of your organic fertilizer. Different organic fertilizer products have various types of manure. The most common ones are poultry manure, cricket droppings, processed cow manure, worm castings, and even bat guano. The ratio of each ingredient impacts the usability and benefits of the fertilizer for specific types of soils and plants.
- Plant Ingredients: Plant products also contribute to the composition of the organic fertilizer. You might have kelp meal, cottonseed meal, as well as alfalfa meal in the package.
- Animal Ingredients: It is common to find feather powder, fishbone powder, ground dried crab and blood products in the mix. It is high in nutrition and is often used to feed the good bacteria in the soil that will turn your fertilizer into readily-available nutrients for the plants.
- Minerals: No organic fertilizer will be complete without a dash of phosphate, ground limestone, sulfate of potash, and greensand to name but a few. They play a major role in enriching the soil long after the organic materials have been used up.
While fertilizers both synthetic and organic are often a necessary part of your gardening, you need to make sure that the fertilizers you’re sprinkling around the roots of your plants are actually needed. Having your soil tested helps you determine the types and quantities of nutrients your garden requires.