If you have ever been confused about the numbers on fertilizers, you are not alone. 8-2-12, 17-7-3, 10-10-10… what do they even mean? Whats the difference? Some plant care guides or articles say to use a specific set of numbers, but then you can’t find it. Other times you have multiple sources telling you different things, then who is right?
Here is a quick guide to help explain the basics and clear up some of the confusion, so you can make a reasonable, informed choice.
What do the Numbers Mean
The three numbers represent the three most important plant nutrients (macronutrients) and how much of them is in each fertilizer. The first number is Nitrogen (N), the second is Phosphorus (P), and the third is Potassium (K). This is also called the N-P-K ratio.
The numbers are always in this order, and they say what percent of each element is in a bag of fertilizer. So a bag of 10-6-4 would be 10% Nitrogen, 6% Phosphorus, and 4% Potassium. That’s 20%. The other 80% will be some combination of other elements the plant needs in smaller amounts (micronutrients), and carrier/filler that makes the fertilizer distribute more evenly.
The main elements do a variety of things for a plant, but in a nutshell, the main purpose of each is:
Nitrogen: Encourages new shoots and leafy growth.
Phosphorus: Helps with root growth and better flowering/fruiting.
Potassium: Promotes overall plant health, vigor, and disease resistance.
You don’t always need the same numbers
Just like people, a well-balanced diet will result in a generally healthy individual. And just like people sometimes use special diets, some plants do better with more or less of certain elements at various parts of the year. You may eat more protein during times when you are trying to build muscle. The same way, you may feed your plant more Nitrogen when it is sparse and needs to fill in. You can change fertilizers depending on what your plant needs at that time.
Location, Location, Location
That article doesn’t know where you live
It is important to know that soils can be very different from state to state, county to county, and even yard to yard. When a book, article, blog, or video recommends a particular fertilizer ratio, consider that a place to start, and not a non-negotiable fact. That source may live in an area with very different nutrient deficiencies and surpluses than you. For example, many Florida soils are already abundantly high in Phosphorus.
Whenever possible, use soil tests and find local sources of knowledge for the most accurate information about your specific fertilizer needs.
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