Understanding Fertilizer

The first step is to get a handle on those three mysterious numbers on all fertilizer bags. You have noticed them haven’t you? You may be familiar with 5-10-5; the most common vegetable food. By law, the numbers represent the percentage, BY WEIGHT of nitrogen (N), phosphorus (P) and potassium (K) respectively.

“WAIT A MINUTE” you say, “if the 5 the 10 and the 5 are percentages, they only add up to 20% of the bag.” I gotta tell you, you’re really on the ball. Guess what, the rest of the bag is filler! The filler is in there to “make the product easier to spread evenly without burning” as the manufacturers will tell you. This is all true but, the ability to adjust the amount of filler vs. nutrients is a device fertilizer producers can manipulate to make a lower quality appear better than it is but, more about that later.


Anyway, the first number, N, or nitrogen shows how much of the fertilizer is devoted to green, upward growth. The second number P, or phosphorus, shows how much is devoted to root growth and flower & fruit production. The last number K, or potassium is an overall building block that benefits all parts of the plant. The easiest way that I’ve found to remember this relationship is: UP-DOWN- ALL AROUND. As illustrated above. This is an easy way to remember the relationship between the three basic plant nutrients. N-P-K. N is Nitrogen for leaf, needle, and blade growth. P is Phosphorus for root growth and flower and fruit production. And last, K is Potash, an important building block for all parts of the plant. Now that you know this you can walk around the garden center and impress your friends by saying things like, “No, no. You don’t want 10-6-4 for you perennial border. It has too much nitrogen. You need more phosphorus for roots and flowers. “ or, “Say Jason, your lawn is kind of sparse. Maybe you need a lawn food with more phosphorus to help it thicken up.”

While you’re there showing off, you’ll probably notice that each manufacturer has their own “secret formula” that they would like you to believe is better than all the rest. Since these products represent a multi-billion dollar business, you better believe that a lot of time and money has been spent to convince you that their product is better than the rest. Your ignorance of fertilizer products is crucial to their success.

Pretty packages and celebrity endorsements are all designed to keep you from asking two key questions.

First, WHAT IS THE FORMULA; 5-10-5,10-6-4, 12-0-0? This I’ve already explained. Remember, these three numbers must, by law, appear on the bag. Often a manufacturer will brag about their formula. What you’ll never hear mentioned is how much their bag weighs. This information is also required to appear on the product but, there are no rules that say how large it must appear. Until you know how much the bag weighs, you can’t possibly know what you’re buying. Remember, the three numbers represent the percentage, BY WEIGHT, of the three major nutrients. This means that a higher number on one fertilizer doesn’t necessarily mean that it has more of a nutrient than another product. In other words, the percentages are meaningless until you know what they are a percentage of. If someone asked you if you’d rather have 10% or 50%, you would naturally ask, “Of what?” 50% of $1 is certainly less than 10% of $10.

So remember, when you’re looking at those three mysterious numbers, the real mystery is, “Where the heck did they hide the weight on this bag?” Until you find out, you have no way to figure out how much of the bag is filler and how much is actual nutrient. % X weight = what you are getting.

While the E.P.A regulates WHAT may be put into these products, there are few rules for HOW CLEARLY the manufacturer explains this information. It’s all tre on the package but it’s up to you to figure it out. The best thing to do is to get to know someone at your local garden center who does ”get it” and have them assist and educate you.

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