Fertilizer Numbers

By Jeff Rugg

October 9, 2019 5 min read

Q: A while back, you wrote that a person could mix two fertilizers and that "each active ingredient in one of the fertilizer packages is also an inactive ingredient to the other fertilizer." Does this mean that if I mix two bottles of nitrogen, one bottle will be inactive?

And you also say, "In your case, an equal amount of the 32-10-20 added to the 15-30-15 becomes a final fertilizer of 23.5-20-17.5." I have seen other mixing examples and mostly the final result/ratio is smaller than the one with which you start. So I wondered, does it make sense to mix fertilizers?

A: To answer your first question, the active ingredients all stay active when fertilizers are combined, but the percentages don't add up the way many people think they will. Some people want to combine two fertilizers to get a super fertilizer, but it just doesn't work that way.

To answer your second question, the fertilizer ratios need to be averaged together depending on how much of each fertilizer is used. If the same amount of each fertilizer is used, then the final ratio is divided in half. If different amounts of each fertilizer are used, then it is much harder to determine the final ratio.

In fertilizers, there are active ingredients that help the plant grow. Each nutrient is listed on the package by percentage of weight. The main three nutrients are always listed in the same order of N (nitrogen), P (phosphorus) and K (potassium). A 100-pound bag of fertilizer that is labeled as 10-15-20 would have 10 pounds of nitrogen, 15 pounds of phosphorus and 20 pounds of potassium. 10+15+20 is 45 pounds of active ingredients. That leaves 55 pounds of inactive ingredients.

Inactive ingredients added to fertilizer include products that keep the fertilizer from drying out or make it easier to use. Some inactive ingredients are coatings to make the fertilizer slow release. Some inactive ingredients are weed killers, or other chemicals that are beneficial, but are not fertilizer. Sand, sawdust, clean or sterile dirt, peat moss, sphagnum moss, ground corn cobs and other products are common inactive ingredients. Inactive ingredients might affect the soil's texture or its ability to hold moisture.

If we have two leftover fertilizer packages and we want to combine them, what will the final fertilizer composition be? For example, let us start with two bags that are both 10 pounds. One is 10-15-20 and the other is 20-20-20. The first bag has 1 pound of nitrogen, 1.5 pounds of phosphorus and 2 pounds of potassium. The second bag has 2 pounds each of nitrogen, phosphorus and potassium. So, if we add the active ingredients, we get 3 pounds nitrogen, 3.5 pounds phosphorus and 4 pounds potassium.

There are 3 pounds of nitrogen in the 20-pound total. So, 3 divided by 20 is 15%. There are 3.5 pounds of phosphorous in the 20 pounds, so 3.5 divided by 20 is 17.5%. There are 4 pounds of potassium in the 20 pounds, so 4 divided by 20 is 20%. The new fertilizer is a 15-17.5-20 — not a 30-35-40 that some people expect.

Some people want to mix fertilizers to get a better one. They think that if they can combine three packages of 20-20-20, they can get a super fertilizer of 60-60-60. They want to add up all the active ingredient percentages, but they forget to add up all the inactive ingredients.

Sometimes it does make sense to mix fertilizers when you have several partial bags left over, but only if the inactive ingredients will allow the fertilizers to mix and be applied properly. It becomes a much bigger math problem to figure out the percentage of active ingredients when you are not mixing the same amount of fertilizer. For instance, what are the final percentages of active ingredients when you add 7 pounds of leftover 20-10-10 fertilizer to a 25-pound bag of 10-15-10 fertilizer? Email me if you want to know the answer.

Email questions to Jeff Rugg at [email protected] To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

Photo credit: Jing at Pixabay

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