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How to Choose the Correct Inorganic Fertilizer

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Chance are good you'll be buying fertilizer for your lawn and garden projects at your favorite retail store. Before you make that purchase, I’d like you to make a couple simple calculations. You might be surprised and save enough to take your spouse out for dinner.

What fertilizer should I apply
Actually, before you even make a calculation, I need to ask you a question. Do you know what fertilizer to apply? I’m sure there are a lot of answers you could come up with; “I always apply the same thing each year”, or “I apply what my neighbor does”, or “I apply what’s on sale”. The best answer however would be that you apply fertilizer based on the recommendations of a soil test you took in the last three years.

Without a soil test, everyone is guessing. You, your neighbor, and I are all guessing at how much nutrients are available in the soil and how much, if any, extra needs to be applied. If the soil has adequate amounts of the three major nutrients, nitrogen (N), phosphorus (P) and potassium (K), you’re wasting your money applying any more.

So here’s the sermon, soil test every three years, and only apply what you need. Also keep in mind that if you live in Johnson County, we have funding to help offset the cost of soil testing. That’s the case for both homeowner and agriculture samples.

Caluculate the amount of fertilizer in the bag
Before we begin calculating, you need to remember that all fertilizers sold are regulated and are required to guarantee the analysis or amount of fertilizer labeled on the bag. The numbers on the fertilizer bag represent the percent of nitrogen, phosphorus and potassium in the bag. The first number represents the percent nitrogen. The second number is the percent of phosphorus. The third number is the percent of potassium. So here are the calculations:

  • Multiply the percentages on the bag by the pounds of the bag and that will tell you the amount of actual fertilizer you are getting.
  • Add all those numbers up and divide that into the cost of the bag and you will get the cost per pound of active ingredient.

It’s no different than a grocery store showing you on their price label the cost per ounce so you can compare one size to another.

Examples of fertilizer price per pound
Here are some examples when I checked prices of fertilizers at a few local sources:

A 10-10-10 bag of lawn fertilizer costs $12 and is in a 40 pound bag. I multiply the 40 pound bag by 10% (the fist 10 on the bag) and find out there are 4 pounds of actual nitrogen. I do the same thing for the second 10 (the amount of phosphorus) and it equals 4 pounds of actual phosphorus. I do the third calculation for the last 10 (the amount of potassium) and of course it’s also 4 pounds of actual potassium. I add the three numbers together and find out I am getting 12 pounds of actual fertilizer to apply to my yard. The bag cost $12 (I didn’t make this up) and I divide by the 12 pounds of active fertilizer and find out it costs $1 per pound of active ingredient.

Look at the comparison to other fertilizer products with a different analysis:

  • 10-10-10 (40 pound bag) @ $12 = $1 per pound active ingredient
  • 12-12-12 (40 pound bag) @ $13 = $.90 per pound active ingredient
  • 46-0-0 (50 pound bag) @ $17.05 = $.76 per pound active ingredient
  • 18-46-0 (50 pound bag) @ $23.80 = $.76 per pound active ingredient
  • 18-24-6 (50 pound bag) @ $36.22 = $1.51 per pound active ingredient

As you can see, there can be a big difference in the actual cost of a pound of fertilizer — over a 100% difference in one of the examples above. You will also notice that some fertilizer products had a little bit of all three major nutrients in the bag. Other products only had one or two nutrients in the bag. That’s extremely important as it relates back to the soil test. If my soil test said I did not need any potassium (the third number on the bag) any products that had potassium were an un-needed expense for me this year. It’s a really simple process of matching the fertilizer with the soil test and then finding the least expensive product.

In addition to chain stores, hardware, and lawn and garden stores don’t forget about local farm co-ops or elevators for fertilizers. They supply farmers with their fertilizers and can help you with the same. Remember, a penny saved is a penny earned.