Backyard Math

Here at Cuyahoga Soil & Water Conservation District, I field questions all the time about fertilizer application rates. So in this month’s spring season blog, I will be reviewing and calculating some real life scenarios that have come my way.

A quick primer on fertilizer math is necessary to begin. Fertilizers are typically sold with a three number code on the front of the bag. Dashes separate the three numbers, and collectively these are known as the fertilizer grade. The three numbers represent the precentage amount of nitrogen, phosphorus, and potassium always in that order. So, cottonseed meal graded as 6-2-1 would contain 6% nitrogen, 2% phosphorus, and 1% potassium.

With that knowledge in hand, we are ready to crunch some numbers!

SCENARIO #1: I have a 26 foot row of bamboo. It’s about two foot wide. And there’s an approximately four foot stretch that’s finally skipped the fence and growing into my neighbor’s backyard. I’m supposed to fertilize with a half pound of nitrogen per 100 square feet. I have a fertilizer that is rated at 8-2-4. Now what?

Step One: Figure out your square footage.

1) (26 linear feet + 4 linear feet) times 2 wide feet = 60 square feet

Step Two: Based on your square footage and nitrogen recommendation, determine amount of nitrogen needed for your scenario.

2) 0.5 pound of Nitrogen per 100 square feet can be written as:

  • 0.5 pounds of Nitrogen times (1)(100/100) = 0.5 pounds of Nitrogen
  • Therefore,
  • 0.5 pounds of Nitrogen times (1)(60/100) = 0.3 pounds of Nitrogen needed

Step Three: Based on the amount of nitrogen needed and your fertilizer on hand, determine how much of that fertilizer to use.

3) So if your fertilizer is 8% Nitrogen, then:

  • 0.08X = 0.3 pounds of Nitrogen
  • 0.08X/0.08 =0.3 pounds of Nitrogen/0.08
  • X = 3.75 pounds of 8-2-4

SCENARIO #2: I have a 120 foot row of asparagus running alongside my house. Per the nursery’s instructions, I am supposed to fertilize with two pounds of 10-10-10 per 100 feet every spring. The only fertilizer I have is 3-4-3.

Step One: Convert 3-4-3 into 10-10-10.

This will need to be a rough conversion because of the extra 1% phosphorus in the 3-4-3 is preventing us from making an exact apples to apples comparison. The good news is that’s okay!

1) To do this, divide the 10 of the 10-10-10 by 3 of the 3-4-3:

  • 10/3 = 3.33
  • To get rid of decimals, for the sake of our lake, and to save money, we’ll round down to the whole number 3.
  • This means you’ll need three times the amount of 3-4-3 to give you something similar to 10-10-10.
  • Thus, the recommended 2 pounds of 10-10-10 becomes 6 pounds of 3-4-3.
  • 2 times 3 = 6 pounds of 3-4-3

Step Two: Account for the extra 20 feet.

The recommendation is for a 100 foot row, but this row is 120 foot. This is a 20% increase from the recommendation. So,

2) 6 pounds 3-4-3 times 1.2* = 7.2 pounds 3-4-3

  • 1.2* represents 120%

Another way to do this as represented in the picture for the blog is to break up the initial 100 foot section into five 20 foot sections and determine how much fertilizer goes to each section, and then add this amount to the total 6 pounds.

3) 6 pounds 3-4-3 ÷ 5 20 foot rows = 1.2 pounds per 20 foot row

  • If you have six 20 foot sections, then:
  • 1.2 + 1.2 + 1.2 + 1.2 + 1.2 + 1.2 = 7.2 pounds
  • Please note that the 1.2 of 120% and the 1.2 pounds per 20 foot section are completely coincidental. In most circumstances, these numbers will not be the same.

If backyard math stills gives you anxiety after this explanation, then please don’t hesitate to reach out to us here at Cuyahoga Soil & Water Conservation District and me in particular.

Blog author: Justin Husher, Horticulture Specialist

Leave a comment