Calculating Liquid
Fertilizer Rates

Calculating liquid fertilizer rates is confusing for many but still a necessary task when using any fertilizer including the best liquid lawn fertilizer. This page originates from a question I received on how to measure the liquid fertilizer for a 40 gallon sprayer to cover 2 acres. 

Regardless of the liquid fertilizer you choose, you need to convert a gallon of liquid to pounds. For the 18-3-6 the customer was using, the manufacturer said one gallon was 10.58 lbs. For properly calculating liquid fertilizer rates you will need to go through a series of math problems to get the information needed to determine how much of the product to apply per 1000 sq. ft.

Below is the email conversation I had explaining how it is done. I picked up after his question:

Note: In my answer to Tommy I was thinking the Growth Products Classic was 16-3-6. I corrected it on this page. The correct Nitrogen content for Growth Products Classic is 18% N by weight and not 16%.


"Hi Tommy, 

If you are referring to Growth Products 18-3-6 for turf the only information the manufacturer gives is on 1000 sq. ft. The product per 1000 sq ft is based the right amount of product for your needs combined with a gallon or two of water to form a solution to cover square footage.

However, it is easy to figure for an acre. There are 43560 sq ft in an acre. You will have to decide how much Nitrogen you will want to apply per 1000 sq ft based on the grass type and how often you want to do it. If you have bermudagrass, then 1/2 to 3/4 lb Nitrogen per 1000 sq ft  is good. If Zoysia, only 1/4 lb or no more than 1/3 lbs of nitrogen is best since it is a lower nitrogen grass. We will calculate it based on 1/3 lbs of N per 1000 sq. ft. 

Understanding Calculating Liquid Fertilizer Rates

Here are important facts to know for Calculating Liquid Fertilizer Rates:

* Product Growth Products weight per gallon is 10.58 lbs. This is often listed on the label. If not on the label you will need to weigh a gallon. 

* Product contents are 18N - 3P - 6K

You will need to determine the lbs of N in each gallon as seen below:

* Lbs of Nitrogen per gallon of product (10.58 X .18 N = 1.904 lbs of Nitrogen per gallon rounded to 1.9 lbs NItrogen per gallon of product)

* The next step is to determine this: if one gallon contains 1.9 lbs of N, how much of one gallon applied to 1000 sq. ft will deliver .33 lbs of N. 

* To determine the amount of product to deliver .33 lbs of Nitrogen per 1000 sq ft.  -  divide .33 lb N / 1.9 lbs of N per gallon = .174 or rounded down to .17 gallon of product.

* Then multiply .17 X 128 ounces in a gallon equals 21.76 ounces or rounded up to 22 ounces of product delivers .33 lbs of N per 1000 sq. ft.

This equals the product label of 22 ounces to deliver 1/3 lb of N.

* Water plus 22 oz of product to cover 1000 sq ft = 2 gallons 

So to summarize: Calculating liquid fertilizer rates requires you to input the size spray tank you have.

If you have a 50 gallon sprayer you will want to spray 22 ounces of product plus water to equal 2 gallons over a 1000 sq ft. To cover a whole acre of grass it would be approximately 860 ounces of product to cover 43000 sq ft.

The 50 gallon tank will only cover about 25000 sq ft, a little over half an acre. The water is the carrier so as long as you are getting the 22 ounces of product per 1000 sq ft you can use as much water as needed, but no less than 1 or 2 gallon of solution per 1000 sq ft. (Solution is the product plus water should come to 1 or 2 gallon with 2 gallons preferable.  

Wow, as I am answering your question, I just got your message on the spray gallon size. Now it is easy to calculate for your sprayer. If you want to apply 1/2 lb of N for Bermudagrass per 1000 sq ft then using the formula above it would take 34 ounces of product. This will give you 8 weeks of release.  For 1 lbs of Nitrogen per 1000 use 68 ounces and will give about 12 weeks of release. Add the product plus water to make 2 gallon of solution to cover 1000 sq ft.

For calculati)g liquid fertilizer rates over an acre, multiply 34 (ounces for 1/2 lb of N) X 43 to get  1462 ounces (over 11 gallons) of product plus water to make 2 gallons of solution for each 1000 sq. ft. to cover the acre. Based on the formulation it will last approximately 8 weeks in the soil. A 40 gallon tank at 2 gallons per 1000 sq ft would cover 20000 sq. ft. 

I hope this helps. Let me know if this helps you.

Russ James

Lawn Care Academy"


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