Calculating Fertilizer Rates

Understanding fertilizer rates can be confusing. While fertilizer bags have printed suggestions on how much to apply, they are largely based on achieving customer expectations, not necessarily what the customer needs for his grass.

The majority of people don't realize that you can accurately adjust the rates depending on the number of applications and the specific needs of your lawn. It is not hard if you understand how to calculate it.

Most fertilizer programs are based on two primary things.

  • The grass type you have planted and its annual nitrogen needs.
  • A fertilization plan using your grass type’s specific Nitrogen needs spread out over a year.

Calculating fertilizer rates are important for delivering the right amount. In this example, let's say the fertilizer you are using has 25% Nitrogen, 4% Phosphorus, and 12% Potassium. It will look like this on the bag: 25-4-12.

For easy metric conversion of any amount below, the following link provides online conversions including metric, imperial, and contemporary units of different countries. Weights, Measures, and Metric Conversions
For easier use, the link opens to a new window so you do not lose this page.

Calculating how much N-P-K is in the bag

Using the analysis above of 25-4-12, lets see how much Nitrogen (25) is actually in the bag. For this example, the bag weighs 25 lbs.

The formula

To get the lbs of Nitrogen: multiply the total weight of the bag times the percent of Nitrogen.

25 X 0.25 = 6.25 lbs of Nitrogen in the bag.

The formula explained

25 (25 lb bag of fertilizer) X 0.25 (25% analysis on the bag) = 6.25 lbs Nitrogen

The same formula can be used to see how much other nutrients are in the bag.

25 lbs X 0.04(P) = 1 lb of Phosphorus in the bag by weight

25 lbs X 0.12(K) = 3 lbs of Potassium in the bag by weight

The bag contains 6.25 lbs Nitrogen, 1lb of Phosphorus and 3 lbs of Potassium. You can use the formula regardless of what the fertilizer analysis is. Simply replace the example's numbers for your bag's weight and percentages.

An important note: If the bag says the 25 lb bag of 25-4-12 will cover 5000 sq. ft., you will be putting down 1.25 lbs of Nitrogen per 1000 sq. ft. at that rate. (6.25 divided by 5) This fertilizer rate will deliver too much Nitrogen in a single application for some grasses such as centipedegrass.

Determining how much to apply per 1000/sq. ft.

Depending on your grass type, the total annual Nitrogen is divided up over several applications a year. If the fertilizer rate calls for an application of half a pound (.5 lbs) of Nitrogen per 1000/sq.ft., you will need to know how much fertilizer to apply to deliver that amount. If you know a simple formula, you can get the fertilizer rate correct regardless of the fertilizer analysis.

The formula

Use the formula below for determining how much fertilizer to apply when you know the specific amount of Nitrogen needed per 1000/sq.ft. Again, the "amount needed" is referring to the pounds of nitrogen per 1000/sq.ft. you need to apply for that application.

Divide the specific amount to be applied per 1000/sq.ft. by the percentage of Nitrogen on the bag and then multiply that answer by 100. Here is what it will look like using the fertilizer analysis 25-4-12:

.5 lb ÷ 25% X 100 = 2 lbs fertilizer

Explained

.5 (1/2 lb of Nitrogen you want to apply over 1000/sq.ft) divided by 25% (Percent of N in bag) X 100 = 2 lbs of fertilizer This means that 2 lbs of fertilizer spread over 1000/sq.ft. will deliver .5 lbs of Nitrogen over 1000/sq.ft.

How much of the other nutrients were applied

Most programs are based on the Nitrogen to be applied and the other nutrients are along for the ride. You will need to know how much of the other nutrients you applied.

To see how much of the other nutrients you applied in the 2lbs of fertilizer, use this formula:

2 lbs fertilizer x .04% P = .08 P (Phosphorus) applied per 1000/sq.ft.

Or

2 lbs of fertilizer over 1000/sq.ft. X the amount of Phosphorus in the bag, = how much Phosphorus you applied.

Use the same formula for (K)

2 lbs fertilizer x .12% K = .24 (1/4 lb) K (Potassium) 1000/sq.ft.

Note: If the soil analysis indicates your soil is low in P or K, and the fertilizer you plan to use doesn't have a sufficient amount of P and K to fix the problem, you have a couple of options.

  • You can still apply your fertilizer, but then purchase fertilizer that contains only P or K, whichever one the soil analysis says is lacking.
  • Find a different fertilizer with the same Nitrogen content, but higher levels of P or K.

Examples would be: 17-17-17 or 20-10-15, 0-25-0 (P only), 0-0-50 (K only). The examples are endless.

A final note: If your fertilizer spreader is not calibrated, you could be putting down the wrong amount, even if you know the right fertilizer rate. Calibrating your spreader is easy. Click on the link below to see how.

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How To Collect A Soil Sample
The first step in understanding what is in your soil is to take a soil sample. Click here to find out how to how to collect a soil sample that will deliver the best results.

Understanding the Soil Analysis Report
Understanding the soil analysis report can be difficult. Click here for an explanation of results commonly found of most reports.

Spreader Calibration Made Easy
Spreader calibration made easy! Knowing if your spreader is actually putting down the right amount is important for professionals and homeowners alike. This page offers all you need to know about calibrating your broadcast or drop spreader.

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