Calculating fertilizer application rates is just math, a simple formula. Here's an explanation for you to reference when you need it.
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1. Gather Facts
Get a soil test and fertilize based on that feedback
You may need less fertilizer than you think you need. Read this post and watch the video on how to conduct a soil test. We recommend Waters Agricultural Laboratories as the best soil testing service for our southeastern soils.
How many square feet is your lawn?
The Yard Area Calculator on our website will help you calculate your square footage.
Know the Fertilizer Rate for your type of lawn
We recommend 3 separate applications of nitrogen (N) fertilizer. Familiarize yourself with the annual recommended nitrogen fertilizer rates for your type of lawn. Divide that annual recommendation into 3 so you know how to proceed with one fertilizer application.
Warm Season Lawn Fertilizer Rates
Fertilize when actively growing in the warm months (summer).
Bermuda
TifTuf: 3-6 lbs N annually
Tifway: 4-6 lbs N annually
Zoysia
Zenith: 2-3 lbs N annually
Zeon: 1-3 lbs N annually
Emerald: 2-4 lbs N annually
Centipede
TifBlair: 1-2 lbs N annually, if at all!
Cool Season Lawn Fertilizer Rates
Fertilize when actively growing in the cool months (fall, winter, early spring).
Elite Tall Fescue
ETF: 2-4 lbs N annually
Look up the nutrient analysis on the fertilizer bag
The nutrient analysis is the three numbers on the bag telling you the amount of nitrogen-phosphorus-potassium, a.k.a. N-P-K.
We recommend our 25 lb bags of 5-10-30 for Centipede and 16-4-8 for Bermuda, Zoysia, and Tall Fescue.
2. Calculate
Example Scenario: You have a Bermuda lawn and intend to apply N at the low end of the range, at the rate of 3 lbs N a year. Separate this into 3 applications of 1 lb N each time. Here’s the math for the first treatment of 1 lb N in May or June using 25 lb bags of our 16-4-8 for a 7,000 sq. ft. lawn.
If you want to learn more, please visit our website TSP fertilizer 50kg/bag.
Notes:
-Calculations are first based on fertilizing a 1,000 sq. ft. area; then for your specific square footage.
-There are different formulas to calculate the fertilizer rate; this one factors weight of the bag at the end.
-You can apply the 4 steps to other fertilizer blends to determine how many bags to purchase.
Step 1: Determine percentage of N in the bag. Take the first number in the ratio, which represents percent N in the bag, and divide it by 100.
16 ÷ 100 = .16 (or 16%) N is in our 25 lb bag of 16-4-8
Step 2: Determine how many lbs of a particular fertilizer you need for 1,000 sq. ft. You want to apply 1 lb N to 1,000 sq. ft. so you divide that number by the percentage of N in the bag.
1 lb ÷ .16 = 6.25 lbs of 16-4-8 should be applied per 1,000 sq. ft.
Step 3: Determine how many lbs of that fertilizer you need for your square footage. Based on 6.25 lbs (the pounds of 16-4-8 needed per 1,000 sq. ft. determined in Step 2), calculate how many pounds are needed for the first application to your lawn, in this case a 7,000 sq. ft. area.
7,000 sq. ft. x (6.25 lbs ÷ 1,000) = 44 lbs of 16-4-8 is how much to apply to 7,000 square feet now
Step 4: Determine how many bags to buy for 7,000 sq. ft. This is where the weight of the bag comes in. Divide the lbs needed by the weight of the bag.
44 lbs of 16-4-8 ÷ 25 lb size = 1.76 bags needed
(round up to 4 and save the rest for your next application)
If you are compost topdressing with Soil3 to build better soil structure and practice organic fertilization, then topdress for one fertilizer application instead of spreading fertilizer.
Further Fertilizing Tips
Separate the annual recommended Nitrogen rate into several applications. We recommend at least 3 "split" applications. At least 3 applications provides a more consistent feeding and safeguards against burning the lawn with too much fertilizer at once.
- Warm Season Lawns: Fertilize with nitrogen (N) through the growing season; starting after spring green up and when soil temperature is above 65 degrees F. Typical months for fertilizing warm season lawns with nitrogen are May, June, July, August.
- Tall Fescue (a cool season lawn): Fertilize at least 3 times a year, during Tall Fescue's growing season in autumn, winter, and early spring. Stop fertilizing Tall Fescue during the summer. A rule of thumb is no nitrogen fertilizer on Tall Fescue during the warm months of May, June, July, and August.
This same calculation can also be used to calculate how many pounds of phosphorus and potassium you are applying, but that isn't typically done unless there is an unusual deficiency in those two elements, with special applications of them required.
Based on the amounts of nitrogen needed for a particular lawn, this same formula can be applied to determine the pounds of nitrogen available in any fertilizer blend you have access to.
The analysis or formula listed on the fertilizer bags is the three big numbers. These give the percentage by weight of the major nutrients contained in that bag. For example, in a bag of 19-26-5 starter fertilizer, 19% of the weight of the fertilizer in the bag is nitrogen (N), 26% is phosphorous (P) as P2O5 equivalent and 5% is potassium (K) as K2O equivalent.
Don’t worry about "fertilizer equivalent" because application should always be given in pounds of N, P2O5 and K2O per area.
To determine the actual amount of each nutrient equivalent in the bag of fertilizer, multiply the weight of the bag by the percentage of that nutrient equivalent in decimal form. So to determine the actual nitrogen content in a 100-pound bag of 19-26-5 starter fertilizer:
100 (pounds) x .19 (percentage of nitrogen) = 19 pounds of actual fertilizer material.
To determine how many pounds of a specific fertilizer material are required to reach a specific amount of one nutrient, divide the percentage of that nutrient (in decimal form) as contained in the bag into the desired number of pounds of that nutrient. For example, in the 19-26-5 starter fertilizer, to determine how much fertilizer is needed to reach a desired rate of 1 pound of nitrogen: 1 divided by .19 = 5.26 pounds. Therefore, 5.26 pounds of fertilizer must be applied to the designated area (let's say 1,000 square feet) to supply 1 pound of nitrogen.
Also note that these are percentages of N, P2O5 and K2O, not the elemental N, P and K. Fertilizer recommendations are always given as the equivalent. Only researchers use the latter elemental values. If interested, to convert oxides to elemental, multiply P2O5 by 0.44 and multiply K2O by 0.83, and to convert elemental to oxides, multiply P x 2.29 and multiply K by 1.2.
Let's work another example. How much 24-6-12 fertilizer, as seen in the label shown above, do you need to apply fertilizer at 1 lb. N/1,000 sq. ft. to an 8,000-sq-ft lawn? Use our formula: required rate of N per 1,000 sq.ft divided by % of N in the analysis (use the decimal form) equals amount of fertilizer product per 1,000 sq.ft. (1/0.24=4.2 lbs per 1,000). Since you need enough for 8,000 sq. ft., then multiply 4.2 x 8 and get 33.6 lbs needed for the job. The same calculation will work for the phosphate percentage and the potash.
Lawns require several fertilizer applications per season for best sustainability. This provides healthy growth, good color and pest and stress resistance. We generally apply between 0.5 and 1 lb of N per 1,000 sq. ft. in an application to the lawn. You may apply about 50% of that value in K2O (potash) and apply P2O5 (phosphate) at a quarter of the N on average. A soil test is still the only accurate route to take for the P, K, pH, etc.
Also see our publication "Louisiana Lawns BMPs" for specific care of the grasses.
Contact us to discuss your requirements of what is 18-46-0 fertilizer used for. Our experienced sales team can help you identify the options that best suit your needs.
