Most lawn fertilizers have a high nitrogen content because of the three nutrients supplied by fertilizers (nitrogen, phosphorous and potassium), that's the one that grass needs most. Nitrogen passes out of the soil quickly, and it's vital for the health and green color of the grass blades. Although a soil test may reveal current nitrogen levels, it's only a fleeting snapshot because this element moves through the soil so quickly that levels can significantly change from soil test to sowing/planting. Essentially, there's no reliable way to test for nitrogen (N), although you can and should test the soil for phosphorous (P) and potassium (K) and use a blend that contains these elements if the soil is deficient.
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The N-P-K ratio displayed on a fertilizer bag tells you the percentage of each of the elements the fertilizer contains, and since you're usually most concerned about nitrogen, that's the percentage you should note when determining how much fertilizer you need per acre of grass. The other bit of information you need is the application rate, which is usually displayed on the bag in pounds/1,000 square feet. If you're fertilizing a large lawn that is an acre in size, use the conversion 1 acre = 43,560 square feet to calculate how many pounds of fertilizer you need and how many bags you should buy.
Multiply the amount of fertilizer you need for 1,000 square feet by 43.56 to determine how much you need for an acre.
Determining the Application Rate
Several factors go into a determination of the application rate for fertilizer, including:
- How often you fertilize.
- What type of grass you have.
- Whether or not you leave grass clippings — which supply nitrogen — on the lawn.
- The overall condition of the lawn.
It's important not to overdo the nitrogen because it will increase the growth rate, and the lawn will need more frequent mowing. You want to add enough to create a vibrant green lawn without having it turn into a field in a couple of weeks — and definitely without burning the grass. Also, excessive nitrogen levels can promote the growth of certain diseases, notably those caused by Rhizoctonia species fungal pathogens.
A good average application rate is 1 pound of nitrogen per 1,000 square feet of lawn. Lawn fertilizer manufacturers may recommend a different rate on the label, and if you choose to go with that, it's a good idea to cut the recommendation in half to avoid problems. If the grass doesn't respond, which would be rare, you can always add more.
Calculating the Actual Nitrogen Content
The N-P-K ratio displayed on the bag, which for Pennington UltraGreen is 30-0-4, tells you the percentage of each of the three elements in the mixture, with the rest of the mixture being filler. If you want to apply 1 pound per 1,000 square feet of actual nitrogen to the lawn, you need 3.33 pounds of this product. You get this number by dividing the percentage displayed on the container into 100 percent. By the same token, if you're using Dr. Earth Nitrogen Fertilizer, which is a 5-0-2 blend, you'd need 100/5 = 20 pounds to supply 1 pound of actual nitrogen.
Once you know how many pounds of product you need, check the weight of the bag and divide that by the number of pounds of actual nitrogen in the bag to get the number of bags you need. For example, a bag of Dr. Earth weighs 18 pounds, so a single bag will supply 18/20 = 0.9, or almost 1 pound of actual nitrogen. You actually need 1.1 bags to get your pound of nitrogen. On the other hand, a bag of Pennington UltraGreen, which has a higher nitrogen content, weighs only 12.5 pounds, but even so, by calculating 12.5/3.33 = 3.75, you find one bag contains almost 4 pounds of nitrogen, so to get a single pound, you only need 1/4 bag.
Fertilizing an Acre of Lawn
So far, all the calculations have been for 1,000 square feet of lawn, but you want to fertilize an acre. Using the conversion 1 acre = 43,560 square feet, you can determine that an acre is composed of 43.56 parcels, each with an area of 1,000 square feet. Multiply the number of bags you need for 1,000 square feet by this number to determine how many bags you need for an acre. For example, you need (1.1 x 43.56) = 48 bags of Dr. Earth and (0.25 X 43.56) = 11 bags of Pennington UltraGreen.