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Know Before You Grow: How to Calculate the Right Amount of Fertilizer

Table of Contents

Nitrogen (N) - Phosphorous (P) - Potassium (K)

Tractor Fertilizing Field

The three basic nutrients in most fertilizers are nitrogen, phosphorus, and potassium. Oftentimes, it may be referred to by the abbreviations, N-P-K.

Nitrogen is important to your lawn as it is important for growth and the color of the grass. Applying nitrogen to a lawn will increase the growth rate and help it have a lush green color.

Phosphorus is important for root system development. It is also important in photosynthesis. Photosyntheses is the process by which plant converts carbon dioxide to oxygen and in order to create carbohydrates for energy.

patchy lawn

Proper fertilization can keep your lawn from looking like this.

Lastly, potassium is important in the development of proteins and starches within the plant. Also, it improves water and nutrient uptake efficiency. Potassium is an important nutrient to increase the grass’s resiliency to stresses like drought and temperature extremes.

If your grass does not get enough of these essential nutrients, it stunts growth and makes it susceptible to stressors like disease, drought, and temperature extremes. A nitrogen deficiency is easily recognizable by the grass turning yellow or orange in patches. Stunted growth is also likely with nitrogen deficiency. Potassium is more difficult to identify, however. The most likely sign is the edges or tips of your grass yellowing. Phosphorus is the hardest to identify, with the only sign being slowed growth.

Too Many Nutrients Can Damage Your Lawn, Too

Where too little nutrients can be detrimental, too much also cause problems. As an example, too much nitrogen burns a lawn. Also, too much phosphorus runs off into streams and lakes and causes nuisance. For example, if you've seen ugly algae blooms–it is probably caused by phosphorus runoff. Also, phosphorus can kill your grass by preventing it from taking up essential micronutrients. Fortunately, too much potassium has little impact on your lawn or environment.

Algae blooms like this are most often caused by excessive Phosphorus runoff into bodies of water.

The best way to determine if nitrogen, potassium and\or phosphorus is needed in your lawn is by conducting a soil test. This soil test kit provides results within 24-48 hours after being received. The results provide guidance on the exact amount of nutrients needed to keep your lawn healthy!

When looking at the results from your soil test, the recommended amounts of nitrogen, phosphorus (P2O5) and potassium (K2O) are listed in pounds per acre or 1000 square feet. Phosphorus and potassium may also be referred to as phosphate (P2O5) or potash (K2O), respectively. When selecting the appropriate fertilizer, understanding the label is important to understand that the right amount of product is used to meet and not exceed the soil test recommendations.

Putting N-P-K Together

On the fertilizer label, you will see the letters N-P-K, or Nitrogen-Phosphate-Potash, along with a number. This number identifies the percentage of each nutrient within that specific bag of fertilizer. For example, in one of the top-rated fertilizers, Pennington Pro Care Fertilizer 15-0-5 .03 LOCKUP there is 15% nitrogen, 0% phosphate, and 5% potash. Therefore, in this 50 pound bag, there is only 7.5 pounds (50lbs x 15%) of nitrogen, 0 (50lbs x 0%) pounds of phosphate, and 2.5 (50lbs x 5%) of potash.

How to calculate fertilizer amounts for lawns

When determining the correct amount of fertilizer, you calculate based on the amount of the most needed nutrient (highest amount). For example, bermudagrass usually needs about 50 pounds of nitrogen and 40 pounds of potassium per acre\year. If using Pennington ProCare fertilizer, you would need to apply a little over 6 ½ bags (50lbs divided by 7.5lbs) of Pennington ProCare (50lb bags) to meet the needed nitrogen levels. In closing, it is always important to conduct a soil test to determine the ideal nutrients needed to make your lawn healthy. In addition, select a fertilizer that will best meet all of the nutrient needs without significantly exceeding the recommended rates. Lastly, the numbers listed on the fertilizer bags are only a percentage of the number of nutrients that make up that particular quantity. Feel free to reach out through our “Ask the Expert” section if you need help interpreting your soil test results or selecting the best fertilizer for your lawn.

How to Calculate Square Footage of Your Yard

Now you know how to calculate the amount of N-P-K in a bag of fertilizer. Before you begin spreading fertilizer, however, you need to calculate the size of your lawn. If you live on a half-acre lot of land, your calculation is rather straightforward. 1 acre of land is 43,560 square-feet. Therefore, a half-acre is 21,780 square-feet. The only thing you have to do now is subtract the square footage of your house and any areas where grass doesn't grow, like your flower beds or under a wooden deck. Here is the formula you'd use:
(sq-ft of lawn) = (sq-ft of lot) - (sq-ft of house) - (sq-ft of other areas where grass doesn't grow)
For the sake of simplicity, let's say your house is 2,000 square-feet and you have no flower beds, patios, or anything like that. In that case, here is your calculation:

step 1: (sq-ft of lawn) = 21,780 – 2,000 – 0

step 2: (sq-ft of lawn) = 19,780

An Easier Way to Calculate Square Footage of Your Yard

Do you have an oddly-shaped lot? Does math give you a migraine. If so, let modern technology come to your rescue! Simply use this web page to calculate your lot's square-footage for you.
1. Enter Your Address

Enter your address. Once you do and the tool finds your address, you'll still need to zoom in to be able to get a good view of your lot.

Draw the boundaries around your lot
Click the “Draw New Area” button. With that button selected, you can now trace around the boundaries of your lot. Once you complete the boundary trace, the area will automatically calculate for you.

Once you complete drawing the boundaries of your lot, the sq-ft automatically calculates for you. In this example, the lot has an area of 19,326 sq-ft.

Finally, we need to subtract the size of the house from the area we just calculated. I happen to know the house on this lot is 2,350 sq-ft so subtracting 2,350 from 19,326 gives us a yard that is 16,976 sq-ft.

Putting it All Together – Calculating Amount of Fertilizer Needed For Your Yard

Let us continue with the examples from above. As we have just calculated, the lawn in question has an area of 16,976 sq-ft. Let's also say that this lawn consists of bermuda grass. As we have already stated, bermuda needs 50lbs of Nitrogen (N) and 40lbs of Potassium (K) per acre per year. We now know everything we need to calculate how much Nitrogen and how much Potassium we should apply to this lawn every year.

We know that we needs 50lbs of Nitrogen for every 1 acre (43,560sq-ft). We know the area of our lawn is 16,976 sq-ft. But how many pounds of Nitrogen do we need for a lawn of that size? To calculate the answer, we just write the formula like what is shown above, cross multiply, then solve for x.

After we cross-multiply the formula above, we get:

848,800 = 43,560x

Now, to solve for ‘x', we just need to divide 848,800 by 43,560. Using the handy calculator that comes on our smartphones, you'll quickly find that the answer is roughly 19.5lbs. So I need to fertilize this bermuda grass lawn with 19.5 lbs of Nitrogen every year.

That's it! Now that you know how to perform the calculations, you can divine the correct amount of fertilizer needs for your lawn.

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