How to Choose and Apply Fertilizer
By Justin Keay
[This article was first published in The Gateway Gardener Winter 2019 issue, and is part of a 4-part series on how to take a soil sample and how to analyze a soil test report and then use that information to purchase and apply the appropriate fertilizer.]
Now that you’ve taken a soil sample, had it analyzed and reviewed the report, it’s time to put that knowledge to work buying the appropriate fertilizer and applying the correct amount.
A Review of N-P-K
A fertilizer bag lists three numbers at the bottom such as 12-12-12 or 28-0-3. These three numbers are the fertilizer grade and reflect the percentage by weight of nitrogen, phosphorous and potassium present in the bag. The nutrients are always listed in this order and are known as N-P-K values. As a reminder, N stands for nitrogen, P stands for phosphorous and K stands for potassium. According to the soil test report “native grasses” will not require a nitrogen application. For the other two selected crops, the soil test report notes that we need to apply .5 lbs. of nitrogen, and 0 lbs of phosphorous and potassium per 1000 square feet. It is important to note that .5 lbs. of nitrogen is not the same as .5 lbs. of fertilizer. If we could find the perfect fertilizer, it would contain only nitrogen and have N-P-K values such as 10-0-0 or 20-0-0. In most circumstances you will not find a fertilizer that matches exactly what you need. The best you can do is select a fertilizer with the closest N-P-K values to what you need. I would look for a fertilizer that has a high amount of nitrogen with the lowest amount of phosphorous and potassium I could find. Lawn fertilizers with N-P-K values such as 28-0-3 or 32-0-5 are easy to find at any hardware store or garden center. Lawn fertilizers are perfectly fine to use in the garden as long as they contain no pre or post-emergent herbicides.
How to Convert Soil Test Information to Practical Equivalents
Often soil test results are intended for farmers or larger areas, so recommendations often come in tons per acre or pounds per 1000 sq. ft., when many homeowners are dealing with much smaller plots and bags of fertilizer. So let’s see how to do some conversions.
Let us use a fertilizer with a 32-0-5 N-P-K value for this example and calculations. This fertilizer is by weight, 32% nitrogen, 0% phosphorous and 5% potassium. Now that we have identified the amount of nitrogen recommended and the fertilizer value we have purchased, we should proceed with a calculation using the following format:
.5 lb Nitrogen/1000 sq. ft. / .32 (N) = 1.5 lbs of fertilizer/1000 sq. ft.
We now know how much fertilizer to apply per 1000 sq. ft., unfortunately most gardens or lawns are not exactly 1000 sq. ft. In order to apply the correct amount of fertilizer you need to know the size of your lawn or garden. This can be approximately measured by pacing off your lawn or garden, one pace is roughly 2.5 feet. Let’s say the short side of your garden measures 12 paces and the long side measure 20 paces. This means your garden is 30 x 50 feet. We multiply these numbers together and we come out with a total of 1500 square feet.
We are almost finished with our calculations. We now know that we need 1.5lbs of fertilizer per 1000 sq. ft., but our garden is 1500 sq. ft. We can solve using a calculation in the following format;
1.5 lb/1000 sq. ft. x 1500 sq. ft. = 2.25 lbs of fertilizer
If you have a scale, you can weigh out the exact amount you need. However, two cups of most standard fertilizer material weighs 1 lb. The 2.25 lbs. of fertilizer we calculated will be approximately equal to 4.5 cups and will be what we need for our 1500 square foot garden. Whether fertilizing a lawn or garden, it is always best to spread small amounts of fertilizer at a time over the entire area. If you proceed in this way, you can ensure that the fertilizer is spread more evenly across the area. If you start spreading too heavily in the beginning you will most likely end up having some areas that are over-fertilized and some areas that receive no fertilizer.
Online Tools to Help with Soil Test Calculations
If you feel like you are back in math class or these calculations are making you pull your hair out, do not fret. There are several online tools that can make this process much simpler. The first is the MU Lawn Fertilizer Calculator http://agebb.missouri.edu/fertcalc/ and the second is a mobile app from University of Connecticut called FertAdvisor. University of Georgia has a more detailed fertilizer calculator http://aesl.ces.uga.edu/soil/fertcalc/ that can be used for a variety of nutrients. To use this more detailed calculator you first must convert the lbs./1000 sq. ft. of fertilizer recommended to lbs. per acre. You can do this simply by multiplying the recommended rate per 1000 square feet by 43.5, as there are 43,560 square feet per acre. This will give you the pounds per acre you need to input into the University of Georgia calculator.
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If you’ve arrived at this point without reading the other articles in this series, you may wish to review them to get a complete understanding of taking a soil sample, having it tested, and analyzing the report. You’ll find them here:
How, Why and When to Test Your Soil
Justin Keay is the MU Extension Field Specialist in Horticulture for St. Louis County and St. Charles County in Missouri.