Garden Guide

Understanding fertilizer recommendations
Jan. 16, 2013 @ 05:00 AM

Taking a soil test before planting any landscape or garden can be invaluable, if someone knows how to use the information received. The last Garden Guide article was written about understanding lime recommendations on a soil test report. This week, our article will focus on the fertilizer recommendations.

In addition to the lime recommendation, a fertilizer recommendation will appear on a soil test report. This recommendation is based on the amount of phosphorous and potassium already in the soil. More specifically, under “Test Results,” P-I and K-I will be listed, which stand for the phosphorous index and the potassium index. The lab will recommend adding these nutrients if either index is less than 50.

The NCDA soil lab does not test for nitrogen. Nitrogen is an essential plant nutrient, but the amount of nitrogen in the environment can rapidly change. Instead the lab will recommend a given amount of nitrogen each year, usually 1 pound per 1000 square feet.

Recommendation will look something like this: 7 pounds, 15-0-14 (the actual amount and grade of fertilizer will differ). The numbers (15-0-14) represent the grade of fertilizer and indicate the percentage of nitrogen, phosphate (P2O5) and potash (K2O) in the material. In our example, the fertilizer would be 15 percent nitrogen, 0 percent phosphate and 14 percent potash.

A common problem occurs when homeowners cannot find the recommended fertilizer. Or, what happens if people have another fertilizer they want to use? First, they will need to calculate the recommended rate of each nutrient:

Recommended rate x (percent nutrient in recommended fertilizer/100) =7 pounds x.15 = 1 lb N

Say someone wants to use ammonium sulfate (21-0-0) for the nitrogen component. To determine how much 21-0-0 to use:

Recommended rate for the nutrient/(percent nutrient in actual fertilizer/100) = 1 pound/.21 = about 5 pounds 21-0-0

Now another fertilizer would be needed to supply the potassium, something like muriate of potash (0-0-60), potassium sulfate (0-0-50) or Sul-Po-Mag (0-0-22). People would go through the same calculations as above to figure out how much of the new fertilizer to use.

The fertilizers in the above calculations are single-nutrient fertilizers; however some fertilizers contain two (15-0-14) or all three nutrients (NPK). For example, 10-10-10 is a complete fertilizer and contains nitrogen, phosphorous (as phosphate) and potassium (as potash). When choosing a fertilizer, try to pick one that is similar to the one stated in the report. For example, if recommended is 10-10-10, then choose 15-15-15 or another 1:1:1 ratio. Be sure to change the recommended rate in order to apply the correct amount of nutrients (use the same calculations as above).

Remember, the total area of the tested region will need to be calculated to determine the total amount of fertilizer to apply. This can be done by breaking up the tested area into geometric shapes and adding the area of the shapes together.

Timing of fertilizer application depends on what is being fertilized – warm-season grasses, cool-season grasses, trees, shrubs, a vegetable garden, etc. Reference NCDA Note 4: Fertilization of Lawns, Gardens, and Ornamentals or contact our Center at (919) 775-5624 for more timing specifics.

Understanding a soil test report is essential to using the information to make economically and environmentally wise fertilizer applications. Those who need help understanding their soil test report can contact our center at (919) 775-5624.

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