Avoid fertilizer disasters by understanding numbers

By David Sprague

OSU Ext. Master Gardener Volunteer

With season changes just around the corner, many of us are getting spring fever and are starting to purchase supplies, including fertilizer, from our local garden centers. I have observed that some people are confused when purchasing fertilizer; specifically as to what the numbers mean on package and how to use them.

When shopping for fertilizer, you’ll notice three numbers on the bag or box separated by dashes or dots, such as 10-10-10. These numbers represent the percentage of primary macronutrients — nitrogen (N), phosphorus (P) and potassium (K) — the fertilizer contains. These three elements are essential for plants to grow. Nitrogen is important in forming chlorophyll and is one of the building blocks of proteins and nucleic acids. Phosphorus is needed for the growth of shoots and roots and is essential for cell division. Potassium is used in rapidly growing tissue and is important in fruit formation.

By federal law, all commercially sold fertilizers have to list the percentage of these elements by weight on the package. The first number listed is nitrogen; the second number is phosphorus; and the third is potassium. So a 100-pound bag of 20-10-5 fertilizer will contain 20 pounds of nitrogen, 10 pounds of phosphorus and 5 pounds of potassium. The remainder of the bag is inert filler material. A fertilizer is considered complete when it contains all three of these elements. A fertilizer is called incomplete when one or more of the elements (N, P, K) is missing. For example, a 10-pound bag of 46-0-0 fertilizer would contain 46 percent nitrogen (4.6 pounds) and no other elements, and is an incomplete fertilizer. An incomplete fertilizer is used when only one or two of the elements is needed to enhance soil fertility.

There are a wide variety of fertilizer formulations to choose from, and using the wrong fertilizer type or amount can be worse for the garden or lawn than no fertilizer at all. The only way to accurately know what the fertilizer needs of your lawn or garden are, is to do a soil test. Soil tests remove the guesswork and prevent the risk of an over- or under-fertilizer application. Our OSU Extension in Mahoning County offers soil tests for $20. A soil test provides a laboratory evaluation of your soil conditions and provides the specific fertilizer recommendations needed for your site based on existing soil nutrient levels, pH (acidity) and specific plant requirements.

For details, visit go.osu.edu/soiltesting.

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