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Home » Farm & Food Events

UNH Cooperative Extension Info Line Question of the Week

By Helen Brody (June 26, 2013)

Q. Can you tell me what the numbers mean on a bag of fertilizer?  Co-operative Extension logo

A. Both processed organic fertilizers such as fish meal, and synthetic fertilizers such as 10-10-10, are identified by their analysis, that is, by the three numbers on the bag or container. The numbers refer to the percentage of nitrogen, phosphorus (expressed as P2O5) and potassium (expressed as K2O) in the fertilizers. For example: a 25 lb bag of 10-10-10 contains 10% nitrogen, 10% phosphate and 10% potash. The rest of the ingredients are referred to as carriers; they “carry” the nutrients and keep the fertilizer from becoming lumpy and unmanageable. Nitrogen, phosphorus and potassium are known as macronutrients, and are necessary in rather large amounts for normal plant growth. Dissolved in water, they are absorbed by the roots.

Fertilizers, whether derived from chemical or organic sources, are most often available in two forms: a granular form that is usually sprinkled on the soil and worked in, and a water soluble form that can be mixed with water and sprayed on plant leaves or applied to the soil. Because most organic materials are of low solubility, N-P-K numbers for reputable organic fertilizers tend to be in the single digit range. This doesn’t mean that they don’t provide ample nutrition over time, just that very little of it will immediately dissolve in water and become instantly available.

Before applying fertilizer to the garden it is best to test the soil. Soil test forms can be obtained from the UNH Cooperative Extension website at

Got questions? UNHCooperativeExtensionEducationCenter‘s Info Line offers practical help finding answers for your lawn and garden questions. Call toll free at 1-877-398-4769, M-F, 9:00 a.m.-2:00 p.m., or e-mail us at

Reprint from Weekly Market Bulletin, June 19, 2o13


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