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The analysis or formula listed on the fertilizer bags is the three big numbers. These give the percentage by weight of the major nutrients contained in that bag. For example, in a bag of 19-26-5 starter fertilizer, 19% of the weight of the fertilizer in the bag is nitrogen (N), 26% is phosphorous (P) as P2O5 equivalent and 5% is potassium (K) as K2O equivalent.
Don’t worry about ‘fertilizer equivalent’ because application should always be given in pounds of N, P2O5 and K2O per area.
To determine the actual amount of each nutrient equivalent in the bag of fertilizer, multiply the weight of the bag by the percentage of that nutrient equivalent in decimal form. So to determine the actual nitrogen content in a 100-pound bag of 19-26-5 starter fertilizer:
100 (pounds) x .19 (percentage of nitrogen) = 19 pounds of actual fertilizer material.
To determine how many pounds of a specific fertilizer material are required to reach a specific amount of one nutrient, divide the percentage of that nutrient (in decimal form) as contained in the bag into the desired number of pounds of that nutrient. For example, in the 19-26-5 starter fertilizer, to determine how much fertilizer is needed to reach a desired rate of 1 pound of nitrogen: 1 divided by .19 = 5.26 pounds. Therefore, 5.26 pounds of fertilizer must be applied to the designated area (let's say 1,000 square feet) to supply 1 pound of nitrogen.
Also note that these are percentages of N, P2O5, and K2O, not the elemental N, P and K. Fertilizer recommendations are always to be given as the equivalent. Only researchers use the latter elemental values. If interested, to convert oxides to elemental, multiply P2O5 by 0.44; multiply K2O by 0.83 and to convert elemental to oxides, multiply P x 2.29; multiply K by 1.2.