The Q&A Archives: Fertilizers

Question: What do the three numbers on the box of fertiler mean? Example 20-10-10.

Answer: These numbers represent the fertilizer analysis. Fertilizer analysis means how much of an element is in a formulation based on percentage by weight. All fertilizers are labeled with three numbers that indicate the guaranteed analysis, or the fertilizergrade. These three numbers give the percentage by weight of nitrogen (N), phosphate (P2O5), and potash (K2O). State regulations require that actual analysis values must be within certain limits of the labeled grade. Often to simplify matters, we say these numbers represent nitrogen, phosphorus, and potassium, or N-P-K. Remember, it is always in that order. An example of how these numbers are applied is if you have a 100 lb. bag of fertilizer labeled 10-10-10, there are 10 lbs. of nitrogen, 10 lbs. of phosphate, and 10 lbs. of potash. The nutrient amounts do not add up to 100% for many reasons. The nutrients actually are in a compound or material that contains other elements. Fertilizers may contain nutrient sources other than N-P-K, conditionersto ensure good physical properties, and light weight absorbents to give slow release of nutrients. Fillers are also used such as limestone. Limestone acts not only as a filler but helps to neutralize the acidifying effects of some nitrogen sources.

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