The Q&A Archives: Fertilizer compostion

Question: Your web page indicates that the numbers used to express
N, P, and K are the weight percent of the respective element. Have I been confused all these years in thinking that the numbers referred to the weight percent of N, P2O5, and K2O?

Answer: You are correct that the percentages given are those of the industry standard: N, P2O5 and K20. Therefore, a fertilizer listed as 5% P205 would have a little less than 5% actual phosphorus, since some of that is the element oxygen. This does not create a problem however because most soil testing labs give their analysis and recommendations in terms of N, P2O5 and K20. So if the soil test report says to put on a certain amount of a nutrient, they are speaking in the same language as the fertilizer bag.

However, those terms can get really confusing for most consumers. Therefore the industry typically refers to the three numbers as N, P and K, respectively. Thus for the consumer, we can simply say N, P, K, and avoid the confusion.

Thanks for the question!

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