Answer: The numbers on a fertilizer bag refer to the percentage of N (nitrogen), P (phosphorous), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers, and potassium keeps the root system healthy. In reality, these elements work in conjunction with one another, but the above will give you a sense of why there are so many different numbers on different types of fertilizers. If you're applying fertilizer to tomato plants, for example, you're not as interested in the plant developing leaves as you are in it flowers and fruit, so you'd use a formulation lower in nitrogen and higher in phosphorous and potassium, such as a 5-10-10 or a 10-20-20. Hope this information helps clear up some of the mystery!
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