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Understanding Fertilizer #s

Updated: Oct 27, 2022


Need to fertilize, but intimidated and confused about the numbers on the bags? Let’s see if we can shed some light and make it easier to understand.


Firstly, you need to figure out why you are fertilizing? This will help determine what you need. Each bag has a label with 3 numbers on it, this represents the percentage (by weight) of the 3 major ingredients required for healthy plants/grass. They will always be listed in the same order: Nitrogen-Phosphorus-Potassium (N-P-K). Each does something specific & should be selected based on your gardening needs.


So, the 1st number is the % of nitrogen in the bag. So in a bag of 10-5-25 has 10% of the weight of the bag in nitrogen. Nitrogen provides plants with the ability to produce more chlorophyll, which in turn allows plants to grow quickly. So if you want your plants to grow tall and your lawn to be a darker green use a fertilizer that is high in nitrogen.


The 2nd # is phosphorus. In a 10-5-25 bag it would contain only 5% phosphorus. Phosphorus aids in root growth & increases the flowering ability & bloom size. High phosphorus should be used when plants are being established in your garden, or when establishing a new lawn or even planting trees.


The 3rd # is potassium. In a bag of 10-5-25 would have 25% of the weight in potassium in the mix. Potassium has many functions. It guards against diseases, aids in drought protection & cold tolerance. You might consider a high potassium at the start of fall & summer as it will aid in protection from temperature extremes or when insects/disease have caused damage to your plants/lawn.


So I know, by now you have added all these numbers and see that it doesn’t equal 100%. What is the rest ... the remainder of the bag is made up of other nutrients & filler product that help to apply everything evenly.


Hopefully, this has cleared up some confusion. The last little and I think the biggest tip I can share that helps to remember what each # does is my catch all phrase, when your looking at the bag and its numbers while in the store just repeat.... “Up, Down, All around” – this has always helped in a pinch.


So if your thinking that this 10-5-25 bag would be the perfect mix to apply right now (fall) you would be correct – great job!



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