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I am following instructions (or attempting to) to plant a pecan tree and one of the steps is to add 5-10-15 fertilizer around the tree. I can find no 5-10-15, but readily find 5-5-5, 10-10-10 (and other varieties) If I cannot find the "right" mix, how do I determine what the 'best mistake' to make would be?

At first glance I would think "compromising" to 10-10-10 would be a good solution. However, some initial research suggests "Too Much" nitrogen and "Too Little" potash are both potentially problematic. But I can find no guidance as to what is the "safe range". If 5-10-15 is "just right" is 10-10-10 "too much/too little" or is that within an acceptable 'range' of 'OK'. The author of the article I'm looking at may believe 5-10-15 to be optimal, but plants would never grow in the wild if the "perfect" blend is required.

So, how do I calculate what is "close enough" when dealing with fertilizer?

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The relative NPK ratios are what's important, not so much the specific amounts. For example 5-10-15 has the same ratios as 1-2-3 and can be used interchangeably provided you compensate for strength. The 5-10-15 is 5x stronger than 1-2-3. Hopefully you can find the right ratio and adjust strength as needed.

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  • so, if I am to spread one pound of 5-10-15 I should use 5 pounds of 1-2-3?
    – Cos Callis
    Jun 6, 2018 at 2:26
  • yes. this is why fertilizers with zeros are helpful.
    – OyaMist
    Jun 6, 2018 at 13:36
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I'm no expert, here is my suggestion. I would stay away from synthetic fertilizers completely and try some of the following: manure, compost, rock dust, fish fertilizer & mycorrhizal.

The best part is, they are all harmless and plant/tree can pick and choose what they need from soil. They naturally improve your soil.

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    You have to be careful with rockdust, due to how it's usually high in calcium (it can raise the pH). Some plants don't like it as much as others, whether or not it's the calcium. I've found that our houseplants didn't like basalt rockdust, anyway, while our outdoor cantaloupes appreciated it. If you get a kind with humic acid added, or if you add it only during composting, that might be safer, but I haven't tried that, yet. I've also found basalt rockdust to increase the need for potassium (adding potassium sulfate with it helped a lot). I haven't tried Azomite, glacial rockdust and such.
    – Brōtsyorfuzthrāx
    Jun 5, 2018 at 3:33

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