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I've heard of at least a few different grades of fertilizer:

Are these standards? Where are they defined? What's the difference between each (and any other grades I missed)?

Edit: I assume most of these have something to do with the levels of purity and contaminants, and I've read the horticultural grade does, but I haven't heard anything on that for the others. As you see in the link above, one person also says that horticultural grade and agricultural grade have to do with size of granules and the number of micronutrients. Other than that, I'm not sure what to think of the rest of these.

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I'm in the UK, have been in the horticultural industry for 30 years plus, and I've never heard these expressions used. All I can tell you is this - if I'm considering buying an fertilizer for any purpose whatsoever, I always examine the label - I'm looking for the NPK readout, and the trace elements (if there are any). N is Nitrogen, P is Phosphorus, and K is Potassium or Potash, both names apply. That means, if I'm looking for a balanced fertilizer, I'll be looking for an NPK read out of something like 7:7:7 - if I'm looking for a high nitrogen one, then it might be 14:2:4 (usually lawn treatments). The NPK is the critical piece of information, along with, in good fertilizers, inclusion of various trace elements, depending on what the fertilizer is intended to be used for.

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