Understanding of NPK percentages as opposed to fertilizer RATIOS is important. NPK %s tell us how much (N)itrogen, (P)hosphorous pentoxide, and (K) potassium oxide (the symbol for potassium is 'K') are in a fertilizer by weight. So a fertilizer that is labeled "All Purpose 24-8-16" is 24% nitrogen, 8% phosphorous, and 16% potassium. 12-4-8 is also a common "all-purpose" fertilizer. It has exactly half the nutrients of 24-8-16, but both are 3:1:2 RATIO fertilizers. Ratios are a way of describing the amount of nutrients in a fertilizer as they relate to each other. Why is this important? It is important because we know that on average, plants use about 6 times as much N as P, and they use about 3/5 as much K as N, and now I will tell you how we can use this information to our plant's advantage.
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When you're employing the 'shotgun' approach and fertilizing w/o a soil test to give you direction, one fertilizer ratio is pretty much as good as the next, though a case can easily be made that even with the shotgun approach, choosing a fertilizer that supplies nutrients in a % multiple as close to a 3:1:2 ratio as possible is probably the wisest and least expensive choice.
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Links
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Allotments
NPK Values
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