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I often see fertilizer NPK ratios that are advertised as "percentages" but the numbers do not add up to 100. My Latin is little rusty but I think per centum means per a hundred. So, in that case why do I see formulas like "13-25-12" which only adds up to 50?

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It is somewhat complex.

You need P (just an example). You may want one of different formulation of P: short release, release on long time, liquid. But also about pH. So you have many formulation (and molecules with P), each with different specific weight. The additional atoms makes just the difference on weight. H maybe will not contribute much, but you have O, C, S, Ca, Mg, Fe, and eventually other atoms (but usually in small quantities).

You may have some components just to make fertilizers more stable (e.g. from humidity), or just being granular: often you do not want powder: wind can blow it away, and you do not want to breathe it.

So the total is never 100%, also for N-only (or P-only, or K-only) fertilizers.

Note: 25 in P doesn't mean P has 25% of weight. It is just equivalent to 25% weight of P2O5. See Wikipedia: Converting nutrient analysis to composition, for some details, which I think very few people care but for trivia questions.

Personally, I would not choose a fertilizer with more "useable weight" (which may be difficult to dose), but I'll look the kind of formulation they have. I have basic soil (high pH), so I prefer acid fertilizers. I have a heavy soil, so long term fertilizers are good (so I can fertilize just every 2 to 4 years). But for liquid fertilizers, where high percentage may be easier to handle.

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  • A commercial grower or farmer who buys fertilizer by the tonne wants the highest concentration, to reduce transport and storage costs. On the other hand a hobbyist may want something that is easier to handle without protective clothing and easier to dilute to a useable concentration without making accurate measurements. – alephzero Nov 24 '20 at 19:12
  • @alephzero: My experience is the contrary. Hobbyist tend to prefer high concentration ("it is better"). Fertilizers doesn't occupy much space, consistent concentration helps the mechanical distributor of fertilizer. Less problems, e.g. liability about birds eating fertilizer: they are not hobbyist which can cover fertilizer with soil, smell in near towns, and all extra requirements about storage/fire. [artisanal bombs are made with fertilizers] – Giacomo Catenazzi Nov 24 '20 at 19:37
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That's because fertilizers contain more than only N-P-K. In your example, 50 percent of the weight is something else than nitrogen, phosphorus, or potassium.

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