I was convinced that liquid fertilizer labeling use the % weight by volume convention. However now I bought a magnesium-sulfur based additive. The label clearly uses % weight by weight. Weight by volume is indicated in brackets (g / liter) and by multiplying the density with the % weight by weight the value in the brackets is calculated.

In this case it is clear that % weight by weight is used. However how can one know if there is only % given without any further information?

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    $\begingroup$ This is a recurrent source of confusion. You can only hope that one was careful and correct (m/m), or the concentration refers to a quite diluted aqueous solution, or be aware of the common use in that field. There is not a convention recommending m/v %, so this was a smooth case. $\endgroup$ – Alchimista Feb 6 at 8:29

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