Road dust

Thus, salt, if not treated properly or put to good use, will become insipid—”lose its saltiness”—and become good for nothing, or what John Stott calls “road dust.”— Sermon on the Mount, page 57

<idle musing>
I like that metaphor, road dust. Growing up and living in the north, you see a lot of that. The roads in the winter are usually sprinkled with a mixture of rock salt and sand (don't get me started on the ethics of that!). Over the course of the winter, the sand breaks the rock salt into tiny little particles. By the end of the winter, its just fine dust that the wind blows hither and yon. But there is enough sand in it that it can't be used as salt. Basically worthless...road dust.
</idle musing>