This won't work. Sure maybe the first year or so. But after you spend thousands of dollars in salt, the rain will just drain it all away. Within 5 years it'll be as good as new unless you dumped literal truckloads and at least 10 cm top layer of salt on it.

There are more efficient and persistent ways of destroying arable land, however I won't share these out of principle. Others however have already said the most obvious: negro does not know how to use it. Small-scale crops and some livestock at most, and that's only the tribes still living in the bushes that have the know-how. Urbanized negroes put in European settlements will just see the supermarket as the source of their food within a generation or two.