. . . . . . . . is the ratio of the increase in osmotic pressure produced by the fertilizer to that produced by the same weight of sodium nitrate (NaNO₃)

The salt index is the ratio of the increase in osmotic pressure produced by the fertilizer to that produced by the same weight of sodium nitrate.
It is calculated by comparing the increase in osmotic potential brought about by the addition of that fertilizer material compared to the increase in osmotic potential when an equivalent weight of sodium nitrate is added to water.