Why are white crusts on the ground a warning sign?

The answer

A white crust often means dissolved salts have been left behind as water evaporated from the surface. High salt levels make it hard for plants to take up water, so crops can wilt even in damp soil. Crusts typically show in low spots or along the edges of wet patches, and they are often linked to poorly drained irrigation. A laboratory test of the soil can confirm whether the problem is salinity, sodicity or something else, because the remedies differ.