White salt crust is forming on the surface of my field, what does it mean?

A white crust on the surface is salt that has moved up with water and stayed behind when the water evaporated. It is a sign of salinity building up, often from salty irrigation water, poor drainage, or watering little and often so the salt never washes down. Get your soil and your irrigation water both tested for EC so you know how serious it is rather than guessing. The basic fix is to give salts a way to leave: improve drainage so water can move down and out, and apply enough water at a stretch to leach salts below the root zone rather than many tiny waterings that just bring more salt to the top. Adding organic matter like FYM or compost and keeping the soil mulched reduces surface evaporation and crusting. If the crust is heavy and crops are failing, the land may need a proper reclamation plan with drainage and possibly gypsum if it is also sodic, so confirm the type with a soil test and your KVK before spending on amendments. Do not just keep flushing with the same salty water, as that can make it worse.