Why is salt so "precious" in the rain forest?

Salt is precious because the elephants need it to survive. It is precious because it is rare, and the elephants are many.

Herds of elephants go inside Kitum Cave at night to obtain minerals and salts. On the plains, it is easy for elephants to find salt in hardpans and dry water holes, but in the rain forest salt is a precious thing. The cave is large enough to hold as many as seventy elephants at a time.