Why did irrigation cause soil salinization in the Karakum Desert?

In the Karakum Desert, irrigation has led to secondary soil salinization—the formation of a salty crust that renders the soil infertile—due to the way water interacts with the arid environment.1

Irrigation water naturally contains dissolved salts. In the desert's hot, arid climate, high rates of evaporation cause this water to evaporate from the soil, leaving the salts behind. When drainage is poor, water cannot move downward through the soil profile to flush these salts away, causing them to accumulate at the surface. Additionally, irrigation can cause saline groundwater to rise toward the surface; as this water evaporates or is transpired by plants, it leaves behind further salt deposits.23

To combat this, improved drainage systems have been constructed to divert salts away from cultivated areas, and leaching techniques are used to wash excess salts out of the soil profile.12