Salination, also spelled salinisation, is the process by which the salt concentration of soil or water increases, most problematically as a result of irrigation in hot climates. Irrigation water always carries dissolved salts; when crops use the water and the rest evaporates, the salts stay behind, accumulating in the root zone until the soil becomes too saline for crops. It is one of the oldest agricultural calamities, implicated in the decline of ancient Mesopotamian farming, and it still threatens millions of hectares today. Management means applying extra water to leach salts below the roots, improving drainage, and breeding salt-tolerant varieties. For food security, salination is a slow emergency, steadily shrinking the world’s productive farmland.