Early in human agricultural history the area around Mesopotamia was rich and excellent farming. Rain usually washes salt deeper into the ground but as an area gets drier the less rain means that salt is built up in the soil and it becomes less productive. This is what happened in parts of the Middle East and continues to happen today in many areas around the world. It is estimated that 1% of the world farming area is made unusable each year due to salt buildup. That is 65,000 acres every day!

(The Great Courses – Understanding Genetics: DNA, Genes and Their Real-World Applications)