NO DOUBT YOU HAVE READ THAT, UNTIL RECENT TIMES, relatively (go back not more than a century), until recent times, the valley of the Nile was considered to have the most fertile soil in the world because, every year at the flood season, the land was so low, was so little above the normal level of the river at the flood season, water overflowed the farm lands. During the two or three months that the lands was flooded, all the slat and alkali that had been accumulating in the soil was leached out of it and washed away, down to the sea.
In the Salt River valley in Arizona (the city of Phoenix is in that valley), about 800 years ago the Indians had a surprisingly high civilization there. They irrigated a great area with the waters of the Salt River. As a matter of fact, the modern irrigation system consists mostly of just a deepening of the remains of the old Indian canals; they were that well engineered. But what happened was this: the water put upon the land dissolved any alkali or salt that was in the soil, and brought it to the surface; and in that hot, dry climate, there was no run-off; the water simply came to the surface and evaporated into the air. So the water was constantly dissolving and bringing to the surface the alkali and slat, and then leaving it there. And over a period of time, the salt and alkali accumulated to the point where the people could no longer raise any crops, because the fertility of the soil was destroyed, and the Indians were starved out about 800 years ago.
The same thing could happen any place else in the world, but it couldn’t happen in the Nile valley because, every year, the flood waters of the Nile flushed out any alkali or salt that had accumulated in the upper part of the soil.