There is so little organic matter in the topsoil due to modern farming practices and it gets worse every year. The result is soil that doesn't retain water, it just runs off. This means that irrigation has to be more and more frequent, and it means that fertilizer runs off (causing algal blooms etc) more than it percolates into the soil
Lots of places to read more about this issue and here's a start: https://www.nhm.ac.uk/discover/soil-degradation.html