[keithly]

The problem is agricultural practices that lead to increased runoff, not the amount of nitrogen naturally occurring in the soil or the amount applied.

https://www.desmoinesregister.com/story/money/agriculture/20...

"Nitrate levels in Iowa's major rivers have increased more than threefold since the 1950s, but have stabilized — and even slightly declined — in recent decades, according to an in-depth review of available research."