Having grown up on a poultry farm, one thing we had plenty of was nitrogen-rich manure, which our crop farmer was only too happy to use on our corn and wheat crops. Why? It saved money on inputs. Using manure every year pretty much obviated the need for large quantities of nitrogen-based fertilizers, since our corn and wheat got their nitrogen in the form of ammonia from the manure every year. The appearance of a soy crop in our field was a rare occasion, with the primary need to alternate once in awhile to keep down corn- or wheat-specific diseases. One thing for sure, we weren’t contributing to excess nitrate runoffs into nearby creeks and water bodies, because there wasn’t a need to over-fertilize with chemicals.
While some farms today are trying to get off the mono-crop bandwagon and rotate crops, many large farms still tend to grow corn or wheat in the same field every year, requiring a lot of nitrogen-based fertilizer. Rotating corn and soy, of course will help the soil and decrease the need for nitrogen-based fertilizers. In fact soy works with nitrogen-fixing bacteria to pull nitrogen out of the air and provide the nutrients (ammonia) that the soy plant requires—a true symbiotic relationship between soy and bacteria.