How about rotating crops with nitrogen fixing plants?
It's common where I grew up to rotate corn and soybeans, and some other crops. Not only are soybeans good at fixing nitrogen on residual root nodules, the rotation reduces some pests that can overwinter in the ground. While the soy rotation helps, both crops need phosphorus and potassium, and additional nitrogen helped the corn yield even better. Corn was the cash crop. Soybeans could be hit or miss on profitability, and they had their own pest problems like spider mites, Japanese beetles, and required more herbicides than corn because corn could outgrow many weeds while the beans would more easily lose soil, water, and sunshine to weeds.
Getting back a little bit more toward the GMO topic, and overlapping with the issues of farm chemicals and commercial fertilizers, it isn't a case where farmers are eager to use lots of chemicals and buy lots of GM seed. That stuff is all EXPENSIVE. Fertilizer is expensive. Fuel is expensive. Chemicals are expensive. Seed is expensive. Equipment is expensive to buy/maintain/repair. Land is expensive. To a great extent, the old-fashioned days of family farming is gone. Some have gotten bigger and are profitable. Many struggle. All it takes is one big bad decision and the business is over. Like my grandfather used to say, "If I won a million dollars, I'd farm until it was all gone!"
