I know what is NOT sustainable agriculture. Exporting grains to china ( or any where else) for their livestock to eat, for their manure to fertilize chinese fields, employing their farm workers, and meat processors. While livestock farmers in North America continue to leave the industry, and more jobs are exported along with the raw material. Not to pick on the Chinese specifically, but their livestock industry, particularly hogs is growing exponentially, while we happily sell them raw material ( grain) for them to value add, rather than feeding it here and selling finished product, meat. The only sustainable model that I see is a mixed farm feeding all production to their own livestock, eventually returning all nutrients and fiber back to the land where it came from. To take it a step further, the waste and byproducts from the ultimate end user, people, is lost forever from the farm that produced it, as it gets buried in landfills or washed away in rivers. Some day, all that waste should come back to the land that it came from too. It already does in many asian countries.

 Another aspect that I don't consider to be sustainable is tenant farmers.  Who will look after their land for future generations better, the owner who farms it, or a long distance renter with a short term lease?