It should be noted here that chemical farming--using applied NPK--itself isn't considered sustainable, even though that's the dominant conventional agricultural practice. There are organic, indigenous sources of all these needed elements in organic farming systems that employ compost and manures. Of course, using these would require a far larger change in how we grow food (80% of arable land grows conventional livestock feed), and what we grow. Much of the equipment, as well, goes to conventional livestock production and is subsidized by the fed. The entire system needs to be more thoughtfully analyzed. We actually want more vibrant ecological farming that tends the land near to where we live...an unassailable food system that builds the health of soil, water, and habitats right in our own communities. I write this as a longtime farmer and food system researcher.