There's more than nitrogen in hay. That's all that was discussed in the previous post about the nitrogen cycle but all primary and secondary nutrients are contained in good hay. Phosphorous, potassium and all of the secondary nutrients and trace minerals are found in varying amounts depending on the hay species and its quality. You can do forage tests and find out just how complete it is if you want to be precise.

This brings up an interesting point: what I'm advocating, seen from an external perspective, is strip mining the hay grower's land to fertilize my own. Unless that grower is dosing his land with balanced nutrients then his land degrades in proportion to my land's improvement. If he does then I am in effect dosing my land with those nutrients too, but it is concealed in the bought in hay. In the end somebody has to pay the bill by digging up rocks and spreading them on ag land or else the whole system grinds down to a low level. The idea that "chemical" fertilizers can be avoided is false. The best you can do is play hide-the-pea by letting someone else use them so that you can pretend to some sort of illusory purity. It may be several steps removed from your immediate use, but the whole edifice rests on manufactured fertility. Without it you regress to third world production levels and food insecurity like the developing world, but more so.

For all of human history lands have been used, abused, exhausted and abandoned to heal over deep time except in a few special places where nutrients are renewed by annual flood, glacial milk (mineral rich melt water), or imported organic matter stripped from other places. There's no free lunch.

Over geologic time lands are renewed, cycling through the geosphere, but they go through a succession of waxing or waning fertility. There's no steady state, no "sustainable" system. Managing ag land can be seen as a faster version of these deep natural rhythms more appropriate for human life spans and species longevity. When done with insight it is natural, but faster.