The U.S has large reserves of phosphate, and potassium is available en-masse from Canada. So that is the "P" and "K" in the N-P-K part of fertilizer coding (where, for example, pure ammonia would be 82-0-0, since it is 82 wt% nitrogen, or Potassium Nitrate would be 14-0-39.

There are probably also lots of places where P and K can be obtained, though not in such concentrated forms, but sufficient to use.

The real kicker in fixed nitrogen. But, when in doubt, copy what the Norwegians and Icelanders did, or what was done in Niagara Falls from the mid 1920's through WW2, or what they did in trail, British Columbia from WW2 until they discovered natural gas in a big way in Alberta. Use electricity to make H2 from water, then react that H2 with N2 to make ammonia. No need for coal or Ngas to make ammonia, whatsoever. And since those are pricey and getting pricier, and CO2 burial is also not going to be cheap, well, the electrically derived ammonia gets around that problem (CO2 garbage disposal) just fine. And there may be some ways to make NH3 directly from electricity, water and N2 in high temperature electrolytic cells, with the promise of slightly better energy efficiency.

With lots of electricity, lots of H2 can be made - about 22 to 25 kw-hr/lb of H2, or about 44 to 50 MW-hr/ton (2000 lbs, not the 2200 lbs in a metric tonne), depending on how hard the cells are run. This also eliminates the largest part of conventional (coal or Ngas based) ammonia plants, which is the purification of the H2 from the water-shift reaction. the H2 coming from electrolysis, once dried of any water, tends to be very pure, simplifies downstream operations, too.

Anyway, this NH3 made from renewable energy won't be cheap, but it will be less expensive than NH3 made from Ngas at current prices (which do not include CO2 trash disposal). And since those prices will rise, as will imported NH3 prices (due to the devaluation of the dollar and rising world prices of Ngas), the renewable approach has the added effect of stabilizing prices, potentially, of this valuable farm input. Plus, it helps take more of the hydrocarbon inputs out of the farm cost equation.

Nb41