"Is it possible that we are all deluded into thinking that we need fertilizer, or will they hit a wall once they have used up the reserves of P and K ( and N if cereals still can't fix it)  in four feet of soil, which may take a while."

I think you hit the nail on the head with this statement.  My guess would be that they are using - relying on - native amounts on mineral nutrients in the soil.  At some point, they will become depleted to the point where they need to add nutrients in the form of manures, fertilizers, etc.  It's a matter of "when".  I would think this is the same process that your ancestors used when they broke up the prairies of the USA & Canada.

As to the nitrogen levels, it might be interesting to have a long-term study of organic matter levels in those soils.  I wonder if they are - over time - depleting the soil O.M. levels, and that is where they are getting at least some of the nitrogen to feed their crops.  Again, at some point, that will no longer be sufficient, and they will have to find another way of doing business.