You sound like a hardcore Doomer. There's no way it could get that bad so quickly after peak oil. True, most petroleum use is in transport, but only a tiny fraction of all transport is foodstuff. And this would get priority over personal transport and fancy consumer goods. Only a century after peak oil would we have so little of the stuff left that we couldn't ship grain and flour into the cities of Europe. And by then, if we still haven't found a substitute, then surely we don't deserve to live on.

It's the same with the industrial-agriculture-is-doomed meme. Pesticides are a tiny volume of petrochemistry. They could be made with other inputs of CHON. Diesel for industrial machinery is again a tiny amount of total consumption. could already be replaced by diester without going mad about biofuel acreage (waste products are enough). The only true problem is natural gas used to make fertilizers (and not petroleum). It's several percent of all NatGas use, and if Peak Gas is a total cliff as expected, then it could bite into those few percents. But since what is needed is actually hydrogen, not natgas, we could still find substitutes, we have decades (like electrolysis from renewables of pyrocracking using solar heat).

Granted, phosphorus is a harder problem, but it is a bit less urgent than peak oil and gas. And I expect when peak oil hits the mainstream (that is, 20 years after it has happened and there is no concealing it anymore), it will change a lot in the way governments are held accountable to the management of these resources. So we are not entirely doomed as a specie. The biggest impacts will be socio-economic, and dense fuel-efficient cities are actually a way to mitigate this.

Pierre