I think you would be hard pressed to feed a nation with organic farms without oil refineries right now.
The War Department did construct 10 ammonia plants which made precursors for explosives, which could also make fertilizer precursors, and they absolutely switched them over after the war.
But ammonia production capacity doubled by 1950, and that wouldn't have happened without a demand side to the story as well. It also turned out that farmers realized nitrogen fertilizers work really, really well.
100 years isn't long when you're changing the global ecosystem.
The Industrial Revolution began in the late 1700s. We're only now beginning to agree on its long-term consequences, and quite a ways off from agreeing on mitigation steps.
I will repeat that I hope Sri Lanka continues this experiment. Experiments like this are the only way we'll get the data we need.
It is absolutely ignorant to believe that organic is better for the environment than non-organic. Oil refineries and heavy industry are needed to support farming enough to feed the world, organic or not. And organic food is more expensive, in terms of resources consumed, not less.
Here's another: If I recall correctly, after WWII the US didn't know what to do with all its munitions factories. They realized that the basic ingredients of bombs were good at fixing nitrogen in the soil, so they made minor conversions to the factories and started producing chemical fertilizers. Fifty years later, Timothy McVeigh was able to buy a truckload of the stuff and bomb a federal building.
I hope Sri Lanka continues this experiment. Could it be that after so many years of conventional agriculture, the soil was too depleted of nutrients to be suitable for food crops? I imagine that things will improve as the land heals.