North Korea as a controlled experiment

Here's two interesting images thanks to Eugenia Kalnay: 

The first is a plot of output (metric tons) of different crops in North Korea over time, the second fertilizer use, also in metric tons. Notice that there is a sharp drop-off of over 70%. This coincides with the 1991 end of perestroika, or the final collapse of the Soviet Union. Around this time, the rapid denationalization of fuel industries in Russia and economic turbulence resulted in an elimination of oil aid from Russia to North Korea. 

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Similarly, the lack of fossil fuels limited the ability to produce and implement nitrogen fertilizer in North Korea, especially compared with other Eastern countries. 

Say what you want about North Korea, but its incredible isolationism has made it well suited to gauging the impact of macroeconomic policy, and here serves to prove the point that availability of fossil fuels, not cropland or technology, is the leading order determiner of agricultural capability.  

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