a random person is a better guardians of his own self-interest than a random do-gooder.

That isn't obvious. D-G's are likely to be qualified to help people, the people they are helping are likely to have a hsitory of not helping themselves sucessfully.

So the study demonstrates that the farmers were better guardians of their own self-interest

Would they have had access to fertilizer at all w/out the govt? Two heads are better than one, again.