Thanks for your comment, Kwabia. By a "public good," I mean something that is non-rival and non-excludable. Under this definition, fertilizer is not even an intermediate public good. That fertilizer use may lower food prices doesn't mean there are externalities: externalities are those effects that are not captured by the price mechanism. The problems with middlemen, bargaining power of production agents, and allocation of credit along political, religious and other grounds that you mention are all problems of government failure. These are some of the downsides that occur when we try to correct a (putative) market failure with government intervention.