I think the passage is talking about a Latin American Indian language, in which there is a concept not included/covered in English: i.e., that a verb is differentiated on the basis that the subject is not human, or that it is.

In English, the passive puts the emphasis on the action, but there is an actor (stated or implied) that would be human in the case of "potatoes are produced"; either the farmer/agribusiness or God produces the potatoes (although that does leave the question of whether God/Higher Sources is/are to be considered not-human).

I think what you need is something different from normal English, along these lines:

'potatoes prosper themselves* (grow)'
'potatoes are prospered* (grow)'

*non-standard English

or cultivated

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Note added at 2 days3 hrs (2007-10-02 17:01:37 GMT)
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Although I don't think that use of the reflexive implies human agency, it would be clearer to say that the potatoes produce themselves, even though it sounds ridiculous.
I'm going to withdraw my answer.