I think we have to be very wary of organisations that allow ideology to lead science instead of the other way around.  The natural / incidental / engineered disctinction is simply a fudge to allow the Soil Association to level a blanket ban on nanomaterials while addressing the criticism that nanomaterials are already present and in and exploited for food production - ice cream and ricotta cheese to name but two. 

The Soil Association tied itself in the same rhetorical knots when it tried to regulate presticides that could be used on organic crops.  Pesticides designed to have minimum impact on farmers and the environment were banned because they were "man made", whilst copper sulphate, a chemical so toxic to such a wide range of organisms that agricultural scientists refer to it as a 'soil sterilant', was endorsed because it is deemed "natural". 

Any distinction that tries to communicate risk needs to be derived from risk measurement.  I don't rebuff the Soil Association's distinctions entirely - they may be useful for discussion - but banning one category over another implies some kind of underlying, tangible difference which simply doesn't exist.

-- Frank Swain (@SciencePunk)