Silicon-based races originally used Durantium, but it was changed to Promethion.  The information about Durantium was never updated.  Only Promethion is needed to build cities.  You do need cities to increase the population limit for planets, but you don't need food, just Promethion.  You should destroy arable land and use the tiles for something better.  If you take over a planet full of farms, you should also destroy those farms.

Also, if you get any event that has choices that give + or - any percentage of food, you can ignore the food part and just look at the other parts of the choice to decide which to choose.  Again, food means nothing to silicon-based races.

Quoting GalCivius,

reply 2

Thanks for the swift answer

I edited my reply with more information.