In the 17th century (and perhaps also the 18th century?) you were importing grain from the Polish-Lithuanian Commonwealth and from Russia (and I've read that you imported the amount of food which was enough to feed between 500,000 and 1,000,000 people, depending on year / period). I wonder when that import was stopped (I know that Russia became the main exporter together with the decline of Poland-Lithuania, but I don't know how long was Russia exporting grain to the Low Countries). Surely you later became self-sufficient, together with the development of farming technology and the increase of the number of polders. But when? Hmmm...

Were there many polders already in the 17th century? When did the number of polders start to rapidly grow?