Being born in the Dutch province of Zeeland and having lived when I grew
upfor about ten years about 10 km away from the Waddenzee in the province of
Groningen, I'll try to answer some of the questions posed.
Indeed, between WWI and WWII the Zuiderzee was closed off from the North
Sea bya dike. Then four polders were made in the Zuiderzee, which was
originally an arm of the North Sea, created in the Middle Ages by the
St.Elisabeth flood. Part of the water of the Rhine mptied in the Zuiderzee.
I do not know, how that fact influence the siltiness of the soil, but from
what I understand of the rivers in Sumer, they brought water from the
mountains too. If that is so, then maybe the fact that the first polder was
prime agricultural land after 10 to 20 years, indicates that the salt may
not take 200 years to leach out. After WWII three more polders were brought
in to production (one was poldered in before the war.) To my knowledge the
silt is gone now.
Zeeland (in the delta of Meuse and Rhine) was a result of the
St.Elisabeth flood as well.