I have read that this is only true for the heavy loam of Northern Europe. The Mediterranean scratch plow was fine for the drier, shallower soils of Italy and Greece. But the much thicker layer of topsoil you get in Northern Europe gets depleted fast if you only scratch the top of it -- you _need_ the iron plow to get that woodland dirt to produce. The tome I read claimed that without the iron plow, Northern Europe was _less_ productive than the Med., but with it, it became more productive.
Anyway, my point was that iron plows are not necessarily advantageous everywhere, though they are certainly useful in Sartar and Peloria. I predict they may not be so functional in, say, Fonrit or Prax. In fact, this may be one reason it's so hard to found colonies in Prax -- the poor bastards, knowing the soil is bad there, take their very finest stuff: iron plows, and ruin the soil thereby, getting piss-poor crop yield, and never understanding why.
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