Well, Montgomery has committed this sin as well: "English farmers gradually increased per-acre grain yields to well above medieval crop yields of twice the seeded amount". I'll leave off the rest of the sentence about early Egyptian crop yields because it is too painful to even contemplate.
The first time I encountered this wildly improbable "fact", I thought, "Self, would _you_ farm if you got a yield of 2-1 for seed? No," responded Self. "Not a fucking chance." Then I thought, "Self, do you think anyone _else_ is dumb enough to farm for a yield of 2-1?" and Self thought about it for a while and replied, doubtfully, "Someone might be that dumb, but they wouldn't successfully reproduce if they spent all their time that way, so it can't possibly be true."
It was the work of seconds -- not even minutes -- to learn the first time that medieval yields were hella higher than 2-1, and that the source of the 2-1 was a poorly understood church storage document that someone mistranslated, misinterpreted and then a lot of people cited without engaging their brains. Which just goes to show that with all the food we have nowadays, it's possible to be much, much, much stupider than you could get away with as a medieval peasant.
This time, a few more seconds on google and wikipedia not only reiterated to me that medieval wheat yields were hella higher than 2-1, but we've got instances on record of people pulling high 20s in bushels per acre. Given that the US wheat yield average is only in the 40s, well, don't be slamming the medieval manor and acting like people were all so smart in the 17th century. I don't have a solid line on seed yield, but it seems clear it wasn't worse than 4-1, ever.
As for the more complex assertions that Montgomery makes about the timing of yield improvements, well, JSTOR has articles that would argue with him about the "gradual" and which centuries it happened in.
Just to be clear: Montgomery is not wrong when he says that soil will flow downhill if you have bad agricultural practices and that has caused problems all over the world at various times. However, he is remarkably unimaginative and uninformed when it comes to _why_ people engaged in bad agricultural practices and his data does not support what he thinks it supports. He has backwards causation all over the place. He repeatedly misrepresents the implications of historic climate variation. And when it comes to demographics, he is breathtakingly innumerate: a population drop of 50% over a couple centuries is remarkably invisible, contrary to what he is at least trying to help the reader infer.
I _like_ that Montgomery doesn't just blindly accept the "tragedy of the commons" explanation. But wow, his alternatives aren't so hot either.
You would think that University of California Press would have insisted this thing be rewritten. In addition to its numerous other problems, it is highly repetitive and simultaneously lacking in detail in the areas I would expect Montgomery's expertise as a geomorphologist to be strongest.