I was reading Peter Wells' Barbarians to Angels (2008), a book arguing that the early Middle Ages were anything but Dark, and he says this:
Of fundamental importance was the development of a new technology of agriculture — the moldboard plow — which vastly increased the efficiency of food production beyond anything in Roman times. This new technology meant that fewer people could produce larger harvests than was possible earlier, thereby releasing many former farmers to work in other, specialized, activities such as manufacturing, trade, and building. (11)
Other technological improvements also contributed to making food production more efficient. The development of the horse collar allowed this faster and stronger animal to replace oxen on some farms as the draft animal pulling the plow. The introduction of the three-field system increased agricultural yields. . . . These three changes — the moldboard plow, the horse collar, and the three-field system — enabled farmers to feel their communities at an unprecedented level of efficiency. (132)
I looked into this problem several years ago but never had any reason to write about it, until Peter Wells inspired me to dive back in. If the experts on Roman agriculture are right, what Wells says, and what all the other textbooks that repeat the same line say, is nonsense. Roman agriculture was actually more intensive and productive than anything practiced in the Middle Ages, and in fact some authorities say the western Europeans did not exceed Roman productivity until after 1850. Here is a fairly typical sample of contemporary writing on Roman agriculture:
The best informed authorities have long acknowledged that Roman farming was both sophisticated and productive, with clear evidence that the ancients had anticipated the critical innovations most responsible for the modern agricultural revolution: seed selection; effective tillage; hoeing and harrowing to destroy weeds; crop rotations; the suppression of bare fallow; the rotation of legumes, whether for human consumption, fodder or green manure; irrigation, particularly of meadows and garden vegetables; artificial leys sown with leguminous fodder crops; housing of livestock; improved manure management; careful grazing management for range and pasture land; and, most decisively, as I have argued in a number of publications, ley farming or convertible husbandry, still the most effective system of intensive mixed farming. . . .If you have read much about modern organic farming you have probably seen ancient Roman works on farming cited approvingly; one gardening book I read noted that using methods described by Columella (c 70 AD) organic vineyards can equal the yields of more "modern" operations, and in a more sustainable way.
Within the highly urbanized and affluent heartland of the Roman empire, our sources and archaeological evidence present a coherent picture of market-oriented intensive mixed farming, viticulture, arboriculture and market gardening, comparable, and often superior, in its productivity and agronomic expertise to the best agricultural practice of England, the Low Countries, France (wine), and Northern Italy in the mid 19th century. GrecoRoman farmers supplied a large urban population equal to, if not significantly greater than, that of early 19th century Italy and Greece, with a diet rich, not just in cereals, but in meat, wine, olive oil, fish, condiments, fresh fruit and vegetables.
Until 19th-century farmers began fertilizing with guano mined from Pacific islands, intensive farming required manure. It could be practiced only where there was a sufficient density of animals and people to created the requisite dung. With enough water, dung and attention, yields per acre could be boosted ten- or twenty-fold. But that was expensive and could only be profitable where there were nearby markets. Traditional agriculture could therefore get into a virtuous cycle with urban populations: more people meant more dung and higher crop prices, which meant more effort could be put into intensive farming, supporting higher populations. This happened in ancient Egypt, Italy and other favored locations; during the Middle Ages it happened in the Low Countries and around Paris and London. Part of the increase in productivity noted by historians of northern Europe came, not from new methods, but from population increase and the rise of urban markets that made it profitable for farmers to work their land more intensively.
This also explains how some parts of the Classical world came to have such primitive agriculture in more recent times. Especially in Greece and Sicily 19th-century travelers marveled at the primitive farming methods they saw and rhapsodized about peasants whose lives had been unchanged since the dawn of time. The more astute puzzled over how ancient Athens or Syracuse could have been sustained that way. The real story is that in some areas that virtuous cycle was broken: the towns were sacked and abandoned, which tanked the markets and cut off the supply of night soil, so the farmers reverted to subsistence methods that required less investment of money and labor; without profitable work, many people moved away to more prosperous regions.
In conclusion, the history of agriculture is vastly more complex than anything your textbooks told you, which should make you question everything else your textbooks told you about technology, society, or economics.