How did medieval manors handle population growth? Was there room for more fields to be ploughed?

As population grew, people starved. This is the Malthusian Trap.

As wealth increased, the population grew until people started starving again.

Most GDP growth was captured not in living standards, but in increased population density.

Most GDP in those days was in the form of food production; food was expensive and consumed most of most people's lives.

When you got new people, you could clear new wilderness, you could conquer new land, you could subdivide your farms. All could increase the total productivity of your land, and hence the total population you could support.

Only when there was surplus food could you produce new specialists. Those specialists in turn could improve your food production and/or generate wealth in other ways.

Then something interesting happened. Some regions of Europe had net fertility below their growth rate, even for extended periods of time. See http://www1.umassd.edu/euro/resources/worldeconomy/1.pdf -- it was starting in Netherlands and later the UK. This is believed to be a mixture of later marriage, separate households for the new families, and early improvements in leveraging non-crop based energy (windmills etc).

Fertility fell, production grew, and wealth increased faster than production.

That then fed into the first industrial revolution, where other non-food energy (coal) was consumed to produce value.

Just adding a little perspective: this question seems to originate along the lines of thinking in terms of the Malthusian catastrophe. But the "growing population" in the Middle Ages did not grow that much:

The population levels of Europe during the Middle Ages can be roughly categorized:

• 400–600 (Late Antiquity): population decline
• 600–1000 (Early Middle Ages): stable at a low level, with intermittent growth.
• 1000–1250 (High Middle Ages): population boom and expansion.
• 1250–1348 (Late Middle Ages): stable or intermittently rising at a high level, with fall in 1315–17 in England.
• 1348–1420 (Late Middle Ages): steep decline in England and France, growth in East Central Europe.
• 1420–1470 (Late Middle Ages): stable or intermittently falling to a low level in Western Europe, growth in East Central Europe.
• 1470–onward: slow expansion gaining momentum in the early 16th century.

1000 –– 40 million
1150 –– 60–70
1300 –– 80-100
1350 –– 75–90
1400 –– 52–60
1450 –– 50
1500 –– 61

You have as reasons for that food levels or starvations, illness and epidemics, short life-spans, especially for child bearing women and newborns aka child mortality. Limitations caused by the economic system, disruptions brought on by war and the ever present knowledge, desire and practice of people to control their reproductive behaviour. Not in the least also caused by the slowly emerging Western European marriage pattern.

It took hundreds of years for the population of Europe to double. And for times of population growth, there were also times of a decline in absolute numbers. Curiously, the Black Death was devastating for the absolute numbers, but explains only a portion of the overall decline and affected the subsequent growth or shrink patterns quite differently in different regions of Europe.

_{Images from "Medieval Population Dynamics to 1500 – Part C: the major population changes and demographic trends from 1250 to ca. 1520" (PDF)}

So, while the population growth was not uniformly 'exploding', when growth did occur substantially, for a long time it was indeed possible to just bring new land under cultivation for peasants. A significant number of people also went for the options of military, clergy, or the growing cities.

Generally speaking, this was a period of warm, dry climate through much of Europe, when enormous amounts of new land were brought under cultivation. People would not bring new land under cultivation for no good reason at all. There were obviously mouths to feed.

In rural areas clearing woods, draining wetlands, and settlement of 'uninhabited' lands with farms, villages, monasteries and markets (towns) were the principle expansion methods of Landesausbau.

This landesausbau coincides with the high middle ages, the period before the black death. It is then a bit of a hen and egg problem of theory whether the population expansion during that time was cause or effect of this internal 'colonisation'. When settlements expanded and forests were cleared the opportunities were there for the population to grow. Or the population grew and was forced to expand settlements and cut down the trees. Until this process hit its limits and instability and contractions followed.

For a simulated model manor the balance of methods for food production need to be considered:

Operating on a Three-Field-System with 2/3 in crops and 1/3 in fallow each year, the effects of changing relative areas of grasslands (livestock pasture) and arable (grain crops) on the output of a 100-acre farm:

Grass  Grain  Fallow  manure  grain total-grain stock-output  total
 acres  acres  acres    tons  yield      output      eq-bu    output in bu
 100     0.0    0.0     ––       ––      ––       1000          1000 
  80    13.3    6.7   > 10.0     27.5    366       800          1166
  77    15.3    7.7     10.0     27.5    421       770          1191
  60    26.7   13.3      4.5     16.5    441       600          1041
  40    40.0   20.0      2.0     11.5    460       400           860
  20    53.3   26.7      0.7      8.9    474       200           674
   0    66.7   33.3      0.0      7.5    500         0           500

_{Assumption: output of livestock products is equivalent to 10 bushels of grain per acre. [grass area in acres, grain area in acres, fallow area i acres, manure in tons per arable land, grain yield: bushels per acre, total grain output bushels, stcok put put in equivalent bushels, total output in bushels]}

So, one reaction for eventually overwhelming growth and hitting the limits of the system is of course: change the system. That was at once possible and seemingly necessary:

Some economics of the manor immediately become apparent when the traditional tools of price theory are employed. The nature of the cost function involved in providing protection, in conjunction with some mobility of labor, determines in theory, for example, the size of a manor. Therefore, as population continues to grow, new manors will eventually be formed when, as a result of population growth in any manor, the marginal cost of providing protection exceeds the value of the lord's share of the marginal product of labor. A frontier movement – or the expansion of settled areas – is therefore the necessary result of a continuous growth in population.

Throughout the high and later Middle Ages a groundswell was rising which eventually would surge through the manorial system, undercutting those basic contractual arrangements which had been its economic foundation. The cause of the upheaval was in two developments: additions to the labor force were now encountering diminishing returns, which changed relative factor prices; and an exchange economy was in the process of developing – first within the local areas between manors, then within regions, and finally interregionally. Following closely the growth of the market came the increasing use of money as a medium of exchange. The use of money further lowered the cost of transactions and widened the market, but introduced problems of its own in the form of a variable general-price level.

Stemming initially from population growth and the resulting frontier movement, the increased potential for trade created the conditions for the establishment of local, then regional, and ultimately interregional markets for produced goods. These changes were accompanied by the rise of towns to serve as central places, and later by the establishment of larger political jurisdictions capable of regulating and protecting commerce.

The rise of a market economy was the result of population growth. Increased reliance upon the market brought with it another agent which affected the nature of contractual relations within the manor – a changing price level. Throughout the later Middle Ages, inflation and deflation were to create additional pressures to change existing contractual arrangements.

Besides accounting for the rise of the market, an expanding population within the manorial economy also impelled the alteration of existing contractual arrangements to adapt to the changing value of labor relative to land. A growing population within fixed boundaries resulted in diminishing returns. Land rose in value as it became scarce relative to labor and the rights to its use became important and valuable. Pressures within the manor to adjust to different factor proportions inevitably ran afoul of current customs, and actual contractual arrangements that developed depended upon the costs of altering customary arrangements

Early in the fourteenth century the ultimate consequences of centuries of population growth caught up with Western Europe. Widespread famines in the early decades were followed by plagues in 1347-1351 which recurred irregularly throughout the rest of the century. The combined result of famine and pestilence was to drastically reduce the population, thus raising the land-labor ratio. Trade and commerce, while substantially reduced in volume, nevertheless continued. The price level rose rapidly immediately after the Black Death but fell slowly thereafter throughout the fifteenth century.

These changed economic conditions again required adjustments in the manorial contractual arrangements. The decline in population left the holdings of many peasants and landlords at least partially vacant. The lords initially attempted to force their surviving tenants to take up vacancies on the old customary terms and resisted with such laws as the Statute of Labourers the increase in real wages consistent with the new economic conditions; such attempts quickly came to nought.

The flight of peasants, the competition between lords anxious to attract tenants, and the stubborn refusal of villeins to obey orders defeated these attempts. Only an effective central coercive authority, as developed in Eastern Europe, could have prevented competition for the now very scare labor.

We have seen that the fundamental institutional arrangements of feudalism and manorialism were sufficient to the needs of a day characterized by anarchy, local autarchy, and differential military capacity. The growth of population radically altered this world by creating a market economy, diminishing returns to labor, and the pressures of a changing price level. The institution of the manor no longer constituted an efficient solution to these problems. Required were new fundamental institutional arrangements that would equate the private rate of return with the social rate of return. Such institutions, of course, do not exist completely, even today. Yet private property in the land (the right of the owner to enjoy, exclude, and alienate his possessions) and a free labor market where each man is able to seek his best alternative were important moves in this directions.
_{–– Douglass C. North & Robert Paul Thomas: "The Rise and Fall of the Manorial System: A Theoretical Model", The Journal of Economic History, Vol. 31, No. 4 (Dec., 1971), pp. 777–803.}

One thing to keep in mind here is that "enough fields to plough" has a few very different dimensionalities. Strictly speaking, in order to live, one does have to eat, but not to work. As long as subsistence is ensured, even for a growing population, the work that needs to be done can also be shared among the growing numbers of workers, giving everyone a better and easier time. The high middle ages is the time before protestantism and capitalism so people were mainly instructed to 'do what's needed and do not sin', compared to 'that's all old news, not working is the biggest sin that really matters for your soul in the afterlife – and besides: for all your hunger in this one'.

Nevertheless, we must keep in mind that subsistence needs were the prime motor of agricultural progress before the Commercial Revolution got going, and direct consumption continued thereafter to play a fundamental role in the economic life of the country. Population growth drove nobles, farmers, and peasants to look for new land and improved techniques: more land and improved techniques in turn supported further population growth. The chain reaction may be quickened by the input of commercial capital and initiative, but does not absolutely need it. Before we consider the commercial components of agricultural development, let us assess as best we can, on the basis of the spotty and hence disputable evidence we now have in hand, the elementary ingredients.

So far as we can tell, the rural population continued to grow throughout Europe until the Great Plague of 1346–48, and in some places (especially in eastern Europe) resumed its growth for a number of years after that crisis. It is probable that by the thirteenth century, if not earlier, accelerated urbanization caused the country to gain proportionately less than the cities. Nevertheless, the absolute size of the agricultural population was large enough to take urbanization in its stride without slowing down considerably its absolute natural increase. Again, the famine of 1315–17 had serious effects, but it was followed by many excellent harvests in the period 1325–45, in some parts of England at least; its repercussions can hardly have lingered thirty years to deliver the under-nourished survivors and their children to the Great Plague, as has been sometimes claimed. Any increase in the incidence of birth control, about which we know virtually nothing except that it was practiced, may have affected the population curve more substantially; for only a very high birth rate could insure continuous demographic growth while the average expectation of life at birth, again in England, was about one half that of today: between thirty and thirty-five years according to the best estimates. (This in turn was better than the ancient Roman average of about twenty-five years, almost equal to China's average in 1946, and not much worse than the English average of barely above forty years in 1838–54.)

The tentative demographic profile we have thus traced agrees with available information on the expansion of cultivated areas, which went on, as a whole, unabated up to the mid-thirteenth century or shortly after and did not come anywhere to a full stop before the mid-fourteenth. The widest development opportunities continued to be found in the east and north central plains, where professional colonization entrepreneurs (locatores, sometimes designated by the prestigious title of magistri indaginis, "searchmasters") often assisted German, Slav, and Magyar princes and prelates in the gigantic task of filling the country with new farming settlements and adding strength and efficiency to older ones. We know very little about personalities and individual achievements, but the basic problems and strategy emerge clearly from twelfth and thirteenth century charters. The entrepreneur had to secure a concession and plan the layout of the future village according to the best economic and military considerations. He had to advertise in the more crowded countries to the west the advantages of receiving sizable plots of fertile land under convenient conditions of tenure; in the early period Flemish and Westphalian farmers responded eagerly, but increasing competition with urban labor markets made recruitment harder in spite of the continuing population pressure. Above all, the locator arranged for the transportation of the immigrants, supported them until the first harvest, prepared temporary shelter in enclosed camps, built churches, mills, and other utilities.

In return for this, he normally received from the overlord a package of rights and privileges resembling those of a vassal: part of the land as his free share, rents and dues from such public conveniences as bakeries, fisheries, inns, and mills, and the hereditary charge of administrator and judge of the new settlement. Whatever the original status of the agricultural promoters (there were noblemen, farmers, and townsmen), their economic role was not much unlike that of the merchant promoters we have met in the industrial field: they gathered capital, invested it at considerable risk, and contributed their technical competence and experience to the success of the enterprise.

There was successful expansion at many other places – not only in peripheral areas such as the subpolar forests of northern Scandinavia and Russia or in the war-torn borderland between Christian and Muslim Iberia, but also in smaller underpopulated pockets such as the swampy "fenlands" of England and the broken coasts of western Corsica – but by the mid-thirteenth century most of the better land in western and southern Europe was either thickly settled or fenced off as hunting, fishing, or grazing reserves. When, around 1300, we find that in certain districts of Normandy, Lincolnshire, or the Tuscan highlands the population was as large as today or even larger, we must conclude that some of the cultivated land was marginal and some was overcropped. It is not sure, however, that Europe as a whole had overstepped what then were the optimum limits of intensive agriculture: evidence is too scattered and sometimes inconsistent to warrant generalizations; moreover, optimum limits ought to be calculated with reference to the normal expectations of the times. In other terms, lords and peasants were accustomed to abysmally lower rewards in subsistence and surplus than modern agriculturists take for granted for the same amount of land and labor; so long as those rewards were obtainable, they would not feel that they had transgressed the law of diminishing returns.

We have pointed out at the beginning that progress in medieval agriculture depended to some extent on technological advances, but to a larger extent on the expansion of the cultivated area; the latter increased total production, but only the former could boost per capita production. Under these circumstances, it seems unfortunate that technological advances tended to slow down at the same time as the land still available for expansion shrank. Nearly all of the basic inventions and improvements in medieval agriculture, from the wheeled plough to the first experiments in three-course rotation of crops, can be traced back to the initial stage of expansion or earlier. To the later period we must ascribe a good number of minor improvements, but only two major ones: the introduction of the windmill and the addition to the plough of mould-boards which turned over the soil at greater depth. In agriculture the main technological contribution of the twelfth and thirteenth centuries was not a continuing flow of fresh innovations and more fruitful practices as in commerce and industry, but the slow propagation of earlier ones. This was undoubtedly an important achievement, for new machines and methods had in every instance to be adjusted to the particular ecology and needs of the place; the fact remains, however, that the average yield of cereals only doubled between the tenth century and the fourteenth, whereas commercial profits and industrial production grew at a much higher rate.

Opportunity existed in agriculture as in commerce, but it was less accessible and, above all, less elastic. Not only were there many villages that remained out of reach of the Commercial Revolution, but every village depended on a delicate ecologic balance that was easily upset by progress. Demographic growth was generally a factor of economic development, but if the population of a specific locality grew too fast and the excess was not absorbed by urbanization and emigration, plots became too small for efficient cultivation, and no adequate provision was made for the livestock whose draft power, fleece or meat and, above all, whose manure were badly needed. If, on the contrary, too much space was allotted to sheep-raising or industrial crops, the community lost its primordial asset, the ability to feed itself. Protected seigniorial forests and ponds, the object of peasants' hatred, were no longer large and well distributed enough to neutralize the accumulative ravages of deforestation, soil erosion, and hydraulic disorder caused by countless generations of men and sheep. The small commercial farmer dreaded abnormally bad and abnormally good harvests almost equally: the former depleted granaries, the latter depressed prices. Liquid capital, generally scarce even among the largest landowners, was not always poured into productive investments such as the purchase of tools and livestock, but often spent for irrational acquisition of scattered additional land, or for those enormous banquets and celebrations that broke the monotony of country life. And even the best administrators miscalculated their chances: the extant accounts of estates seldom match the high yields optimistically suggested by manuals of agriculture, and some of them, especially after the mid-thirteenth century, show declines.

Still the worst degree of destitution in the thirteenth century does not seem as stark as that of many serf and slave peasants of the tenth; and increased distances between the very rich and the very poor are the normal result of ages of growth, including our own. All told, there is no reason to depart from the tentative diagnosis we suggested in regard to population growth, expansion of cultures and technological progress: continuing improvement throughout the age of the Commercial Revolution, with constant or increasing acceleration up to the mid-thirteenth century, but with a tendency towards a slackening pace in the last decades of that century and the first half of the fourteenth.

The retardation of agricultural development in the last hundred years before the Great Plague of 1346–48 had an attenuated parallel in a number of disturbances that affected commercial and craft development to some extent. Demography is the sector where the parallelism was closest: many towns seem to have slowed down or arrested their growth after 1250, and even Florence, after attaining her medieval peak around 1300, underwent a very slight population decrease. The adverse trend, however, was not so pronounced or generalized that it may not be explained by intensified birth control or various local causes. The other unfavorable occurrences were still more localized, or external to the main economic process: wars in France, in the Levant, and between or inside towns; taxation and forced loans connected with the wars; bank failures that did not, however, prevent the formation of new banks on the ruins of the old ones; declining profits in ordinary trade, but with lower risks; contractions in the Flemish woollen industry and the Lucchese silk industry that may have been offset by the expansion of younger textile centers; depletion of certain silver mines that was probably compensated by exploitation of other veins. It has been claimed that these and other attritions, combined with the retardation of agricultural growth, prepared the ground for the more serious blow of the Great Plague and its consequences.

It is not our task to follow up the general crisis that brought the Commercial Revolution to a halt after 1348, and it would be impossible to examine in a few lines the debated problems of its origin, impact, limits, and duration. In the writer's opinion, the commercial attrition and agricultural saturation that preceded the crisis were not incurable. They might have been solved if they had not been compounded by three agents of secular depression. These may have been latent before 1346-48 but came to the fore in the second half of the thirteenth century: protracted and destructive wars in western Europe and the entire continent of Asia; a sudden return of periodically recurring, hemisphere-wide epidemics that went on for almost three hundred years; and a new pulsation of climate that made it harder for nature to repair the wounds inflicted by man. War, plague, climatic change: these had been the major disasters that broke the economy of the Roman Empire. Happily, the Commercial Revolution had built up economic strength and resilience far beyond the classic peak of golden mediocrity; hence the economy of medieval Europe declined somewhat, but did not fall.
_{–– Robert S. Lopez: "The Commercial Revolution of the Middle Ages 950–1350", Cambridge University Press: Cambridge, New York, 1971.}

The main ways are through improved field management, clearing woodland, and adoption of dairy.

Crop rotation involves the evolution from two-field to three-field crop rotation starting in Charlemagne's time; followed by the adoption of four-field crop rotation starting in Belgium in the 18th century. The change to three-field crop rotation both reduces the fallow land from 50% to 33% and improves the fertility of all the land. This dramatically increases the crop yield. Four-field crop rotation completely eliminates the fallow need, and improves fertility still more - another big increase to land fertility.

Through the entire time the great forests of Europe are slowly being cleared for farmland, excepting the woodlots needed for fuel, hunting, navy and construction. The clearing is completed in different parts of Europe at quite different times: fairly early in France - giving it a very large relative population right through the Napoleonic era - and very late in much of Germany - even in 1809 there remain vast tracts of forest in Bavaria between the villages.

Finally, the adoption of dairy in NW Europe and particularly the Low Countries dramatically increases the productivity of the land. Marginal land that is completely unsuitable for harvested crops can be highly productive as dairy pasture.