Donald G. Baker
Department of Soil, Water, and Climate
University of Minnesota
St. Paul, Minnesota


The word revolution used in the title indicates that three sudden and radical changes occurred to agriculture. I believe you will agree that the changes were radical but two of the three were far from sudden. So, obviously, use of the word revolution is applied with a certain license.

In each of the three so-called revolutions, technology and climate played major roles. The technology in the form of an implement, an instrument or a process creating the revolution is usually relatively easy to cite. However, the part climate played is more difficult to pinpoint. Climate is like civilization itself, we can look back upon it and recognize changes that have occurred, but definitive explanations as to causes and timing remain elusive.

With this proviso - - let us proceed.

(This document is still under construction. Figures will be added as we receive permission and as time is available. References to sources of most of the documents not locally produced are listed at the end if you wish to see them before we can make them available here.)

1. Pre-History

Previous to the domestication of plants, man was a hunter and a gatherer and most probably stored little food. As determined by anthropologists who tested their theories on wild plants in the mid-east, Fig. 1, where progenitors of our modern small grains still exist, a gatherer more or less unconsciously seeks plants that have larger seeds, more seeds per ear, and a compact inflorescence (Evans, 1980). The loose inflorescence of oats and the relative difficulty in gathering it may explain why oats was probably the last of the small grains to be domesticated, Fig. 2.

The main point I wish to make is that man was measuring yield, consciously or unconsciously, in terms of yield per human effort or yield per time involved. This then can be considered as the first yield expression.

2. The First Agricultural Revolution

The domestication of plants, possibly the greatest single milestone in man's history, Fig.3, is generally accepted as the point where plants retained their seed upon maturity. Thus, threshing is required as well as the intentional sowing of the grain, since the succeeding crop no longer occurs as a result of the natural shedding of the seed due to movement of the plant, for example, by the wind or passing animals. Over time, yields have been expressed in three different ways and a fourth may come into use in the near future. They are shown in Table 1.

Table 1. Crop yield expressions and approximate period of use.

Yield/human effort* Pre-Domestication of plants
Yield/seed sown 3-8,000 B.C. to 1000-1500 A.D.
Yield/land area 1000-1500 A.D. to present
Yield/critical factor future use?

The first agricultural revolution and the associated dawn of civilization apparently coincided with the warming of the earth centered around 5-6000 years ago, Fig. 4, following the end of the Pleistocene, the last "ice age". The ice age ended about 8-10,000 years ago. The climate change that occurred following this ice age was a definite improvement and undoubtedly played an important part in this first revolution. If for nothing else, it meant that attention could be devoted to more than keeping the "home" fires or perhaps the "cave" fires burning.

With plant domestication it was no longer possible to rely on naturally scattered grain for regeneration of the crop. Sowing became essential along with the necessary self-discipline required to hold some grain aside as seed for the next year. Along with this self-discipline came a greater awareness of the weather and its powerful influence on crop yields as noted, for example, in the famous Biblical story of Joseph and his plans for the 7 full years and the 7 years of famine (according to Biblical scholars occurring between 2000-1700 B.C.) . Sacrifices to the gods were no longer sufficient. In other words the domestication of plants also brought about a domestication of man, a civilizing influence, if you wish (Evans, 1980). Ever since agriculture and civilization have gone hand in hand.

With agriculture, a whole new set of conditions came into being, generally associated with the dawn of civilization, Table 2, and a new criterion of determining yield was established. That is, yield of grain compared to the amount sown. This is the measure mentioned in the Bible, also by Roman writers, and by the ancient Chinese. At the time of Christ wheat yields were about 3 or 4 to 1, but on good, fertile soils they could be considerably higher (Evans, 1980). By comparison, the average Minnesota wheat yield today is approximately 100 to 1. Although this comparison is not exactly fair, since the seed of modern wheat plants is also larger and may have other advantages as well. The return with commercial corn today is about 800-1000 to 1.

Agricultural implements initially may have been weapons which served a secondary purpose as a tool to scratch the surface. From these rudimentary beginnings there developed the hoe and the plow. The plow is considered to be the most important agricultural implement since the beginning of history. First there was a foot plow, shown at the top of Fig. 5, also called "digging sticks" and used as shown in Egyptian tomb paintings, Fig. 6, then the "ard plow", also termed a "traction plow," Fig. 7, which was at first human powered and required two to service it as illustrated, Fig. 8. (There is some discussion whether these instruments should be termed plows, since they lack two parts, the colter and the moldboard, often used to differentiate a plow from similar instruments.) A somewhat advanced "ard plow" is illustrated in a Chinese drawing, Fig. 9, dated about the sixth century A.D.

Table 2. Events of the first agricultural revolution.


3. The Second Agricultural Revolution

The next method of yield measurement introduces us to the second great agricultural revolution and provides us with a fascinating story that few people even in agriculture are aware of. Like the first revolution, it rests upon a favorable climate and upon two special features - an improved plow and the horse. Together these three, climate, plow, and horse, created a revolution in agriculture that can be compared to the post-World War II agricultural revolution, though it extended over a much longer period. This second revolution took place beginning about 500 or 600 A.D. (Burke, 1978) and is centered upon the Medieval period. It is little known because of a peculiarity of many historians that is only belatedly being corrected. For too long historians wrote with a view limited to man's conflicts and his artistic and literary accomplishments. The fact that technology and those who contribute to it are seldom recognized is a phenomenon of long standing in the western world (Gimpel, 1976). This was noted by Plato, and even Leonardo da Vinci felt the scorn of the intellectuals of his day who considered him little better than a manual worker or technician. The existence of these two groups, that is, the literary or artistic individual and the scientist or technician existing side by side but with virtually no contact, was a frequent subject of the late English author C.P. Snow. He wrote of the gulf existing between the two groups, or "cultures" in his terminology, and noted that the distance between Greenwich Village in New York and the Massachusetts Institute of Technology in Boston is like an ocean. As a matter of fact members of Greenwich Village, New York, are much closer intellectually to Chelsea, London's home of the arts, while the M.I.T. staff in Boston are closer to South Kensington, London's science center, than they are to each other (Gimpel, 1976). In other words the literary or artistic types simply neither understand nor appreciate the scientist and technician.

a. The "Dark Ages"

The historian of the past seldom considered the technology or science of an age with more than a passing reference. For this reason we have the Medieval period dismissed until recently as the Dark Ages. In fact it was anything but that, except perhaps immediately after the breakup of the Roman Empire. (In this regard it is interesting to read what the Encyclopedia Britannica, 1945 printing, has to say about this period: "Dark Ages was a term formerly used to cover the whole period between the end of classical civilization, [that is, Greek and Roman period] and the revival of learning in the 15th century. Use of the term [Dark Ages] implied an exclusive respect for classical standards in literature and art and a corresponding disparagement of all that was achieved between the decline of ancient culture and the work of Renaissance scholars, writers, and artists."). In many respects the Medieval period has outshone even the Renaissance, which the conventional historian had convinced us was the real flowering of man's intellect. The foundations of our present technologically oriented society were not laid in the Italian Renaissance or the English Industrial Revolution but in the Medieval period.

Based on the generally accepted view of medieval man it is difficult to realize that he was surrounded by machines (Gimpel, 1976). Water power was developed as it had not been by the Greeks and Romans. The Domesday Book of William the Conqueror records a total of 5624 water mills in the England of 1086 A.D. There apparently were two reasons for the lack of mills in Rome and Greece. One was the dependence upon slaves. The second reason, and perhaps the more important of the two, is the Mediterranean climate which does not provide sufficient summer precipitation for a constant stream flow, as noted in Fig. 10, comparing monthly precipitation at Athens and Rome with that of the Twin Cities.

Water power in the Medieval period was not always limited to a static power source as, for example, a mill adjacent to a dam. Sometimes the mills were mounted on barges permitting their movement as either business or stream flow varied. Medieval man also made use of wind power. Windmills may have been introduced from the plateaus of ancient Persia (modern Iran and Afghanistan) (Gimpel, 1976). Wind was a power source well adapted to the level plains of northern Europe. Again this was a power source little used if at all in Greece or Rome. Finally and most surprisingly was the use of tidal power, an energy source currently under investigation by our Department of Energy, or at least it was during the energy emergency of the 1970's.

Medieval man was an artisan in the broadest sense, that is, an artist and a mechanic. Churches such as the great cathedrals of Europe having stained glass windows such as those of Sainte Chapelle, Paris, Fig. 11, or the south aisle of Winchester Cathedral, England, Fig. 12, which were built essentially within the period 1180 to 1250 A.D. (Clark, 1948). They show better than any words the skill and artistry of the craftsmen. The engineering and technical knowledge demonstrated by these structures is that of accomplished engineers and artisans, they cannot be the product of a "dark age," while the development of power sources demonstrates the mechanical ability of Medieval man. Modern historians are now having to revise history, and the term Dark Ages has been dropped in favor of the Medieval period, subdivided into Early, Middle, and Late.

b. The Climate

The conventional historian also overlooked a number of factors relative to man's agricultural progress in the period from about 600 or 800 to 1200 A.D. First, it was a period that was climatically advantageous for agriculture, Fig. 13. In fact, it is known as the "Medieval Climatic Optimum" because the climate was both warmer and drier than it had been for some time either before or afterwards. This warmer climate may also have reduced forest expansion or even caused it to retreat. Because of the higher temperatures, crops could be grown at greater elevations. For example, in northern England during the World War II food emergency plowing campaign of 1940-44 elevations were reached which had not been under the plow since the Medieval period (Gimpel, 1976).

c. The Implements

As already noted, the first agricultural tools were probably the hoe followed by the foot and "ard" plows. Then came the Mediterranean "scratch" or swing plow, Fig. 14. The scratch plow, essentially a sharpened stick with handles for guidance and a pole for attachment to an animal or a human, was adapted to light or coarse textured soils. It is shown in this delightful wooden model of oxen and plow found in an Egyptian tomb of about 2000 B.C., Fig. 15. It may even have been in use three to four thousand years before Christ. In order to prepare an adequate seedbed with this plow it was necessary to crisscross the field as shown in Fig. 16. As a result the field shape was usually square.

A statement attributed to Daniel Webster can be interjected at this point: "In tillage is the beginning of all art." Since the presence of art implies that there is a civilization, several important ideas can be gleaned from this statement. First, agriculture, as evidenced by tillage, not only requires skill but is an obvious manifestation of civilization. Today it is not as common as it once was to define an artist as "one versed in the practice of a fine manual occupation, as sewing." Nor was it common to restrict the definition of art to only the fine arts. We have only to consider the former names of some universities to remember that art and an artist can be broadly defined: Michigan State and Iowa State universities once carried the name Agricultural and Mechanic Arts, just as Texas A and M and Oklahoma A and M do today. And up to a few years ago a major high school in St. Paul was named Mechanic Arts High School. Even architecture was taught there.

Sometime in the sixth century A.D. a different plow appeared which carried two extremely important features: a knife called a "colter," which could cut through heavy roots, and a mold-board which lifted the cut soil to one side, Fig. 17. These two features serve to define a plow and separate it from other instruments. These features, the knife and the moldboard, wrought major changes in Medieval agriculture, especially in combination with the horse and the very essential horse collar which probably entered the scene a century or two later.

Wet fields could be plowed and the furrows running the full length of a field improved the drainage. With this plow deeper rooted vegetation could be removed and the heavier (finer textured) more inherently fertile soils of northern Europe could also be worked. The previously forested lands were now entered, and they became the "new lands" of the Medieval period. It is of interest that most European cities with "new" in their name, such as Neuchatel, Switzerland, and Neumunster, Germany, were formed at this time. Indeed, chronicles describing the removal of forests and the settling of people in northern Europe, have been compared to the stories of the opening of the American West (Burke, 1978). A word was even developed to describe what was taking place in France at that time. It was "essart", now spelled "assart," (which can be found in unabridged English dictionaries) which means the grubbing up of the trees.

This new plow, in addition to being heavier, was also longer as more than two animals were frequently used to pull the plow. This necessitated a major change. For more efficient use of the plow, the shape of fields was changed from square to rectangular, Fig. 18, since even with a front wheel, Fig. 19, and Fig. 20, shown in some Medieval illustrations (Burke, 1978), the plow was obviously more difficult to handle at turns than the "scratch" or swing plow. Another change wrought by this plow was the development of cooperatives (Burke, 1978). The plow, together with the animals to power it, represented major investments. Thus, in most cases the investment required could only be accomplished through the peasants banding together in cooperatives and sharing ownership.

Because the long and narrow fields could no longer be readily criss-crossed, the furrows made were not acceptable seedbeds so the harrow was soon developed. Examples of harrows include three: the first used brush held in place by stones, Fig. 21, the second consists of wooden spikes, Fig. 22, even though it was in use in early 19th century, and the last a roller which performed a task similar to the harrow, Fig. 23. I found this roller in southwestern France a few years ago. Upon close inspection, I discovered it consisted of a Roman column.

d. The Horse Collar

Probably of equal importance to the heavy wheeled plow was the development of the horse collar. A collar similar to one used on camels was introduced from the east, perhaps from Bactria (ancient Afghanistan), reaching Europe around 800 or 900 A.D. This device permitted the exploitation of the horse. It is hard to believe but neither the Greeks nor Romans (who represented the "Classic period" for historians) were able to fully exploit the horse because only a variation of the ox yoke was originally used on the horse. A yoke suitable for oxen is shown in Fig. 24 (from a Russian book). The yoke not only pressed on the jugular vein of the horse, but it succeeded in choking the horse if the load were more than about 1000 pounds. In fact the Theodosian code of 438 A.D., the Roman Law under Emperor Theodosius, decreed that a horse should not pull a load greater than 1000 pounds. It was not until 1910 that a French cavalry officer tested this weight limit and determined that a horse would indeed choke if forced to pull a load of that size using a yoke (Gimpel, 1976). Fig. 25 illustrates a modern horse collar. Lovers of Greek art celebrate the genius of Greek sculptures because the horse looks so "noble". Actually the "noble" horse, with its head held high, did so to prevent choking itself.

The Romans had also failed to harness horses so they could work in line. For example, Roman chariots were pulled by two, three, and occasionally four horses with the horses always abreast, never in line. It is interesting to know, too, that the Romans were apparently slow to develop a four-wheeled wagon in which the front axle could be swiveled. If true this probably explains the straight road system that they developed. It appears that the Romans either adopted or reinvented a wagon in which the front axle could be turned, Fig. 26, an invention of the first century B.C. attributed to the Celts (Williams, 1987). With the horse collar and new harness the number of horses in line, not abreast, could now be increased.

Another event that aided in the exploitation of the horse was the development of the lowly horse shoe and nails, Fig. 27. This obviously helped greatly and permitted field work to be done under a wider range of soil and weather conditions, since the shoe gave greater traction and helped prevent hoof rot.

With the introduction of the horse collar, horseshoe, and nails, a remarkable series of events occurred once the horse could be truly exploited, Table 3. The horse, in contrast to the ox, is 50% faster and has greater endurance, working two to three more hours per day (Gimpel, 1976). Thus, in a sense, the limiting factor became the amount of land that could be farmed. Therefore, the new way to express yields became yield per unit land area, bushels or pounds per acre, and so on.

Because more land could now be exploited, the 2-field Roman system of one field fallow and the other in crop was replaced by the 3-field system. With 3 fields only one-third of the land was now in fallow, thus releasing more land for crops. Another advantage of the 3-field system was that a greater variety of crops was possible with a marked dietary improvement. Thus, with two plantings and two harvests a better distribution of labor and a decreased susceptibility to weather and crop losses became a part of the new agriculture (Gimpel, 1976). A further change was the nearly universal growth of oats as a crop for the horses.

It was about the time of this new plow that the unit of land called an acre came into use. It was defined as the amount of land that one horse or two oxen could plow in one day.

The so-called "climatic optimum" which more or less coincided with and was in part responsible for the advances made during the Medieval period ended abruptly. The succeeding century was simply miserable. Not only did the climate become cold and wet in Europe but the 14th century ushered in a hideous famine (1315-1317), the Hundred Years' War (1338 - 1453), the Black Death (1347-1350), and a series of peasant revolts in England as well as the Continent.

Table 3. Events of the Second Agricultural Revolution (Medieval Period)

HORSE 1. 50% Faster
  2. Greater Endurance
  3. Horseshoe + nails
  4. Hitch in line
YIELD -- Yield Per Unit Area
SOCIAL CHANGES 1. Cooperatives
  2. Villages Formed
  3. Population Increase
PHYSICAL CHANGES 1. Field Drainage
  2. Harrow
  3. Field Shape
  4. 3-Field System a. Diet Improvement
    b. Spread of Risk
    c. More Land
  5. Finer Texture Soils Can Be Worked
  6. Deforestation
  7. Oats a Universal Crop

4. The Third Agricultural Revolution

From today's vantage point it is hard to believe that there was little change in agriculture from the Medieval period until about the middle of this century. Sure, tractors were taking over from the horse, and the binder, reaper, and threshing machine were reducing the work required. Nevertheless, the U.S. horse population didn't reach its peak until 1914, and crop yields were not all that much better than they were in the Medieval period of about 700 years earlier. Nor had tillage methods changed much: as the "minute man's" plow of 1775 shows us, Fig. 28, and as shown in these 20th century scenes: Fig. 29, an "ard" plow still used in Italy, the scratch plow in Ecuador, Fig. 30, and two scenes from Korea in 1951, Fig. 31, and Fig. 32. The third scene from Korea, Fig. 33, illustrates a Korean horse of diminutive stature. The small size is due either to malnutrition or a purposeful breeding to obtain miniature horses for children of the imperial family some centuries ago.

This third revolution was the most abrupt of the three. The delay in the application of the accumulated technology was caused by World War II and then the Korean Conflict. But the effects of this revolution are immediately apparent when viewing yield data. Yields throughout the advanced countries, with England's wheat yields as an example, Fig. 34, show a similar post World War II major increase in yield.

It is my contention that a particular climatic anomaly is in part responsible for the recent economic problems faced by Midwestern agriculture in particular. The long term corn yield record of Minnesota, Fig. 35, will be used to demonstrate this. The first portion, 1866-1938 shows a yield averaging only about 30 bushels per acre, which is not much better than a very good yield in the Medieval period (if corn had been grown then). This was followed by a 42 bushel per acre average yield from 1939-1951 when some of the new technology such as commercial fertilizers and hybrid corn began to be applied. This was then followed by a yield trend, 1952-present, that has shown an increase equaling nearly 2 bushels per acre per year as technology became fully adopted. It can be assumed that the trend lines are due to technology (or lack of it) and that the variation about the three trend lines is due to weather. In a very real sense the yield of a crop, as illustrated here, represents an integration of the climate for a given season. In other words, yield can upon occasion serve as proxy evidence of climate.

In Fig. 36 is shown the variation of the annual yields about the trend line. The yield depressions in 1976 and 1988, both drought years, and the wet season of 1993, are very evident. But when viewed not in absolute terms as departures from the appropriate trend line, but as a percentage variation from the general trend lines, Fig. 37, it becomes evident that the yield depressions of 1934 and 1936 were relatively more serious for farmers of that time than the yield depressions of 1976 and 1988 were for today's farmers.

According to this long term yield record the usual state of affairs is seen to be one with large yield variations due to a "hostile" climate, while the "benign" climate from about the late 1940s to the early 1970s, with decreased yield variation, was the unusual feature, an anomaly. The heart of this "benign" climate period was about 1952-1964 as shown in Fig. 38, which is based on the variation of annual temperatures for 13-year periods. It shows just how rare this "benign" period was. It was truly an anomalous event. In other words, the modern farmer should plan for the expected climate, which is one of large yield variation from year to year.

It is my belief that this "benign" period helped bring about an enthusiasm, and indeed a general euphoria which became evident in the 1960s among Midwestern farmers in particular and in U.S. agricultural circles in general. This optimism and a willingness to take greater risks in terms of investment in land was increasing, especially among the younger farmers who had experienced no major adversity up to this point. They were too young to have experienced the 1930s. After all, the weather has been remarkably tranquil during much of their tenure, and yields increased almost every year as the application of technology increased. So unique was this "benign" period that some "experts" felt that technology had even overcome the effects of weather. As a result, the climate, the general economy, and even technology were setting agriculture up for a dizzying ride, first up and then down.

Unbeknownst to all but the keenest of observers was the fact that the "benign" climate had ended in the early 1970s and was returning to the more usual or "hostile" climate of former years. The first indication of this in Minnesota was the early frost of 1974, and three years of declining precipitation that culminated in the drought of 1976.

Soon to follow and a natural culmination of these events was the abrupt contraction of land values. And with a more "hostile" climate, that is, a more variable one, it meant that yields and, therefore, income were no longer reliable and the high land prices could no longer be supported. Thus, the agricultural depression of the 1980s was ushered in.

5. A Fourth Agricultural Revolution?

The third revolution may run its course or it may receive a boost from biotechnology. But with or without the application of a new technology, a fourth method of yield measurement may be used in the near future. It is the ratio of yield to a critical factor other than land. As the critical factor in the past has gone from human effort, to the amount of seed sown, to the amount of land used, it may soon change, for example, to the nitrogen, the phosphorus, or the energy expended. Perhaps the best one would be an economic one, since it also requires a superior bookkeeping system. Thus, the next yield expression might become yield per dollar spent.

Figure Captions

1. The Near East where civilization apparently began. This area now comprises the modern states of Israel, Jordan, Syria, southeastern Turkey and Iraq. It is also known as the "Fertile Crescent". (From p. 59 of Smith, 1995).

2. The inflorescences left to right of barley, oats, and wheat.

3. The seven primary centers of agricultural development with the approximate time periods when plants and animals were first domesticated. (From p. 13 of Smith, 1995.)

4. The average air temperature variations for the past 18,000 years. (From Fig. 184., p. 481, Ahrens, 1994.)

5. Foot plows or digging sticks (top), swing, scratch or spade plows (center and bottom). (Fig. 7, p. 71, from Curwen, 1953.)

6. A view of a digging stick in use in Egypt. (From p. 8 of Burke, 1978.)

7. The ard plow (center). (From Fig. 4, p. 48, Steensberg, 1977.)

8. The ard plow in use. (From Fig. 6, p. 48, Steensberg, 1977.)

9. A Chinese ard in use. (From Fig. 7, p. 48, Steensberg, 1977.)

10. Comparison of average monthly precipitation at two Mediterranean climate stations, Athens and Rome, and the continental climate of the Twin Cities.

11. The stained glass windows of Sainte Chapelle, Paris, France.

12. The south aisle of Winchester Cathedral, England.

13. The average temperature variations of eastern Europe for the last 1200 years. (From Fig. 18.5, p. 481, Ahrens, 1994.)

14. Foot plows or digging sticks (top), swing, scratch, or spade plows (center and bottom). (Fig. 7, p. 71, from Curwen, 1953.)

15. Model of oxen and scratch plows found in an Egyptian tomb dating about 2000 B.C. (From p. 9 of Burke, 1978.)

16. The scratch or swing plow common to the Mediterranean area. The field shape associated with these plows was square. (From p. 62 of Burke, 1978.)

17. The deep plow shown with the colter (knife) and moldboard. (From p. 42 of Gimpel, 1976.)

18. The medieval wheeled plow with knife (colter) and moldboard and the typical rectangular field associated with this plow. (From p. 63 of Burke, 1978.)

19. The medieval heavy wheeled plow with knife (colter) and moldboard with four oxen and three plowmen in 11th Century Britain. (Fig. 13, p. 84 of Curwen and Holt, 1953.)

20. A print of agricultural life in medieval Britain. In foreground is the heavy, wheeled plow and just above it is the essential harrow. (From p. 64 of Burke, 1978.)

21. The bush harrow. Note the log to hold the brush and branches in place. (From p. 114 of Vince, 1983.)

22. A wooden drag harrow used in the early 19th Century in Britain. (From Fig. 24 of Lawson, 1982.)

23. A heavy stone roller used to serve the same purpose as a cultipacker. Photographed in France in 1976. The stone roller is actually a Roman column.

24. An oxen yoke, upper right, attached to a scratch or swing plow. (From Fig. 53, p. 92 from Krasnov, 1987; a Russian book on early agricultural implements. I am unable to provide book title as it is entirely in Russian.)

25. The modern horse collar shown on a team of plowing horses. (From p. 96 of Hall, 1992.)

26. Wagon

27. Horseshoes and nails. (From Fig. 128 of Lawson, 1982.)

28. The "Minute Man" of Lexington and Concord who met the call to arms in 1775. Appropriately there is a simple plow (without colter or moldboard) at his side.

29. A team of oxen pulling an ard plow in modern Italy. (From Fig. 8, p. 75 of Curwen and Hatt, 1953.)

30. A crude stick or swing plow photographed in the Andean plateau, Ecuador, in 1961. (From p. 45 of Hall, 1992.)

31. Plowing in a Korean rice paddy with an ox and a scratch or swing plow, 1951.

32. Plowing in a field near Taegu, Korea, 1951, with an ox and a primitive plow. In the background is an American F-84 taking off with jato assist.

33. Korean farmers with wagons pulled by diminutive horses, 1951.

34. Yield of wheat in England since the Medieval period. (From Evans, 1980.)

35. Average yield of corn (maize) in Minnesota, 1866-1995. (From Baker et al., 1993.)

36. Yield departure in bushels per acre from the appropriate trend lines shown in Fig. 35. (From Baker et al., 1993.)

37. Yield departure in percent from the appropriate trend lines shown in Fig. 35. (From Baker, et al., 1993.)

38. The 13-year running standard deviations of the eastern Minnesota average annual temperatures. A 13-year period was selected because it is the length with the minimum standard deviation of the many lengths tested. (From Baker et.al.,1993.)

Literature Cited

1. Ahrens, C.D. 1994. Meteorology Today. 5th Ed. West Publishing Co., Minneapolis/St. Paul. Pp. 592.

2. Arnold, J. 1977. Farm Wagons And Carts. David and Charles, North Pomfret, Vt. Pp. 144.

3. Baker, D.G., D.L. Ruschy, and R.H. Skaggs. 1993. Agriculture and the Recent "Benign Climate" in Minnesota. Bull. Amer. Meteor. Soc. 74:1035-1040.

4. Burke, J. 1978. Connections. Little, Brown and Co., Boston. Pp. 304.

5. Clark, S.A. 1948. Today in Cathedral France. Robert M. McBride and Co., New York. Pp. 301.

6. Curwen, E.C. 1953. Prehistoric farming of Europe and the Near East. Henry Schuman, Inc. N.Y. 329 pp.

7. Encyclopedia Britannica. 1975.

8. Evans, L.T. 1980. The natural history of crop yield. Amer. Scient. 68(4):388-397.

9. Gimpel, J. 1976. The Medieval Machine. The Industrial Revolution of the Middle Ages. Holt, Rinehart and Winston, N.Y. Pp. 274.

10. Hall, A. 1992. Ploughman's Progress. Farming Press, Ipswich, England. 147 pp.

11. Krasnov, A. 1987. (Title in Russian, unable to translate.)

12. Lawson, D. 1982. Hand to the Plough. Ashgrove Press, Sevenraks, Kent, England. Pp. 148.

13. Smith, B.D. 1995. The Emergence of Agriculture. W.H. Freeman and Co., N.Y. Pp. 231.

14. Steensberg, A. 1977. The husbandry of food production. Pp. 43-54. In The Early History of Agriuculture, Ed. Clark, J.G.G., E.J. Jope, and R. Riley. Oxford Univ. Press. Pp. 213.

15. Vince, J. 1983. Old Farms, An Illustrated Guide. Schocken Books, N.Y. 160 pp.