Since the dawn of agriculture, human farming has reduced the nutrient supply in many of the Earth’s soils. Nutrient depletion, especially of nitrogen (N) (which is the nutrient responsible for, inter alia, the deep green colour of leaves, as well as for the size and protein content of cereal grains, and without which plants cannot grow), limits plant growth and hence crop yields.1

The history of agriculture is a struggle against this fact. Before the growth of cities, nutrient depletion took place on only a modest scale, because most of what plants extracted from the soil returned to it after shorter or longer stays in animal and human alimentary canals and tissues. However, with cities, human societies began systematically to export nutrients from farming and grazing land. Some were returned, and all over the world human excrements have been collected and distributed to farmers as fertilizers. This practice is mentioned by Homer in The Odyssey (700 BC), and can be traced in Europe throughout the centuries until the introduction of the water closet around the year 1900. In Denmark, the State Railways (DSB) ran special wagons for transporting ‘night soil’ from Copenhagen to the surrounding agricultural areas up until the beginning of the 20th century.

A remedy for N deficits that was known already in ancient times and recommended by Classical agricultural authorities such as Theophrastrus, Varro, Cato, Columella, Palladius and Virgil (in his agricultural poem Georgics), was the growing of various plants from the pea family (legumes), especially vetch, lupins, broad beans, peas and lentils, whose associated Rhinzobium bacteria fix and convert atmospheric nitrogen (N2) into NOx, thus making it accessible to other plants which cannot make direct use of atmospheric nitrogen, of which there is an abundance (78% of the atmosphere, by volume).

In the Middle Ages, as cities grew more populous after the long standstill in the wake of the fall of the Classical world, the nitrogen supplement from peas, vetches and lupins became incapable of compensating for the losses. The agriculture of the 13th and 14th centuries in Mediaeval Europe, which had the enormous burden of feeding a large number of big cities such as London, Paris, Cologne, Milan, Florence and Naples, as well as Cordoba, Seville and Granada in densely populated Arab Spain, suffered from a lack of nutrients, especially nitrogen, and was entering a negative, self-destructive downward spiral with diminishing returns. The Black Death (1347–52), which halved the population of Europe, solved the problems for a while, simply by reducing the pressure on agriculture. The medical nature of the Black Death is well known, but the reasons for the plague’s unrivalled success have been eagerly discussed. The most likely explanation for the truly impressive death rates seems to be that the Black Death hit a population that had been weakened by protracted under- nourishment as a result of an agriculture undermined by increasing nitrogen deficits.2

In the 16th century the European population was recovering, and by 1700 three European cities—Paris, London and Istanbul—each had more than half a million inhabitants (McNeill, 2000, p. 282) and many other cities, including Moscow, Berlin, Stockholm and Copenhagen, were growing rapidly. The popu- lation of Copenhagen—the centre of the absolutist Dano-Norwegian–Schleswig- Holsteinian monarchy—was approaching 100 000 by the middle of the 18th century, and was the largest aggregation of people ever seen in this part of Europe (Kjærgaard, 1994, p. 62). The nitrogen problem was more acute than ever—it was a question of life or death.

The Rise of Clover 1000–1800

Despite its critical influence on the evolution of agricultural productivity and hence on the economy in general, the problem of nitrogen supply has received little attention from historians. Here I should like to discuss the problem with special reference to the so-called Agricultural Revolution, which broadly speak- ing began in Europe in the 17th century and was essentially a nitrogen revolution. Among the many European countries that experienced more or less identical problems, I have singled out Denmark for special investigation.

Clover (red)2153775
Peas63 715

Table 1. Nitrogen, phosphorus and potash in clover (red), peas and lupines (kg/ha). Note: an important supplementary quality of clover is its value as cattle feed. In this respect also it is much better than its competitors. Source: elaborated from Ilsøe (1975, p. 563).

Where manure was used, the annual nitrogen supplement received by ploughed land around 1700—some 500 000 hectares—was about 30 kg per hectare, corresponding to 10–15% of the average nitrogen application given to Danish arable land today.3 The grazing area, which was much larger (1.9 million hectares) than the ploughed lands, received no N supplement in the form of stable manure. Here the nitrogen supply was even lower, about 20 kg per annum per hectare derived from the atmosphere by free-living nitrogen-forming organisms and from nitrogen fixation through spontaneously growing legumes, including wild clover. The situation was unstable. Increasing demands for agricultural products from a growing and financially well-provided urban sector, which did not return all the nutrients imported from the countryside, meant that the size of the outlying grazing areas had to be continually increased, for instance by clearing forest land and inclusion of fragile areas unfit for agriculture, merely to keep the yield on the ploughed land constant. The disproportionality between grazing and cultivated (ploughed) areas became increasingly critical, and in addition hydrological and erosion problems appeared as the combined result of forest clearance and inconsiderate inclusion of vulnerable areas under agriculture. A full-scale ecological crisis, whose core was lack of nitrogen, was developing. However, Danish agriculture was saved, and so was European agriculture as a whole. The name of the saviour was domesticated clover, which by far exceeded the other nitrogen-fixing plants known from antiquity (Table 1).

In the second millennium clover was a new crop. Barley, wheat, oats and rye, together with the Classical nitrogen-assimilating crops, had been cultivated for thousands of years, ever since the beginnings of agriculture. Clover, which was found everywhere in Europe and in the Middle East, remained a wild plant and was not domesticated until much later, probably at the beginning of the second millennium in Moorish Andalusia, which had upheld the scientific tradition from antiquity, and which in every way was the most important centre of agricultural innovation during the Middle Ages (Bodlens, 1981).

From Moorish Spain, where a field of clover was described by the great Mediaeval scholar and botanist Albertus Magnus before 1270 in such a fashion that there can be no doubt that what he saw was domesticated clover, cultivated clover came to Christian Spain, a rapidly expanding power whose fortunes clover followed through Europe. By the middle of the 16th century we find clover in Lombardy, where it was enthusiastically praised by the agronomists Agostino Gallo (Vinti giornate della vera agricultura) and Camillo Tarello (Ricordod’agricultura), and in the Netherlands, both under Spanish rule. By 1583, domesticated clover had reached France, where it achieved a wide distribution during the 17th century. Around 1620, clover seed was exported from the Netherlands to England. With Sir Richard Weston’s famous book Discours of Husbandrie used in Brabant and Flanders, a description of the miraculous Flemish clover farming with which Sir Richard had become acquainted during a sojourn in Ghent, Bruges and Antwerp in 1644, a wave of enthusiasm arose for clover in the British Isles. At the close of the 17th century, cultivated clover had spread over most of England. Domesticated clover was documented in Germany from around 1645; before another hundred years had passed, it was known throughout the German-speaking countries.

In the year 1710 the new crop reached what was then the Danish island of Fehmarn on the northern coast of modern Germany. This first attempt to gain a foothold in Denmark was short-lived, but in 1730 clover was back in Fehmarn, this time for good. In the meantime, cultivated clover had reached the duchies of Schleswig and Holstein, which were under Danish rule. Within the borders of present-day Denmark the first certain evidence of domesticated clover dates from 1732, when clover was sown in Tisvilde in the northern part of Zealand, not far from Copenhagen (Figure 1). This first attempt was not successful, but within a few years clover made its appearance in other parts of Denmark, and it took a firm hold. At a rate without parallel before the 20th century, the new crop spread over the fields of Denmark from the end of the Denmark clover was spread to Sweden and Finland.

Figure 1. The expansion of cultivated clover in Europe, 1000–1750. Source: Kjærgaard (1994, p. 73); reproduced with permission from Cambridge University Press.

In the beginning, when clover was a new crop in Denmark, only small experimental areas were sown. The seed was expensive and bought in small portions that often had to be procured from afar. It took only a few years, however, to work up a local clover-seed production from a small portion of foundation stock. As clover rapidly proved to have an extremely high yielding capacity, greater boldness was displayed and the clover area was enlarged. Within a few years, small experimental areas no bigger than gardens had turned into huge expanses of clover. In 1779 the farmers of Nybølle in the parish of Hillerslev on the island of Funen received a prize of 20 rix-dollars for having sown no less than 100 tønder, i.e. roughly 50 hectares, with red clover seed. At the big main farm at Gjorslev Manor in south-east Zealand, scepticism about introducing clover prevailed for a long time. Finally, in May 1794, it was decided to give it a try, and 50 kg of red and white clover seed were bought on Funen. The result was apparently most satisfactory, for the very next year Gjorslev bought almost 750 kg of red clover seed, enough to sow 100 tønder. In the 1780s and the 1790s, clover fields covering 30–50% of the land area of a farm became a matter of course. In some cases the proportion was even higher.

A survey of the introduction of domesticated clover into the kingdom of Denmark (excluding Norway and the duchies of Schleswig and Holstein) between 1732 and 1805 can be followed on the four distribution maps (1775, 1785, 1795 and 1805, respectively Figures 2, 3, 4 and 5). These maps are based on investigations of all preserved 18th-century manor archives and a number of other sources; they give an immediate impression of the tremendous success of clover spreading from a few epicentres on the island of Funen over the whole country. The only place where virtually no clover was to be found at the beginning of the 19th century was in western Jutland. The reason was not lack of interest in clover, but a harsh climate and wet, acid soils. With the draining of the soils, the correction of the acidity by marling and the improvement of the microclimate by the planting of forests, clover also gradually reached western Jutland. This took place at the end of the 19th century.

The rise of domesticated clover from its first beginnings in southern Spain until the 19th century, by which time it had conquered Europe, changed agriculture. Some years ago the English historian C.P.H. Chorley estimated that the increase of production in European agriculture between 1750 and 1880 was around 175% (Chorley, 1981, p. 92). Of this enormous increase he estimated that as much as two-thirds could be attributed to the effect of the increased cultivation of nitrogen-fixing legumes, particularly clover. Chorley’s figures were based on very rough estimates drawn from a limited number of sources, and they looked rather fanciful when published. It seems, however, that detailed investigated cases such as the Danish one fully confirm Chorley’s bold estimates. On the island of Funen and adjacent islands, the amount of clover in the crop rotation was, in 1805, between 30 and 50%. The annual quantity of nitrogen added by clover on Funen and adjacent islands during these years was about 15 000 tonnes, corresponding to as much as 50 kg of nitrogen per hectare of arable land—in other words, more than double the average quantity added 50 years earlier, and more than enough to double agricultural production in a system where lack of nitrogen was the ultimate constraint.

Figure 2. Domesticated clover in Denmark, 1775. Source: Kjærgaard (1994, p. 77); reproduced with permission from Cambridge University Press.

Figure 3. Domesticated clover in Denmark, 1785. Source: Kjærgaard (1994, p. 78); reproduced with permission from Cambridge University Press.

Figure 4. Domesticated clover in Denmark, 1795. Source: Kjærgaard (1994, p. 79); reproduced with permission from Cambridge University Press.


Figure 5. Domesticated clover in Denmark, 1805. Source: Kjærgaard (1994, p. 80); reproduced with permission from Cambridge University Press.

Grain production in Denmark is estimated to have almost doubled in the last decades of the 18th century, from four or five million tønder to eight or nine million per annum. Moreover, since clover is an excellent and abundant cattle fodder, the effects of the nitrogen revolution were not limited to grain. Forage production increased dramatically, and the critical disproportion between grazing and cultivated areas was reversed. The trend was towards smaller, more efficient clover fields instead of endlessly expanding, low-yielding grazing areas which destroyed forest land and caused erosion. With clover it became possible to produce more on less land; the agricultural production system could be stabilized, so could the forest land, and the latent environmental catastrophe was avoided. The Danish cattle population is estimated to have increased by one- third during the period 1770–1805, from about 450 000 to about 600 000, and the production of butter and milk soared.

When historians discuss the reasons for the 18th-century development of agriculture, the nitrogen factor is often forgotten. In Denmark, discussions about the Agricultural Revolution tend to gravitate around enclosure, serfdom and the significance of the introduction of the owner-occupier system. However, the Danish Agricultural Revolution of 1750–1800 was, like its European counter- parts, essentially a clover-driven nitrogen revolution. To disregard the bio- technological nitrogen factor when discussing 18th-century agriculture is tantamount to disregarding coal when discussing the Industrial Revolution.

The Century of Clover: 1800–1900

If the 19th century was the century of coal, it was also the century of nitrogen-as- similating crops, mainly clover in most of Europe, in parts of the United States (US), and in New Zealand. In southern Europe and in some parts of the US, alfalfa (lucerne) and certain other crops such as sainfoin were also important. Nitrogen-assimilating crops provided food for cattle and nitrogen for plants, not only for cereals, but also for more demanding crops such as potatoes and beet. Indeed, the potato was totally dependent upon clover. We often hear about the expansion of potatoes in Europe in the 19th century, and we learn how the potato made it possible to feed the growing populations (Salaman, 1949). The nitrogen for the potatoes came from clover and other nitrogen-assimilating crops, with a little help from South American guano.4 Without the expansion of clover there would have been no potatoes for feeding the European people, and potatoes would have remained an exotic luxury crop, reserved for the upper class, as they were in the 17th century. The American agronomist N.L. Taylor has claimed, that “clover has had a greater influence on civilization than the potato” (Taylor, 1985, p. 4). Indeed, without clover there would have been no potatoes—and without clover the tripling of the European population between 1750 and 1900 (from 140 million to 401 million) would not have been possible. Around 1900, clover-generated nitrogen supplemented by guano supported one in two Europeans.

N.L. Taylor has not been the only one to praise clover as a plant which has changed the world. In 1761 Philip Ernst Lüders, founder of the Königliche Dänische Acker-Academie (Royal Danish Agricultural Academy) in Glücksburg in the duchy of Schleswig, declared that clover “would transform our barren, northern country into a new Canaan flowing with milk and honey” (Lüders, 1761, p. 22). And he proved to be right quite literally. Clover permitted an increase in the number of cattle—in other words it provided milk. It also gave honey as its expansion increased the biological niche of the honeybee. Honey production in northern Europe increased dramatically during the last decades of the 18th century and reached an unprecedented level in the 19th. Clover made the Old World, which had always been so sour, a lot sweeter5 been unjustly overlooked, because the rise of clover honey in the Old World coincided with the rise of sugar from the New World.6

Honey was only one of many indirect effects of clover. Another one was the transformation of the landscape. The white and the red clover fields gave new colours, just as they gave new smells. In June, when clover blossomed, the countryside was transformed into a wonderful flower garden. The dreamy, romantic 19th-century landscape with its red, white and green fields, humming bees and endless herds of cattle was created by clover. In Denmark, the new idyllic landscape was eagerly adopted by writers (not least the two most famous and influential of them all, the fairy-tale writer Hans Christian Andersen and the philosopher Søren Kierkegaard), painters and music composers, and thus be- came an indelible feature of the country’s national identity. Even the disease pattern was influenced by the introduction of clover. Malaria, which was a prominent disease in Europe in the ecological system prevailing prior to the period of major changes, was chased away by clover in northern Europe. The most common malaria mosquito here, Anopheles atroparvus, is zoophilic, prefer- ring cattle and other domestic animals to humans. The green revolution, which allowed a drastic rise in the number of domestic animals, improved the zoophilic mosquito’s chances of finding animals to bite. This proved fatal to malaria, because cattle do not develop the disease, and its life cycle was severed. In the 1770s, malaria was still one of the most common diseases in northern Europe, but in the 1860s it was practically eradicated, thanks to the green nitrogen revolution. Only in southern Europe, where there are anthropophilic mosquitoes, was malaria able to survive into the 20th century, until it was eradicated with chloroquine and other synthetic anti-malarial drugs (Kjærgaard, 2000, pp. 19–20).

The Fall of Clover

In the 1930s the fields of Europe were blossoming with clover. Few will have imagined that the days of clover were numbered—certainly not the prominent members of a Danish ‘national committee’ who in 1936 solemnly declared domesticated red clover to be a ‘national plant’. The decisive year was 1909, when the German chemist Fritz Haber worked out how to extract nitrogen from the air through ammonia synthesis (McNeill, 2000, pp. 24–25). In 1913, Haber and Carl Bosch, another chemist, developed the so-called Haber–Bosch method, a process for mass production of nitrates directly from the air without using clover (or other nitrogen-fixing plants) as converters. In the 1920s and 1930s the Haber–Bosch method was used on only a limited scale—roughly corresponding to the application of guano in its heyday in the 19th century. However, after the Second World War the industrial production of nitrogen expanded dramatically. Worldwide production of an insignificant four million tonnes of artificial fertilizers (nitrogen, phosphorus and potassium put together) per year in 1940 had become 40 million tonnes in 1965 and, by 1990, 150 million tonnes, a develop- ment which has made clover disappear from the fields of the globe. The consequences for post-Second World War landscape and post-Second World War War society have been two-fold. The landscape has become dominated by monoculture and has lost its sublime romantic charm. As far as world society is concerned, it has become dependent on artificially produced nitrogen—roughly two billion, i.e. one-third of all the people on the globe, live on artificially produced nitrogen, extracted by the high-energy Haber–Bosch method. And clover, having changed the world, has retired to biological farms, where it survives, awaiting the day when the enormous energy costs connected with the Haber–Bosch method are no longer acceptable, due to the many indirect effects of energy production on climate and environment, or, quite simply, impossible, due to the end of the oil age, i.e. the end of cheap and abundant energy. If and when this day comes, domesticated clover, the greatest triumph of Arab agricultural science, will be ready to serve the world once again.


The author wishes to express his gratitude to David Hohnen for translating this paper.


  1. This and the following paragraph draw heavily on McNeill (2000, pp. 22–23); on the importance of nitrogen for plant growth, see Smil (1990, p. 423).
  2. This statement is based on ongoing personal research. The only discussion of the difficult nitrogen question in Mediaeval agriculture is in Loomis (1978). Loomis demonstrates the increasingly destructive lack of nitrogen in the 13th and 14th centuries; he does not, however, make the explicit connection between the Black Death and nitrogen deficits.
  3. Kjærgaard (1994, p. 22). Much of the following is based on this book, to which no further references will be given.
  4. From 1840, when oceanic transport became cheap enough, Chilean and Peruvian guano, which contains 7% nitrogen, 14% phosphorus and 2% potash, was shipped to Europe and North America. The peak was reached in the years 1848–75 when more than 20 million tonnes of South American guano was spread on the soils of Europe and North America. From 1880 the importance of guano declined (Madsen-Mygdal, 1937–38, volume I, p. 491).
  5. Honeybees and clover like to travel together, and clover does not travel at all without the honeybee, as the New Zealand case so convincingly demonstrates. There were no bees in the original fauna of New Zealand, and as clover is self-sterile the plants cannot propagate without bees. The result was that in the early 19th century clover growing failed repeatedly in New Zealand, although the first generation of the plant always grew beautifully. Finally, the missing link, honeybees, was remembered and introduced on the remote islands. Immediately New Zealand got a huge production of clover—and of honey—and the country became, as Lüders would have put it, a new Canaan flowing with milk and honey (Crosby, 1986, pp. 239–240).
  6. The average consumption of refined sugar and syrup per inhabitant increased 15-fold in Denmark between 1745 and 1798, from 1/6 of a pound to 2 pound (Sveistrup & Willerslev, 1945, p. 364). Similar figures apply to the rest of Europe.


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