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The Medieval Arab Agricultural Revolution
The Arab conquests of the 7th and early-8th centuries led to the creation of a political entity which stretched from the Indus Valley to Iberia and parts of West Africa. In time political unity was followed by important changes in spheres such as religion, language, culture, science, technology, trade and agriculture. These changes survived the later slow political fragmentation of the region and contributed to the formation of a world which in many ways differed markedly from that of pre-Islamic times. Very important changes occurred in the agricultural sector at least until 1100 CE and in some areas over a longer period. Together they are sometimes referred to as the Arab Agricultural Revolution.

Agriculture evolved in three ways: (1) the technologies for the capture, channeling, lifting, storage and spreading of irrigation water, nearly all inherited from the ancient world, were diffused much more widely than in ancient times and used  more intensively, sometimes in new combinations;  (2)  crops, nearly all of which originated outside the Islamic world, only a few of which had already appeared in limited areas in the lands conquered by the Arabs, were carried throughout the Islamic world and most were grown wherever soil, water and climate would allow their cultivation;  and (3) the improvements in irrigation combined with the new crops made much land more productive, including areas which had not previously been cultivated, and increased markedly the productivity agricultural labour by raising yields, introducing crops of higher value, and  using  labour (and land) over longer periods of the year.

The history of irrigation in West Asia appears to go back at least as far as the history of agriculture, and over the millennia which followed many technologies were invented to control floods, store water, drain marshlands and irrigate cultivated lands. The diffusion of these, however, seems to have been spotty, so that in many parts of the ancient world they were  unknown or little used. Furthermore, the centuries leading up to the rise of Islam saw a marked decline in irrigation in almost all regions the Arabs were to conquer. This began in the western part of the Roman Empire as population dropped from c. 250 CE onwards, cities shrank, rural life became more insecure, and the money economy almost disappeared. With the catastrophic Plague of Justinian in the late sixth and seventh centuries decline spread to Byzantium, the Sasanian Empire and the Arabian peninsula; in some of these regions political decay and  various natural disasters further curtailed irrigated agriculture. After the collapse of the dam of Ma’rib in the late-6th century, none of the great ancient works of irrigation in the Hadramaut was in operation. In 626 CE a huge flood on the Tigris and Euphrates could not be contained and destroyed the important Nimrud dam, many other irrigation works, and much agricultural land.

After the Arab conquests of the 7th and early-8th centuries, and the great movements of peoples across the early Islamic world, the irrigation losses of the late ancient world were in large measure recovered and important advances were made. These were based mainly on known technologies which were diffused more widely, often into areas where they appear to have been unknown, and indeed over virtually all of the Islamic world. As landowners and farmers found a wider range of possibilities for irrigation available, they could choose those techniques which were best suited to local environments. Moreover, the greater choice of technologies allowed for new combinations which could intensify irrigation, extend its duration, and allow its spread into new regions, large and small. The lengthening of the period of irrigation through a greater part of the year was of special importance since, as will be seen below, many of the new crops introduced  had originated in tropical regions and required intensive watering through the hot, dry summer months which characterized the climate of most of the Islamic world. Of the new crops which were diffused in the early-Islamic world, Andrew Watson has been able to study seventeen. These include four crops which had already been introduced before the Arab conquests, namely rice, sugar cane, cotton and sorghum. Their pre-Islamic distribution appears to have been very limited: it was  confined entirely to the eastern part of the lands to be conquered by the Arabs, and within this large region it was limited to areas with warm winters, a constraint which seems to reflect the incomplete acclimatization of these plants to the cooler, non-tropical  climate of the region. This acclimatization was to come in the centuries following the Arab conquests. Twelve other crops appear to have been first introduced into the Islamic world after the Arab conquests, many of them from the Indus Valley where they had arrived some centuries earlier and undergone some acclimatization. These were sour (or Seville) oranges, lemons, limes, bananas, plantains, coconuts, watermelons, spinach, colocasia, eggplants and mangoes. And finally, one new crop appeared which seems not to have been known anywhere in the pre-Islamic world: this was  the artichoke, which was probably developed from the cardoon in Islamic times, possibly in the Maghrib. Though these 18 crops were of varying economic and social importance, they all were diffused very widely across the vast territories of the early-Islamic world, seemingly coming to be grown almost wherever their cultivation was allowed by soil, water and climate. The wider choice of crops allowed farmers to choose cropping patterns which were more flexible, better adapted to local conditions and more profitable.

The appearance of the new crops, many of them summer crops in Islamic lands, and the availability of irrigation water in many places over most of the year were responsible for a considerable intensification of agriculture. In most places in the ancient world rainfed farming yielded one crop every second winter; during the remainder of the two-year period, the land was fallowed. With the improvements in irrigation it became possible to produce four or more crops in a two-year period – indeed such results could be obtained in some areas which had never been cultivated or where farming had long been abandoned. Though such startling changes were not uncommon, it is probably true that in most places the gains were less spectacular – but still of considerable importance. These improvements in the productivity of both agricultural land and labour were connected in complex ways to many developments outside the agricultural sector. Most obviously, they were linked as both cause and effect to demographic increase, to a marked rise in  urbanization of the early-Islamic world, to the growth in both local and long-distance trade, and to the increasing monetization of both rural and urban economies. Although these connections are difficult to study in detail, their importance is evident. Had the Arab agricultural revolution not occurred, the early-Islamic world would have been a very different place.