* Historical Data of Rice * China has a long history of rice cultivation. It is recorded in ancient books such as Guanzi and Lu Jiaxin Yu that "five grains" were sown in the Shennong era in the 27th century BC, and rice was listed as one of the five grains. The record of "Jade Age is beneficial to ordinary rice, so it can be planted in low humidity" in Historical Records Xia Benji shows that in the 20th century BC, our people began to struggle with nature, dredge the "Nine Rivers" and develop rice in the "low humidity" zone. About 4200 years ago, rice cultivation has been extended from the middle and lower reaches of the Yangtze River to the middle reaches of the Yellow River. In the Warring States period, due to the application of iron farm tools and plows, intensive cultivation was started, and large-scale water conservancy projects were built to develop rice, such as Zhangshui Canal in Hebei (445-396 BC), Dujiangyan in Sichuan (256 BC) and Zheng Guoqu in Shaanxi (246 BC). Terraced fields first appeared in Sichuan in the Western Han Dynasty. In 1930s, Qi Yao Min Shu written by Jia Sixie in the Northern Wei Dynasty was devoted to the cultivation techniques of rice and upland rice. During the Jin Dynasty, Guang Zhi also recorded the development of green manure, the increase of organic fertilizer sources and the improvement of paddy soil fertility. It reflects that the rice planting technology at that time had reached a certain level. After Wei, Jin, Southern and Northern Dynasties, the economic center of China gradually moved southward. During the Tang and Song Dynasties, Jiangnan became the national rice production center, and Taihu Basin was the rice production base. The rice needed by Beijing's military and civilians depends on Jiangnan water transport. At that time, due to the emphasis on water conservancy construction, reclamation of rivers and lakes, improvement of farm tools, soil fertility, double cropping of rice and wheat and variety renewal, a relatively complete planting and cultivation system had been initially formed in the rice-growing areas in the south of the Yangtze River. Rice saved the cultivation of rice in China civilization. As a big agriculture in China, it rose around the Northern Song Dynasty. Since the Song Dynasty introduced high-quality varieties of Zhancheng rice from Vietnam, rice has really shouldered the heavy responsibility of carrying Chinese civilization. Rice is a crop with a hot climate and needs a lot of water resources, which is very suitable for planting in the middle and lower reaches of the Yangtze River and the Pearl River Delta. The yield of this crop is very high, and the seeds sown by farmers can usually harvest about 20 times of rice. This is obviously considerable compared with the yield of wheat which is four times that of seeds. Coupled with the warm and humid climate in the south of the Yangtze River basin, it can be planted for two or even three seasons a year, which means that rice can be planted several times more than wheat in the same area of land. This is a great boon to the people of ancient China who were constantly in turmoil due to the food crisis. More importantly, the land where rice is planted does not need to be fallow, and it can be cultivated continuously every year as long as nutrients are properly supplemented, and the yield will not decrease. Compared with planting wheat, it is necessary to lay fallow for one or two years, which greatly alleviates the contradiction between man and land. In addition, rice grows in tens of centimeters of water all the year round, which avoids the salinization problem caused by repeated irrigation in wheat fields, greatly prolongs the cycle of soil fertility aging, and invisibly prolongs the cycle of agricultural output decline. Since the Northern Song Dynasty, the economic center of China's ancient dynasty has officially moved from the Yellow River valley to the Yangtze River valley. With the decline of wheat agriculture, Chinese civilization has regained its vitality in rice agriculture, avoiding the fate of repeating the mistakes of ancient Egypt and the two rivers civilization. /kloc-the watershed 0/000 years ago If the Song Dynasty is regarded as the watershed of wheat economy and rice economy, we will find that after rice took over the agriculture in China, the change cycle of the unified dynasty in China was longer than in the past. From the establishment of the centralized unified dynasty by Qin Shihuang to the establishment of the Northern Song Dynasty, China experienced ten changes in Qin Dynasty, Western Han Dynasty, Eastern Han Dynasty, Three Kingdoms, Western Jin Dynasty, Eastern Jin Dynasty, Southern and Northern Dynasties, Sui Dynasty, Tang Dynasty, Five Dynasties and Ten Countries, which lasted more than 1 180 years, with an average of only 100 years in each dynasty. If the short-lived dynasties of the Northern and Southern Dynasties and the Five Dynasties and Ten Kingdoms are counted separately, this figure is even smaller. From the Northern Song Dynasty to the demise of the Qing Dynasty, there were five dynasties: the Northern Song Dynasty, the Southern Song Dynasty, the Yuan Dynasty, the Ming Dynasty and the Qing Dynasty, which lasted more than 950 years, with an average of nearly 200 years in each dynasty. It can be seen that dynasties changed very frequently before the Northern Song Dynasty, which had something to do with wheat agriculture in the Yellow River basin. After the Northern Song Dynasty, rice production in the Yangtze River basin was regarded as the foundation of the empire's survival, which obviously greatly improved the health of the empire. It is worth mentioning that among the five dynasties after the Northern Song Dynasty, the Yuan Dynasty existed for the shortest time, only 70 years, which was related to the national oppression policy of the Mongolian and Yuan rulers. On the other hand, it is probably because the rulers of the Yuan Dynasty came from grasslands, so they focused on the north and neglected the transfer to the economic center of the south. According to the historical population data, the population of China never exceeded 60 million before the Northern Song Dynasty, but it increased sharply after the Northern Song Dynasty, reaching 1 billion, and reached 400 million at the end of the Qing Dynasty. As the basis of population growth, the change of major food crops is undoubtedly decisive. The transformation from wheat to rice has also brought profound changes to China's political structure. For more than 3,000 years before the Northern Song Dynasty, China's population and economy were concentrated in the Yellow River Basin, and the social and economic prosperity in the Central Plains had considerable advantages over the nomadic people in the north. Therefore, although nomadic people invaded the south repeatedly, they were never able to go deep into the empire, never be able to gain a foothold in the Central Plains, and were quickly driven back to the northern grasslands. However, after the Northern Song Dynasty, because the economic center of gravity shifted to the Yangtze River basin, a large number of people in the Yellow River basin moved southward, which made the Central Plains empty and gave nomadic people the opportunity to enter the Central Plains. The feud between Northern Song Dynasty and Liao Dynasty, Southern Song Dynasty and Jin Dynasty is a portrayal of this situation. Subsequently, Mongolia and Manchu became the rulers of China. This never happened before the Northern Song Dynasty. We also missed the opportunity. In the history of the world, the crisis of one era can often be an opportunity to open the next era. People in ancient western Europe, like people in China before the Northern Song Dynasty, often faced the crisis of insufficient food supply. Since the decline of the Roman Empire, Europe has been divided into large and small nation states. Long-term food shortage has led to frequent civil strife and wars. They are not as lucky as we are, and there is no rice emergency, so they take various ways to solve the problem of survival, including vigorously developing industry and commerce and exchanging industrial products for distant food. Hunger led Europe to the world in a down-to-earth way and finally found the road to industrialization. By contrast, we seem lucky. When our civilization faced the crisis of survival, when our ancestors faced hunger, we successfully managed to cope with this crisis by using rice, which brought the historical development back to the original track. From then on, we still stay at home and continue our life, guarding an acre of land from generation to generation, because we believe that as long as we try our best to cultivate intensively, the plants on this land can bear fruits that feed us. Whether it is business or industry, it seems unnecessary or even idle. We always guard the most practical agriculture. As a result, the opportunity came in the barren wheat field and soon flowed away from the fertile rice ditch.

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