In the 3rd century BC, Gong Yu, Shan Hai Jing, Shilu Chunqiu and other ancient documents have recorded the types of citrus and grapefruit. Han Yanzhi's Annals of Citrus in the Southern Song Dynasty is still recognized by the international citrus circle as the first masterpiece in history. According to the research of modern scholars, most citrus varieties from China were introduced to Japan in the Tang and Song Dynasties. Hunan's natural conditions are suitable for the growth of citrus, and Daoxian Wild Orange, a marginal wild species of wide-skinned citrus, is produced in Hunan Province. At the same time, the history of planting citrus is very long. Citrus seeds were found in the cultural relics unearthed in Mawangdui, Changsha (AD 193), which is the oldest physical specimen at home and abroad. In 1970s after the founding of New China, the citrus production in Hunan Province has made great progress. According to the statistics of 1982, there are more than 10,000 mu of citrus orchards 165438+ in Hunan province, and the output is122,000 t, which is more than 200,000 t of t, 1984. Both the area and output are in the leading position in the country. Since 1980s, citrus has been further developed, orange orchards have mushroomed and the output has increased rapidly.

2. Relationship between citrus, pomelo and geological soil types.

China began to study the relationship between citrus and pomelo and geological soil types from the1980s. For example, the former Geophysical Exploration Team of Hunan Bureau of Geology and Mineral Resources and Team 4 18 respectively studied citrus planted in carbonate soil in Shaoyang City and Lianyuan City. Through the analysis of rocks, soil, fruits and other samples and the mathematical statistics of their data, they think that the most suitable area for planting citrus is the soil with sandy marl, siltstone and sandy shale as its parent rocks. Li of Chengdu Institute of Geology and Minerals studied the correlation between Sichuan agricultural geological background and citrus yield and quality. In this study, we not only found the restrictive effects of large elements (such as Ca and Mg) and trace elements (such as Zn and Cu) on citrus, but also found the special effects of Cl, Sr, R b and other elements on citrus. Bing Lin, Zhejiang Geological College, etc. In the "Zhejiang Agricultural Geological Research Report", it is concluded that there is Huangyan, the hometown of citrus in Zhejiang Province, where the citrus roots are developed, the branches and leaves are lush, the yield is high and the quality is good. It is mainly planted in the gully region on both sides of Chengjiang River and at the junction of mountainous area and plain, especially in the fluvo-aquic soil where the parent material of soil is mainly marine sediments, followed by flood alluvium. Pear-shaped Wendan pomelo in Yuhuan County was produced in the soil with glutenite, shale, siliceous mudstone and tuffaceous sand shale as its parent materials in the late Jurassic. According to the research of Tong et al. of Hunan Institute of Geology [1], the soil types of Cretaceous purple clastic rocks and Quaternary alluvial soil are the best. In recent years, Jiangxi Geological Survey has conducted agricultural geochemical research on the famous Xinfeng tangerine, and it is considered that "the soil with Cretaceous-Tertiary terrigenous clastic rocks (red beds) as soil-forming parent rocks is one of the main soil types for producing high-quality navel oranges" [2]. According to the geological and geochemical background investigation of high-quality citrus in Hubei by Chen Dexing of China Geo University, "Permian-Jurassic clastic rocks and Cretaceous purple clastic rocks should be selected as soil types for planting high-quality citrus" [3].