Analysis of the Relationship Between the Northern Limit of Chinese Orchid Distribution and Soil

The North China Plain covers about 300,000 sq km and is a long-term subsidence area. It requires large amounts of sediment to fill it, which makes it an ideal destination for the confluence of great rivers. Fortunately, there are many loess terraces along the edges of the plain, with loose soil that is easily eroded and collapsed by river water, and the water of the Yellow River, Huai River, Hai River, Luan River, Dawen River, Ding River, Hutuo River, Wei River and others washes through and converges here. The Yellow River alone carries 1.3-1.6 billion tons of sediment each year from the Loess Plateau into this depression. Sandstorms also played a certain role; in ancient times North China had records of one cun to one chi of loess falling in a single day and night. The apex of the Yellow River fan-shaped plain is near Zhengzhou, and the Yellow River's course changes are also near Zhengzhou, that is, below Longmen the Yellow River becomes a shifting river. In recorded history, there were 26 major course changes of the Yellow River and 12 flood channels (such as Zeng Zhaoxuan's map of lower Yellow River changes), which silted up the fan-shaped plain generally one time over. Ancient lakes and marshes were filled flat, and famous cities were buried by silt, such as the county seat of Julu in Hebei Province, which was buried more than one zhang deep underground in 1108. The raising of the fan-shaped plain by silt forced the Huai River to move south and the Hai River to flow north. The sediment carried by the Yellow River is mostly calcareous, with high fertility, and there is a saying that "it both irrigates and manures, growing my grain and millet," but drought, waterlogging and alkali damage are prominent. The great plain lacks acid soil, so Chinese orchids cannot survive, which conforms to Wu Yingxiang's argument. (Wang Tao)