Soil and national orchid distribution north line

The North China Plain covers an area of about 300000 kn2 and is a long-term subsidence area. It needs a lot of sediment filling, which is a good place for the confluence of the big river. Fortunately, there are many loess terraces on the edge of the plain, which are easy to be eroded and destroyed by rivers, such as the Yellow River, Huaihe River, Haihe River, Luanhe River, Dawen River, Dinghe River, Hutuo River and Wei River. Only the Yellow River carries 13-1.6 billion tons of sediment from the Loess Plateau every year to fill this depression. Sandstorms also played a role. In ancient North China, there were records of one inch to one foot of loess day and night.

The crest of the Yellow River fan-shaped plain is near Zhengzhou, and the Yellow River is also diverted near Zhengzhou, that is, the Yellow River becomes a swinging river below Longmen. It has been recorded in history that the Yellow River has been diverted 26 times, and 12 flooded roads (such as Zeng Zhaoxuan's map of the lower reaches of the Yellow River) have generally silted up the fan-shaped plain. Ancient lakes and marshes were silted and famous cities were silted up. For example, Julu County in Hebei Province was buried in more than one deep underground in 1108. The silting height of the fan-shaped plain forces the Huaihe River to move to the south and the Haihe River to the north. Most of the sediment carried by the Yellow River contains calcium and high fertility, there is a saying that "dung and dung, grow my millet", but drought, waterlogging and alkali damage are prominent. The Great Plain lacks acidic soil, and Guolan can not survive, which is in line with Wu Yingxiang's argument. (Wang Tao)