Purple soil

It is developed in calcareous purple sand shale parent soil in subtropical area. The whole section is uniform purple or purplish red, and the layers are not obvious. Mainly distributed in subtropical areas of China, mainly in Sichuan basin. Purple soil is formed under frequent weathering and erosion, and its process is characterized by strong physical weathering, weak chemical weathering and lime leaching. Purple soil is shallow, generally less than 50 cm, and few people exceed 1 m, generally containing calcium carbonate, which is neutral or slightly alkaline. The content of organic matter is low and phosphorus and potassium are rich. Purple soil is one of the important dry farming soils in southern China, because it is loose, easy to disintegrate, rich in mineral nutrients and high in fertility. Except for the top of the mountain or steep slope, it has been reclaimed and planted. Due to soil erosion and drought and water shortage, terraces and reservoirs should be built to develop irrigation water sources. Opening up fertilizer sources and increasing the content of soil organic matter and nitrogen are also important measures to improve its productivity.

Purple soil is mainly distributed in hills and mountains where dark purple mudstone, shale and reddish purple glutenite are exposed in Cretaceous. The area accounts for 4.6% of the city's total soil area and 5.4% of the city's mountainous (arid) soil area. Due to the strong physical weathering of the parent rock, the vegetation on the purple soil is sparse, the soil erosion is very serious, and the soil-forming environment is very unstable, resulting in the soil development has been stuck in the young stage, and the whole section inherits the color of the parent rock, showing purple or reddish purple. The thickness of soil layer is greatly influenced by topography. Generally, the soil layer in the middle and upper part of the hillside is very thin, and the soil layer at the foot of the slope is slightly thick. According to the characteristics of parent material, urban purple soil can be divided into calcareous purple soil and acidic purple soil.

(1) calcareous purple soil subclass. Soil-forming parent materials are mainly weathered slopes and residues of Cretaceous purple sandstone and purple glutenite, which are mainly distributed in the low hills on both sides of the southwest valley and at the bottom of the basin, interspersed in the transition area from red soil subclass to yellow red soil subclass, accounting for 60.2% of purple soil classes. According to the type of parent material, this subclass is divided into two soil genera: purple sand soil and red purple sand soil. Purple sand soil is saturated with salt base, the whole profile is calcareous, the soil is slightly alkaline, and the pH value is 7.5 ~ 8.0. The base saturation of red-purple soil is lower than that of purple soil, and the clay content is higher than that of purple soil. Except for the calcareous reaction in the parent material layer, the upper soil has always been slightly acidic with a pH value of 5.6-6.5.

(2) Acid purple soil subclass. The parent material is the weathered slopes and residues of non-calcareous red-purple sand shale, which are mainly distributed in low mountains and hills. Due to the loose lithology of the parent rock, it is easy to be physically weathered and the soil erosion is serious. The soil layer is shallow, the pedogenesis is weak, and the texture is mostly clayey clay, containing gravel, loose and unstructured. It is acidic-slightly acidic.