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Asia’s soils are related mainly to climate and vegetation. In some areas the origins of soils, perhaps from volcanic action or from materials carried by streams, may be more important. Such volcanic or alluvial soils are especially fertile.
The tundra soils of the northernmost part of the continent are acidic and infertile. Many of these areas are underlain by permanently frozen subsoils that never thaw during the brief summers. Subsoils of this type, known as permafrost, cover a very large area in the northern part of Siberia.
South of the tundra, the soils of the taiga are also acidic and relatively infertile. Somewhat less acidic and more fertile soils are found in the mixed forests and the broadleaf forests farther south.
Prairie and black chernozem soils are south of the forests. Because these soils developed where there is limited precipitation, their desirable minerals have not been absorbed or washed away, a process known as leaching. These soils are among the most fertile in the world. The best farmland of Russian Asia occurs largely on black soils and on the more inferior soils of the mixed and broadleaf forests.
The unleached soils of the semiarid and arid areas of the continent are often fertile, except where they are too saturated with salts or alkaline minerals. The availability of water for irrigation largely determines their use. Continued irrigation, however, may increase the concentration of salts or alkaline minerals and make the growing of crops impossible.
The soils of the rainy tropics are generally infertile. High precipitation and high temperatures cause most of the valuable minerals to be leached from the soil. Less leaching occurs in the rainy-and-dry tropics and the humid subtropics.
Many of the red and yellow soils of the humid subtropical area of China have been improved by thousands of years of care, which has included the use of compost, or rotted plant refuse. In some semiarid regions of China, however, natural vegetation with deep roots—which kept the soils in place—was cleared for food crops that lacked sufficient root systems and caused the topsoil to become terribly eroded.