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For different systems, the sources of SOM are different.

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______ refer to the newly-produced and accumulated materials during soil formation, such as iron discs, iron, manganese and lime concretions, coating and saline spots. The new formations are very important characteristics to judge the nature, composition, and genesis of the soil.

The five natural factors of soil formation are: parent material, organism, ______ , relief and time.

According to their stacking characteristics and modes of transport, parent materials are divided into residual parent materials and ______ .

The soluble salts in the soil are transported to the lower layer or leached out of soil when the groundwater level is decreased through precipitation or anthropogenic irrigation, ditch drainage. This process is called desalination.