The essential feature of soil is rock weathering.
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Soil fertility is soil productivity, which refers to the ability of soil to produce plant products.
Soil is an important environmental factor, which is the buffer zone and filter of environmental pollutants.
为了避免陷入虚拟变量陷阱，如果一个定性变量有m类，则要引入m个虚拟变量。（ ）
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