When uncultivated land without erosion is cultivated

the 210Pbex in the depth of tillage

which decreases exponentially with depth

can be mixed in the plough layer. Subsequently

the 210Pbex of the plough layer would gradually diffuse into the plough pan layer owing to the concentration grads of 210Pbex between them

and finally a steady state would come forth as a result of balance between diffusion and decay of the 210Pbex. This process can be considered as one-dimensional diffusion from a thin diffusion source layer with unlimited width to semi-infinite media. This paper describes the process in a quantitative way. The time of the process from initial cultivation to the steady state is about 10 years (y=10.37 a). As in the case of cultivated purple soil of Neijiang in Sichuan province

the downward diffusion coefficient of soils of the plough pan layer is D=0.676 cm2·a-1

which is achieved according to the 210Pbex depth distribution of the soils. Subsequently

the 210Pbex inventory of the plough pan layer of different time after initial cultivation are calculated

and ultimately the process of 210Pbex distribution in the non-eroded purple soils after cultivation is figured out.