Fig. 1 -II shows the concentrations of soil aqua regia extractable Cd
under different treatments at RSES. Measurements on soils stored
since 1990 showed that the concentration of aqua regia extractable
Cd initially varied little among treatments. The extent of soil contamination was examined by comparing the concentrations of aqua regia
extractable Cd with the maximum allowable concentration (the MAC
value for Cd: 0.3 mg kg − 1). The aqua regia extractable Cd concentration in treatments other than ONPK and OM did not extend to the
MAC level. Aqua regia extractable Cd in topsoil of the OM and ONPK