① pH ≤ 5.5:Cd ≤ 0.30 mg/kg
② 5.5<pH ≤ 6.5:Cd ≤ 0.34 mg/kg
③ 6.5<pH ≤ 7.5:Cd ≤ 0.47 mg/kg
④ pH>7.5:Cd ≤ 0.55 mg/kg