It's ok. I got the answer (in a permaculture group):

1 centimole = 1/100 of molar mass of the element or compound.
So, for each element and compound 0.02 c.mole/kg is a different value.

Aluminium's molar mass is 26.9815 g/mol, therefore 0.02 c.mole/kg equals 5 mg/kg
while 8280 mg/kg equals 30.7 c.mole/kg