dave said:Soils or medium with a high cation exchange capacity (abbreviated CEC on soil tests) means it can attract and hold onto large amounts of cations with the opposite charge. This translates to soil that can hold nutrients rather then leaching them right out.
Vermiculite has a good CEC value while perlite has almost zero. This means that generally speaking a potting soil mix with vermiculite will hold nutrients better than soil without it. The same is not true of perlite.
This almost implies that vermiculite is 'good' and perlite is 'bad'--
Really, the goodness or badness of either is multifactoral and dependent on the desired outcome(s)...sometimes it is desirable to hold cations and sometimes it really isn't.
Not all charged particles are 'nutrients'...and not all 'nutrients' are positively charged
Cation= positively charged particle (i.e. Na+)
Anion= negatively charged particle (i.e. Cl-)
Vermiculite does have a high CEC ~100-150 meq/100 g (incidentally, significantly greater than any of the soil types listed in this guide from Purdue)
for more than most people want to know...