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.
